Parallel processing in visual perception and memory: What goes where and when?

Parallel Processing in Visual Perception and Memory: What Goes Where

and When?

MARIA L. BERG AND JAMES G. MAY University o f New Orleans

This article begins with reviews of parallel processing models in the areas of visual perception and memory, pointing out kinds of information purported to be processed in each, and the overlap in the physiological substrates involved. Next, some pertinent literature having to do with the linkage between perception and memory is reviewed (e.g., visual memory for what or where), concluding that there exists a serious lack of research and knowledge of how different perceptual processes may lead to facilitated, distorted or impaired memory in different forms of storage. Some possible scenarios are presented concerning how perceptual information might be interfaced with memorial mechanisms, and some working hypotheses are considered. Finally, a new paradigm is outlined that examines the linkage between local and global perceptual processing and explicit and implicit learning. This paradigm combines the global precedence paradigm of Navon (1977; 1981) and the sequence learning paradigm of Nissen and Bullemer (1987). Convincing arguments indicate that global stimuli are mediated more quickly via one perceptual stream (the M-cell pathway), but can be processed more slowly by another (the P-cell system). Local aspects of the stimuli are exclusively mediated by the P-cell system. The results of two experiments employing iterations of stimulus sequence, in which sequence learning is possible and measur- able in terms of reaction time changes over trials are presented. The second experiment indicates that information thought to be mediated by the M-cell pathway results in incidental sequential learning, while other information thought to be mediated by the P-cell pathway does not. Spatial filtering of the visual stimuli reveals that low spatial frequencies are necessary for sequence learning to occur. The issue of whether this learning is implicit or explicit is also discussed. Ideas for future research, exploring this new area of interest, are proposed. Current knowledge of perceptual and memorial deficits in special populations are considered in an attempt to identify new areas of investigation.

. . . men combine together several ideas appre-

hended by diverse senses or by the same sense at

different times or in different circumstances, but

observed, however, to have some connexion in Na-

ture either with respect to co-existence or succes-

sion; all which they refer to one name and con-

sider as one thing.

George Berke ley , 1733

Current Psychology: Developmental �9 Learning ~ Personality ~ Social Fall 1997AVinter 1998, Vol. 16, Nos. 3/4, 247-283.

248 Current Psychology / Fall 1997/Winter 1998

It is evident from Berkeley's statement that the notion of parallel processing of information, perceived through the senses and stored in memory, is not a recent concept. Many of today's scholars concerned with how we acquire or store information feel they have more substantial knowledge of the mechanisms involved because of the increased volume and rigor of scientific investigation. But these strides have come with the cost of greater specialization and limitations on the scope of the problems studied. In Berkeley's time, to understand how we acquire and use knowledge involved an appreciation of sensation, perception, attention, emotions, motivation and memory. The extensive literature on each of these topics has forced modem psycholo- gists to specialize, and knowledge regarding these areas has tended to be segregated into domains of inquiry wherein information and conceptualizations are often commu- nicated only within the bounds of the area. Thus, vision scientists, in the main, have been concerned with which physiological processes mediate different aspects of visual scenes, while memory researchers have dealt more with the types and forms of storage than with what is actually stored. Those concerned with perception rarely ask where or how that information is retained for later use, and those interested in memory have not characterized the content of what is stored in terms bearing a straightforward relationship to early processing constraints. The present investigation was motivated by the notion that a knowledge of how information is packaged at the input might lead to how information may be differentially stored in memory. It was also influenced by three other notions recurrent in perception and memory. The first has to do with the fact that parallel processing of information is thought to occur at both stages; the second is the correspondence between the physiological substrates of two processing streams in the visual system and putative memorial processes; and the third concerns the idea that information is processed unconsciously as well as consciously. In an effort to fully elaborate the rationale for the experiments performed to date, evidence bearing on each of these notions is detailed below.

PARALLEL PROCESSING IN VISION

During the last three decades physiologists and psychophysicists have been exploring the properties and pathways of distinct but parallel channels of information processing in the visual system. In physiology, this distinction begins as early as the retina, wherein the macula contains mechanisms that mediate high levels of visual acuity, and the periphery, which has poor acuity but provides information about the position of objects in the environment and their movement. Another distinction can be seen in the ganglion cells projecting from the retina. Observations as early as the mid- 1800s noted that the optic nerves split into two pathways, the geniculostriate and geniculotectal pathways. Recording of single cell activity in the optic nerve and tract in the cat led to differentiation of different types of ganglion cells labeled by their distinct reactions to visual stimuli (Enroth-Cugell and Robson, 1966). One classification of these neurons involved X- and Y-cells. X-cells had two concentric, antagonistic receptive field areas and were described as either excitatory or inhibitory in the center with the surround mediating the opposite response. These cells were said to sum the

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inputs received from earlier retinal cells in a linear fashion. Y-cells, on the other hand, have more complex receptive field properties. They have larger receptive fields and thicker axons that conduct action potentials significantly faster than those of X-cells. Y-cells respond transiently, but with shorter latencies, to stimulation, responding mostly to the onset and offset of a stimulus, while X-cells react more slowly with a sustained response to the presence of a stimulus. Y-cells also respond more to low spatial frequencies and quickly moving stimuli, where X-cells respond to higher spatial frequencies and slowly moving or static stimuli (Enroth-Cugell and Robson, 1966; Cleland, Dubin and Levick, 1971). Cleland, Dubin, and Levick (1971) argued that the terms sustained cells and transient cells were more descriptive terms for the two types of ganglion cells and also demonstrated that the properties of these cells were present in the cells of the lateral geniculate nucleus (LGN) and the visual cortex of the cat as well. Ganglion cells with similar properties have been found in the visual system of the macaque monkey (Lennie, 1980). The M-cells of the macaque, which have similar characteristics to the Y-cells in the cat, project from the retina to the magnocellular layers of the LGN and the P-cells, which show similar characteristics to the X-cells in the cat, project to the parvocellular layers of the LGN. These findings of such distinct properties of the cells in the visual system have led researchers to the belief that the two types of ganglion cells have very different functions in visual processing. The M- cells, having larger receptive fields, larger axons and responding transiently to stimuli, are thought to be responsible for orientation to novel stimuli throughout the visual field, while P-cells are responsible for the identification of the details of the stimuli that have been attended to and fixated upon. These two cell types have also been differentially described anatomically. The M-cells project from the retina directly to the superior colliculus and indirectly via the lateral geniculate nucleus to the visual cortex. The P-cells project only through the lateral geniculate nucleus to the visual cortex. These two processing streams remain segregated through several cortical stages and are believed to serve different visual abilities. The P-cells are thought to mediate color, texture, and form perception, while the M-cells are used to process motion and spatial position. Depth information has been suggested to derive from an interaction of these two streams (DeYoe and Van Essen, 1988).

Psychophysical investigations in human subjects have been influenced by the physiological distinctions observed in cat and nonhuman primates. The general strategy has been to manipulate stimuli so that optimal conditions are achieved for exciting various classes of cells defined physiologically in monkeys. For example, Tolhurst (1975) measured RTs to sinusoidal gratings of high or low spatial frequencies when contrast was close to threshold. He found that subjects' RTs to low spatial frequencies grouped at either the onset or offset of the stimulus, but for high spatial frequencies their RTs were distributed throughout the time that the stimulus was present. This demonstrated the transient response to low spatial frequencies and the sustained response to high spatial frequencies. Harwerth and Levi (1978) also looked at RTs to gratings over a range of spatial frequency and contrast values. They found that RTs decreased with an increase in contrast; however, they also observed that over a range of spatial frequencies the relationship between RT and contrast curves appeared biphasic. They sug-


gested that the change in shape of the curve reflected the change in detection by the transient and then sustained channels. Such psychophysical research has shown that transient channels as opposed to sustained channels respond preferably to low spatial frequencies, are more sensitive to flicker and abrupt stimulus onset (Breitmeyer and Julez, 1975), and respond with a shorter latency (Breitmeyer, 1975). Sustained channels respond preferably to high spatial frequencies and respond in a sustained fashion to stimulation.

It has been suggested that the processing of temporal and pattern information is accomplished by two separate but interactive subsystems in the visual system with different spatiotemporal response characteristics (Tolhurst, 1973). Cleland et al.'s findings that the cell types remained differentiated at cortical levels led to what some have termed the sustained and transient channel theory of human visual perception (Breitmeyer and Ganz, 1976; and Weisstein, Ozog and Szoc, 1975; and Livingstone and Hubel, 1988), which postulates that there are channels mediated by the P and M cells in the human visual system that are responsible for different kinds of visual processing. These theoretical papers emphasize the notion of parallel processing and interactions between parallel mechanisms to explain numerous and diverse perceptual phenomena. These theories have led to additional lines of investigation implementing these mechanisms in our understanding of: visual masking (Breitmeyer, 1980); spatial location (Solman and May, 1990); form perception and attentional abilities (Treisman, Cavanaugh, Fischer, Ramachandran, and von der Heydt, 1990); real (Livingston and Hubel, 1988) and apparent (Breitmeyer, May, and Williams, 1988) motion; figure- ground organization (Livingston and Hubel, 1988); saccadic suppression (Burr, Morrone and Ross, 1994; Uchikawa and Sato, 1995) and visual persistance (May, Martin, MacCanna and Lovegrove, 1988). The picture that emerges from this considerable literature is a visual system composed of at least two parallel streams. One stream serves as a global "early warning system," conveying information about the spatial position and movement of objects throughout the visual field, and provides a perception of the temporal relationships inherent in dynamic visual scenes. This system is thought to respond reflexively and prior to directed attention (pre-attentively) to objects and spatial locations. The other stream, concerned with more local processing in the center of the visual field, is involved in the identification, discrimination, and analysis of objects. It is guided by the prior input of the global mechanism which is said to have temporal precedence.

In summary, visual perceptual mechanisms are presently characterised by two physiologically based parallel processing streams, each mediating different, yet well-defined, physical attributes of visual scenes. As we will see below, compared to parallel processing models in memory, the strength of visual perceptual processing theories derives from knowledge of what kinds of information are mediated by each parallel tract. This distinction has been more difficult to draw from the memory literature.

PARALLEL PROCESSING IN MEMORY

The idea that there are different types of learning and memory has been argued since

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Tolman presented opposition to the behaviorists' stimulus-response learning (Tolman, 1932). Once the idea of different types of learning was established, then the existence of multiple memory systems came up for debate. Since the first of these debates, which argued the separation of short-term and long-term memory, researchers have been attempting to identify how many systems make up human memory, how these systems function and how they interact. In a very short time, multiple forms of memory has become the major theme of memory research. Though many different memory systems have been proposed, ideas of what constitutes a memory system is still open for discussion. Three criteria presented by Schacter and Tulving (1994) are: an intact memory system enables the performance of a particular class or category of tasks regardless of the specific informational contents of the task; the memory system can be described in terms of its features and its relation to other systems; and the tasks for which it enables performance must differentiate it from other systems. Based on these criteria Schacter and Tulving identify five memory systems in current memory literature: procedural memory; perceptual representation; semantic memory; primary or working memory; and episodic memory. As with most of the memory systems literature, the criteria proposed by Schacter and Tulving concentrate on how information is processed into memory and not on what information is processed into memory. The definitions of the five major memory systems also reflect this tendency.

Procedural memory is thought to be the system that is involved in automatic, unconscious processes of skill learning and performance. It is characterized by gradual learning over time and knowledge that is expressed through performance rather than recollection (Squire, 1994). Procedural learning encompasses motor skills, perceptual skills, cognitive skills, habit formation, classical conditioning and priming and has been referred to in the literature as nondeclarative memory, which in some cases is also called implicit memory.

The four other memory systems are thought to be cognitive processing systems (Schacter and Tulving, 1994). Working memory can only hold information for a short period of time. Rehearsal and active processing must occur for information to pass from working memory to long-term memory. Most people understand working or short-term memory as processing information before it reaches long-term memory. Therefore short-term and long-term memory would be temporally separate. However, if one also thinks of short-term or working memory as the memory system that holds information that is brought from long-term memory, used, worked with, reprocessed and then stored again, one must recognize that these two memory systems would have to be working in parallel.

The perceptual-representation system (PRS) is a memory system having to do with domain specific subsystems that process the form and structure of words and objects separate from the meaning and associative properties of these stimuli (Schacter, 1994). The properties of this system have been observed through priming, which is the facilitation of performance, on a number of specific tasks, by a singular, previous encounter with a particular stimulus. This process is nonconscious and occurs independently of recall or recognition (Cave and Squire, 1992; Shimamura, 1986; Tulving and Schacter, 1990). The PRS is thought to be divided into three subgroups, which include visual


word form, structural description, and auditory word form. Further study has shown hemispheric differences in visual word form processes. Marsolek, Kosslyn, and Squire (1992) found that both within-modality and case-specific visual priming for words is greater when stimuli are first presented to the right hemisphere. However, this is not the case for tests of explicit memory. Marsolek et al. suggest that this represents at least two separate systems that encode the visual representations that produce priming. One system, which is more effective in the right hemisphere is better at processing form-specific information, and one not as effective in the right hemisphere does not distinguish among distinct instances of word forms.

The episodic and semantic memory systems have to do with the type of information that is processed into long-term memory. Episodic memory is defined as comprising memory for specific events and depends upon temporal and/or spatial contextual cues for retrieval (Tulving, 1972; Jacoby and Witherspoon, 1982; Butters, Salmon, and Heindel, 1994). Semantic memory stores general knowledge such as dates and definitions. Interestingly, Jacoby and Witherspoon (1982) compared the episodic-semantic theory of memory to the dissociation of perceptual identification performance and recognition memory. They presented the idea that recognition could depend on a memory trace of a single presentation of a stimulus, but enhanced perceptual identification reflects the "priming" of a more abstract semantic representation. They also proposed that it is this priming of semantic information that makes learning without awareness possible.

The study of humans with amnesia has led to yet another distinction in memory m the implicit/explicit distinction. Implicit memory is defined as an unconscious, passive process that results in knowledge that is abstract, powerful, and unavailable to conscious awareness (Reber, 1967; Reber and Alien, 1978). Explicit memory involves conscious or intentional recognition of previously studied information. Though implicit and explicit memory are often referred to as separate systems of memory, it is yet unknown whether they depend on the same underlying memory system or different underlying systems (Schacter and Tulving, 1994). Researchers have found that amnesic patients exhibit impaired performance on explicit tasks while their performance on implicit tasks remains relatively unimpaired (Graf and Schacter, 1985; Graf, Shimamura, and Squire, 1985; Jacoby and Witherspoon, 1982; and Schacter, Church, and Treadwell, 1994), Impaired explicit but relatively spared implicit memory has also been shown in patients with Alzheimer's disease and people with Huntington's disease (Butters, Salmon, and Heindel, 1994), and in people with major depression (Danion, Willard- Schroeder, Zimmermann, Grange, Schleinger, and Singer, 1991; Roediger and McDermott, 1992). These findings argue for separate underlying mechanisms of two types of memory, one that is automatic and one that is intentional.

Throughout the multiple memory systems literature, researchers have presented the idea of dichotomous memory systems such as; episodic and semantic memory (Tulving, 1972), and declarative and nondeclarative memory (Squire and Zola-Morgan, 1988). Though these memory systems have been shown to act separately (spared abilities in amnesics), they do not seem to be mutually exclusive or contradictory to each other, as the definition of a dichotomy would assume. On the contrary, most researchers present

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the idea that, though there could possibly be many memory systems and subsystems, these systems act in parallel to process information efficientlylsuch that one could be consciously learning a list of words while unconsciously making associations between those words. One could also be consciously learning to respond to stimuli while unconsciously learning a sequence in the presentation of those stimuli.

Though many systems and subsystems have been identified to explain different memorial processes, the main separation between different systems seems to be if the processes occurred with or without conscious awareness. The terms implicit or explicit, declarative or nondeclarative and conscious or nonconscious have all been used as descriptive terms for this main separation of memory systems. One can thus group the aforementioned memory systems as implicit (procedural memory, perceptual representation, and perhaps, most of the time the processes of short-term memory) and explicit (episodic and semantic memory). Grouping memory systems by the conscious or nonconscious processes seems to map well with physiological findings.

PHYSIOLOGICAL SUBSTRATES

The separation of the magnocellular and parvocellular pathways in the visual system does not end at the LGN. Livingstone and Hubel (1988) presented extensive physiological research that shows separate pathways of visual information up through the higher visual areas of the cortex. The parvocellular cells project to layer 4X~ of the striate cortex, then to layers 2 and 3 and from there to visual areas 2 and 4 (Zeki, 1980; Van Essen, 1985; De Yoe & Van Essen, 1988). The cells from the magnocellular geniculate layers project to layer 4Ctx of the primary visual (striate) cortex (V1), then to layer 4B, which then projects to visual area 2 (V2), visual area 3 (V3), and to cortical mid-temporal area (MT). There are also projections from V3 to V4 and MT. Neurons in V4 project to both the infereotemporal cortical areas and to the parietal cortical areas, but neurons in MT project only parietally. Livingstone and Hubel suggest that, perhaps as early as the output stage of V1, there is also a subdivision of the parvocellular system that may have some integration with cells from the magnocellular system. These researchers describe the magno system as being responsible for movement perception, depth cues and linking of stimuli, such as figure/ground discrimination and the parvo system as responsible for shape discrimination, color differentiation, and orientation discrimination.

Animal models of amnesia and other lesion studies have provided physiological evidence for different types of memory. Animal researchers have proposed that there are two memory systems--one that is facilitated by the hippocampus and surrounding structures, and one that does not rely on these areas. In the past twenty years, researchers have identified the hippocampal formation along with the entorhinal cortex, per- irhinal cortex and parahippocampal cortex and also the medial thalamus, as the structures that, when damaged, cause severe memory deficits. These deficits, however, are now thought to occur mostly in one type of memory, explicit or conscious memory. The discovery that these structures were involved began with an attempt to replicate, in monkeys, the brain damage that had occurred in a severely amnesic patient (patient


H. M.). Monkeys whose amygdalae alone were removed were slow to learn the association between a visual stimulus and a reward, but with the removal of the amygdala and the hippocampus the monkeys could not perform the task at all. Further study showed that the removal of the amygdala and the hippocampus had created a form of global amnesia in the animals (Mishkin and Appenzeller, 1987). With such amnesic monkeys, it has been shown that there is spared learning of visual-pattern discrimination presented with repeated trials, though the animals fall completely with trial- unique objects (Mishkin, 1954; Zola-Morgan, Squire, and Mishkin, 1982; Mishkin, Malamut and Bachevalier, 1984). However, further study showed that it was not the combination of the hippocampus and the amygdala that was causing the amnesia but the combination of the hippocampal lesion and damage to the cortical areas surrounding the amygdala (Squire, 1992; Zola-Morgan and Squire, 1990; Zola-Morgan, Squire, and Amaral, 1989).

Monkeys with lesions of the hippocampus and surrounding cortical structures (H+ lesions) are impaired on such tasks as simple object discriminations and concurrent object discriminations, which require several object pairs to be learned together (Zola- Morgan and Squire, 1990). They are also impaired on delayed nonmatching to sample (Mishkin, 1978; Zola-Morgan and Squire, 1985). Monkeys with H+ lesions can succeed, however, on certain skill-based or habit-based tasks such as pattern discrimination and the 24 hr. concurrent discrimination task (Malamut, Saunders, and Mishkin, 1984; Mishkin, Malamut and Bachevalier, 1984; Zola-Morgan and Squire, 1984). In this way the physiology of the spared as well as impaired memory in amnesiacs can be confirmed using animal models.

Other evidence for the distinction of a hippocampal based memory system and a nonhippocampal based memory system came from the double dissociation of the caudate nucleus and the fornix for maze performance of rats. Packard, Hirsh, and White (1989) found that lesions of the fornix impaired performance on a win-shift task in a radial maze where lesions of the caudate actually improved performance on this task. However, on a win-stay task the opposite occurred. Lesions of the caudate impaired performance on this task, where lesions of the fornix did not.

Exploring which areas are affected in such diseases as Alzheimer's and Huntington's have led researchers to postulate which areas are involved in implicit learning. Neo- cortical areas that are damaged in Alzheimer's disease, such as association cortex may be critical for word priming and neostriatal areas, which are damaged in Huntington's disease such as the basal ganglia may be critical for perceptual-motor skill learning (Butters, Salmon and Heindel, 1994). Relative to caudate and posterior neocortical damage, volume loss in the structures of the mesial temporal lobe was found to be related to poor recognition memory. Damage to the caudate affected perceptual / lexical processing of words (Ostergaard and Jernigan, 1993). This evidence indicates that there may be many brain areas involved in implicit learning and memory.

Positron Emission Tomography (PET) studies have also revealed physiological evidence for the conscious and nonconscious distinction in memory. Findings show significant reduction in cerebral blood flow in the right extrastriate cortex during word- stem completion priming (Squire, Ojemann, Miezin, Petersen, Videen and Raichle,

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1992). This same region was found to be activated by the visual features of words, nonwords, nonsense letter strings, and letter-like shapes (Petersen, Fox, Posner, Mintun, and Raichle, 1988; Petersen, Fox, Snyder, and Raichle, 1990). This area operates prior to the analysis of meaning and prior to the involvement of the hippocampal complex that is responsible for explicit/declarative memory (Squire, 1994; Squire and Zola- Morgan, 1991).

NEUROPSYCHOLOGICAL EVIDENCE

As previously mentioned, the study of preserved memorial abilities in patients with organic amnesia has resulted in evidence supporting at least two memory systems, one that involves conscious processes and one that involves nonconscious processes. How- ever, this evidence seems to be based on experiments using many different types of tasks to explore which tasks amnesiacs can perform (relative to normals) and which they cannot. If an amnesiac can perform a task, it is said to be a task that employs nonconscious processes. If an amnesiac cannot perform a task it is said to employ conscious processes. However, close examination of the literature reveals that there are specific differences between the tasks that amnesiacs can perform comparable to normals and those in which they are deficient.

Amnesiacs have demonstrated normal learning on such tasks as rotary pursuit (Brooks and Baddeley 1976; Cermak, Lewis, Butters, and Goodglass, 1973), visual maze learning (Brooks and Baddeley, 1976), reading mirror inverted words (Cohen and Squire, 1980; Moscovitch, 1982) puzzle solutions (Brooks and Baddeley, 1976; Cohen, 1984) and sequence learning (Nissen and Bullemer, 1987). Each of these tasks are not specific to the memory for a specific object but involve the learning of a spatial and/or temporal relationship. Tasks at which amnesiacs perform poorly, such as recognition and recall, are tasks that involve memory for specific stimuli or memory for "what" has been previously encountered regardless of where and when it was encountered. These deficits are perhaps better understood by looking at neuropsychological evidence that categorizes certain perceptual deficits.

Neuropsychological evidence has been found for the separation of the "what" and "where" pathways. Humans with parietal lobe damage, or damage to the dorsal visual stream, have difficulty on a variety of spatial tasks, such as localizing points in space (Holmes, 1919; Cole, Schutta, Warrington, 1962; Ratcliff and Davies-Jones, 1972), determining the depths of objects (Danta, Hilton and O'Boyle, 1978; Benton and Hecaen, 1970; Cartoon and Bechtoldt, 1969), recognizing the orientation of lines (Benton, Hannay and Varney, 1975; Meerwaldt and Van Harskamp, 1982), compre- hending geometric relations (Franco and Sperry, 1977), detecting and processing motion (Hess, Baker and Zihl, 1989; Vaina, LeMay, Bienfang, Choi, and Nakayama, 1990), and rotating objects mentally (Ditunno and Mann, 1990; Ratcliff, 1979). Pa- tients with parietal lobe damage do not, however, have difficulty identifying objects.

Individuals with right temporal lobe damage seem to have difficulty with spatial memory (Pigott and Milner, 1994). This damage affects one's ability to detect changes in a scene when the locations of objects are switched, to detect when an object's


position has been moved laterally and when an object has been removed from a scene. It also disrupts the ability to remember complex spatial patterns, such as nonsense designs (Jones-Gotman, 1986).

The functioning of the "what" pathway has been revealed through the study of patients with agnosia. There are two types of agnosia that have to do with deficits in visual recognition--apperceptive agnosia and associative agnosia. Patients with apperceptive agnosia usually have damage to the occipital lobe (Farah, 1990) and have little or no ability to discriminate between objects or to copy or match simple shapes. They also seem only to perceive extremely small or local aspects of contour (Benson and Greenberg, 1969). Patients with associative agnosia retain the ability to group local aspects into a perceptual whole; however, if asked to draw the same image that they have perceived from memory, the patient with apperceptive agnosia would be unable to do so. This difficulty is not considered a general memory deficit because the patient is able to provide definitions of what objects are (Ratcliff and Newcombe, 1982). The lesions involved in this disorder are generally located in the occipitotemporal region.

Differences in the processing of global and local information have also been demonstrated in patients with prosopagnosia. Prosopagnosia is the inability to recognize faces. Studies have shown that prosopagnosics have a deficit in forming new facial recognition but have normal preferences for target faces (Greve and Bauer, 1990; DeHaan, Bauer and Greve, 1992). Because the patient could recognize all of the stimuli as faces, along with other global attributes such as age, sex, and emotional expression (Tranel, Damasio, and Damasio, 1988), but could not identify to whom the face belonged, the experimenters concluded that at least some of the results of perceptual processing are available to the patient and can form the basis for certain nonconscious discriminations (Greve and Bauer, 1990). Patients with prosopagnosia demonstrate a severe deficit in processing local detail though they seem to demonstrate some spared global processing. Prosopagnosia has been related to damage to the occipital lobes (Greve and Bauer, 1990) and more specifically to the posterior sector of the right hemisphere (Geffen, Bradshaw and Wallace, 1971; Yin, 1970).

Differences for processing of global and local information have also been demonstrated by patients with brain damage who are asked to make reaction time responses to hierarchical stimuli, which are stimuli made up of small items arranged in a particular spatial array to form a recognizable larger item. The principle of global precedence will be discussed further in a later section of this article. Neuropsychological tests involving this paradigm have revealed that lesions of the left hemisphere disrupt the ability to perceive local but not global aspects of an item, where right-hemisphere lesions disrupt the ability to perceive global but not local aspects of a stimulus (Delis, Robertson, and Efron, 1986). These lesions are usually found in the temporal region, part of the ventral visual-processing system, which is consistent with the idea that this area is specialized for object recognition (Robertson, Lamb and Knight, 1988).

These findings show strong parallels between the areas of visual processing and memory. The finding that damage to cortical areas surrounding the amygdala disrupts object discrimination relates well to the fact that it is this area that receives projections from the parvo pathway. The disruption of tasks requiring memory for spatial position

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and temporal order seen with dorsal stream damage is commensurate with M-cell input of such information to that region.

L I N K I N G P R O P O S I T I O N S

Early notions of the transfer of information from perceptual stages to memory involved the concept of a short-term sensory storage (STSS - Sperling, 1960) stage wherein "visual or auditory snap shots" were held for brief periods (< 1 sec.) prior to, or while, attended aspects of the scene were transferred to a short-term memory stage (STM - Peterson and Peterson, 1959), where it was estimated to be accessible for less than 1 min. when active rehearsal was prevented. The properties of STSS were origi- nally investigated with stimuli that could be processed as verbal material (Sperling, 1960), but a later series of studies on visual persistence employed nonlinguistic measures (Meyer and Maguire, 1977; May, Brown Scott and Donlon, 1990). These studies found that the duration of persistence was determined by the spatial frequency content of visual stimuli.

Investigations of STM have used, almost exclusively, verbal material, and little attention has been given to the spatial frequency content of stimuli presented visually. Consolidation and long-term storage of information in STM is thought to be facilitated by rehearsal, and chunking of items as they are transferred into long-term memory (LTM) stores. This conceptualization of the perceptual-memorial linkage had appeal because of its generality, but the lack of specificity has led to much confusion as to how certain types of visual information are transferred, and descriptions of the various forms of LTM reviewed above are based on cognitive models, as opposed to visual models of memory content.

Mishkin and co-workers (for review see, Mishkin and Appenzeller, 1987) have made the greatest contribution to our understanding of how perceptually defined aspects of a situation are stored in memory. They propose two memory systems in the primate brain concerned with encoding objects (the what) and their spatial position (the where). The perception of these two stimulus attributes appear to depend on the integrity of inferior temporal and the parietal cortex, respectively. With wall-designed behavioral experiments they have determined which areas of the "what" pathway serve perceptual processing (object discrimination) and which areas support the memory for objects (object recognition). This is an important distinction for the definitions of perception and memory. Object discrimination involves immediate differentiation of object properties, while object recognition denotes storage and retrieval of associations between objects. The "where" pathway is thought to store information about the spatial position of objects. Limbic system structures appear to be involved with retrieving the association of objects with their locations, and other sensory representations of those objects.

The areas involved in the "where" pathway are also known to be required for other perceptual abilities. One of those areas, MT, has been shown to be necessary for the perception of global dot motion, the ability to discern direction of motion without form (Newsome, Britten, and Movshon, 1989). Since motion information can be conceptu-


alized as a sequence of more than one "where," it might be the case that the "where" pathway processes information concerning position in time as well as space. Thus, the "where" pathway may also discern the sequential order of events and the sequential position of objects over time. Johansson (1979) has argued that much of perception involves detecting changes in the quality, quantity, or position of objects over a particular time period. He points out that the temporal boundary for the perception of object motion exists on the same continuum, but at a higher level, with the perception of an object in different spatial positions at different times. At high rates of perceptual change, moving objects may not be observed, but their motion path may define a more global figure. At slower rates, the objects are visible, but their sequential positions are difficult to retain. He argues that the traditional icon model of visual memory, which treats visual scenes as a series of static "snap shots" stored in memory over time, should be replaced by a perceptual vector analysis that accounts best for the integration of the displacement of related elements. However, it may be the case that both types of memory exist in parallel: memory for the sequence of icons and memory for the motion of related elements.

Most vision involves the detection and identification of objects as well as the tracking of moving objects, and/or the tracking of the change in position of objects relative to the moving observer. All of these abilities appear to be continuous despite frequent interruptions of the visual scene imposed by blinks and eye movements. Research and theory regarding two phenomena, the attentional blink (AB -Shapiro, Driver, Ward and Sorensen, 1997; Chun and Potter, 1995; Maki and Padmanabhan, 1994; Raymond, Shapiro, and Arnell 1992, 1995; Shapiro, Raymond and Arnell. 1994; Weichselgarmer and Sperling, 1987; Broadbent and Broadbent, 1987) and repetition blindness (RB - Kanwisher, 1987; Hochhaus and Johnston, 1996; Bjork and Murray, 1977; Egeth and Santee, 1981; Kanwisher, 1991; Kanwisher, Driver, and Machado, 1995; Luo and Caramazza, 1996; Mozer, 1989; Chun and Cavanaugh, 1997)suggests that perception of events leads to the formation of two representations of objects---object types and object tokens (Kanwisher, 1987; Kanwisher and Driver, 1992). Object types are specific instances of more abstract object token categories. Thus, perceiving an object (e.g., a) leads to the formation of a type (a) and a token (A), and this token becomes linked to other instances of the type providing spatiotemporal information (e.g., spatial position, sequence, direction of motion, etc.) about the objects in a dynamic scene. An attentional blink occurs when attention is captured by a target that is detected among a series of distractors. If another target is presented less than 500 msec. after the presentation of the detected target, the probability of its detection is significantly lower. Attentional blinks are observed with orthographically dissimilar targets (e.g., when x and o are targets), and the effect appears to depend solely on the proximity of events in a sequence (Raymond et al. 1992; Shapiro et al. 1994). Repetition blindness refers to the finding that similar results occur with repeated targets that are the same (e.g, a and a). Detection for the second target is significantly less compared to detection of dissimilar targets if the second target occurs within about 500 msec of the first. It has been explained in terms of the failure to form two object tokens for each type, suggest-

Berg and May 259

ing that event perception depends on token formation. Recent experiments (Shapiro, Driver, Ward and Sorensen, 1997) combined these two paradigms to determine whether a target (T2), rendered unreportable through an attentional blink (produced by T1), would influence detection of a repetition of T2 (T3). Their findings indicated that T3 detectability was decreased, relative to nonrepeated targets (indicating RB), when T2 escaped the attentional blink (was detected), but increased (indicating priming) when T2 was not detected. Since T2 and T3 were orthographically dissimilar (A and a respectively), this study suggests that RB and priming occur at different processing stages. If T2 is detected, an object token is formed and T3 is linked to that token as if T2 and T3 are a single occurrence of the actual stimulus represented by the token. RB represents a failure at the token formation stage. Priming has been shown to depend on type formation, and the fact that it occurs when T2 is not detected implies that type formation can occur in the absence of token formation. RB depends on the conscious perception of T2, but priming does not.

In another recent study of RB (Chun and Cavanaugh, 1997), targets were presented in the center of two separate apparent motion streams moving in opposite directions. Targets could be repeated in adjacent spatial positions within or between motion streams. Significantly more RB was found within a motion stream, than between motion streams. The authors suggest that motion streams can serve as global objects, despite the change in local object properties within the stream.

It is interesting to think about which of the visual pathways might mediate these various effects. Since the attentional blink does not seem to depend on object properties, it could be due to reflexive attentional mechanisms thought to be mediated by the "where" pathway. Thus, the reduction of T2 visibility may stem from an interruption in sequential processing. Since RB and priming do depend on the formation of types and tokens that are related to object properties, these effects might occur in the "what" pathway. Thus, a failure in sequential processing can occur in either pathway because of AB (loss of T2) or RB (loss of T3). Facilitation of sequential processing can occur in either pathway because of priming in the "what" pathway or segregation into motion streams in the "where" pathway.

One early controversy about different types of information processing centered around automaticity and intentional learning. Hasher and Zacks (1979) argued that certain fundamental aspects of the flow of information (spatial position, temporal order, and frequency-of-occurrence) are processed automatically, without intention, and do not benefit from practice. They suggested that these processes are genetically "prepared," are evident early in life and change little during development. These processes were contrasted with those that are "effortfur' and require control of a limited capacity attentional mechanism. These operations involve rehearsal and mnemonic activities that give rise to improvements with practice and development. Hasher and Zacks cited experimental evidence to show that memory for spatial position, temporal relationships, and frequency-of-occurrence does not differ under intentional, as opposed to incidental testing conditions. In refutation of the notion of automaticity, Naveh-Ben- jamin (1987; 1988) presented conflicting data that indicated significantly different recall and recognition for spatial position under intentional conditions, age and prac-


rice. If the proposal that learning of spatial and temporal relations is possible both implicitly and explicitly is true, it is not surprising that intentional conditions may augment, to some extent, the considerable learning that occurs automatically. The proposal put forth by Hasher and Zack is consistent with the notion of "what" and "where" pathways and suggests that the "where" pathway may involve more implicit or automatic processing of spatial and temporal information.

PERCEPTUAL-MEMORIAL SCENARIOS

The evidence for parallel processing in both visual perception and memory suggests that it may be the case that different kinds of information extracted at perceptual stages might be stored in different kinds of memory, and the consideration of physiological substrates for these putative memory systems supports such a notion. Mishkin's "what" pathway appears to feed information about objects and their properties into the hippocampal memory system, which is thought to support declarative memory. The "where" pathway appears to feed information about spatial position, motion, and habit formation into a memory system involving the striatum, but this information might eventu- ally be integrated with information about object properties in the hippocampus. It is not clear, however, whether these two perceptual-memorial streams differ with regard to consciousness during information processing. Perceptual studies suggest the "where" pathway may involve preattentive, unconscious, and automatic processing of information preceding directed attentional (cognitive) processing in the "what" pathway, but some memory studies support the contention that implicit learning may influence the declarative system as well (Berry and Broadbent, 1984; and Broadbent, FitzGerald, and Broadbent, 1986). It is also not clear whether the sequence of perceptual events is stored in both pathways or there is segmentation of sequence information in one or the other pathway.

Suppose a task involves introduction of various objects presented sequentially in different spatial positions. One could ask subjects to recall the order in which positions were filled without reference to what filled them. One could ask for the sequence of objects without reference to where they occurred. Or one could ask where objects were presented and in what order. One scenario might be that the sequence for position of perceptual events is retained in the "where" stream, while information about the sequence of objects is retained in the "what" pathway. If this is the case, the order of spatial position would depend on the "where" pathway, and the sequence of objects would depend on the "what" pathway. Knowledge of what went where and when would require both pathways. Another scenario might be that the "where" pathway records spatial position and order (temporal) information, but the "what" pathway only records which objects are presented. The sequence of spatial position would require only the "where" pathway, but questions about object sequence, and what went where and when, would require both pathways.

Berg and May 261

SEQUENTIAL PATTERN ACQUISITION

One task that has been used to study implicit learning involves the repetition of a sequence of stimuli to which differential responses are required. A simple example of such a task involves pressing one of four keys in reaction to stimuli presented in one of four locations in a display (4 choice reaction time). If the sequence of stimulus positions is predictable over a block of trials (e.g., n=8), mean reaction time (RT) over a succession of blocks might be expected to decline with repeated exposure to the sequential pattern. Since considerable warm-up effects are observed in choice RT even with totally random sequences, the decline in RT alone cannot be taken as evidence for sequence learning per se. If, however, totally random blocks are presented after extensive exposure to iterative blocks, increases in mean RT for those blocks, relative to RT for immediately preceding iterative blocks, can be taken as evidence of learning the sequence. The question of whether this learning occurs explicitly or implicitly is addressed by interrogating subjects as to the existence of a pattern. If, after exposure to the sequence, subjects can verbally report the pattern, an explicit knowledge of the sequence is indicated. Evidence of sequence learning from RT measures in the absence of evidence of sequence learning from verbal report has been taken as an indication of implicit learning.

Sequence learning has been noted as possibly the best paradigm for studying implicit learning processes--not only because it is fast paced, leaving subjects little time to consider explicit learning strategies, but also because the task demands are low so that it is not necessary for subjects to use the sequence information to perform the task, and the structure within the study material is of low salience (Cleeremans, 1993). Since Nissen and Bullemer's first study using a sequential learning paradigm to explore implicit learning, many studies have been done using this paradigm to explore how people learn a sequence of stimuli (Cohen, Ivry, and Keele, 1990; Willingham, Nissen and Bullemer, 1989; Lewicki, Hill and Bizot, 1988; Lewicki, Czyzewska, and Hoffman, 1987).

Some take issue with the tests of conscious knowledge of the sequence employed when using the sequential learning paradigm (Perruchet and Amorim, 1992; Perruchet and Gallego, 1993). Perruchet and Amorim (1992) showed a close parallelism of conscious knowledge of fragments of a repeating sequence and performance (which had not been shown previously) using tests of free recall and recognition. However, other researchers have argued that the tests used by Perruchet and Amorim were flawed (Cohen and Curran, 1993; Willingham, Greeley, and Bardone, 1993) in that they were not clearly free of possible implicit contributions.

One problem that makes the investigation of conscious and/or unconscious learning difficult is that of individual differences. Feldman, Kerr, and Streissguth (1995), using the same paradigm of Nissen and Bullemer (1987), found that declarative but not procedural learning scores correlated with scores on the WAIS-R full-scale IQ test. This correlation between explicit knowledge and IQ was also found previously by Reber, Walkenfeld, and Hernstadt (1991) using series-completion problem solving (an implicit task) and artificial grammar learning (an explicit task). This finding may

262 Current Psychology / Fall 1997~Winter 1998

explain the problem researchers are having defining whether a certain task is explicitly or implicitly learned in normal subjects. However, the question of whether learning is implicit or explicit becomes more complicated with the discovery of these individual differences.

Other studies using the sequential learning paradigm have explored the role of attention. The question is: if something is learned without conscious awareness, does this mean subjects do not need to pay attention to it? Nissen and Bullemer (1987) demonstrated that subjects' performance did not improve over trials when a memory- intensive secondary task was performed concurrently. It has also been shown that sequence learning under attentional distraction interacts with sequence complexity (Cohen, Ivry, and Keele, 1990, Frensch, Buchner, and Lin, 1994). Thus, attention is needed to learn even moderately complex sequence information, even though the learning of this information may not require awareness.

EXPERIMENT 1

In an effort to determine whether sequence learning was different when subjects performed discrimination tasks that might be performed with either the "what" or "where" pathways, orientation discrimination was performed with sine-wave gratings of high or low spatial frequency. Since, at the early stages of visual processing, the inputs to the "where" pathway are more reactive to low spatial frequencies, discrimination of such stimuli might be assumed to activate that pathway more. Since the "where" pathway is less sensitive to high spatial frequencies, orientation discrimination using high spatial frequencies might be expected to involve the "what" pathway to a greater extent. A version of the Nissen and Bullemer (1987) paradigm was used in which iteration of the same sequence was repeated for a number of trials and then a new randomization was introduced. To address the question of whether sequence acquisition in the two pathways might differ in terms of the memory system employed, measures of explicit learning were obtained after completion of the task. In an effort to determine the time course of implicit and explicit learning, either 32 or 62 trial blocks were used for each spatial frequency level.

METHODS

Subjects

Forty university students and members of the community served as subjects. Twenty- four were female and sixteen were male. All were naive as to the hypothesis under investigation. Subjects ranged in age from 18 to 32. All had normal or corrected to normal visual acuity (20/20). Only one subject was left-handed.

Stimuli and Apparatus

Stimulus presentation, reaction time recording and analysis of responses were car- ded out with a microprocessor (High Tech, Model 486). The stimuli were constructed

Berg and May 263

with an image processing system (Data Translation, Model DT 2861) and were dis- played on a high resolution monitor (Sony, Model PVM 1343 MD) at a rate of one per 750 ms. The stimuli consisted of orthogonal oblique gratings of either high (10.9 c/ deg) or low spatial frequency (SF) (0.65c/deg). These gratings were either oriented 45 ~ or 315 ~ from vertical. The Michaelson contrast of the stimuli was 64 percent for both the low and high frequency stimuli. The stimuli subtended visual angles of 5.8 ~ vertically by 7.4 ~ horizontally. Subjects sat 230 cm. from the computer monitor.

The Stimulus Conditions

The four stimuli were used to create four different experimental conditions: low Spatial Frequency (SF), short; low SF, long; high SF, short; and high SF, long. Subjects were randomly assigned to one of the four groups. Each group participated in only one of the experimental conditions. Subjects first performed a practice session that consisted of nine trial blocks of a random sequence of stimuli. Subjects were randomly assigned to respond to the orientation of either the high spatial frequency gratings or the low spatial frequency gratings, and this assignment was also used when the experimental task was performed. The experimental task was a choice reaction time (RT) task in which subjects indicated the orientation of the gratings (left arrow key = 315 ~ or right arrow key = 45~ RTs were recorded to the nearest 1.0 ms. Two of the groups participated in "short" conditions; the other two in "long" conditions. The short conditions consisted of 32 sets of 8 stimulus presentations. The last 2 sets of 8 stimuli were in a random sequence, while the other 30 sets of 8 stimuli were a repeated sequence representing each of the stimuli four times in each set. The long conditions consisted of 62 sets of 8 stimulus presentations, also with a repeated sequence of 8 throughout, except for the last 2 sets. The same repeating sequence was used for all of the experimental conditions. At the end of the session the subjects were asked if they had noticed a pattern. If subjects responded that they had, they were asked what the pattem was and their responses were recorded by the experimenter. If subjects indicated no knowledge of a pattern, they were told that there had been a repeating sequence of 8 stimuli during the task and asked if they could tell the experimenter what that sequence was, and again responses were recorded by the experimenter. This acted as a "probe" to determine whether subjects had explicitly learned the trial sequence.

RESULTS

Means of the reaction times in successive blocks of 16 were obtained for each subject, yielding 16 and 31 data points for the short and long conditions, respectively. These data were submitted to two separate mixed analysis of variance designs (SF condition with trial blocks as repeated measures), which revealed only a significant main effect for trial blocks for the short trial block data (F[15,270] = 2.23; p<0.006). It was apparent that large individual differences existed in terms of RT. In an effort to reduce between subject variance, the data were normalized by expressing the data in trial blocks 2 through 15 or 31 as percentages of the mean for the first trial block.

264 Current Psychology I Fall 1997/Winter 1998

These data are depicted in Figure 1 for the low and high frequency conditions (open circles = iterative blocks, closed square -- new randomization, vertical lines = + 1.0 SE). The data for the iterative trials were submitted to two mixed analyses of variance designs (SF condition with trial blocks as repeated measures) for each trial block condition (short and long). No significant effects were uncovered in either analysis, providing little evidence for sequential learning. However, similar analyses for the last iterative block and the final block revealed significant main effects for trial blocks for both the short (F[1,18] = 19.35; pc 0.0003) and long (F[1,18] = 11.06; p<0.003) conditions, and this may be taken as strong evidence for sequence learning. The lack of significance for the SF condition main effects and the lack of significant interactions in these analyses provides no support for differential sequential learning for the SF conditions.

The probe data revealed that some explicit knowledge of stimulus sequence occurred. These results are summarized in Table 1. More subjects in the short conditions (n=8) than in the long condition (n=3) reported detecting no sequence, while the number of subject detecting no sequence was more similar for the high (n=6) and low SF (n=6) conditions. The number of subjects reporting the correct sequence was similar for the short (n=6) and long (n=7) trial blocks length, and the low (n=7) and high (n=6) SF conditions.

In an attempt to examine the relationship between the RT and probe measures of sequential learning, the percent differences between the last two trials for each subject were submitted to a mixed analysis of variance design (explicit learning by length by SF condition) which revealed significant main effects for SF condition (F[1,32] = 5.77; p'0.02), explicit learning (F[1,32] = 17.43; p<0.0002) and a significant two-way interaction between SF condition and explicit learning (F[1,32]; p<0.005). The significant interaction is depicted in Figure 2. Tests subsequent to analysis of variance revealed that RT indices of sequential learning were: significantly greater within the high SF group for subjects who had acquired explicit knowledge of the sequence (p<0.0001); and also significantly greater for these same subjects in the high SF group as compared to the low SF group, even if the latter had acquired explicit knowledge (p<O.O01).

DISCUSSION

Although sequence learning was not evidenced by a systematic decline in RT over iterative trials, the fact that it occurred can be inferred from the significant increase in RT from the last iterative trial block to the final trial block. Such learning occurred with both SF conditions and with both short and long trial conditions. The fact that the SF manipulation did not result in differences in sequence learning could imply that such learning is mediated with equal efficiency by the "what" and "where" pathways, or it could indicate that the particular paradigm did not produce differential processing by the two pathways. It could be that the choice of SF levels was not sufficient to create differential processing by the two pathways. The failure to find increased differences in explicit learning with the SF manipulation can be interpreted in similar

Berg and May 265

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TABLE 1 Subjects' Attempts to Identify Sequence, Experiment I

Low SF, Short High SF, Short

Subject # Response Subject # Response

1. RLRLRRLL* 1. RLRLRRLL* 2. 2. LRLRLRRR 3. 3. 4. 4. 5. 5. 6. 6. 7. 7. 8. 8. 9. 9.

10. 10.

RLRLRRLL* RRLLRR RLRLRRLL*

RLRLRRLL* RRLRRLRLL

RRLRLLR RLRLRRLL* RLRLRRLL*

LRLRRRL

Low SF, Long High SF, Short

Subject # Response Subject # Response

1. RLRLRRLL* 1. 2. RLRLRRLR 2. RLRRRLLL 3. RRLRRLR 3. RLRLRRLL* 4. RLRLRRLL* 4. RLRLRRLL* 5. RLRLRRRL 5. LRLRLLRR 6. RRRLRRLLL 6. RLRLRR 7. LRLR 7. LLRRLRR 8. RLRLRRLL* 8. RLRLRRLL* 9. 9.

10. RLRR 10. LRLRRRLL

Notes: Subjects' attempts to identify the sequence used in Experiment 1 for each experimental group. The correct sequence was RLRLRRLL. Asterisks indicate correct responses.

fashion. Either it implies that implicit and explicit learning occurs in both pathways or the stimulus manipulation was not sufficient to produce differential processing. There is some indication that the latter case is true. If differential processing occurred it should be the case that RT to the low SF stimuli was significantly lower than RT for the high SF conditions. This main effect was not significant in the initial analysis of the unnormalized RT data. Another possibility is that the task could be performed using either pathway and subjects differed with regard to which pathway was employed. Some subjects might have used local differences in orientation and contrast (object recognition), while others might have employed a strategy involving where the stripes were pointing (spatial location), regardless of the SF condition.

Another possibility is that the sequence learning that occurred in the present study could have been motor learning as opposed to the learning of stimulus sequence. That is, subjects could have been learning a series of motor responses or they could have been learning the sequence for the stimuli that elicited such motor responses. Since the

Berg and May 267

FIGURE 2 RT Differences for High and Low Spatial Frequency Groups

Notes: RT differences as a function of whether probes indicated that subjects had explicitly learned the sequence or not for both the high and low spatial frequency groups from Experi- ment 1. Vertical error bars represent + 1.0 SE.

stimulus manipulation (low and high SF) did not effect sequence learning, the possibility that these results are due to motor learning cannot be ruled out.

Explicit knowledge of sequence did not lead to significantly larger RT indices of sequence learning in the low SF conditions, but it did result in significantly increased RT differences in the high SF conditions. If the "where" pathway was more involved in processing the sequence of the low SF stimuli, explicit knowledge did not facilitate


RT measures of sequence learning. This finding is reminiscent of the contention by Hasher and Zacks (1979) that memory for spatial position does not benefit from intentional instruction. If the "what" pathway was employed in processing the sequence of the high SF stimuli, explicit knowledge did facilitate RT measures of sequence learning. This finding is reminiscent of the findings of Naveh-Benjamin (1987; 1988), that memory for spatial position can be improved with explicit knowledge.

The fact that RT or verbal report evidence for sequence learning did not differ in the short and long trial block conditions provided little information about the time course of implicit and explicit learning. Individual differences in these types of learning may have existed. The fact that 14 of the 40 subjects exhibited explicit knowledge of sequence raises another concern with the current paradigm. The orientation discrimination task required very little concentration or attention, and this might have allowed considerable cognitive resources to be allocated to monitoring the incidental aspects of the task. A more complex task involving greater attentional demands might have led to less of an ability to learn the sequence explicitly. In an effort to design a paradigm that would address these concerns, more traditionally cognitive tasks were considered.

GLOBAL PRECEDENCE AND CONSISTENCY EFFECTS

The major goal of this article is to explore ways in which information, perceptually processed in the "what" and "where" pathways, might be stored differentially in the explicit and implicit memory systems. A popular approach to the study of parallel processing in vision is based on the notion of local and global processing. The principle of global precedence contends that global information is processed first followed by the analysis of local detailed information (Navon, 1977;1981). Support for this idea comes from choice reaction time (RT) studies, which show that RT for global aspects is faster than RT for local detail. In this paradigm, in addition to measuring global precedence, it is also possible to examine the consistency effect. This is the finding that, when attending to local stimuli, RTs are slower when the global and local stimuli are inconsistent (the local stimuli are different from the global), but this does not occur when attending to the global stimuli. The fact that global information interferes with local processing but not vice versa, has been interpreted as further support for the priority of global information in the visual system (Navon, 1977; 1981) and for the idea of parallel processing (Hughes, Layton, Baird, and Lester, 1984; Lamb and Robertson, 1989; Lennie, 1980).

Much of the research pertaining to global precedence has explored how manipulation of the stimuli affects responding. The advantage of the global elements has been found to diminish with sparsity of local elements (Martin, 1979), and spatial filtering (Badcock, Whitworth, Badcock and Lovegrove, 1990, and LaGasse, 1993). All of these manipulations result in reductions of low spatial frequency amplitude. In addition, whole field flicker (Lovegrove, Lehmkuhle, Baro and Garzia, 1990) was found to diminish global precedence effect by slowing responses to global elements. The fact that removing low spatial frequencies inhibits the global precedence effect links this

Berg and May 269

effect with the "what" and "where" pathways. Badcock, Whitworth, Badcock, and Lovegrove (1990) proposed that the slowing of RTs to global stimuli when low spatial frequencies were removed was a result of those stimuli being processed by the slower, high-frequency, sustained channels (the "what" pathway). In this experiment the consistency effect was also absent with the removal of low spatial frequencies. Badcock et al. interpreted this as a lack of priming or interference from the global, low spatial frequency, transient channels (the "where" pathway). May, Gutierrez and Harsin (1995) continued this research by looking at RTs in relation to delays between the presentation of global and local elements of stimuli. They found that, as delays increased, the global precedence effect was attenuated, then absent and then replaced with a local precedence effect. The consistency effect, however, did not follow the same time course which, they argue, suggests that the consistency effect is the result of higher level processing.

The global precedence paradigm offers some advantages in addressing the linkage between perception and memory. Discriminating between the global figures depends on the ability to detect differences in where local elements are located, but is unaided by the identity of these elements. Discrimination of the local elements depends on detecting differences in the form of these elements, and is unaided by the redundancy of elements in many locations. If subjects do perform these two tasks with different pathways, then it may be the case that incidental learning about the sequence of "what" (local elements) or "where" (global figures) may be inferred from performance over repeated trials.

The removal of low spatial frequencies through spatial filtering has been found to slow RTs to global aspects of visual stimuli presumably by forcing these aspects to be processed through the slower, high frequency sensitive, p-cell pathways (Badcock et al., 1990). To address the question of whether or not sequence learning is mediated by the m-cell pathway, the present experiment combined the paradigm used by Nissen and Bullemer (1987) with that of Badcock, Whitworth, Badcock and Lovegrove (1990). Choice RTs, for the global or local aspects of a sequence of compound stimuli that were filtered to remove low spatial frequencies and nonfiltered stimuli that were matched in contrast, were obtained. Learning was possible (iterative sequence) in all conditions, and the sequence used was the same for all groups. After all trials were completed subjects were asked to replicate the pattern in the sequence of stimuli, which provided an index of explicit memory. If sequence learning occurs in both "what" and "where" pathways, RT differences would be expected for both the local and global conditions. If sequence learning occurs both implicitly and explicitly, both RT and verbal report measures should reveal knowledge of the sequence. If sequence learning is confined to the "what" pathway, RT differences would be expected for both local and global conditions, but if sequence learning is confined to the "where" pathway, RT differences would be expected only in the unfiltered global condition.


FIGURE 3 Low Spatial Frequency Filtered Stimuli

Notes: Examples of low spatial frequency filtered stimuli used in Experiment 2. When viewed from the appropriate distance on the display monitor, only spatial frequency content above 12 c\d are visible.

METHODS

Subjects

Forty university students and members of the community served as subjects. Twenty- four were female and sixteen were male. All were naive as to the hypothesis under investigation. Subjects ranged in age from 18 to 40. All had normal or corrected to normal visual acuity (20/20). Only one subject was left-handed.

Stimuli and Apparatus: Stimulus presentation, reaction time recording and analysis of responses were carried out as in Experiment i. The stimuli consisted of global

Berg and May 271

FIGURE 4 Unfiltered, Low Contrast Stimuli

Notes: Examples of unfiltered, low contrast stimuli used in Experiment 2. These stimuli were matched in contrast to the filtered stimuli illustrated in Figure 3.

characters (either arrows pointing to the left or to the right), made up of local characters, smaller arrows that were either consistent (pointing in the same direction) or inconsistent (pointing in the opposite direction) as the global character. One set of four stimuli were filtered to remove the low spatial frequencies (Figure 3). After two- dimensional Fourier Analysis, a ramp filter (15 dB/octave) was used. The amplitude spectra of the images were high passed to contain spatial frequencies above 12.0 c/deg. For another set of four stimuli the contrast of the stimuli was lowered to match, as closely as possible, the contrast of the filtered images (Figure 4). The filtered stimuli had a space averaged luminance of 16.7 cd/m 2, and the low contrast stimuli had a space averaged luminance of 16.91 cd/m 2. The Michaelson contrast of the stimuli was


47.0 percent for both the filtered stimuli and the low contrast stimuli. The local stimuli subtended visual angles of 0.27 ~ vertically by 0.32 ~ horizontally, and the global stimuli subtended visual angles of 1.96 ~ vertically by 3.36 ~ horizontally. Subjects sat 230 cm. from the computer monitor.

The Stimulus Conditions

The eight stimuli were used to create four different experimental conditions. Sub- jects were randomly assigned to one of these four groups. Each group participated in only one of the experimental conditions. Because previous research has shown that explicit memory performance is correlated with intelligence, but implicit memory performance is not (Reber, Walkenfeld, and Hemstadt, 1991), subjects were given the Kaufman Brief Intelligence Test for later correlational analysis. They then performed a practice session, which consisted of nine trial blocks of a random sequence of the filtered stimuli. Subjects were randomly assigned to respond to the direction of either the global or local elements of the stimuli, and this assignment was also used when the experimental task was performed. The experimental task was a choice reaction time (RT) task in which they indicated the direction that the arrows pointed (left or right) for a given level (global or local). In the global conditions subjects were asked to attend to the global stimuli and report the direction of the arrow as quickly as possible, pressing the right or left arrows on the keyboard with the index and ring finger of their preferred hand. During the local conditions the subjects were asked to attend to the local stimuli and report the direction that they pointed as quickly as possible. RTs were recorded to the nearest 1.0 ms. Each condition consisted of 48 sets of 8 stimulus presentations. The last 2 sets of 8 stimuli were in a random sequence, while the other 46 sets of 8 stimuli were a repeated sequence of 8 representing each of the stimuli twice. The same repeating sequence was used for all of the experimental conditions. At the end of the session the subjects were told that there had been a repeating sequence of 8 stimuli during the task. They were then asked to type this sequence using the L and R letter keys on the computer keyboard to indicate the sequence. This acted as a "probe" to determine whether subjects had explicitly learned the trial sequence.

RESULTS

Reaction Time Analysis

Mean reaction time data were derived as in Experiment 1, yielding 25 data points/ subject. Mixed analysis of variance (groups with trials as repeated measures) produced significant main effects for groups [F (1,36) = 9.74; p<0.0001] and trials [F (23,828) = 3.91; p< 0.0001) but no significant two-way interaction. The global precedence effect is evident in the differences in mean RTs between the nonfiltered groups, and filtering diminished this effect by slowing the RTs to the global level of the stimuli. A similar analysis using the last iterative and final trial blocks yielded significant main effects for group [F(1,36) = 3.1 1; p< 0.03] and trials [F(1,36) = 6.02; p< 0.01]. A significant

Berg and May 273

Table 2 Subjects' Attempts to Identify Sequence Used, Experiment 2

Local, Unfiltered Global, Unfiltered

Subject # Response Correlation Subject # Response Correlation

1. LRLRLLRR 1.00 1, LLLRLRRL 0.26 2. LLRRLRLR 1.00 2. LRLRLLRR 1.00 3. LRLRRLLR 0.50 3. RLRLRRLL 0.50 4. LLRRLRLL 0.78 4. LRLLRLRR 0.50 5. LLLRRLRR 0.50 5. RRLRLLRL 0.50 6. RRLRLRRR 0.58 6. LRLRLLRR 1.00 7. RLRRLLRL 0.50 7. RLRLRLRL 0.50 8. RRLRRRLL 0.78 8. RLRLRRLL 0.50 9. RRLRLLRL 0.50 9. RLRLRRLL 0.50

10. RLLLLLRR 0.26 10. RLRLRRLL 0.50

Local, Filtered Global, Filtered

Subject # Response Correlation Subject # Response Correlation

1. LLLRRLRL 0.78 1. LRLRRLRL 0.50 2. RLRLRRLL 0.50 2. LRLRLLRR 1.00 3. RLLRRLLL 0.78 3. RLRLRRLL 0.50 4. LRLRRLRL 0.50 4. RLRRRLRL 0.78 5. LLLRLRLR 0.78 5. RRRLLLRL 0.50 6. LLRLRLRR 0.50 6. LLRRLLRR 0.50 7. RRLLLRRR 0.26 7. RLRLRRLL 0.50 8. RLLLRRLR 0.50 8. RRLLRLLR 0.50 9. RLRLRRLL 0.50 9. RRLRLLRL 0.50

10. RLRLRR 10. RLLRRRLL 0.50

Notes: Subjects' attempts to identify the sequence used in Experiment 2 and the crosslag correlations for each experimental group. The correct sequence was LLRRLRLR.

two-way interaction between trials and groups was also significant [F(1,36) = 3.19; p< 0.03]. The data were normalized as in Experiment 1 and are presented in Figure 5 for local and global judgments of filtered and nonfiltered stimuli. Generally, RTs fluctu- ated over trials but did not systematically decrease from trial block 2 to trial block 23. In both the global and the local nonfiltered conditions RTs increased with the presentation of a random sequence of stimuli during the last block of trials, which is indicative of sequence learning. This increase in RTs did not occur in either of the filtered conditions. The data were submitted to a mixed analysis of variance (groups with trials as the within group variable), which revealed a main effect for trials [F(1,792) = 2.73; p< 0.0001]. No other main effects or interactions were significant A similar analysis was performed on the last iterative and final trial blocks, revealing a significant trials effect [F(1, 36) = 7.24; p< 0.01] and a significant two-way interaction between trials and groups [F(1,36) = 3.60; p< 0.02]. Tests subsequent to this analysis (Newman Kuel's) indicated that sequence learning occurred in the global, unfiltered group (p<


0.02) only. This evidence for sequence learning, however, does not reveal whether or not learning was implicit or explicit. The.post-trial responses were analyzed to explore this question.

Sequence Knowledge

The attempts to specify the sequence for each subject is presented in Table 2. Examination of the responses indicates that subjects could have the sequence correct with various orders. For example, the sequence used was LLRRLRLR, and the response of subject # 1 was LRLRLLRR. To quantify performance, a crosslag correlation was performed on the these data, comparing the sequence used with the responses reported by the subjects. For subject # 1 a maximum correlation of 1.00 occurred at the fifth lag. Maximum correlations ranged from 0.26 to 1.00. Two subjects in the global nonfiltered group, two subjects in the local nonfiltered group and one subject in the global filtered group provided maximum correlations of 1.00, indicating explicit knowledge of the sequence. One subject (#30) provided incomplete data and was dropped from the analysis. To determine if explicit knowledge of the sequence correlated with RT evidence of sequence leaming, a regression analysis was performed on the crosslag correlations and the changes in RT over the last two trial blocks. Correlations were low (r = 0.07) and nonsignificant (n.s.).

Intelligence

The percentile rank scores on the Kaufman Brief Intelligence Test ranged from 13 percent to 96 percent. IQ was poorly correlated with RT differences on the last two trial blocks (r = 0.008, n.s.), but significantly correlated with explicit learning scores (r = .36, p < .05). This later relationship is depicted in Figure 6.

DISCUSSION

The finding that large and significant differences in nonnormalized RT occurred between the unfiltered local and global conditions indicates that this paradigm involved differential processing of local discriminations by the "what" pathway and global discriminations by the "where" pathway. Under these conditions sequence learning as evidenced by RT measures occurred only when the "where" pathway was employed. Spatial filtering of the stimuli, a manipulation designed to force subjects to employ the "what" pathway, disrupted sequence learning when subjects performed the global task. It is unlikely that this sequence learning derived from motor learning in that the stimulus manipulation disrupted learning only for the global conditions.

The crosslag correlation scores indicated that only 5 of the 40 subjects exhibited strong explicit learning (r = 1.00): 2 in each of the unfiltered groups and 1 in the global filtered condition. Thus, the use of a more complex task reduced explicit learning compared to Experiment 1. This finding is consistent with the suggestion that sequence learning in the present study occurred implicitly first, but a few subjects at-

Berg and May 275

FIGURE 5 Normalized Mean RT, Experiment 2

120

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5 10 15 20 25

T

110 F I L T E R E D / lOO O~ TT(~ T ~l

0 5 10 15 20 25

120 -

1 1 0 -

1 0 0 -

9 0 -

8 0 -

7 0

120

110

100

90

80

70

LOCAL

UNFILTERED

, , ; , ]

5 10 15 20 25

FILTERED

O

5 10 15 20 25

TRIAL BLOCKS

Notes: Normalized mean RT as a function of trials for local and global judgments of filtered and nonfiltered stimuli from Experiment 2. Vertical error bars represent + 1.0 SE.

tained enough conscious awareness of the sequence to remember it explicitly. The suggestion that sequence learning is implicit, at first, is also consistent with the findings of Willingham, Nissen and Bullemer (1989), that sequence learning was apparent from RT measures after 100 trials, but subjects did not report being aware of a sequence until they had responded to over 300 trials. However, the finding could also support the idea of parallel, interactive functions of implicit and explicit memory (Reber, Allen and Regan, 1985; Berry and Broadbent, 1988) in that the task was learned largely implicitly, though the nonimplicit process of recognizing the sequence


Figure 6 IQ as a Function of Crosslag Correlation

z

1.0

[- ,e~ 0.8

S i 0.6

0.4

0.2

r = 0.36, p < 0.05

�9 LL �9 GL 0 LF [] GF

O 0 0

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i i i i

20 40 60 80

IQ PERCENTILE

i

100

Note: The solid line represents a first order polinomial fit to the data.

may occur and assist in learning. The findings that a significant correlation between implicit learning (RT differences between trial blocks 23 and 24) and explicit learning scores (crosslag correlations) was not obtained, and that explicit measures of learning were correlated with intelligence, but implicit measures were not, further support the idea of parallel, interactive functions of implicit and explicit memory. The significant correlation between intelligence and explicit memory scores suggests that it was the subjects with higher IQ scores that learned explicitly. Thus, it is perhaps one's level of cognitive ability that mediates the interaction of explicit processes with implicit processes.

Berg and May 277

This research has resulted in evidence that the manipulations which curtail m-cell inputs to the "where" pathway disrupts sequence learning. This is apparent in both the significant evidence of sequence learning for the global aspects of the stimuli and the significant lack of learning for the global task with low spatially filtered stimuli. Sequence learning mediated by the "where" pathway appears to be largely implicit, though some subjects demonstrate explicit knowledge of the sequence. Explicit learning scores correlate with I.Q., and this implies that perhaps there is an interaction of implicit and explicit memory that is mediated by one's cognitive ability.

GENERAL DISCUSSION

The major finding of this article is that, under some conditions, a stimulus manipulation intended to bias which perceptual processes are used to perform a task can determine whether incidental learning occurs. This provides data in support of the notion that certain kinds of learning, in this case sequence learning, require certain stimulus inputs for optimal processing. The data also suggest that the sequence learning occurred implicitly, and this supports the notion that learning in the "where" pathway occurs implicitly. Explicit learning of sequence is clearly possible, as evidenced in Experiment 1, but is less likely when the task demands are high. The strategy employed in this experimental effort parallels the strategies that were pioneered by visual psychophysicists in their attempt to discern the functional roles of the p- and m-cell pathways.

Future extensions of this approach might benefit from numerous other stimulus manipulations that have been shown to favor processing in the "what" pathway. Since m-cells require abrupt onsets or offsets for vigorous activation, ramping stimuli gradu- ally on and off should stimulate m-cells poorly, but should provide considerable stimulation for p-cells. Livingston and Hubel (1987; 1988), suggested that another way to "silence" m-cell activity is to present stimuli in isoluminance, since m-cells do not respond to purely chromatic differences. Another manipulation related to color differences in these cells concerns the fact that a subpopulation of m-cells is tonically suppressed by red light (Wiesel and Hubel 1966; DeMonasterio and Schein 1980; and Dreher, Fukada, and Rodieck 1975). Psychophysical experiments comparing performance with different colored surrounds have successfully disrupted the m-cell response (Breitmeyer, May, and Heller, 1991). Yet another tact has to do with whole field flicker. Since m-cells respond vigorously to rapid luminance modulation, some researchers have used such stimulation to "jam" m-cell processing while subjects perform RT tasks (Lovegrove, Lehmkuhle, Baro, and Garzia, 1990). The use of such stimulus manipulations in learning and memory paradigms may well shed further light on the contributions of the perceptual streams to different memory systems.

Another line of evidence that could yield converging support for the notion that different perceptual pathways feed into different memorial systems could come from the study of memory in special populations with perceptual disabilities. Brannan and Williams (1987) found that children with reading disabilities were not able to utilize information provided by a cue to target location in a target detection task if the cue


preceded the target by less than 50 msec., whereas normal readers could utilize such information at shorter temporal separations. Likewise, it has been shown that children with reading disabilities require more time than either normal reading children or adults to make accurate judgments when asked to specify which of two simple words were presented first (Brannan and Williams, 1987; May, Williams, and Dunlap, 1988). Given that the m-cell stream is a likely candidate for the mediation of temporal order, these results suggest that disabled readers may have a slow or sluggish "where" pathway. In two studies, Solman and May (1990) compared the size of spatial discrepancies, in locating letter and nonletter stimuli in space, by good and poor readers. In the first experiment, spatial discrepancies increased with eccentricity for both groups, but the rate of increase was significantly greater for the poor readers. The second study required that two spatially and temporally located targets be localized on each trial. Results similar to Experiment 1 were obtained. In both experiments the children were also asked to report what letter had been presented on the trials involving letters. The groups did not differ with regard to identification accuracy. Future experiments with sequential learning and explicit/implicit differentiation with poor readers may provide more support for the hypothesis in question.

The rate at which stimuli were presented was held constant in the present experiments, but it may be informative to explore this variable more extensively. The ventral processing stream should be able to process information at much higher rates, and this may potentiate sequence learning in the "where" pathway, while making it less likely in the "what" pathway. Conversely, very slow rates of stimulus presentation may result in better sequence learning in the "what" pathway and disruptions of such learning in the "where" pathway.

ACKNOWLEDGMENTS

This research was supported with funds from the Ernest and Yvette C. Villere Chair for Research in Neuroscience endorsed by the University of New Orleans and the Louisiana State University Neuroscience Center of Excellence.

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Parallel processing in visual perception and memory: What goes where and when?

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