Preattentive visual search and perceptual grouping in schizophrenia

ELSEVIER Psychiatry Research 79 (1998) 151-162

PSYCHIATRY

RESEARCH

Preattentive visual search and perceptual grouping in schizophrenia

Vaughan J. Carr * , Sally A.M. Dewis, Terry J. Lewin

Discipline of Psychiatry, Faculty of Medicine and Health Sciences, Universi@ of Newcastle, Universiry Drive, Callaghan, NS W 2308, Australia

Received 19 July 1997; revised 9 March 1998; accepted 22 March 1998

Abstract

To help determine whether patients with schizophrenia show deficits in the stimulus-based aspects of preattentive processing, we undertook a series of experiments within the framework of feature integration theory. Thirty subjects with a DSM-III-R diagnosis of schizophrenia and 30 age-, gender-, and education-matched normal control subjects completed two computerized experimental tasks, a visual search task assessing parallel and serial information processing (Experiment 1) and a task which examined the effects of perceptual grouping on visual search strategies (Experiment 2). We also assessed current symptomatology and its relationship to task performance. While the schizophrenia subjects had longer reaction times in Experiment 1, their overall pattern of performance across both experimental tasks was similar to that of the control subjects, and generally unrelated to current symptomatology. Predictions from feature integration theory about the impact of varying display size (Experiment 1) and number of perceptual groups (Experiment 2) on the detection of feature and conjunction targets were strongly supported. This study revealed no firmevidence that schizophrenia is associated with a preattentive abnormality in visual search using stimuli that differ on the basis of physical characteristics. While subject and task characteristics may partially account for differences between this and previous studies, it is more likely that preattentive processing abnormalities in schizophrenia may occur only under conditions involving selected ‘top-down’ factors such as context and meaning. 0 1998 Elsevier Science Ireland Ltd. All rights reserved.

Keywords: Attention; Mental processes; Visual perception; Schizophrenia; Reaction time

*Corresponding author. Tel.: + 61 2 49211270; fax: + 61 2 49211282; e-mail: [email protected]

0165-1781/98/$19.00 0 1998 Elsevier Science Ireland Ltd. All rights reserved. PZZ SO165-1781(98)00035-3

152 %J. Carr et al. /Psychiatry Research 79 (1998) 151-162

1. Introduction

1.1. Feature integration theory

An influential model of visual perception pro- poses an early, preattentive stage of global or holistic processing followed by a stage of local or analytic processing. Feature integration theory (Treisman and Gelade, 1980) provides a detailed articulation of this model. In this theory two functional levels of visual processing are distin- guished (Treisman, 1983, 1988). At the first or preattentive level, separate sensory features (e.g. color, orientation, size) are coded automatically, in parallel and without focused attention - a non-effortful process that is believed to be unaf- fected by a variety of factors, including arousal, mood and practice. This leads to the formation of ‘feature maps’ which are organized into groups, textures or homogenous areas. These ‘maps’ pro- vide a preliminary organization of the stimulus field and prepare stimulus inputs for optimal serial processing. The second level is the feature integration stage in which focused attention is used in serially scanning the stimulus field - a controlled, effortful process. Features that are registered within the same central ‘fixation’ can then be ‘conjoined’ and integrated into a single percept, the nature and relation of the feature conjunctions determining the identity of the object. Within this model of perception, the pheno- menon of preattentive grouping refers to the preliminary perceptual segregation of the stimulus field prior to the application of focused attention (Treisman, 1982). It enables efficient processing through the direction of focused attention to groups rather than single items in the stimulus field.

1.2. Preattentive processing in schizophrenia

If patients with schizophrenia have an abnormality in the preattentive stage of perceptual information processing, then it is possible that they may use serial processing instead of parallel processing and show deficient preattentive per-

ceptual grouping. Individuals in the early stages of schizophrenia have reported loss of perceptual stability, fragmentation of percepts, and inability to interpret ‘wholes’ as meaningful gestalts (Mc- Ghie and Chapman, 1961; Chapman, 1966). As an apparent consequence of these phenomena, they have reported a sense of needing to apply con- scious, deliberate effort in the assembly of percepts (McGhie and Chapman, 1961; Chapman, 1966). These subjective reports suggest dysfunction in the preattentive, automatic stage of processing, necessitating the use of consciously controlled serial processing for perceptual assembly operations that would normally be conducted automatically and in parallel. A view reflecting this formulation was put forward by Venables (19841 and subsequently elaborated into an information processing model of schizophrenia (Carr and Wale, lY86).

Some empirical evidence suggests that schizophrenia is associated with an impairment in the early stages of perceptual information processing, specifically a deficit in perceptual organization (Cox and Leventhal, 1978; Place and Gilmore, 1980; Wells and Leventhal, 1984). The extent to which preattentive processes vs. focused attention contribute to perceptual organization has not been resolved. However, it is likely that preattentive processes underlying perceptual organization involve at least two sets of phenomena. One of these is the grouping of stimulus components on the basis of physical characteristics (e.g. color, orientation, size, etc.), or ‘bottom-up’ processing; another involves ‘top-down’ processing which reflects the influence of prior experience on the organization of stimulus components (Silverstein et al., 1996). The latter implies a potential influence of memory-related factors, such as context, meaning and attentional strategies, on perceptual organization (Silverstein et al., 1996). A further complication in determining whether information processing dysfunction in schizophrenia involves abnormalities in preattentive mechanisms, focused attention, or both is that global and local processing may be initiated at the same time (Heinze and Munte, 1993) and, under the influence of ‘top-down’ factors, may

KJ. Carret al. /Psychiaty Research 79 (1998) 151-162 153

interact with each other (Friedman-Hill and Wolfe, 1995).

While one influential view is that schizophrenia involves a normal parallel processing system with defective serial processing (Callaway and Naghdi, 19821, others have proposed a defective parallel processing system with, as a consequence, serial processing inappropriately applied and thus over- loaded (Venables, 1984), inefficient or otherwise disordered (Carr and Wale, 1986). Although sev- eral studies have suggested that preattentive processes in schizophrenia may be abnormal (Cox and Leventhal, 1978; Place and Gilmore, 1980; Schwartz et al., 1988, 1990; Shelley et al., 1991; Huey and Wexler, 19941, no consistent evidence has emerged to indicate whether the problem is one of ‘bottom-up’ processing, ‘top-down’ processing or both.

In testing the disordered preattentive processing hypothesis of schizophrenia, it would seem to be fundamental to determine whether schizophrenia, patients show any abnormalities in the stimulus-based aspects of perception, or ‘bottom-up’ processing. In a study of texture discrimi- nation using visual target detection tasks, Lieb et al. (1994) reported that schizophrenia subjects made fewer correct responses and were slower than normal control subjects in detecting an ‘L target in an array of ‘ + ’ distracters. This result, which was interpreted as supporting a preattentive abnormality in schizophrenia, has since been partially replicated in a group of adolescents at genetic risk for schizophrenia (Lieb et al., 1996). In contrast, Mori et al. (19961, using an experimental paradigm based on the feature integration work of Treisman and colleagues (Treisman and Gelade, 1980; Treisman, 1983, 1988>, were unable to demonstrate a preattentive abnormality in schizophrenia patients. Discrepancies between the findings of Lieb et al. (1994) and Mori et al. (1996) may be less marked than they first appear. In the initial experiment of Lieb et al. (1994, p. 53) briefly presented stimuli were used, whereas the overall conclusion from their subsequent search based-task was that ‘given enough time, texton (feature) detection seems to be quite reli- able in schizophrenics’.

1.3. The current study

It was in an attempt to resolve the question of whether schizophrenia patients show deficits in the stimulus-based aspects of preattentive processing that we undertook a series of four experiments within the framework of feature integration theory, the first two of which are described in this article. In general terms, it was predicted that schizophrenia patients would be more reliant on serial processing strategies and would show deficits in a perceptual grouping task. Based on the limited literature available when this project began, and preliminary data from pilot studies, two specific hypotheses were examined in the present study:

1. Schizophrenia subjects will be more likely: (a) to use serial processing strategies in a visual search task normally performed using parallel processing strategies; and (b) to change to parallel processing as the task proceeds (Ex- periment 11.

2. The performance of schizophrenia subjects on a perceptual grouping task will show little or no effect of the number of groups in the stimulus array whereas normal control subjects will show a non-linear reduction in search time as groups become larger and fewer (Experiment 2).

In addition, on the basis of the prediction that perceptual abnormalities would be mediating vulnerability indicators (i.e. trait variables that varied with clinical state; Dawson et al., 1994; Nuechterlein et al., 1994), we hypothesized that in a sample of clinically stable outpatient schizophrenia subjects these abnormalities would be more pronounced in subjects with more severe psychotic symptoms (delusions, hallucinations and disorganization).

2. Materials and methods

2.1. Subjects

Thirty schizophrenia subjects and 30 age-, gender-, and education-matched normal control sub-

154 KJ. Carr et al. / Psychintry Research 79 (1998) 151-162

jects participated in this study, as paid volunteers. Schizophrenia subjects fulfilled DSM-III-R crite- ria for schizophrenia (American Psychiatric Asso- ciation, 1987) and were recruited from community mental health clinics. Those with other psychiatric disorders, including current drug and/or alcohol abuse, organic mental disorders and mental retardation were excluded. Control subjects were recruited from community employment agencies and had no current or past psychiatric disorder. All subjects were aged between 18 and 60 years with normal or corrected to normal visual acuity and normal color vision.

2.2. Clinical assessment

Diagnosis of schizophrenia was made using the Structured Clinical Interview for DSM-III-R, SCID-P (Spitzer et al., 1990a). Current symptomatology in the schizophrenia subjects was assessed at interview using the Scale for the Assess- ment of Positive Symptoms, SAPS (Andreasen, 1989a), the Scale for the Assessment of Negative Symptoms, SANS (Andreasen, 1989b) and the extended version of the Brief Psychiatric Rating Scale, BPRS (Lukoff et al., 1986). Normal control subjects were interviewed using the SCID, Non- Patient Edition (Spitzer et al., 1990b). Both subject groups were administered the Symptom Checklist-90-Revised, SCL-90-R (Derogatis, 19831, four scales of psychosis proneness (Chap- man et al., 1976, 1978; Eckblad and Chapman, 1983; Mishlove and Chapman, 1985) and the Na- tional Adult Reading Test, NART (Nelson, 1991). The doses of antipsychotic drugs in schizophrenia subjects were recorded and converted to chlorpromazine equivalents (Victorian Drug Usage Advisory Committee, 1993).

2.3. Assessment of attentionalprocesses

The two experimental tasks outlined below are based on the visual search tasks described in Treisman (1982). Stimuli were presented on a computer display using programs written with Mi- cro Experimental Laboratory (MEL) software (Schneider, 1990). Each subject’s reaction time, response selection and accuracy were recorded.

2.4. Experiment 1

This task was designed to assess parallel and serial information processing. Each display con- tained either a target with a unique feature not shared by the distracters (feature target), which has been demonstrated to elicit target ‘popout’ (parallel detection), or a target sharing a feature with the distracters (conjunction target), which required serial scanning of the display (serial detection).

2.4.1. Stimuli The distracters in this task were red Hs and

green Xs. There were two types of targets: in the conjunction condition the target was a green H, differing from distracters by the conjunction of color and shape; in the feature condition the target was either a blue H or blue X, or a red S or green S, thus sharing either the feature of color or shape singularly with the distracters. In the feature condition trials the subject was asked to search for either a ‘blue letter’ or an ‘S’ without knowing which would be present. Display size varied between 2,4, 16 and 36 items arranged in regular rectangular matrices (e.g. 2 X 2, 4 X 4, 6 x 6) with a 1.3:1 ratio of width to height. Each item subtended a visual angle of 0.57”. The aver- age distance of items from the fixation point was 2.14” for all display sizes. Viewing distance was 60 cm. Target positions were chosen so that the mean distance from fixation to targets equaled the mean distance from fixation to distracters in each set. Targets occurred with equal frequency in each quadrant.

2.4.2. Procedure Each trial began with the presentation of a

white fixation cross in the center of the screen. After 1000 ms the fixation cross was replaced by the stimulus display which remained present until the subject responded. Subjects were instructed to search quickly and accurately for the known target amongst a number of distractor items (1,3, 15 or 35, arranged in random sequence) and to respond by pressing one of two buttons: ‘yes’ if a target was detected; and ‘no’ if a target was absent from the display. This procedure was re-

KJ. Carret al. /Psychiatry Research 79 (1998) 1.51-162 155

peated over three blocks of 64 trials each: two (target present vs. target absent) x four (display sizes) X eight (presentations). Subjects were provided with 12 practice trials at the beginning of the session for familiarization. Parallel processing is reflected by similar detection durations regard- less of the number of distracters whereas serial processing is evidenced by increased detection durations as the number of distracters increases.

2.5. Experiment 2

This task examined the effects of perceptual grouping on visual search for objects defined by either features or conjunctions.

2.5.1. Stimuli The same target and distractor stimuli were

used as in Experiment 1, but in this case the display size was fixed at 36 items and the number of homogenous groups of distracters was varied. The same matrix of 36 positions was used, with the red Hs and green Xs arranged in varying numbers of homogenous groups: 18 groups of two items (i.e. horizontal pairs of identical items); nine groups of 2 X 2 identical items; four groups of 3 X 3 identical items; or a single large group of identical items (all green Xs or all red Hs). In target-present displays, the green H (conjunction) target, or the blue or S disjunctive (feature) target replaced one item with equal frequency in each quadrant in each type of group.

2.5.2. Procedure Each trial proceeded as in Experiment 1 and

subjects were required to search as quickly and accurately as possible for a known target amongst a display of 36 items arranged in varying numbers of homogeneous groups (1 X 36, 2 X 18, 4 X 9, 9 x 4, 36 x 1) and to respond as in Experiment 1. Twelve practice trials were again provided at the beginning of the session. This procedure was repeated over three blocks of 80 trials: two (target present vs. target absent) X five (homogenous groups) X eight (presentations). The perceptual organization of the display normally influences search time if serial scanning is used, such that search time for a conjunction target decreases

non-linearly as the groups become larger and fewer.

Subjects participated in Experiments 1 and 2 in separate sessions of approximately 1 h in duration. Rest breaks were provided at least every 15 min.

2.6. Data analysis

The dependent variable for both experiments was Reaction Time (RT) measured in millisec- onds (ms). For Experiment 1, separate repeated measures analyses of covariance (ANCOVAsl were conducted for target-present and target-absent conditions. Each of these ANCOVAs in- volved one between-group factor (subject group) and three within-group factors (display size, block, target type). Trend contrasts were used to ex- amine differences between the four display sizes, representing the linear and quadratic components of trend. Simple comparisons were used to ex- amine differences between each of the three blocks (of trials/conditions) and between the two target types (feature vs. conjunction targets). The covariates in each analysis were antipsychotic medication (chlorpromazine equivalents) and illness duration (years), which were set at the mean scores on these variables for each member of the control group (following Cohen and Cohen, 1983). With respect to the subject group factor, overall differences between the schizophrenia and control groups were examined first, and then the control group was compared with three overlap. ping subgroups of schizophrenia subjects defined as follows: hallucinations present (Hal, n = 17) vs no hallucinations (NHal, y1 = 13); delusions present (Del, n = 13) vs. no delusions (NDel, n = 17); negative symptoms moderately severe (Neg, 12 = 12) vs. no (or few) negative symptoms (NNeg, n = 18). These subgroups were defined using the SAPS/SANS symptom dimensions and cutpoints shown in Table 2. For the data analyses involving subgroup comparisons, Bonferroni adjustments (a/3) were used to control for the number of statistical tests.

A similar analysis strategy was used for Experi- ment 2, with each of the ANCOVAs involving one between-group factor (subject group) and

156 KJ. Carr et al. /Psychiahy Research 79 (1998) 151-162

three within group factors (number of groups, block, target type).

3. Results

3.1. Sample characteristics 3.2. Experiment 1

As shown in Table 1, the schizophrenia and control subjects were matched for age, gender and level of education. There were no significant differences between the two groups in the proportion receiving social security benefits, level of intelligence as measured by the NART, or lifetime history of substance abuse. The schizophrenia group had a lower proportion of employed individuals and higher levels of self-reported psychiatric symptoms as measured by the SCL-90-R.

The clinical characteristics of the schizophrenia subjects are summarised in Table 2. It can be seen that this was a chronic schizophrenia group with substantial illness duration, multiple hospital admissions and moderate levels of persistent negative and psychotic symptoms, although symptoms

Table 1 Sample characteristics

of disorganization were rare. Antipsychotic drug doses were somewhat higher than those recom- mended more recently but are not atypical for outpatient maintenance therapy.

3.2.1. Overall differences between schizophrenia and control subjects

Fig. 1A presents a breakdown of mean RT by display size, target type and subject group for the target-present condition in Experiment 1. The corresponding repeated measures ANCOVA revealed a significant main effect for subject group (F,,,, = 5.72, P < O.OS>, with schizophrenia subjects taking longer to respond than control subjects (mean RT: 1127.53 ms vs. 928.28 ms). Time taken to detect conjunction targets was significantly longer than that for feature targets (mean RT: 1146.52 ms vs. 909.29 ms, F,,57 = 50.05, P c 0.001) and there was a significant overall linear increase in RT as display size increased (FI 57 = 301.12, P < 0.001). Consistent with predictions

Variable Schizophrenia subjects (n = 30)

Control subjects (n = 30)

Statistical analysis

Matching variables Mean age [years] (SD.) 37.90 (10.43)

Gender 14 F, 16 M

Mean education [l-7] (SD.) 3.70 (1.32)

Non-matching variables % Employed 6.7%

% Receiving social security benefits 80.0%

Mean NART (S.D.) 100.37 (11.11)

SCL-90-R Paranoid ideation (S.D.) 0.84 (0.73) Psychoticism (S.D.) 0.91 (0.68) Global Severity Index [GSI] (S.D.) 0.89 (0.58)

% Lifetime history of substance 34.5% abuse/dependence

*P < 0.05; **P < 0.01; ***P < 0.001; ns, non-significant.

37.20 (9.49)

14 F, 16 M

3.80 (1.49)

26.7% x2 = 4.32*

63.3% x2 = 2.05, ns

103.29 (9.24) t c58j = 1.09, ns

0.32 (0.45) t (5*) = - 3.33*** 0.20 (0.39) t (58) = -49.7*** 0.40 (0.40) tcss, = - 3.84* * *

26.7% x2 = 0.43, ns

VJ. Carr et al. / Psychiatty Research 79 (1998) 151-162 157

Table 2 Clinical characteristics of the schizophrenia subjects (n = 30)

Variable

Age of onset of psychotic symptoms (years)

Illness duration (years)

Antipsychotic dose (chlorpromazine

Mean S.D.

24.27 5.45

14.06 9.79

520.50 502.31 equivalents)

Number of psychiatric admissions

Time since most recent admission (years) [n = 271

Brief Psychiatric Rating Scale [BPRSI (total)

SAPS/SANS symptom dimensions” Negative

Disorganized

Psychotic Delusions Hallucinations

7.76

3.38

11.63 7.83

1.71

0.44

1.77 1.33 2.20

% Above SAPS/SANS symptom cutpoint ( 2 2) Negative 40.0%

Disorganized 3.3%

Psychotic Delusions Hallucinations

46.7% 43.3% 56.7%

13.60

4.24

1.10

0.61

1.44 1.40 1.97

aAndreasen et al. (1995).

from feature integration theory (Treisman and Gelade, 19801, the linear increase with display size was more marked for conjunction targets than feature targets (display size x target type interaction: Fl,57 = 150.46, P < 0.001). Across the three experimental blocks there was a reduction in mean RT (1089.97 ms vs. 1003.79 ms vs. 989.96 ms), with block 1 being significantly different from block 2 (Fl,57 = 16.41, P < 0.001) and block 3 <F,,5, = 8.91, P < 0.01). There was also a significant display size x target type x block interaction (Fl,57 = 11.60, P < 0.01) with the extent of differences in RT between conjunction and feature targets in the linear trend across display size more marked in block 1 than block 3. Both the schizophrenia and control subjects performed this task according to feature integration theory, as

Target Present

Target Absent

/

500’ 2 4 16 36

Display Size

Fig. 1. Mean reaction time CRT, ms) by display size, target type (feature vs. conjunction) and subject group (schizophrenia vs. control subject) for the target-present (1A) and target-absent (1B) conditions in Experiment 1.

demonstrated by the absence of subject group X

display size X target type interactions. However, the schizophrenia subjects’ visual search for targets (either feature or conjunction) was more influenced by increasing display size than the control subjects’ visual search (subject group X

linear trend across display size interaction: F1,57 = 7.04, P < 0.01).

Fig. 1B presents a breakdown of mean RT by display size, target type and subject group for the target-absent condition in Experiment 1. Consis- tent with the target-present condition, the repeated measures ANCOVA revealed a significant main effect for subject group (F,,,, = 8.92 P < 0.011, with schizophrenia subjects taking longer to respond than the control subjects (mean RT: 2006.31 ms vs. 1546.53 ms>. Time taken to search

158 KJ. Corret al. /Psych&my Research 79 (1998) 151-162

for conjunction targets was significantly longer than that for feature targets (mean RT: 1883.29 ms YS. 1669.55 ms; F1,57 = 13.62, P < 0.0011 and there was a significant overall linear increase in RT as display size increased (FI,5, = 330.62, P < 0.001). While the linear increase with display size was more marked for conjunction targets than feature targets (display size x target type interaction: FI 57 = 98.55, P < 0.000, search times for conjunction and feature targets were both influenced by increasing display size, which was not the case in the target-present condition (see Fig. 1A). Across the three experimental blocks there was a reduction in mean RT (1893.24 ms vs. 1742.10 ms vs. 1693.92 ms>, with block 1 being significantly different from block 2 (F,,57 = 12.75, P < 0.001) and block 3 (F,,,, = 22.15, P < 0.01). There was also a significant display size x block interaction (FI,57 = 10.69, P < 0.01) with the linear trend across display size more marked in block 1 than block 3. For the target-absent condition there were no other differences between the schizophrenia and control groups.

3.2.2. Subgroup comparisons The follow-up comparisons between the control

subjects and the schizophrenia subgroups, defined on the basis of symptomatology, revealed a similar pattern of results to the overall analyses reported above for the target-present and target-absent conditions. However, in the target-present condition of Experiment 1 there was a significant but modest subgroup (Hal vs. NHal) x display size (quadratic trend) X target type interaction (FI,57 = 6.52, P < 0.05) due primarily to an increased RT in the NHal subgroup for feature targets in the largest display size. In the target- absent condition there was a significant subgroup (Hal vs. NHal) x display size X target type x block interaction (F1,57 = 6.71, P < 0.05). The schizophrenia subjects without hallucinations had a similar pattern of performance across blocks. However, for those with hallucinations the extent of differences in RT betvveen conjunction and feature targets in the linear trend across display size was more marked in block 1 than block 3.

3.3. Experiment 2

3.3.1. Overall differences between schizophrenia and control subjects

Fig. 2A presents a breakdown of mean RT by number of homogeneous groups in the display, target type and subject group for the target-present condition in Experiment 2. The corresponding repeated measures ANCOVA revealed no significant main effect for subject group, nor any interaction effects involving subject group. Time taken to detect conjunction targets was significantly longer than that for feature targets (mean RT: 1474.17 ms vs. 988.10 ms; F1,57 = 179.95, P < 0.001) and there was a significant overall curvilinear increase in RT as number of groups increased (linear trend: F,,,, = 130.19, P < 0.001; quadratic trend: FI,57 = 136.63, P < 0.001). As predicted (Treisman and Gelade, 19801, the curvilinear increase in RT as the number of groups increased (and group size decreased) was more marked for conjunction targets than feature targets (linear trend x target type: F,,,, = 69.88, P < 0.001;

quadratic trend x target type: FI,57 = 81.15, P < 0.001). Across the three experimental blocks there was a reduction in mean RT (1361.13 ms vs. 1177.78 ms vs. 1154.49 ms), with block 1 being significantly different from block 2 (F1,57 = 35.37, P < 0.001) and block 3 (F1,57 = 22.59, P < 0.01). There were also eight significant interactions involving block. The extent of differences in RT between feature and conjunction targets was more marked in block 1 than in block 2 (F1,57 = 9.30, P < 0.01) and block 3 (FI,57 = 8.76, P < 0.01). Likewise, the slope (or linear component) of the increase in RT as the number of homogeneous groups increased was greater in block 1 than in block 2 (F,,,, = 6.71, P < 0.051. The remaining five interactions were modest and reflected a de- crease in the influence of number of homogeneous groups on RT in block 3 compared to blocks 1 and 2.

Fig. 2B presents a breakdown of mean RT by number of homogeneous groups, target type and subject group for the target-absent condition in Experiment 2. The repeated measures ANCOVA

W. Carr et al. /Psychiatry Research 79 (1998) 151-l 62 159

Target Present

1,&y&L___-:

Target Absent

6001 1 4 9 16 36

Number of Groups

Fig. 2. Mean reaction time (RT, ms) by number of homogeneous groups in the display, target type (feature vs. conjunction) and subject group (schizophrenia vs. control) for the target-present (2A) and target-absent (2B) conditions in Ex- periment 2.

revealed no significant main effect for subject group, and only one trivial interaction involving subject group. Time taken to search for conjunction targets was significantly longer than that for feature targets (mean RT: 2807.99 ms vs. 2230.09

ms; F1,s7 = 55.91, P < 0.001) and there was a significant overall curvilinear increase in RT as number of groups increased (linear trend: F1,57 = 239.58, P < 0.001; quadratic trend: Fl,57 = 133.43, P < 0.001). Again, consistent with feature integration theory (Treisman and Gelade, 19801, the linear increase with greater numbers of groups was more marked for conjunction targets than feature targets (linear trend x target type: Fl,57 = 54.21, P < 0.001). Across the three experimental blocks there was a significant reduction in mean

RT (2828.69 ms vs. 2471.70 ms vs. 2256.74 ms; block 1 vs. block 2, F1,57 = 26.93, P < 0.001; block 1 vs. block 3, Fl,57 = 39.44, P < 0.001; block 2 vs. block 3, Fl 5, = 10.31, P < 0.01). The extent of differences in RT between feature and conjunction targets also decreased significantly over the three blocks (block x target type interactions: block 1 vs. block 2, Fl,57 = 6.92, P < 0.05; block 1 vs. block 3, F,,,, = 29.93, P < 0.001; block 2 vs. block 3, F1,57 = 10.12, P < 0.01).

3.3.2. Subgroup comparisons The comparisons between the control subjects

and the schizophrenia subgroups revealed no significant differences in the target-present condition in Experiment 2. However, the corresponding comparisons in the target-absent condition revealed two subject group X number of groups X

target type interactions. Relative to the control and NNeg subjects, the Neg subgroup was more influenced by the effect of perceptual grouping when searching for targets in the feature condition (linear trend across number of groups x

target type interactions: Neg vs. control, F,,57 = 10.86, P< 0.01; Neg vs. NNeg, Fl,57 = 7.79, P< 0.05). That is, the pattern of response for feature targets in the Neg subgroup more closely resem- bled that obtained for conjunction targets, indi- cating a distracting effect of perceptual grouping in the Neg subjects.

4. Discussion

The two principal hypotheses tested in this study were not confirmed. There was no evidence that schizophrenia subjects used serial processing strategies in a task normally performed using parallel processing. This is consistent with the findings of Mori et al. (1996) using a similar paradigm and with Silverstein et al. (in press). Neither was there evidence that schizophrenia subjects were uninfluenced by perceptual grouping. If these findings are replicable, this suggests that the feature integration theory paradigmsused in these experiments are extremely robust and apply as much to chronic medicated schizophrenia subjects as they do to normal individuals.

The lack of differences between the groups

160 KJ. Carret al. /Psychiatry Research 79 (1998) 151-162

cannot be attributed to a poor experimental de- sign or low power, since the major predictions from Treisman’s group (Treisman and Gelade, 1980; Treisman, 1982; Treisman and Schmidt, 19821 were strongly supported, namely, display size X target type interactions (Experiment 1) and number of perceptual groups x target type interactions (Experiment 2). In addition, for both experiments, more subtle effects were also de- tectable, such as the range of significant interactions involving target type and block, in which the differences between feature and conjunction targets became less marked across successive experimental blocks. Such findings suggest that, with practice, tasks that otherwise require serial search strategies tend to become somewhat more automatic. However, at larger display sizes (Experi- ment 1) and when there were more perceptual groups (Experiment 2), there remained marked differences between the feature and conjunction targets even after three blocks of experimental trials, with generally similar patterns being found for schizophrenia and control subjects.

The findings in the present study need to be evaluated within the context of the growing body of schizophrenia studies that have demonstrated impairments in preattentive processing using a variety of experimental paradigms, including backward masking (e.g. Green et al., 1994; Lieb et al., 19961, span of apprehension, and continuous performance tests (e.g. Nuechterlein et al., 1986; Ito et al., 1997). However, while impairments have often been observed, there is considerable variation between studies and within experimental paradigms as to the nature and extent of these impairments. The type of impairment expressed by different individuals during different stages of illness may be a function of chronic@, clinical stability, medication status, or a host of other subject characteristics, including symptom sub- types. Such factors may contribute to differential performance across experimental conditions or they may simply influence overall response times. In Experiment 1, for example, the schizophrenia subjects had significantly longer reaction times than the control subjects in both the target-present and target-absent conditions, but apart from that ‘displacement’ the experimental profiles for

the two groups were reasonably similar (see Fig. 1). Task constraints may also contribute strongly to the apparent differences found between studies. For example, while Lieb et al. (1994) and Mori et al. (1996) both examined feature detection in schizophrenia, the former study used briefly presented stimuli (and, hence, rapid de- tectability), while the latter study employed a visual search paradigm (with varying display sizes).

Possible explanations for a failure to demonstrate the predicted subject group effects include the fact that the schizophrenia subjects in this study were clinically stable, those with current substance abuse problems were excluded, and we statistically controlled for differences in antipsychotic medication and illness duration. It is possible that the predicted effects may be demonstra- ble in subjects who are acutely unwell. Silverstein et al. (1996), for example, demonstrated a deficit in perceptual organization in acutely psychotic poor premorbid schizophrenia subjects but not in poor premorbid outpatient subjects. The finding of a preattentive deficit by Lieb et al. (1994), using a different but conceptually related paradigm, could be related to antipsychotic drug dose which was almost four times higher in that study than in the present investigation (1973.8 vs. 520.5 chlorpromazine equivalents 1. It is possible that high doses of such drugs could impair the schizophrenia subjects’ abilities to perform such visual search tasks. Lack of comparability between schizophrenia and control groups in terms of age, gender and level of education is unlikely to account for differences in study findings since none of these variables differed significantly between subject groups in either the Lieb et al. (1994) study or the present one. Moreover, in the present study the schizophrenia and control subjects were very carefully matched and there were no significant group differences in intelligence or lifetime history of substance abuse/dependence.

One of the additional strengths of this study was that we carefully assessed current psychotic symptomatology and undertook a series of follow- up analyses contrasting the performance of subgroups of schizophrenia subjects with each other and with the control group. On the basis of these analyses. we can be reasonably confident that the

KJ. Carr et al. /Psychiatry Research 79 (1998) 151-162 161

lack of differences on the experimental tasks between the schizophrenia and control subjects was not due to differential task performance associated with different patterns of current symptomatology. That is, one subgroup’s performance did not negate that of the other subgroups, with similar experimental profiles being observed among schizophrenia subjects with and without hallucinations, delusions and moderately severe negative symptoms. However, an absence of disorganization symptoms in the patient sample did not permit us to evaluate the effect of this variable on visual search performance. Since Knight and Silverstein (1998) report impaired perceptual organization in association with disorganization symptoms in schizophrenia, the relative absence of these symptoms in the present sample may have accounted for the failure to find symptom- related effects in the experimental tasks.

None of the subgroup comparisons in Experi- ment 1 provided support for the first hypothesis. The finding in Experiment 2 that perceptual grouping appeared to interfere with the performance of negative symptom subjects in the feature-target-absent condition indicates that such subjects may be impaired in their ability to use perceptual groups in efficient visual search under conditions in which recognition is not possible. Nevertheless, as shown in Fig. 2, the main findings from Experiment 2 refute the hypothesis that schizophrenia subjects are impaired in their ability to use perceptual groups based on physical characteristics of the stimuli. The finding in relation to negative symptoms may reflect not only intact perceptual grouping in these subjects but a failure to disengage attention from a compelling aspect of the stimulus display, namely the grouping of elements in the display.

In summary, this study revealed no firm evidence that schizophrenia is associated with a preattentive abnormality in visual search using stimuli that differ on the basis of physical characteristics (i.e. bottom-up processing). While this is at variance with some studies that have reported preattentive abnormalities, and subject and task characteristics may partially account for these differences, it is more likely that preattentive processing abnormalities in schizophrenia may

occur only under conditions involving selected ‘top-down’ factors such as context and meaning.

Acknowledgements

This study was supported by grants from the National Health and Medical Research Council and the University of Newcastle. The authors would like to thank Hunter Mental Health Ser- vices, the Commonwealth Employment Service, the individuals who participated in this research, and Dr Steven Silverstein of the University of Rochester, NY, for his helpful comments on an earlier draft of this report.

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Preattentive visual search and perceptual grouping in schizophrenia

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