Acta Psychologica 76 (1991) 149-163 North-Holland

149

Peripheral visual changes and spatial attention

Anthony Lambert * University of Durham, Durham UK

Robert Hockey University of Sheffield, Sheffield, UK

Accepted June 1990

Three experiments are reported investigating the attentional effects of peripheral visual changes. In agreement with previous work, experiment 1 demonstrated facilitatory and inhibitory effects of a peripheral visual change on the latency of peripheral target detection. However, after a few minutes practice the facilitatory effect disappeared entirely. The inhibitory effect, though slightly reduced in later blocks, remained significant. Hence, the two effects are dissociable and not inter-dependent as argued by Maylor (1985). In experiments 2 and 3 the perceptual salience of the peripheral cue was manipulated. With a low energy, barely noticeable cue there was no reduction in either facilitation or inhibition as a function of practice. In contrast, the attentional effects of cues higher in energy tended to diminish with practice. Theoretical implications of these data are discussed.

A number of recent studies have investigated how spatial attention is affected by a sudden visual change in the periphery (Posner and Cohen 1984; Maylor 1985; Lambert et al. 1987). Posner and Cohen (1984) reported that a peripheral visual change can produce two opposing effects on attention, which they termed facilitation and inhibition. These effects succeeded one another, and were both finely time locked to the onset of the visual change (termed a ‘cue’ in their experiments). Facilitation was observed if a target was presented 150 msec or less after the onset of the cue. This effect was short lived, and was replaced

* Correspondence address: A.J. Lambert, Dept. of Psychology, University of Auckland, Private Bag, Auckland, New Zealand.

OOOl-6918/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

150 A. Lambert, R. Hockey / Peripheral visual changes

by inhibition on trials where a target was presented 300 msec or more after cue onset. The way in which these two effects were measured experimentally was as follows.

In one of Posner and Cohen’s (1984) experiments subjects viewed a display comprising three outline boxes, one central, one to the left, and one to the right of centre. Each trial began with a brief peripheral display change or ‘cue’. One of the peripheral boxes was brightened for 150 msec. Following this subjects made a speeded manual response to the onset of a bright dot in the centre of one of the boxes. On 60% of trials targets appeared in the central box. On 10% of trials targets appeared in the left, and on 10% of trials targets appeared in the right box. The remaining 20% were catch trials in which the peripheral box brightened but no target was presented. Note that the location of the peripheral ‘cue’ was unrelated to target location. (This is in contrast to previous studies, such as Jonides (1981), which have looked at the attentional effects of spatially informative peripheral cues.) Overall, subjects responded most quickly to targets presented in the central box, presumably because of both the fovea1 location and higher probability of a central target. Speed of responding to peripheral targets showed an interaction between spatial location (cue and target on the same or opposite sides) and the interval between cue onset and target onset. At brief intervals (150 msec or less) responses were quicker when the target appeared on the same side as the cue, compared with cue and target on opposite sides. This was the facilitation effect. At longer intervals (300 and 500 msec) this was reversed and responding was quicker when the target appeared on the opposite side to the cue - the inhibition effect. Facilitation was interpreted in terms of a tendency of peripheral visual changes to automatically summon attention (cf. Jonides 1981). Inhibition was seen as an opposed process reflecting a reduced tendency to respond to previously sampled sources of stimula- tion. Such a process may ensure that novel locations are sampled, and attention is not overcommitted to a particular location.

Posner and Cohen reported several further experiments exploring more detailed characteristics of facilitation and inhibition, in an at- tempt to unravel their functional significance for the control of spatial attention. The same pattern of effects emerged when the peripheral box was dimmed rather than brightened. This showed that the effects could be driven by a luminance change, rather than simply by a luminance increment. In another experiment both peripheral boxes were bright-

A. Lambert, R. Hockey / Peripheral visual changes 151

ened. At brief intervals responses were not significantly faster than responses to uncued trials in the single cue condition. However, at longer intervals responses were inhibited relative to uncued trials in the single cue condition. From this they suggest that inhibition does not arise from orienting to the periphery, but from the energy change occurring at the cued position. However, this conclusion was consid- ered tentative for two reasons. Firstly, at brief intervals in the double cueing experiment there was a small RT difference consistent with a facilitatory effect, but this did not attain statistical significance. Sec- ondly, the conclusion that inhibition can occur in the absence of a prior facilitation effect was difficult to reconcile with data reported by Maylor (1985). Maylor argues that inhibition is dependent on the prior occurrence of covert orienting to the periphery, indexed by the facilita- tion effect. In a very similar double cueing experiment Maylor found that the double cue reduced the facilitation effect by approximately half, and also reduced the inhibition effect. In a further experiment Maylor required subjects to carry out secondary tasks involving eye movements, in addition to the primary luminance detection task. It was found that a secondary task which did not abolish the facilitation effect did not abolish the inhibition effect either, whereas a task which did abolish facilitation also abolished inhibition. Taken together the Maylor studies seem quite convincing in suggesting that inhibition is not simply dependent on the occurrence of peripheral visual stimulation, but is dependent on the individual having covertly oriented attention towards the source of stimulation, as indexed by the presence of the facilitation effect.

The primary aim of the experiments reported below was to explore the relationship between facilitation and inhibition. Are these effects inextricably linked, suggesting a single underlying process? Or can each effect occur independently of the other, suggesting separable underly- ing processes? This question was addressed in the light of a pilot experiment carried out in this laboratory (Lambert 1990), which showed that both facilitation and inhibition effects tend to reduce with time on task (over a 20minute period of testing). This reduction in facilitation and inhibition was explored as a way of addressing the dissociability question. If Maylor (1985) is correct in assuming that inhibition is dependent on prior orienting then inhibitory effects should always be preceded by facilitatory effects. If on the other hand Posner and Cohen (1984) are correct in assuming that the two effects are dissociable, and

152 A. Lmnbert, R. Hockey / Peripheral visual changes

that inhibition is driven primarily by the occurrence of peripheral stimulation, then observation of inhibition in the absence of the early facilitatory effect remains possible. Since findings from the first experi- ment were somewhat discrepant with previous work, two further ex- periments were carried out. These attempted to resolve the discrepancy by exploring how the perceptual salience of a peripheral visual change modulates its effects on spatial attention.

Experiment 1

Method

Subjects Eighteen adult volunteers took part. The proportion of male and female subjects

was not recorded.

Apparatus A Hewlett Packard 984X desk-top computer fitted with a HP98035A real time

clock was used for display presentation and timing.

Display The experimental display was modelled on that used by Posner and Cohen (1984)

and comprised three dark blue outline boxes, one central, one to the left and one to the right of centre. Each box subtended approximately 1.7 O. The inner edge of each side box was approximately 5.3” from fixation. A small fixation cross subtending 0.4” x

0.4” was present in the centre of the central box. The target stimulus was a white filled box subtending 0.4” x 0.4” presented in the centre of either the left or right box. Thus, the inner edge of the target was approximately 5.95 o from fixation. The cue comprised a transient colour change lasting 100 msec in one of the lateral boxes: dark blue to yellow then back to dark blue. Since this involved a change in luminance as well as hue the cue was experienced as a flash in one of the lateral boxes.

Procedure Each trial began with presentation of a cue for 100 msec. After varying amounts of

delay a target was presented in one of the lateral boxes. Ten percent of trials were catch trials in which a cue was presented but no target. Subjects were instructed to press a button located on the right hand side of the keyboard with the index finger of their right hand as quickly as possible in response to the onset of a target. Subjects were informed concerning presentation of the peripheral cues. They were instructed to respond to the target stimuli, and not to the cues. They were also warned about the inclusion of catch trials to minimise the tendency to ‘anticipate’ rather than respond to target occurrence. If subjects responded on a catch trial or responded before target

A. Lambert, R. Hockey / Peripheral visual changes

INITIAL BLOCKS X65- .

O-O CUED

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50 100 300 500 1000

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FINAL BLOCKS

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Fig. 1. Simple detection latency as a function of cueing and SOA, during the initial and final blocks of experiment 1. Open circles = cued trials; filled squares = uncued trials.

onset the computer emitted an 85 msec warning ‘beep’. Depression of the response key caused the target to disappear. Following this there was an interval that varied randomly between 850 msec and 1350 msec before onset of the next cue. Five different intervals between cue onset and target onset (stimulus onset asynchronies - SOA’s) were used: 50 msec, 100 msec, 300 msec, 500 msec and 1000 msec.

Subjects were presented with five blocks of 110 trials each. Ten of these were catch trials. The remaining 100 trials were randomised with respect to trial type, and equally divided with respect to SOA, cued and uncued trials and target location (left or right).

Prior to commencing the experiment subjects carried out lo-15 practice trials to familiarise them with the task.

Results and Discussion

Results from the experiment are shown in fig. 1. Median response latencies were calculated for each condition within each block of trials. In all three experiments reported here, the effects of time on task were assessed by comparing performance in the first two trial blocks with that in the final two blocks. The data were entered into a three way analysis of variance with the following factors: Cueing (cued, i.e., cue and target on same side vs. uncued, i.e., cue and target on opposite sides, trials), SOA (50,

154 A. Lambert, R. Hockey / Peripherd visual changes

100, 300, 500 and 1000 msec), and Time on Task (Initial Blocks vs. Final Blocks). There was a main effect of Cueing, which shows that overall, latencies were slower on cued than uncued trials (335 msec vs. 325 msec), F(1, 17) = 6.00, p < 0.05. There was a main effect of SOA which showed that overall, response latencies decreased across the five SOA’s (SOA 50 - 346 msec; SOA 100 ~ 343 msec; SOA 300 - 340 msec; SOA 500 ~ 311 msec; SOA 1000 - 311 msec), F(4, 68) = 16.73, p < 0.001. This main effect seems best interpreted as a warning signal effect. In addition to acting as a direct spatial cue, the peripheral flash acts as a general warning signal for target onset. The finding that simple reaction time decreases as a function of increasing foreperiod is certainly consonant with previous work on this topic (Niemi and N;iHt;inen 1981). The warning signal effect interacted with Time on Task, F(4, 68) = 3.46, p -C 0.025. This interaction (see fig. 1) shows that the shape of the warning signal function changed across trial blocks. In the initial two blocks latencies decreased monotonically as a function of SOA. In the final two blocks minimum latency was reached at SOA 500, and there was then a small increase in latency for SOA 1000.

The crucially interesting aspects of the data centre on the interactions of Cueing with SOA and Time on Task. There was an interaction between Cueing and SOA, F(4, 68) = 20.85, p < 0.001. This shows that at brief SOA’s (50 and 100 msec) latencies were faster on cued than uncued trials (facilitation), whereas at the three later SOA’s latencies were slower on cued than uncued trials (inhibition ~ see fig. 1). The facilitatory effect was most marked at SOA 50, and the inhibitory effect was most marked at SOA 500. Cueing also interacted with Time on Task, F(1, 17) = 6.11, p i 0.025. This interaction shows that the overall advantage for uncued trials was more marked in the final blocks (320 msec vs. 334 msec) compared with the initial trial blocks (331 msec vs. 336 msec). The question asked of this experiment - How does the balanced of facilitatory and inhibitory effects of a peripheral cue alter as a function of time on task? - is answered by the three way interaction between Cueing, SOA and Time on Task. This interaction was significant, F(4, 68) = 4.71, p < 0.005, and is illustrated in fig. 1.

In agreement with our pilot observations, the pattern of effects altered with Time on Task. During the initial blocks there was a facilitation effect at the two short SOA’s. r-tests revealed that the effect was significant both for SOA 50 (p < 0.005, one tailed) and SOA 100 (p c 0.05, one tailed). In the final blocks the facilitation effect disap- peared entirely. In contrast, the inhibitory effect at later SOA’s was present at both the beginning and end of the experiment. t-tests showed that the inhibition effect was significant at all three later SOA’s (300, 500, 1000) in the final blocks (p < 0.005, one tailed in all three cases). A closer examination of performance in the final two blocks showed that there was no facilitation at early SOA’s in either block 4 or block 5. There was a tendency for inhibition to decrease between blocks four and five for all three later SOA’s. However, r-tests showed that the inhibitory effect remained significant for all three SOA’s in block 5 ( p -C 0.05 one tailed, in all three cases).

This dissociation between facilitation and inhibition constitutes the main finding of experiment 1. Although our pilot study showed that both facilitation and inhibition may reduce with practice, in the present experiment the inhibitory effect was present to a significant degree at both the beginning and end of the experiment. Hence, facilita- tion and inhibition appear differentially susceptible to the effects of practice. While the

A. Lambert, R. Hockey / Peripheral visual changes 155

facilitatory effect disappeared entirely, the reduction in inhibition was small and confined to the final block of trials.

This suggests that Maylor (1985) is incorrect in assuming that the inhibitory effect of a peripheral display change is dependent on prior covert orienting to the cued location having taken place. The present data show that inhibition can occur indepen- dently of facilitation at early SOA’s. Hence, as Posner and Cohen (1984) originally suggested, the processes underlying facilitatory and inhibitory effects of a peripheral cue do appear separable.

Before discussing these results further, a methodological point should be clarified. This concerns possible confounding of attentional effects with eye movements in this experiment. Since eye movements were not monitored this raises the possibility that latency advantages may arise not from attentional effects, but from failures of fixation, bringing a target location nearer to the fovea. There are several reasons why this seems unlikely. In previous studies of covert attention it has been found that subjects are generally successful in following fixation instructions. For example, in studies using a centrally presented, spatially informative cue Posner et al. (1980) reported that eye movements greater than one degree occurred on less than 4% of trials. The pattern of results was unaltered by exclusion of these trials. Similarly, Mueller and Findlay (1987) found that saccadic eye movements occurred on less than 0.5% of trials: again the pattern of effects was unaltered by exclusion of these trials. In both the Mueller and Posner studies just cited, spatially informative cues were used. In this kind of situation performance could potentially be improved by moving the eyes towards the most likely target location. Since this incentive was not present when spatially uninformative cues are used, the occurrence of eye movements seems even less likely in the present experiment. It will be remembered that reminders to maintain fixation were presented at the end of each ‘trial block. It is also worth noting that the use of clearly supra-threshold targets which were easily visible in peripheral vision also lessens the likelihood that subjects will deviate from fixation in order to bring a target location nearer to the fovea.

While demonstrating that facilitation and inhibition are indeed dissociable, the results also prompt a further question. How can the results be reconciled with the work of Maylor (1985) and Maylor and Hockey (1987) in which no practice effects were found? The most obvious difference between the present experiment and those of Maylor concerns the nature of the peripheral display change. The peripheral cue used by Maylor seems to have involved a fairly subtle peripheral display change. She writes that: ‘Subjects reported that they were unaware of the cueing procedure although they did notice that the three boxes, particularly the central one, tended to flicker throughout the experiment.’ Phenomenally, the cue was experienced as a subtle peripheral display change (Maylor, personal communication).

In contrast, the cue used in the present experiment involved a salient visual change. The colour change from dark blue to yellow was experienced as a clearly noticeable peripheral flash at one of the lateral locations. It seems clear that the transitory change in luminance produced by the cue was considerably larger in our experiment compared to Maylor’s. Accordingly, experiment 2 was an attempt to reproduce more closely the perceptual conditions of Maylor’s experiments, by altering the nature of the peripheral display change. In experiment 2 the cue involved a transitory colour change from green

156 A. Lambert, R. Hockey / Peripheral visual changes

to cyan, then back to green. The subjective luminance of these two colours was very similar, and phenomenally the cue was experienced as a subtle, barely noticeable peripheral visual change. If the salience of the peripheral cue is the critical factor mediating the difference between our results and Maylor’s, then experiment 2 should reproduce the pattern observed by Maylor, in which practice effects were not apparent.

In addition to this empirical rationale for experiment 2, the manipulation of perceptual salience is of theoretical interest. Naively, one would expect that clear, perceptually salient peripheral visual changes would produce the strongest and most reliable effects on spatial attention. Hence, reducing the perceptual salience of a cue should also weaken the attentional capture (i.e. facilitation) effect. However, our comparison with previous work suggested that the attentional effects of a subtle cue may be more resistant to reduction with practice. As will be seen the results bore out the latter alternative.

Experiment 2

Method

Subjects Eight female and one male adult volunteer took part.

Apparatus This was the same as for experiment 1.

Display This was the same as for experiment 1, with the exception that the three boxes were

cyan rather than dark blue, and the cue comprised a 100-msec colour change to green in one of the lateral boxes.

Procedure This was the same as for experiment 1.

Results und Discussion

The number of catch trial errors and anticipations made by subjects was acceptably low (1.9% and 3.5% respectively).

Median response latencies were calculated for each condition, within each block of trials. As before, the effects of time on task on performance were assessed by comparing performance in the initial two blocks of trials with performance in the final two trial blocks. The data were entered into a three way analysis of variance with the factors Cueing (cued vs. uncued trials), SOA (50, 100, 300, 500 and 1000 msec), and Time on Task (initial blocks vs. final blocks). There was a main effect of SOA, F(4, 32) = 10.53, p -C 0.001. Once again the warning signal effect interacted with Time

A. Lambert, R Hockey / Peripheral visual changes 157

INITIAL BLOCKS

O-O CUED n -m UNCUED

50 100 300 500 1000

SOA (msec)

FINAL BLOCKS 365

O-O CUED

‘;; n -= UNCUED

:

50 100 300 500 1000

SOA (msec)

Fig. 2. Simple detection latency as a function of cueing and SOA, during the initial and final blocks of experiment 2. Open circles = cued trials; filled squares = unwed trials.

on Task, F(4, 32) = 2.79, p < 0.05: in the initial blocks the warning signal effect stemmed chiefly from an overall drop in latency between SOA 500 and SOA 1000; while in the final blocks the effect stemmed mainly from a sharp overall drop in latency between SOA 300 and SOA 500 (see fig. 2).

As before, the main interest of the experiment centres on the Cueing effect and its interactions with SOA and Time on Task. There was no main effect of Cueing and no interaction of Cueing with Time on Task (both F < 1). The interaction between Cueing and SOA was highly significant, F (4, 32) = 14.13, p < 0.001 (see fig. 2). However, the interaction between Cueing, SOA and Time on Task did not approach significance, F < 1. It is clear from fig. 2 that neither the facilitation effect observed at brief SOA’s (50 and 100 msec), nor the inhibition effect observed at longer SOA’s (500 and 1000 msec) shows any change as a function of Time on Task in this experiment.

These results support the view that the perceptual salience of a peripheral visual change can modulate its attentional effects. However, the pattern of effects is some- what counter-intuitive. The attentional effects of perceptually subtle visual changes appear stronger, in being resistant to the practice effect observed in experiment 1. This prompts two related questions. Firstly, why should the attentional effects of a periph- eral cue diminish with practice? And secondly, why should the attentional effects of less salient cues be more resistant to reduction with practice?

158 A. Lambert, R. Hockey / Penpheral visual changes

A study reported by Lambert et al. (1987) suggest one possible answer to these questions. This experiment showed that both the facilitatory and inhibitory effects of a peripheral cue are sensitive to the probabilistic relation between cue location and target location. When targets were much more likely to appear on the uncued than on the cued side (p = 0.8 vs. p = 0.2) latencies were still somewhat shorter on the cued than the uncued side, at brief SOA’s (50 msec and 100 msec). However, when subjects were explicitly made aware (by instructions) that targets were more likely on the uncued side this effect was suppressed. Hence, when subjects were explicitly aware that orienting attention towards the cued side was a sub-optimal strategy (because targets were likely to appear on the uncued side) they were able to suppress the rapid orienting process. Similarly, when targets were much more likely on the cued side, the inhibitory effect was suppressed. Hence, neither facilitation nor inhibition reflect entirely automatic processes. Each can be over-ridden in response to task demands.

Diminution of facilitation and inhibition observed in our pilot study and in experiment 1 may be due to the same general process - the tendency of subjects to match performance with the statistical structure of tasks. It is well established that observers are adept at matching their attentional strategy to the precise structure of perceptual tasks such as the one studied here (Lambert 1987; Lambert and Hockey 1986; Shaw and Shaw 1977). In an important sense both the facilitatory and inhibitory effects of the cue are detrimental to performance. Optimal performance would be achieved by spreading attention evenly between the two equally likely locations. However, as with any task, optimal performance may not be attained instantly. At the beginning of an experimental session a peripheral visual change can produce both facilitatory and inhibitory effects on spatial attention. One can follow the interpreta- tion previously offered in explaining this: i.e., peripheral visual changes initially ‘capture’ attention, and then produce an opposed inhibitory effect arising from a tendency to sample new visual locations. However, neither process is entire/y auto- matic, since both can be affected by specific task demands (Lambert et al. 1987). Diminution of facilitation and inhibition as a function of practice may arise from the general process of adapting to the precise structure of a task: i.e. matching perceptual performance with the equal probability of target presentation at cued and uncued locations.

However, as explained above, the study of Lambert et al. (1987) suggested that being aware of the cueing procedure itself may be necessary for suppression of the associated spatial attentional effects. We proposed that the attention capturing effect of a new event in the periphery may be automatic in a sense that is analogous to the concept of the default option in computing systems: i.e., the process will be executed unless there are explicit instructions to the contrary One can see that the system may have evolved in this way because in most situations it is advantageous to pay some attention to new peripheral perceptual events. On the other hand, it seems desirable that the system should also have some flexibility, so that the individual is not always compelled to attend to peripheral events that have been classified as irrelevant to the current task. The somewhat artificial situation studied here is one in which the peripheral ‘cue’ can be clearly classified as irrelevant to the target detection task. However, the subject may need to be clearly aware that an irrelevant peripheral display change is occurring for suppression of its unwanted attentional effects to occur. This

A. Lamberr, R. Hockey / Peripheral visual changes 159

may explain why facilitatory and inhibitory effects reduced with practice in our pilot experiment and in experiment 1, but not in experiment 2 or in the experiments of Maylor (1985) or in more recent experiments reported by Maylor and Hockey (1987).

Although this interpretation successfully reconciles the results of our own experi- ments and others in the literature, it is somewhat post hoc. It was also the case that the manipulation of cue salience across experiments 1 and 2 was less than ideal, involving variation across both the hue and luminance dimensions. In view of this a third experiment was carried out. In this study the energy of the peripheral visual change forming the cue was manipulated directly. The attentional effects of a high energy (i.e. perceptually salient) and a low energy (i.e. perceptually subtle) cue were compared. The low energy cue was identical to stimuli employed in pilot work (Lambert 1990). Over four experiments employing a two forced choice procedure the accuracy of detecting this peripheral change ranged from 56% to 58%. Detection of a stimulus rather lower in energy than the high energy cue approached 100%. Two predictions were made: firstly, that the attentional effects of the low energy cue would show no change as a function of practice; and secondly that the attentional effect of the high energy cue would reduce with practice.

Experiment 3

Method

Subjects Six male and 10 female adult volunteers took part.

Apparatus

This was the same as for experiments 1 and 2.

Display

This was similar to experiments 1 and 2. To enable direct manipulation of the cue energy, three changes were introduced. Firstly, the outline boxes were displayed in white, rather than in the colours of experiments of 1 and 2. Secondly the dimensions of the display were slightly different (this enabled direct comparisons with the experi- ments reported in Lambert, 1990). Each outline box subtended approximately 2.6”. The inner edge of each side box was approximately 5.7” from fixation. Hence, the inner edge of the target was approximately 6.8” from fixation. Thirdly, the nature of the peripheral cues was slightly different. A cue comprised the transient inscription for 50 msec of a right angle comer immediately inside each comer of one of the lateral boxes. In the Low Cue condition each of the four right angles comprised 3 new display points. In the High Cue condition each right angle comprised 11 new display points. Phenomenally, the High Cue condition appeared as a clear brightening at the comers of a lateral box. In contrast, the display change in the Low Cue condition appeared barely noticeable. In a related series of experiments reported in Lambert (1990) a two forced choice procedure assessed the accuracy with which subjects detected peripheral

160 A. Lmnbert, R. Hockey / Peripheral visual changes

display changes. Over four experiments a stimulus identical to the Low Cue was detected with an accuracy of approximately 57%. Detection of a stimulus rather lower in energy than the High Cue approached 100%.

Procedure This was the same as for experiments 1 and 2.

Results and Discussion

One subject was excluded from analysis, through making an excessive number of anticipations. The number of anticipations made by this subject was 4 and 5 standard deviations away from the mean for the rest of the group, in the initial and final blocks respectively. The overall number of anticipations and catch trial errors made by the remaining subjects was acceptably low (1.5% and 3.4% respectively).

Median response latencies were calculated for each condition within each block of trials. As before, the effect of time on task was assessed by comparing performance in the initial two blocks with the final two trial blocks. Data from the Low Cue and the High Cue conditions were entered into separate analyses of variance with the factors: Cueing (cued vs. uncued trials), SOA (100 msec vs. 600 msec) and Time on Task (initial blocks vs. final blocks).

Low Cue condition There was a main effect of SOA, showing that overall, latencies were shorter at the

longer SOA (356 msec vs. 381 msec), F(l, 14) = 17.18, p < 0.005. As before, this main effect interacted with Time on Task, F(1, 14) = 5.94, p < 0.05. This interaction (il- lustrated in fig. 3a) shows that the warning signal effect of the cue was rather more pronounced in the initial compared with the final blocks.

As before, Cueing interacted with SOA, F(l, 14) = 10.53, p < 0.01 (see fig. 3a). No other effect approached significance in this analysis (all F < 1). It is clear from fig. 3a that neither the facilitatory effect of the cue at SOA 100 msec, nor the inhibitory effect at SOA 600 msec diminished with Time on Task. The magnitude of both effects was virtually identical in the initial and final blocks.

High Cue condition Data from this condition are shown in fig. 3b. The only effect to reach significance

in analysis of variance was the main effect of Cueing, F(1, 14) = 14.53, p < 0.005, which shows that, overall, latencies were slower on cued than uncued trials (377 msec vs. 361 msec). As fig. 3b shows, this effect tended to occur at both SOA’s: the interactions between Cueing and SOA and between Cueing, SOA and Time on Task both fell far short of significance. The interaction between Cueing and Time on Task was marginally significant, F(l, 14) = 3.33, p < 0.10, suggesting an overall reduction in the cueing effect with time on task. Closer examination of the data revealed that for SOA 600 msec there was a significant reduction in inhibition between the initial and final trial blocks (25 msec vs. 10 msec, t(14) = 2.23, p < 0.025, one tailed).

Hence, the results were largely in agreement with our predictions. In the Low Cue

(4 INITIAL BLOCKS 161

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SOA (msec)

FINAL BLOCKS 400 1

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Fig. 3. Simple detection latency as a function of cueing and SOA, during the initial and final

blocks of experiment 3. Data from the Low Cue condition are shown in panel (a), and data from

the High Cue condition are shown in panel (b). Open circles = cued trials; filled squares = unwed trials.

162 A. Lambert, R. Hockey / Peripheral visual changes

condition facilitatory and inhibitory effects were found at short and long SOA’s respectively, and the magnitude of these effects did not alter with Time on Task. In the High Cue condition there was an inhibitory effect at SOA 600 msec, which reduced significantly with Time on Task.

These findings further support the proposal that the attentional effects of a peripheral visual change are modulated by its perceptual salience. The results are also consistent with the view that subjects need to be clearly aware of the cue, in order for suppression of its unwanted attentional effects to occur. In this experiment perceptual salience was manipulated more systematically than in experiments 1 and 2, by altering the number of display points comprising the cue. As noted earlier, pilot work with a two alternative forced choice procedure demonstrated that detection of the low energy cue was considerably less accurate (approximately 57%) than detection of the high energy cue (approximately 100%).

A surprising feature of the High Cue condition was the presence of an inhibitory effect, rather than facilitation, at the early SOA. The most likely reason for this is that metacontrast masking may occur when the energy of the peripheral stimulus change is high. Initially, Posner and Cohen (1984) rejected an explanation of the inhibitory effect in terms of masking, since (a) in previous studies metacontrast masking has not typically had any effect on response latency tasks, (b) metacontrast masking would be expected at the early (100 msec) SOA, where Posner and Cohen observed facilitation rather than inhibition. Conversely, the inhibitory effect has a relatively long time course, which extends well beyond the range of forward masking effects (Maylor and Hockey 1987; Spencer et al. 1988). The present results suggest that discounting metacontrast masking as an important factor may only be warranted when the cue is low in energy. With a high energy cue, masking may impair responding to a target that appears about 100 msec after cue onset.

Conclusion

In summary, the experiments reported here suggest three main conclusions.

(1) The facilitatory and inhibitory attentional effects of a peripheral cue are separable. In experiments 1 and 3 inhibition was observed at the later SOA’s, in the absence of an early facilitation effect. This is inconsistent with Maylor’s (1985) proposal that inhibition is dependent upon prior covert orienting to the cued location. Rather, the mere occurrence of visual stimulation appears sufficient for inhibition to occur.

(2) The attentional effects of a peripheral cue are modulated by the nature of the visual change forming the cue. If the cue is perceptually salient, its ‘automatic’ attentional effects diminish after a few minutes practice. This may be due to the general tendency of subjects to match

A. Lambert, R. Hockey / Peripheral visual changes 163

their performance with the precise statistical structure of a task - tending to equalise performance across the two equiprobable target locations. Remember, that in this situation the ‘cue’ is simply a repetitively occurring, irrelevant display change.

(3) Subjects may need to be clearly aware of the irrelevant visual change for this suppression of its unwanted attentional effects to occur. When the cue was made perceptually less salient (experiments 2 and 3), there was no reduction in either facilitation or inhibition with practice. In this sense the perceptually subtle peripheral display change captured attention mire effectively than the perceptually salient cue.

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