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Spatio-temporal distribution of facilitation and inhibition of return arising from the

reflexive orienting of covert attention.

Alexander Collie1,2,, Paul Maruff 1,2, Murat Yucel 1,2, James Danckert 1,2 & Jon Currie1,3

From 1. The Neurophysiology and Neurovisual Research Unit,

Mental Health Research Institute of Victoria,

Parkville, Victoria, Australia.

2. School of Psychological Sciences,

La Trobe University,

Bundoora, Victoria, Australia.

3. Brain Research Unit, Drug and Alcohol Services,

Westmead Hospital, Westmead,

Sydney, Australia.

All correspondence should be addressed to: Paul Maruff Neurophysiology and Neurovisual Research Unit Mental Health Research Institute of Victoria Parkville Victoria Australia 3052

Telephone -61 3 9388 1633 Fax -61 3 9387 5061 Email paul@neuro.mhri.edu.au

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Abstract

Currently, there is debate regarding both the spatial and temporal relationship between

facilitation and inhibition of return (IOR) components of attention, as observed on the

covert orienting of visual attention task (COVAT). These issues were addressed in a

series of experiments where the spatial and temporal relationships between cue and target

were manipulated. Facilitation occured only when the SOA was short and there was

temporal overlap between cue and target. IOR occurred only when SOA was long and

there was no temporal overlap between cue and target. Facilitation encompased the cued

location and all locations between the cue and fixation, while IOR arose for the entire

cued hemifield. These findings suggest that the facilitation and IOR found on COVATs

which use non-informative peripheral cues are separable and stimulus driven processes.

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Introduction

The covert orienting of visual attentional task (COVAT), developed by Posner (1980),

has been influential in the development of psychological theories of visual attention. In

almost all COVAT studies that have used non-informative peripheral cues, response

times (RT) to targets at cued locations are faster than RTs to uncued targets when the

time between the onset of the cue and the onset of the target (stimulus onset asynchrony;

SOA) is less than approximately 250msec (facilitation; see Table 1). At SOAs greater

than 250msec, RTs to cued locations are slower than RTs to uncued locations (inhibition

of return [IOR]; see Table 1). While IOR to cued locations is almost always observed to

follow facilitation on the COVAT when target onset is indicated by the appearance of a

new object or item (see Table 1), there are still unresolved issues regarding the nature of

reflexive attentional shifts to COVAT displays. One of these concerns the nature of, and

relationship between, facilitation and IOR.

The relationship between facilitation and IOR

The debate concerning the relationship between facilitation and IOR centres on the

question of whether these phenomena are two parts of a single biphasic process or

independent processes. While the majority of researchers have observed that IOR at long

SOAs always follows facilitation at short SOAs (ie; the biphasic effect; Lambert &

Hockey, 1991; Posner et al, 1985; Posner et al, 1987; Rafal et al, 1989; Rafal & Henik,

1994), other groups have found that RTs to cued targets are slower than RTs to uncued

targets at both short or long SOAs (Berlucchi et al, 1989; Tassinari et al, 1989; Tassinari

et al, 1994; Tassinari & Berlucchi, 1996). Importantly, these different patterns of

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COVAT performance have led to very different theories about how attention is oriented

covertly. For example, the biphasic effect has been interpreted to reflect that facilitation

arises at short SOAs because the attentional focus is attracted reflexively to the location

of the peripheral cue (Jonides, 1981; Rafal et al, 1989; Rafal & Henik, 1994). The

inhibition observed on the COVAT has been proposed to reflect either a bias against

returning the attentional focus to a previously examined location (Reuter-Lorenz et al,

1996), or a motor bias against responding in the direction of the cued location (Klein &

Taylor, 1994; Rafal & Henik, 1994).

In contrast to the biphasic model, some research finds very little or no significant

facilitation at short SOAs. In fact, IOR has been found for SOAs ranging from 0 to

4000msec (Tassinari et al, 1989; Tassinari et al, 1994; Tassinari & Berlucchi, 1996).

This inhibition has been inferred to occur as a consequence of the COVAT requirement

that subjects maintain fixation at the central location. While peripheral cues initiate the

preparation of a saccade toward the cued location, the COVAT requires that this saccade

be inhibited. As a consequence, the motor system is biased against responding to targets

appearing at previously cued locations. The magnitude of IOR is assumed to be constant

across the entire cued visual field. More specifically, this motor theory proposes that

facilitation observed at short SOAs arises only because there is a summation of light

energy when there is temporal overlap between the cue and target (Tassinari et al, 1989;

Tassinari et al, 1994; Tassinari & Berlucchi, 1996), while the inhibition observed at long

SOAs occurs due to a motor bias such as that described above. A review of covert

orienting studies that have used non-informative peripheral cues and onset target

detection accords well with this observation (see Table 1). The motor hypothesis of IOR

is also supported by findings from attentional paradigms which require saccadic and

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manual responses to previously cued locations, and from temporal order judgement tasks

(see Taylor & Klein, 1998 for review; Gibson & Egeth, 1994a; 1994b; Klein & Taylor,

1994). In contrast, attentional models are unable to account for IOR occurring at early

SOAs, such as that found by Tassinari and colleagues. For example, the spotlight model

(described below) proposes that targets appearing in the cued location will be facilitated

at short SOAs.

An analysis of studies that have investigated reflexive attentional shifts in response to

uninformative peripheral cues indicates that the interaction between SOA, the temporal

relationship between cue and target, and the target duration can account for the presence

or absence of facilitation and IOR in nearly all COVAT experiments conducted to date

(see Table 1). Taken together, these results suggest that facilitation and IOR are

separable processes, and that the relative speed of RTs to targets appearing at the cued

location for any SOA (and therefore the presence of a RT advantage for targets appearing

at either the cued or uncued location) is dependent on the interaction between the two

processes.

In a recent study of reflexive orienting, we identified a set of conditions necessary for

facilitation to occur at short SOAs (Maruff et al, 1999). Facilitation required temporal

overlap between cue and target and the target to remain visible until the response was

made. Without both of these conditions no significant facilitation occurred at the short

SOAs. Interestingly, this facilitation was not restricted to the early SOA as proposed by

Tassinari and colleagues and by most current models of reflexive orienting. Instead, RTs

to targets appearing at the cued location were reduced by approximately 20msec across

all SOAs when there was temporal overlap between the cue and target. At short SOAs

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this caused RTs to cued targets to become significantly faster than RTs to uncued targets,

and this was hypothesised to reflect the effects of facilitation.

In a series of recent studies, Tassinari and colleagues failed to find significant facilitation

in their covert orienting experiments even under conditions in which there was temporal

overlap between cue and target at short SOAs. More specifically, Tassinari et al (1994)

observed significant IOR at 0msec SOA, and consequently inferred that the onset of IOR

precedes the onset of facilitation, a proposition which conflicts directly with the results of

most recent covert orienting studies (see Table 1). One explanation for Tassinari et al’s

(1994) results was offered recently by Lupianez and Weaver (1998), who suggested that

their findings are due to a change in the task requirements at 0msec SOA. In Tassinari et

al’s experiments, at both long and short SOAs, subjects were required to detect a

luminance increase (target onset). However, trials where cue and target appeared

simultaneously and at the same location (0msec SOA) could easily have been confused

with trials where the target is delayed and the cue appears alone. Effectively, subjects

were then required to discriminate between these two types of trials, which resulted in

slower RTs to trials where cue and target appear simultaneously and at the same location.

Therefore, the early inhibitory effect observed at 0msec SOA by Tassinari and colleagues

(1994) is most likely due to a change in task requirements rather than the effects of IOR.

Lupianez and Weaver (1998) also point out that discriminating 0msec SOA trials where

cue and target appear in the same location is more difficult than discriminating 0msec

SOA trials where cue and target appear in opposite hemifields, another factor which

could account for Tassinari et al’s (1994) observation of an early inhibitory effect.

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When considered together, the results of recent covert orienting experiments suggest that

temporal overlap between cue and target results in a reduction in RT at both short and

long SOAs. The reduction in RT under overlapping cue-target conditions at long SOAs

found by Maruff et al (1999) may be accounted for by two alternative explanations. If

the continued presence of the cue at the subsequent target location means that IOR does

not arise, or is delayed such that it is not evident at long SOAs, then the 20 msec

reduction observed by Maruff et al (1999) must be due to the increased preparation time

prior to response. In this case, it would be argued that neither facilitation nor IOR are

affecting RTs at long SOAs. Alternatively, it may be that the continued presence of the

cue causes facilitatory processes to remain active, and that the effect of this facilitation at

long SOAs is to cancel out the IOR that has arisen by that time, such that RTs to both

cued and uncued targets are equal. In this case, it would be argued that both facilitation

and IOR are having an effect on RT at long SOAs. Both explanations account for the

finding that significant IOR occurred only at later SOAs when there was no temporal

overlap between cue and target. Specifically, the former hypothesis would propose that

inhibition has time to arise when the cue is only present at the subsequent target location

for a short period of time. The latter hypothesis would propose that IOR is observed

because facilitation is unable to mask the effects of IOR when temporal conditions are

not optimal for the appearance of facilitation.

These observations suggest that facilitation arises because of the physical properties of

the spatial cues themselves, consistent with the energy summation hypothesis of Tassinari

and colleagues. However, the Tassinari hypothesis contradicts conventional models of

reflexive attention which propose that facilitation arises because of attentional shifts, the

speed of which depend on the direction and distance over which the shift must occur

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(Posner et al, 1987; Shepherd & Muller, 1988; Henderson, 1991; Rafal & Henik, 1994).

One method of assessing whether facilitation arises from sensory or attentional processes

is to investigate whether the spatial distribution of facilitation extends beyond the

location where a spatial cue is presented. Tassinari’s hypothesis would predict that

facilitation does not extend beyond the cued location, whereas conventional models of

attention would predict that it does.

The spatial distribution of facilitation and IOR.

This observation leads to a second unresolved issue concerning the nature of reflexive

attentional shifts; the spatial distribution of facilitation and IOR. Although a number of

different models have been proposed to account for the distribution of attention following

a spatial cue, these have been based mainly on performance on COVATs which measure

voluntary shifts of attention (Posner & Cohen, 1984; Eriksen & Yeh, 1985; Tsal, 1983;

Henderson, 1990; Henderson & MacQuistan, 1993; Eriksen & St.James, 1986; LaBerge,

1983) or on other attentional paradigms such as the flanker task (LaBerge, 1983). As a

consequence, no single model can account for the spatial distributions of both facilitation

and IOR following peripheral cues. For example, studies which propose that the spatial

distribution of attention is akin to a spotlight with a fixed area (Posner et al, 1987; Tsal,

1983) have based such models on COVATs which use discrimination responses and

probabilistic symbolic cues or probabilistic peripheral cues (Shulman et al, 1979: Posner,

1980; Tsal 1983; Eriksen & Yeh, 1985). On the other hand, the gradient model of

attention proposes that the spread of attention over the visual field is fixed, and that the

areas immediately surrounding the cued location are allocated the most attention while

those on the boundary receive the least. This model is based primarily on the results of

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studies which require letter discrimination following non-predictive cues (Henderson &

MacQuistan, 1993; Henderson, 1990), or which require identification of a central letter in

the presence of distractors (ie. flanker task; LaBerge, 1993). A model that seeks to

integrate both the spotlight and gradient models, the zoom lens model, contends that the

attentional focus has a variable size, limited resources and requires time to constrict and

expand. Therefore, the focus of attention can take the form of a spotlight and be focused

closely around attentional fixation, or it can zoom out to encompass a wider area. It is

generally assumed that in a resting state this type of zoom lens acts much like the

gradient model, having graduated levels of attention moving out from the centre (Eriksen

and St James, 1986). The zoom lens model is based on the performance found when

subjects were required to discriminate letters in the presence of distractors following

predictive spatial cues appearing at the target locations (Eriksen & St James, 1986) or

when subjects were required to detect targets following probabilistic central symbolic

cues or predictive peripheral cues (Shepherd & Muller, 1989). While detection

paradigms have also been used to investigate these models, studies employing such

techniques use either probabilistic central symbolic cues (McCormick & Klein, 1990) or

analyse their data in such a way that makes accurate interpretation difficult (see above

critique of Tassinari et al, 1994). Therefore, none of these models are based on data from

investigations of purely reflexive shifts of attention. As a result, none of the attentional

models proposed to date are able to account for both the facilitation and IOR components

of reflexive attentional shifts.

The motor theory makes different predictions than any of these attentional models. First,

the motor theory attempts only to account for the IOR component of reflexive attentional

shifts. The motor theory proposes that attention is allocated en masse to the uncued

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visual hemifield, and that inhibition of the cued hemifield results (Tassinari et al, 1994;

Tassinari et al, 1989). Thus, objects appearing in the cued visual field are inhibited

relative to objects appearing in the uncued visual field at long SOAs. In contrast, the

attentional models described above propose that the orienting of attention is a dynamic

phenomenon that is readily affected by changes in the immediate visual environment.

The aim of the current set of experiments was to investigate the spatial and temporal

distributions of both the facilitation and IOR components of reflexive attentional shifts.

The first experiment replicated and extended the findings of Maruff et al (1999) to ensure

that the temporal relationship between cue and target also constrained the appearance of

facilitation and IOR at 18 degree locations. Experiment 2 investigated the spatial

distribution of facilitation and IOR using both optimal and non-optimal temporal

conditions and compared RTs to targets that appeared at either 9 or 18 degrees

eccentricity following cues presented at 9 or 18 degrees. Finally, Experiment 3

investigated the time course of the facilitation and IOR components of reflexive

attentional shifts.

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Experiment 1

The first experiment sought to replicate and extend the findings of Maruff et al (1999),

who found that significant facilitation occurred only at short SOAs when there was

temporal overlap between cue and target, whereas significant IOR occurred only at

longer SOAs when there was no temporal overlap between cue and target. However,

Maruff et al (1999) investigated performance at an eccentricity of 10 degrees. Therefore,

before inferences about shifts of attention can be based on the comparison of

performance between 9 and 18 degree locations (see Experiment 2), it is important to

ensure that facilitation and IOR also depend on the cue-target temporal relationships

when stimuli are presented at eccentricities greater than 10 degrees.

Methods

Subjects

Fourteen subjects (6 males) aged between 21-53 years (mean = 26.07 years, SD = 10.75

years) participated in the study after giving informed consent. All subjects had normal or

corrected-to-normal vision. All subjects were naive to the purpose of the experiment,

although some had experience with reaction time tasks.

Apparatus

All testing was conducted in a sound attenuated, dimly lit ocular motor laboratory. An

IBM compatible 486 computer and a colour monitor were used to present visual stimuli

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and record responses. The stimulus presentation and response recording software are

accurate to +/- 1 msec and the stimulus parameters used here were identical to those used

by us in all of our previous studies (e.g. Maruff et al, 1999; Maruff & Currie, 1995).

Subjects responded using a hand held microswitch. Throughout the experimental session

three landmark circles remained visible on the computer screen. The background was

black with a luminance of 0.05 cd/m3. Each landmark circle subtended 1.5 degrees of the

visual angle. The centre of the middle landmark circle was in the centre of the display.

The centres of the two peripheral landmark circles were 18 degrees from fixation. Each

landmark circle was coloured green with a luminance of 40.2 cd/m3. Spatial cues were

presented by drawing a second green circle, with a luminance of 60.6 cd/m3, around one

of the three display circles. Therefore, cue onset was marked by a simultaneous increase

in luminance and line width. The target was a small filled red circle (0.6 degrees) that

could appear in either the left or right landmark circle. The luminance of the target was

63.2 cd/m3.

Procedure

Subjects were seated and instructed to hold the response button in their preferred hand.

They were instructed to fix their eyes on the central landmark circle of the COVAT

display but to attend to all three landmark circles and to respond, as quickly as possible,

to the appearance of a target in any one of the circles. Subjects were instructed that

although they may see spatial cues at different locations, these had no meaning and that

their task was to detect the onset of a target in any of the marked locations as quickly as

possible. In previous studies examining the effect of cue expectancies on target detection

we have found that COVAT data is most reliable when subjects are provided with no

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information about cue-target relationships (Maruff & Currie, 1995; Danckert et al, 1998).

Therefore, subjects were not instructed about the possible locations of targets, the

possible cue duration or the probability with which the target would appear at any

location. Eye movements were monitored by the experimenter using an infrared scleral

reflectance technique which sampled eye position at 1kHz (Maruff et al, 1999). Trials

where eye movements occurred were excluded from the testing session and rescheduled

automatically to the end of the block of trials. Trials where saccadic drift of greater than

0.5 degrees occurred were removed from the analysis and rerun in later blocks if the

percentage of trials removed was greater than 5% (Muller & Rabbitt, 1989). The timing

of a COVAT trial is shown in Figure 1. Each trial began with an inter-trial-interval (ITI)

that could vary randomly between 1010 and 1090 msec. Following the ITI, a spatial cue

was presented. The spatial cue could have a duration of 50msec (non-overlapping cue) or

remain present until a response was made (overlapping cue). Spatial cues could appear at

either of the two peripheral landmark circles with equal probability. The SOA varied

between intervals of 150 and 850 msec. The target then appeared inside one of these

locations and remained visible until the subject responded or until five seconds had

elapsed. Targets appeared at the cued or uncued location with equal probability, but were

never presented at the centre landmark circle. Within a single block of 176 trials, 16

trials where no target followed the cue were also included. These catch trials were

included to discourage anticipatory responses. Subjects completed two blocks of trials in

separate testing sessions giving 40 RTs for each combination of cue duration, cue type

and SOA.

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Data analysis

For each subject the distributions of data were inspected and RTs less than 100msec and

greater than 1000msec were defined as anticipatory responses or abnormally slow

responses and excluded from the data analysis. In this experiment, each subject was

presented with 32 catch trials. The median percent of responses on these trials (ie false

alarm rate) was 3% (range 0% -5%). When catch trials were further analysed in

overlapping and non-overlapping cue conditions, median false alarm rates were 4% and

2%, respectively. The mean number of the remaining 320 trials that were excluded

because of abnormally slow responses or saccadic drift of greater than 0.5 degrees from

fixation was 0.9% (range 0.3% - 1.3%). For each subject, the mean RT for each

experimental condition was calculated. As the major hypotheses were tested by the

presence or absence of significant differences between RTs to targets appearing at the

cued and uncued location, these differences were compared with a series of within groups

t-tests. The differences also gave the magnitude of facilitation and IOR under each

experimental condition. Facilitation was inferred to have occurred when RTs to targets

appearing at the cued location were significantly faster than RTs to targets appearing at

the same location in the uncued field. Inhibition of return was inferred to have occurred

when RTs to targets appearing at the cued location were significantly slower than RTs to

targets appearing at the same location in the uncued field (Posner & Cohen, 1984;

Maylor, 1985; Tassinari et al, 1994; Egly et al, in press). Therefore, facilitation always

had a positive value and IOR a negative value.

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Results

The group mean RTs for each cuetype at each SOA under the non-overlapping and the

overlapping cue conditions are shown in Table 2 (Experiment 1), as are the magnitude of

the facilitation and IOR associated with each SOA for the non-overlapping and

overlapping cue conditions. For the non-overlapping condition, there was no significant

facilitation or IOR for the 150msec SOA (t(13)<1), but significant IOR at the 850msec

(t(13)= 4.3, p=0.001) SOA. For the overlapping cue condition, there was significant

facilitation at the 150msec SOA (t(13) = 3.9, p = 0.005), but no significant facilitation or

IOR at the 850msec (t(13) = 1.6, p = 0.1) SOA.

Discussion

The pattern of facilitation and IOR observed with cues and targets at 18 degrees

eccentricity in Experiment 1 was identical to that found previously at 10 degrees

eccentricity (Maruff et al, 1999). Facilitation occurred only when the SOA was short and

there was temporal overlap between cue and target. IOR occurred only when the SOA

was long and there was no temporal overlap between cue and target. Table 2 shows that

RTs to uncued trials were nearly identical in the overlap and non-overlap conditions

whereas under the overlap condition, RTs to cued trials decreased by approximately

20msec at both the short and long SOA. Therefore, a required condition for facilitation

and IOR to be observed is that the cue appears in the same spatial location as the

subsequent target. This suggests that the cue is the source of facilitation at short SOAs,

and also that the cue differentially effects the presence of IOR. Specifically, if IOR

begins when attention leaves the cue and returns towards fixation, then the continued

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presence of the cue either delays the departure of attention, or holds attention at the cued

location, both of which would result in a reduction in the magnitude of IOR.

Alternatively, it may be that the continued presence of the cue results in facilitatory

processes remaining active at long SOAs, and overcoming the effects of IOR that have

arisen by then.

The absence of facilitation at 150 msec SOA with a non-overlapping cue in Experiment

1, when considered in light of previous research which has shown that facilitation can be

observed at SOAs as short as 50 msec (Posner & Cohen, 1984; Lambert & Hockey, 1991;

Rafal et al, 1991), suggests that cue offset results in the withdrawal of attention from the

cued location. This early withdrawal of attention also leaves enough time for IOR to be

initiated at longer SOAs. In the current study, the RTs were consistently slower than

those observed in our previous experiments (Maruff et al, 1999), although the magnitude

of facilitation and IOR was equivalent. The slower RTs probably reflect the greater

eccentricity of targets in the current study, and suggest that the effect of retinal

eccentricity should be considered when attempting to describe the spatial distribution of

either facilitation or IOR.

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Experiment 2

The aim of Experiment 2 was to investigate the spatial distribution of the facilitation and

IOR components of reflexive attentional shifts. The comparison of RTs to 18 degree

targets following 9 degree cues with RTs to 9 degree targets following 18 degree cues

will provide some insight into the way in which facilitation and IOR are distributed

across the visual field. If facilitation is purely a sensory phenomenon, as Tassinari and

colleagues propose (Tassinari et al, 1989; Tassinari et al, 1994; Tassinari & Berlucchi,

1996; Berlucchi et al, 1989), then it should occur only at locations where the cue and

target appear simultaneously. However, if facilitation arises because of attentional

processes (Posner, 1980; Tsal, 1983; LaBerge & Brown, 1989; Eriksen & St.James,

1986; LaBerge, 1983; Henderson & MacQuistan, 1993) then it should occur at other

locations within the same hemi-field as the cue, though the gradient model would predict

reductions in the magnitude of that facilitation at distances further from the cue. If IOR is

dependent on facilitation as the biphasic model predicts, then its spatial distribution

should mirror that found for facilitation. The absence of any correlation between the

spatial distributions of facilitation and IOR would suggest that the two processes are

independent. Experiment 1 and Maruff et al (1999, Experiment 1) showed that the

magnitude of IOR at long SOAs was decreased if there was temporal overlap between the

cue and target. A likely explanation for the reduction in the magnitude of IOR at long

SOAs is that the facilitatory processes arising from spatial cues which overlap the target

in time can overcome the IOR that occurs when there is no temporal overlap between cue

and target (Maruff et al, 1999). Therefore, it should be possible to investigate the spatial

distribution of facilitation at long SOAs by determining the extent to which temporally

overlapping cues can overcome IOR at locations in the same visual field.

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Method

Subjects

Twenty eight subjects (10 males) aged between 19-32 years (mean = 23.01 years, SD =

5.67 years) participated in the study after giving informed consent. All had normal or

corrected-to-normal vision. Subjects were naive to the purpose of the experiment

although some had experience with RT tasks. No subject had participated in Experiment

1.

Apparatus and Procedure

The COVAT display, materials and recording procedure were identical to that described

for Experiment 1. However, in this experiment five landmark circles remained visible

on the computer screen (see Figure 1). Each landmark circle subtended 1.5 degrees of

the visual angle. The centre of the middle landmark circle was in the centre of the

display. The centres of the two peripheral landmark circles in each visual field were 9

and 18 degrees from fixation. Subjects were instructed to fix their eyes on the central

landmark circle of the COVAT display but to attend to all five landmark circles and to

respond, as quickly as possible, to the appearance of a target in any one of the circles.

One group of 14 subjects performed a version of the COVAT where the cue remained

present until the subject responded for the 150msec SOA and the cue duration was 50

msec for the 850msec. The second group of 14 subjects performed a version of the

COVAT where the cue duration was 50msec for the 150msec SOA and the cue remained

present until the subject responded for the 850msec SOA. In all conditions the target

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remained visible until the subject responded or until five seconds had elapsed. Within a

single block of trials, when the cue appeared at the 18 degree location, the target could

appear at the 18 degree location in the same hemifield (20 trials), at the 9 degree location

in the same hemifield (20 trials) at the 9 degree location in the opposite hemifield (20

trials) or at the 18 degree location in the opposite hemifield (20 trials). When the cue

appeared at the 9 degree location, the target could appear at the 18 degree location in the

same hemifield (20 trials), at the 9 degree location in the same hemifield (20 trials) at the

9 degree location in the opposite hemifield (20 trials), or at the 18 degree location in the

opposite hemifield (20 trials). Catch trials (10 trials for each cue condition interspersed

evenly across cue locations), where no target appeared after the spatial cue, were also

included in a block of trials to discourage subjects from making anticipatory responses

(180 trials per block). To complete an experiment, each subject performed four blocks

of trials over two testing sessions. Therefore, each subject gave a total of 40 trials for

each combination of cue location, target location and SOA. Rest periods of 2 to 3

minutes were given between blocks of trials.

Data analysis

The mean number of trials excluded for abnormally slow responses or saccadic drift of

greater than 0.5 degrees from fixation was 0.19% (SD = 0.1%). In this experiment

subjects were presented with 80 catch trials. The median percent of responses on these

trials (ie false alarm rate) was 4% (range 0-12%). False alarm rates were further analysed

according to cue/target temporal relationship and cue eccentricity. When there was

temporal overlap between the cue and the target, median false alarm rates for cues

appearing at 9° and 18° were 3% and 8%, respectively. When there was no temporal

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overlap between the cue and the target, median false alarm rates for cues appearing at 9°

and 18° were 1% and 4%, respectively. For each subject, the mean RT of included trials

for each experimental condition was calculated. Once again the major hypotheses were

tested by the direction and significance of any difference in RTs to targets appearing at

the cued and uncued locations. However, in calculating these difference scores it was

noted that previous research has found that RTs to targets varies as a function of their

distance from fixation (Haines & Gilliland, 1973). In addition, a comparison of

Experiment 1 in the current study with that of Maruff et al (1999, Experiment 1)

indicated that RTs to 18 degree targets were generally slower than to 10 degree targets.

Pilot studies conducted on the current display indicated that RTs to uncued targets

appearing at the 18 degree location were on average 40msec slower than RTs to targets

appearing at the 9 degree location. Therefore, to control for the effect of retinal

eccentricity when calculating the magnitude of facilitation and IOR, RTs to targets

appearing at each location in the cued field were subtracted from RTs to targets appearing

at the mirror symmetric location in the opposite visual field. For example, RTs to targets

appearing at the 18 degree location in the cued visual field were subtracted from RTs to

targets appearing at the 18 degree location in the opposite field.

Results

The group means of mean RTs to each target location following each cue location at both

SOAs under the optimal and non-optimal conditions are shown in Table 2 (Experiment

2). Table 2 also shows the magnitude of facilitation and IOR for 9 and 18 degree targets

following 9 and 18 degree cues under the temporal overlap and no temporal overlap

conditions. For the 150msec SOA, when there was temporal overlap between the cue and

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target and the cue appeared at the 18 degree location, RTs to targets appearing at the 18

degree location in the cued field were significantly faster then RTs to targets appearing at

the 18 degree location in the uncued hemifield (t(13) = 5.85, p<0.001). Similarly, RTs to

targets appearing at the 9 degree location in the cued field were significantly faster than

RTs to targets appearing at the 9 degree location in the uncued field (t(13) = 4.22,

p<0.01). The facilitation found for targets appearing at the 18 degree location was

significatnly greater than for targets appearing at the 9 degree location (t(13) = 5.6;

p<0.01). When the cue appeared at the 9 degree location, RTs to targets appearing at the

18 degree location in the cued field were not significantly different from RTs to targets

appearing at the 18 degree location in the uncued field (t(13)<1). However, RTs to

targets appearing at the 9 degree location in the cued field were significantly faster than

RTs to targets appearing at the 9 degree location in the uncued field (t(13) = 3.44,

p<0.01). When there was no temporal overlap between the cue and target at the

150msec SOA and the cue appeared at the 18 degree location, there was no significant

difference between RTs to targets appearing in the cued or uncued location at either the

18 degree or 9 degree locations. Similarly, when the cue appeared at the 9 degree

location there was no significant difference between RTs to targets appearing in the cued

or uncued location at either the 9 or 18 degree locations.

For the 850msec SOA, when there was no temporal overlap between cue and target and

the cue appeared at the 18 degree location, RTs to targets appearing at the 18 degree

location were significantly slower than RTs to targets appearing at the 18 degree location

in the uncued field (t(13) = -2.9, p < 0.01). Similarly, RTs to targets appearing at the 9

degree location in the cued hemifield were significantly slower than RTs to targets

22

appearing at the 9 degree location in the uncued visual hemifield (t(13) = -4.03, p<0.01).

For the 9 degree cue, RTs to targets appearing at the 18 degree location in the cued field

were significantly slower than RTs to targets appearing at the 18 degree location in the

uncued field (t(13) = -4.80, p<0.001). Similarly, RTs to targets appearing at the 9 degree

location in the cued field were significantly slower than RTs to targets appearing at the 9

degree location in the uncued field (t(13) = -3.87, p<0.01).

When there was temporal overlap between the cue and target at the 850msec SOA and

the cue appeared at the 18 degree location, there was no difference between RTs to

targets appearing at the 18 degree location in the cued and uncued fields or between RTs

to targets appearing at the 9 degree location in the cued and uncued fields. For the 9

degree cue, there was no significant difference between RTs to targets appearing at the 9

degree location in the cued and uncued fields. However, RTs to targets appearing at the

18 degree location in the cued field were significantly slower than RTs to targets at the 18

degree location in the uncued field (t(12) = -3.2, p<0.01).

Discussion

The results of Experiment 2 indicate that facilitation at the short SOA was not restricted

to the cued location. When the cue appeared at the 18 degree location, significant

facilitation was found for targets appearing at both the 18 and 9 degree locations in the

same field. However, the magnitude of this facilitation was larger for targets appearing

at the 18 degree location. When the cue appeared at the 9 degree location, facilitation

was observed only for targets appearing at the 9 degree location in the same field. Taken

23

together, these results suggest that facilitation arises as a consequence of attentional shift

rather than reflecting sensory processes, which would be restricted to the cued location.

Reflexive attentional shifts initiated by peripheral cues extend from fixation to the cue

but not beyond the cue to more lateral locations.

For the long SOA, significant IOR was observed for targets appearing at both the 9 and

18 degree locations irrespective of the eccentricity of the cue. This result is consistent

with the hypothesis that IOR arises due to a motor bias (Klein & Taylor, 1994) against

responding to targets in the cued hemifield (Tassinari et al, 1989; Tassinari et al, 1994;

Tassinari & Berlucchi, 1996). It also suggests that the motor bias is reduced as targets

appear further from fixation. Importantly, the different spatial distributions found for

facilitation and IOR suggest strongly that the two processes are separable, and may be

independent.

The results also show that the magnitude of IOR at long SOAs was reduced when there

was temporal overlap between cue and target. This is consistent with the findings of

Experiment 1. At the 850msec SOA, when cues were presented at 18 degrees, no

significant IOR was found at either the 18 or 9 degree locations. When the cue was

presented at the 9 degree location at the 850msec SOA, no significant IOR was found at

the 9 degree location. However, significant IOR was still found for targets appearing at

the 18 degree location. These results suggest that the facilitation of RTs that occurs with

temporal overlap between cue and target can cancel out the effect of IOR at long SOAs

and that the spatial distribution of this facilitation at long SOAs is the same as that found

for significant facilitation at early SOAs. The results also suggest that facilitation is a

unitary process that is independent of IOR. For the 150msec SOA, when there was no

24

temporal overlap between cue and target, the magnitude of facilitation was small and

non-significant for all combinations of cue location and target location. This is consistent

with the hypothesis that facilitation is not the result of sensory processes restricted to the

cued location. The absence of facilitation at short SOAs when there was no temporal

overlap between cue and target is also consistent with previous studies which have found

no, or small non-significant, facilitation under the same conditions (Tassinari et al, 1994;

Bowman et al, 1993; Possamai, 1986; see Table 1).

25

Experiment 3

The results of Experiments 2 suggest that the spatial distributions of facilitation and IOR

are different, and may reflect the independence of these two processes. Facilitation

spreads from the cued location to fixation, but IOR is evident across the entire visual

field, with medial locations receiving more inhibition than more lateral locations. The

aim of Experiment 3 was to the determine the temporal properties of the spatial

distributions of facilitation and IOR by manipulating SOA and the temporal relationship

between cue and target, and comparing RTs to targets appearing at the 18 and 9 degree

locations following 18 degree cues. Experiment 2 found that facilitation was greater for

18 degree targets than 9 degree targets following 18 degree cues at 150msec SOA. If

facilitation moves from fixation toward the peripheral cue then at 50msec SOA this

pattern should reverse and the magnitude of facilitation for 9 degree targets should be

greater than for 18 degree targets. Alternatively, if facilitation begins at the cued location

and then moves toward fixation, the magnitude of facilitation should be greater at the 18

degree location irrespective of SOA, and the magnitude of facilitation for 9 degree targets

should increase with SOA. Experiment 2 also showed that the magnitude of IOR was

greater for 9 degree targets than for 18 degree targets irrespective of the cued location.

Therefore, if IOR arises because of a motor bias, its magnitude should be consistently

greater for 9 degree targets than for 18 degree targets as SOA increases beyond 750msec.

Experiment 2 and a previous study (Maruff et al, 1999) have suggested that the effects of

facilitation are not restricted to early SOAs, but are also active at longer SOAs. More

specifically, Experiment 2 and Maruff et al (1999) showed that facilitation can overcome

IOR at long SOAs when there is temporal overlap between the cue and the target. If

26

facilitation is a unitary process independent of IOR, then the nullifying effect of

facilitation on IOR should remain evident as SOA increases beyond 750 msec. Similarly,

Experiment 2 and Maruff et al (1999) showed that neither facilitation nor IOR occurred at

short SOAs when there is no temporal overlap between cue and target. Two factors can

account for these results: the hypothesis that facilitation is a result of sensory processes

that are not active in the absence of a continuous cue, and the finding that IOR takes

longer than 150 msec to have an effect. Experiment 3 also investigated whether these

effects remained at SOAs ranging from 50 to 150 msec.

Subjects

Twenty subjects (4 males) aged between 20-40 years (mean = 23.6 years, SD = 8.43

years) participated in the study after giving informed consent. All subjects had normal or

corrected to normal vision on self report. All subjects were naive to the purpose of the

experiment. None of the subjects had participated in Experiment 1 or 2 but all had

participated in reaction time studies previously.

Apparatus and Procedure

The COVAT display, materials, recording procedure and cue durations were identical to

that described for Experiment 2. Spatial cues appeared only at the 18 degree location,

although cues and targets were presented to the left and right visual fields with equal

probability. On half of the trials there was temporal overlap between the cue and target

and on the other half there was no overlap between cue and target. Within a single block

of trials, when the cue appeared at the 18 degree location, the target could appear at the

27

18 degree location in the same hemifield (30 trials), at the 9 degree location in the same

hemifield (30 trials), at the 9 degree location in the opposite hemifield (30 trials) or at the

18 degree location in the opposite hemifield (30 trials). Catch trials (5 trials at each

location), where no target appeared after the spatial cue, were also included in a block of

trials to discourage subjects from making anticipatory responses (total 20 catch trials).

In each block of 140 trials, SOA varied pseudorandomly between 50, 100, 150, 750, 800

and 850msec and the location of cues and targets varied pseudorandomly between visual

fields. As the size of facilitation and IOR for 9 degree targets following 18 degree cues

was found to be small in Experiment 1, the number of trials per condition for each subject

was increased to provide the additional statistical power necessary to investigate these

potentially small effects. Therefore, to complete the experiment each subject performed

28 blocks of trials in fourteen testing sessions. These testing sessions were carried out

over a one month period. Each subject performed three testing sessions per day with an

median of four days between testing sessions (range 2-7 days). This gave a total of 80

trials for each combination of cue duration, target location and SOA. Rest periods of 2 to

3 minutes were given between blocks of trials.

Data analysis

In this experiment, each subject was presented with 560 catch trials. The median percent

of responses on these trials for the experiment overall (ie false alarm rate) was 3% (range

0% -5%). When catch trials were analysed according to overlapping and non-overlapping

cue conditions, median false alarm rates were 4% and 3%, respectively. The mean

number of the remaining 3360 trials that were excluded for abnormally slow responses or

saccadic drift of greater than 0.5 degrees from fixation was 1.19% (range 0.07% - 2.2%).

28

For each subject, the mean RT of included trials for each experimental condition was

calculated. These means were submitted to a cue duration (overlapping, non-overlapping)

x cuetype (cued, uncued) x target location (18°, 9°) x SOA (50, 100, 150, 750, 800, 850)

repeated measures analysis of variance (ANOVA). The major hypothesis was tested

using ANOVA to determine the significance of a cue duration x cue type x target location

x SOA interaction. Because different patterns of performance were expected between the

three short SOAs (50, 100, 150) and the three long SOAs (750, 800, 850), data from these

two sets of SOAs were analysed separately. As a very large number of trials were

necessary to investigate the hypotheses in Experiment 3, it is possible that there was a

change in performance across time, although the random presentation of trials ensured

that any such changes would have affected all trials equally. However, to determine

whether there was a change in performance across testing sessions, a facilitation and IOR

value was calculated for each subject for each of their fourteen testing sessions.

Facilitation and IOR were calculated for trials in the overlapping and non-overlapping

conditions across all SOAs less than 150msec and separately across all SOAs greater than

750msec. Each value was submitted to a one-way repeated measures ANOVA with trial

block number treated as the independent variable.

Results

The group means of mean RTs to each target location following each cue location at both

SOAs under the optimal and non-optimal conditions are shown in Table 2 (Experiment

3). ANOVA indicated that interaction between cue duration x cuetype x target location x

SOA was significant (F(15,285) = 4.2; p<0.05). For short SOAs, when the cue appeared

at the 18 degree location and there was temporal overlap between cue and target,

29

significant facilitation was found when targets appeared at the 18 degree location. There

was no significant difference between SOAs in the magnitude of this facilitation.

However, when targets appeared at the 9 degree location the magnitude of facilitation

increased significantly from the 50msec SOA to the 100msec SOA (t(13) = 4.24; p<0.01)

and from the 100msec SOA to the 150msec SOA (t(13) = 5.34; p<0.01). When there was

no overlap between cues and targets, no significant facilitation or IOR was found for any

of these early SOAs at either target location.

For long SOAs, when the cue appeared at the 18 degree location and there was no

temporal overlap between the cue and target, significant IOR was found when targets

appeared at both the 9 or 18 degree locations. There was no difference in the magnitude

of IOR between the SOAs for targets appearing at either the 9 degree or 18 degree

location. However, the magnitude of IOR was larger for 9 degree targets (mean =

22.03msec) than for 18 degree targets (mean = 14.7msec) when averaged across all SOAs

(F(2,18) = 83.6; p<0.001). When there was temporal overlap between cue and target no

significant facilitation or IOR was found for any of these long SOAs at either target

location. Finally, there was no significant effect of testing session on the magnitude of

facilitation averaged across the 50, 100 and 150msec when there was temporal overlap

between cue and target (F(8,12) < 1, mean facilitation = 19.9, range 15.5 - 25.1) and no

significant effect of testing session on the magnitude of IOR averaged across the 750, 800

and 850msec when there was no temporal overlap between cue and target (F(8,12) <1,

mean = 21.3, range = 17.5 - 24.5).

30

Discussion

These results show that the spatio-temporal distribution of the facilitation and IOR

components of reflexive attentional shifts are quite different. Initially, facilitation occurs

only at the cued location and spreads from there toward fixation as the time between cue

and target increases. IOR is always greater for the location closer to fixation, and

although the magnitude of IOR at both the 9 and 18 degree locations increases with SOA,

the spatial distribution does not change. These results show that the spatio-temporal

distributions of facilitation and IOR are qualitatively different and provide further support

for the hypothesis that the two components of reflexive attentional shifts are separable.

Examination of Table 2 (Experiment 3) also reveals that the continued presence of the

cue at long SOAs results in a lack of IOR, consistent with the results of Experiment 2 and

previous research (Maruff et al, 1999). One explanation for this finding is that IOR fails

to be generated, or is delayed, when the cue remains on. Alternatively, it may be that the

continued presence of the cue results in facilitation of the cued location even at long

SOAs, which is manifested as a reduction in the magnitude of IOR so that RTs to both

cued and uncued target locations become equal.

Perhaps surprisingly given recent research which has found that the magnitude of

location-based, as well as object-based and static-display IOR, diminishes with practice

(Weaver, Lupianez & Watson, 1999), there was no effect of practice on COVAT

performance in Experiment 3. However, the current result is not without support, as both

Muller & Von Muhlenen (1996) and Pratt & McAuliffe (1999) have reported a lack of

practice effects on the magnitude of IOR in static displays, such as that used in the

current study.

31

General Discussion

The results of the current experiments suggest that facilitation cannot be described

adequately as reflecting only sensory processes at the cued location. Instead, they

suggest that facilitation arises because attentional shifts are made toward the peripheral

cue. These attentional processes are restricted initially to the cued location but as the

time between cue and target increases they spread from the cued location toward fixation.

Experiment 2 found that facilitation did not extend to locations that were more lateral

than the cued location. These results are consistent with the zoom lens or focusing model

of attention, where the attentional focus is initially attracted to the location of the

peripheral cue, but spreads to other locations in the same field as time passes. Unlike

facilitation, the spatial distribution of IOR extends across the entire cued field, although

its magnitude is greater for targets which appear closer to fixation. In addition, the

spatial distribution of IOR is not altered when SOA increases beyond 750msec. These

data are consistent with a motor view of IOR.

The difference between the spatial distributions of facilitation and IOR strongly suggest

that the two processes are separable, and that they do not arise because of the temporally

defined ‘biphasic’ effect of peripheral cueing (Rafal et al, 1989; Rafal & Henik, 1994).

Instead, the current data and that of a previous study (Maruff et al, 1999) suggest that

non-informative peripheral cues give rise to facilitation and IOR separately. The effect of

these two separable processes on RT depends upon the SOA and the temporal

relationship between cue and target, and are restricted to targets which appear in the cued

field. Responses to targets appearing at the cued location are approximately 20msec

faster when there is temporal overlap between cue and target (compared to the non-

32

overlapping condition) (Table 2; see also Maruff et al, 1999). While this reduction in RT

occurs equally across SOAs, its consequences for the appearance of facilitation at short

SOAs and IOR at long SOAs are different. More specifically, the reduction in RT that

occurs with temporal overlap between cue and target at short SOAs results in RTs to

targets appearing at the cued location becoming faster than RTs to targets appearing at

the uncued location. However, at long SOAs this reduction leads RTs to targets

appearing at the cued location to become equal to RTs to targets appearing at the uncued

location. Conversely, when there is no temporal overlap between cue and target and

SOAs are short, RTs to targets appearing at the cued and uncued location are equal,

whereas at long SOAs RTs to targets appearing at the cued location are significantly

slower than RTs to targets appearing at the uncued location. These results suggest that

the appearance of facilitation or IOR depends upon the SOA, the temporal relationship

between cue and target and the interaction between facilitatory and inhibitory processes.

Similarly, these results suggest that IOR and facilitation may be independent processes,

however, further empirical investigation needs to be undertaken before such a claim can

be substantiated.

While the effects of facilitation are evident at long SOAs under optimal conditions

(temporal overlap between cue and target), the effects of IOR are absent at short SOAs

under optimal conditions (no temporal overlap between cue and target, Experiment 1, 2

& 3; Maruff et al, 1999). This suggests that IOR takes longer than 150msec to take

effect. As a consequence, the reduction in RT that occurs when there is temporal overlap

between cue and target is sufficient for facilitation to appear at SOAs less than 150msec.

However, at longer SOAs the RT advantage associated with temporally overlapping cues

and targets can only reduce the magnitude of the IOR. Evidence to support the

33

hypothesis that peripheral cues give rise to both facilitatory and inhibitory processes is

seen in the results of the current Experiments.

These results are consistent with some aspects of previous studies. For example,

Shepherd & Muller (1989; Experiment 2) investigated the spatio-temporal distribution of

reflexive attention by using predictive peripheral cues at SOAs which extended from

50msec to 500msec. While in this study the magnitude of costs and benefits were

calculated relative to a neutral cue, their raw data (Table 3) can be recalculated in the

same way as in the current study. At short SOAs (50msec) facilitation was restricted to

the cued location (18 degrees) and spread from this location to other locations in the cued

visual field (8 degrees) as SOA increased to 200 msec. No facilitation was observed for

targets appearing at 18 degrees following cues at the 8 degree location at any SOA.

These results are generally consistent with the current findings although the magnitude of

facilitation reported by Shepherd & Muller (1989) was larger than that found in the

current study. In addition, they found no IOR at the 500msec SOA despite using a cue

duration of 50msec. However, Shepherd & Muller (1989) used predictive peripheral

cues, which have been shown to confound reflexive attentional shifts with voluntary

attentional shifts (Rafal & Henik, 1994; Maruff & Currie, 1995). For example, it has

been observed that the magnitude of facilitation found at early SOAs increases when

predictive peripheral cues signal attentional shifts compared to when non-predictive

peripheral cues are used (Maruff & Currie, 1995; Maruff et al, 1999; Danckert et al,

1998; Jonides, 1980). A number of studies using predictive peripheral cues of a brief

duration (50-70msec) have reported significant facilitation at SOAs of 150msec or

greater (Maruff & Currie, 1995; Maruff et al, 1995). Finally, the voluntary attentional

component of predictive peripheral cues can overcome the IOR that is found at SOAs

34

greater than approximately 250msec (Maruff et al, 1995; Rafal & Henik, 1994; Posner et

al 1982; Egly et al, in press). Confirming these findings, Shepherd and Muller (1989)

also found no significant IOR at their 500msec SOA. Therefore, Shepherd & Muller’s

(1989) findings cannot be considered to be contrary to the current results.

The current results support the model of IOR proposed by Klein and Taylor (1994; see

also Taylor & Klein, 1998), and Tassinari and colleagues (Tassinari et al, 1989; Tassinari

et al, 1994; Tassinari et al, 1996; Berlucchi et al, 1989), which postulates that IOR arises

as a motor bias against responding to stimuli appearing in the cued hemifield. The motor

bias model of IOR can account for the fact that inhibition was found to effect the entire

cued hemifield in the current study, regardless of cue location. In contrast, an attentional

model of IOR would propose that only the cued location would be inhibited, while targets

appearing in other locations in the cued hemifield would not be inhibited. In regards to

the debate concerning how IOR is generated (see Taylor & Klein, 1998 for a review), two

possible explanations for a motor bias have been proposed: (1) that IOR arises as a result

of response inhibition, in particular, the inhibition of a saccade toward the cued location;

and (2) that IOR is a result of activation of an oculomotor program to fixate the cue.

While previous research suggests that the latter may be the more likely of these proposals

(Rafal et al, 1989; see Taylor & Klein, 1998 for other evidence), the current results are

unable to make such a distinction.

With respect to the spatial distribution of the IOR component of attention, Tassinari and

colleagues (Tassinari et al, 1989; Tassinari & Berlucchi, 1996; Tassinari et al, 1994;

Berlucchi et al, 1989) have proposed that IOR is always greatest at the cued location, but

extends to other locations in the cued field. The current results do not support this view.

35

Instead, they suggest that IOR is always greatest at locations closer to fixation

irrespective of where the cue appears (provided it is in the same hemi-field). Tassinari

and colleagues results may have been confounded by their method of data analysis, where

they combine RTs to targets appearing at medial and lateral locations in a number of their

studies in order to calculate facilitation and IOR (Tassinari et al, 1993; Tassinari et al,

1994; see Tassinari & Berlucchi, 1996 for a review). The current results indicate that as

much as 40msec difference can occur between RTs to targets appearing at 9 and 18

degree locations in the uncued visual field, irrespective of the location of any preceding

cue. Tassinari and colleagues' calculations do not take into account this eccentricity

effect, which results in the relationship between the magnitude of IOR and and the effect

of retinal eccentricity being obscured.

A recently proposed attentional model, that of attentional momentum, must also be

considered in relation to the current results (Pratt, Spalek & Bradshaw, 1999). The

attentional momentum model proposes that targets appearing along the path on which

attention is travelling receive a RT benefit relative to targets appearing off the path of

attentional momentum. Thus, in the standard COVAT paradigm, facilitation occurs at

short SOAs because attention is moving toward the cue when the target appears, whereas

IOR occurs at long SOAs because attention is first drawn to the cue, and then has enough

time to begin to move back in the opposite direction towards central fixation. Therefore,

at long SOAs responses to targets appearing in the uncued hemifield would be faster than

targets appearing in the cued hemifield as targets in the cued hemifield are off the path of

attentional momentum. The attentional momentum model adequately accounts for the

IOR observed in the current study. However, this model would predict that targets

36

appearing lateral to the cued location in the cued hemifield would receive facilitation

relative to targets appearing in the uncued hemifield at short SOAs. The results of

Experiments 1 and 2, where no facilitation was observed for such lateral targets at short

SOAs, therefore argue against an attentional momentum account of covert orienting.

A number of issues arise from the current results. One important issue is why IOR

doesn’t occur when the cue remains present at long SOAs. While it appears that the

continued presence of the cue means that facilitatory effects are sufficient to remove any

effect of IOR on RTs, it also seems clear that IOR is not cancelled out. Evidence to

support this proposal can be seen in the results of Experiment 2, where under non-optimal

temporal conditions at 850 msec SOA, the facilitatory processes overcomes the IOR at 9

degree target locations regardless of cue location, whereas IOR at the 18 degree target

location is only overcome when there is an 18 degree cue. This masking pattern is

identical to the pattern of facilitation observed at 150 msec SOA when there was no

temporal overlap between cue and target. The continued presence of IOR outside these

facilitatory boundaries suggests that the peripheral cue does not cancel out the motor bias

at a longer SOA, but is sufficient to overcome it.

Another issue that needs to be raised is the conflict between the current results, where

brief cues (that do not overlap the target) fail to produce facilitation, and results observed

in discrimination paradigms, where facilitation is commonly observed with such cues (eg;

see Henderson & MacQuistan, 1993). While it may be that successful performance of

discrimination tasks requires more or different aspects of attention than performance of

simple detection tasks, the current results provide little insight into what such differences

may be, or exactly what effects these differences have on the orienting of attention.

37

Finally, it must be noted that the current study employed experimental methods which

were similar to that used by Tassinari et al (1994). However, there are a number of

important differences between the current experiments and those of Tassinari. First, at no

stage in the current study was a 0 msec SOA employed, which is a critical distinction

given Lupianez and Weaver’s (1998) critique of Tassinari’s use of such an SOA. Second,

the effects of retinal eccentricity were accounted for when determining the magnitude of

facilitation and IOR in the current study through the comparison of mirror symmetric

locations. Tassinari and colleagues (1994) failed to take these effects into account. Third,

and perhaps most importantly, the current study employed catch trials where Tassinari et

al (1994) did not.

Conclusion

The current results suggest that the facilitation and IOR components of reflexive

attentional shifts are at least separable, and may be shown to be independent upon further

investigation. While abrupt onset peripheral cues initiate a motor bias against

responding to targets appearing in the same field (see Klein & Taylor, 1994), the rise

time of this motor bias is greater than 150msec and it’s effects can be nullified if there is

temporal overlap between cue and target. The process by which the motor bias is

masked is not a cancellation of the bias. Instead, when there is temporal overlap between

the cue and the target, attentional processes initiated by the continued presence of the cue

act against the motor bias. The cue can hold attention at its location until the target

appears, thereby allowing faster RTs. Initially, the area facilitated by the continually

present cue is restricted to the cued location. As the time between cue onset and target

38

onset increases, the distribution of attention spreads from the cue across locations

between the cue and the fixation point. However, it does not spread beyond the cued

location to include more peripheral locations. This spread of attention always begins at

the cued location and is consistent with the metaphor of an attentional zoom lens or a

variable attentional focus. Initially, the size of this focus is under the control of the cue

but it increases in size quickly. This increase in size occurs only toward fixation. In

addition, the magnitude of facilitation is always greatest at the cued location and is

reduced at other locations within the attentional focus.

When there is no temporal overlap between cue and target, facilitation does not arise and

therefore cannot act against the effect of the motor bias. This has different effects for

when there are short and long intervals between cue and target, respectively. More

specifically, as the motor bias takes longer than 150 msec to arise, neither facilitation nor

IOR are observed when the time between the cue and target is short. At longer cue-target

intervals, the motor bias has had time to take effect and IOR is observed. While the

spatial distribution of the motor bias remains constant as time between cue and target

increases, with medial locations in the uncued visual field displaying greater IOR than

lateral locations, the magnitude of the motor bias increases as time between cue and

target increases.

39

Acknowledgements

The authors would like to acknowledge those people who reviewed this manuscript and

provided many valuable suggestions. In particular, we would like to thank Raymond

Klein for his helpful and incisive comments on earlier versions of the manuscript. We

would also like to thank Bradley Gibson and an anonymous reviewer for their comments.

40

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47

Table 1. Summary of findings in previous COVAT studies.

STUDY

CUE DURATION

SOA

FACILITATION

INHIBITION OF RETURN

DATA FROM

Posner & Cohen (1984) Experiment 1

150

50 100 150 200 500 600

15* 17* 20*

0* -15* -50*

Figure 32.2

Posner & Cohen (1984) Experiment 2

[1] 150 [2] 150

100 650

25* -30*

Figure 32.3

Maylor (1985) Experiment 1a

[1] 100 [2] 100

100 300 500

25* -8* -25*

Figure 1

Possamai (1986) 35 50 75 100 250 500

5* 10* 5*

-10* -20*

Figure 2

Maylor & Hockey (1987) Experiment 1 (random condition)

[1] 100 [2] 100

100 300 500

15* - 8* -25*

Figure 1

Berlucchi et al., (1989) Experiment 1

5 300 900 1500 4000

-43.5 -38.3 -18.1 -10.6

Table 1

Tassinari et al., (1989) 5 200 600 1500 5000

-41.4 -55.9 -47.5 -2.3

Table 1

Lambert & Hockey (1991) Experiment 1 Initial blocks

100 50 100 300 500 1000

25* 10*

- 5* -35* -25*

Figure 1

Lambert & Hockey (1991) Experiment 2

100 50 100 300 500 1000

25* 20*

-3* -25* -25*

Figure 2

Lambert & Hockey (1991) Experiment 3

100 100 600

10*

-15*

Figure 3

Rafal et al., (1991)

On until response

50 100 150 200

18* 70* 75* 80*

Figure 2

Bowman et al., (1993) 83 100 400 700

-1* -20* -30*

Figure 5

Tassinari et al., (1994) Experiment 1

5

0 65 130 300

-56 -40.5 -39 -37.5

48

900

-38.8

Table 1

Tassinari et al., (1994) Experiment 2

SOA + 300 0 65 130 300 900

4.9 3.2

-12.9 -6.8 -7.8

Table 2

Tassinari et al., (1994) Experiment 3

130 0 65 130 300 900

-40 -3.6 -24.1 -24.3 -28.9

Table 3

Tassinari et al., (1994) Experiment 4

130 0 65 130 300 900

3.0

-13.4 -11.9 -20.9 -22.7

Table 4

Muller & vonMuhlenen (1996) Experiment 1 (Static condition)

[1] 100 [2] 100

100 250 400 1000 1300

9

-1 -6 -14 -18 -21

Table 1

Reuter-Lorenz et al., (1996). Experiment 4

[1] 200 [2] 300

1000 1300

-92*

Table 4

Egly et al., (in press) 200 50 150 750 1000

45* 25*

-45* -45*

Figure 1

Lupianez et al., (1997) Experiments 1a, 2a, 3a, 4a.

50 100 400 700 1000 1300

19 -26 -50 -41 -32

Table 1

Maruff et al., (1999) Experiment 1

50 On until response

150 350 850 150 350 850

1.5 19.5

-18.6 -28.2 -1.5 -3.9

Table 1

Maruff et al., (1999) Experiment 2

50 550

150 850 150 850

24.1

-8.6 -23.6 -23.6

Table 2

Berger et al., (1999) Experiment 2

50 100

50 100 150 200 250 300 600 50 100 150 200

4* 12* 12* 10* 10* 4* 42* 36* 22* 8*

-12*

49

200

250 300 600 50 100 150 200 250 300 600

10* 10* 28* 20* 20* 12*

-20* -2* -10* -24*

Figure 3

Note: Table 1 describes those studies where target location is not predicted by the previous location of the cue, and the subject is required to detect onset targets. Neither discrimination nor detection tasks with predictive cues are included in this table. To be included in Table 1, studies were required to have RT as their primary dependent variable. All studies in Table 1 calculated facilitation and inhibition by subtracting RT to a validly cued target from RT to an invalidly cued target. For the Cue Duration column the numbers in square brackets indicates the cue number. When more than one cue was used, the second cue always appeared at fixation in these studies. * = exact data not available, so effect size was estimated from figure or table indicated in last column.

50

Table 2: Summary of experimental results. Group mean RTs (+SD) and cue effect sizes for all conditions in Experiments 1,2 and 3. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- cue/target cue 18O target 9 O target cue effect size Exp relationship SOA location cued uncued cued uncued 18O target 9 O target ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1 overlap 150 18 413.2 (20.1) 433.1 (22.4) 19.9 850 18 388.3 (19.7) 386.2 (21.3) -2.1 no overlap 150 18 438.7 (23.7) 436.2 (19.8) -2.5 850 18 410.7 (20.5) 389.6 (16.7) -21.1 2 overlap 150 18 406.5 (24.1) 427.1 (24.3) 350.9 (16.7) 360.9 (24.5) 20.6 10.0 9 427.3 (22.3) 425.4 (29.8) 345.2 (23.4) 361.7 (29.8) -1.9 16.5

850 18 453.4 (29.6) 449.6 (26.7) 383.1 (25.6) 390.4 (27.8) -3.8 -7.3 9 449.3 (27.2) 424.2 (25.2) 369.1 (29.2) 364.3 (19.8) -25.1 -4.8 no overlap 150 18 433.5 (25.4) 441.1 (26.7) 379.8 (31.1) 384.6 (24.1) 7.6 4.8

9 419.3 (23.4) 422.4 (19.1) 365.5 (25.3) 360.4 (27.5) 3.1 -5.1 850 18 414.9 (22.3) 402.3 (17.9) 364.7 (27.1) 344.7 (23.5) -12.6 -20.0

9 414.5 (23.1) 402.2 (21.4) 373.6 (27.6) 354.5 (23.0) -12.3 -19.1

3 overlap 50 18 416.7 (21.3) 439.2 (30.8) 391.1 (21.3) 385.3 (28.3) 22.5 -5.8 100 18 411.3 (27.6) 430.2 (24.8) 360.2 (23.1) 372.4 (29.7) 18.9 12.2

150 18 394.1 (18.6) 415.6 (24.5) 346.5 (14.5) 366.6 (24.7) 21.5 20.1 750 18 405.6 (21.3) 411.6 (32.1) 349.2 (24.7) 351.1 (28.9) 5.0 1.9 800 18 400.7 (19.8) 405.2 (13.7) 353.4 (34.0) 357.5 (20.5) 5.2 4.1

850 18 404.1 (29.6) 402.2 (24.6) 355.4 (29.1) 358.6 (20.7) -1.9 3.2

no overlap 50 18 436.5 (21.4) 439.3 (32.4) 380.3 (13.5) 383.2 (16.4) 2.9 2.9 100 18 427.3 (23.5) 427.1 (23.2) 376.3 (18.9) 373.9 (23.4) -0.2 -2.4

150 18 411.7 (24.5) 415.3 (19.7) 357.6 (25.6) 365.1 (34.1) -3.6 7.5 750 18 434.3 (21.3) 420.3 (29.5) 394.6 (25.4) 373.3 (26.1) -14.1 -21.3 800 18 425.3 (20.0) 408.2 (19.1) 390.7 (19.6) 367.7 (27.1) -17.1 -23.0

850 18 411.9 (12.4) 399.1 (23.8) 372.4 (25.7) 350.6 (18.6) -12.8 -21.8 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------

51

1010-1090 msec ITI Landmark circles 9° and 18° left and right of centre.

Overlapping: remains on Cue Non-overlapping: 50msec

150msec or SOA 850msec

Target On until response

53

Figure Legend

Figure 1: The timecourse of events in a COVAT trial in the current study. Figure 1

describes the COVAT display and timing of events for Experiments 2 and 3 of the

current study. In this example, the target appears in the cued location. In Experiment 2,

cues and targets could also appear in the other three peripheral locations, while in

Experiment 3 cues could appear in both 18 degree locations and targets in any of the four

peripheral locations. In Experiment 1, only three landmark circles were used; one each at

18 degrees left and right of fixation and one at the centre of the display. ITI = inter-trial

interval; SOA = stimulus onset asynchrony.