Ignace T. C. Hooge

CONTROL OF FIXATION DURATION IN A SIMPLE SEARCH TASK

To obtain insight into the control of fixation duration during visual search, we had 4 subjects perform simple search tasks in which we systematically varied the discriminability of the target. The experiment was carried out under two conditions. Under the first condition (blocked), the discriminability of the target was kept constant during a session. Under the second condition (mixed), the discriminability of the target varied per trial. Under the blocked condition, fixation duration increased with decreasing discriminability. For 2 subjects, we found much shorter fixation durations in difficult trials with the mixed condition than in difficult trials with the blocked condition. Overall, the subjects fixated the target, continued to search, and then went back to the target in 5%–55% of the correct trials. In these trials, the result of the analysis of the foveal target was not used for preparing the next saccade. The results support a preprogramming model of the control of fixation duration. In a simple search task, control of fixation duration appears to be indirect.

Adjustment of fixation duration in visual search

To investigate whether fixation durations are adjusted to the duration of a foveal analysis task, we designed a search task in which each stimulus element yielded information about the position of the target. We asked subjects to look for the target by making eye movements in the direction indicated by each stimulus element. We explicitly asked the subjects to make the eye movements in the correct direction, but they did not always do this. They made only 65-80% of the eye movements in directions indicated by the stimulus elements. From these results we conclude that fixation durations are not solely determined by the immediate visual stimulus and that subjects encounter difficulties when trying to increase fixation durations to values that would enable them to direct saccades accurately. In a second experiment we shortened the presentation time in order to provide an incentive for the subjects to speed up search. Shortening the presentation time did not affect fixation duration. Therefore, we suggest that fixation duration is controlled by a mechanism that uses estimations of the foveal analysis time of previous fixated stimulus elements.

Delayed Attentional Engagement in the Attentional Blink

Observers often miss the 2nd of 2 visual targets (first target [T1] and second target [T2]) when these targets are presented closely in time; the attentional blink (AB). The authors hypothesized that the AB occurs because the attentional response to T2 is delayed by T1 processing, causing T2 to lose a competition for attention to the item that follows it. The authors investigated this hypothesis by determining whether the AB is attenuated when T2 is precued. The results from 4 experiments showed that the duration and magnitude of the AB were substantially reduced when T2 was precued. The observed improvement in T2 report did not occur at the expense of T1 report, suggesting that processing of T1 was already completed or was at least protected when the cue was presented. The authors conclude that, during the AB, there is a delay between detection and the selection of target candidates for consolidation in short-term memory.

Faces retain attention

In the present study, we investigated whether faces have an advantage in retaining attention over other stimulus categories. In three experiments, subjects were asked to focus on a central go/no-go signal before classifying a concurrently presented peripheral line target. In Experiment 1, the go/no-go signal could be superimposed on photographs of upright famous faces, matching inverted faces, or meaningful objects. Experiments 2 and 3 tested upright and inverted unfamiliar faces, printed names, and another class of meaningful objects in an identical design. A fourth experiment provided a replication of Experiment 1, but with a 1,000-msec stimulus onset asynchrony between the onset of the central face/nonface stimuli and the peripheral targets. In all the experiments, the presence of an upright face significantly delayed target response times, in comparison with each of the other stimulus categories. These results suggest a general attentional bias, so that it is particularly difficult to disengage processing resources from faces.

Inhibition of return is not a foraging facilitator in saccadic search and free viewing

The ability to search and scan the environment effectively is a prerequisite for spatial behavior. A longstanding theory proposes that inhibition of previously attended loci (Inhibition of return; IOR) serves to facilitate exploration by increasing the likelihood to inspect new areas instead of returning to locations that have been inspected before. In this eye movement study we tested whether we could find evidence in favor of this hypothesis. Here we report that IOR does occur during search and free viewing, because we found increased fixation times preceding return saccades (eye movements that return to previously fixated locations). Meanwhile we observed no influence of IOR on the search strategy. Rather than the predicted low number we found many return saccades. Therefore, IOR does not serve as a foraging facilitator in saccadic search and free viewing. We hypothesize that IOR is an intrinsic aspect of shifting attention and gaze direction and furthermore that it is not always advantageous to prevent return saccades.

Crowding degrades saccadic search performance.

The identity of a target is more difficult to acquire when it is surrounded by distracters. The purpose of the present experiments was to investigate the implications of this crowding phenomenon for performance and eye movements in a real-life task as search with eye movements. The participants searched for a target in a one dimensional search strip. Above and below this search strip additional elements were added. In three conditions, the similarity of these mask elements to the search elements was varied. The spatial extent of crowding is known to increase with target-mask similarity [Nazir, T. A. (1992). Effects of lateral masking and spatial precueing on gap-resolution in central and peripheral vision. Vision Research, 32, 771-777, Kooi, F. L., Toet, A., Tripathy, S. P., & Levi, D. M. (1994). The effect of similarity and duration on spatial interaction in peripheral vision. Spatial Vision, 8(2), 255-279]. One condition did not contain masks. In a visibility experiment, we firstly validated this crowding manipulation. In the search experiment, we subsequently found that with increasing crowding search times were up to 76% longer. Eye movements were also affected. The number of fixations and fixation duration increased and saccade amplitude decreased with increasing crowding. We conclude that in order to understand eye movements in (everyday) tasks that require active exploration of the visual scene, crowding should be taken into account.

Brief Report : Eye Movements During Visual Search Tasks Indicate Enhanced Stimulus Discriminability in Subjects with PDD

Subjects with PDD excel on certain visuo-spatial tasks, amongst which visual search tasks, and this has been attributed to enhanced perceptual discrimination. However, an alternative explanation is that subjects with PDD show a different, more effective search strategy. The present study aimed to test both hypotheses, by measuring eye movements during visual search tasks in high functioning adult men with PDD and a control group. Subjects with PDD were significantly faster than controls in these tasks, replicating earlier findings in children. Eye movement data showed that subjects with PDD made fewer eye movements than controls. No evidence was found for a different search strategy between the groups. The data indicate an enhanced ability to discriminate between stimulus elements in PDD.

Hypervigilance for fear after basolateral amygdala damage in humans

Recent rodent research has shown that the basolateral amygdala (BLA) inhibits unconditioned, or innate, fear. It is, however, unknown whether the BLA acts in similar ways in humans. In a group of five subjects with a rare genetic syndrome, that is, Urbach–Wiethe disease (UWD), we used a combination of structural and functional neuroimaging, and established focal, bilateral BLA damage, while other amygdala sub-regions are functionally intact. We tested the translational hypothesis that these BLA-damaged UWD-subjects are hypervigilant to facial expressions of fear, which are prototypical innate threat cues in humans. Our data indeed repeatedly confirm fear hypervigilance in these UWD subjects. They show hypervigilant responses to unconsciously presented fearful faces in a modified Stroop task. They attend longer to the eyes of dynamically displayed fearful faces in an eye-tracked emotion recognition task, and in that task recognize facial fear significantly better than control subjects. These findings provide the first direct evidence in humans in support of an inhibitory function of the BLA on the brains threat vigilance system, which has important implications for the understanding of the amygdalas role in the disorders of fear and anxiety.

Coarse-to-fine eye movement strategy in visual search

Oculomotor behavior contributes importantly to visual search. Saccadic eye movements can direct the fovea to potentially interesting parts of the visual field. Ensuing stable fixations enables the visual system to analyze those parts. The visual system may use fixation duration and saccadic amplitude as optimizers for visual search performance. Here we investigate whether the time courses of fixation duration and saccade amplitude depend on the subjects knowledge of the search stimulus, in particular target conspicuity. We analyzed 65,000 saccades and fixations in a search experiment for (possibly camouflaged) military vehicles of unknown type and size. Mean saccade amplitude decreased and mean fixation duration increased gradually as a function of the ordinal saccade and fixation number. In addition we analyzed 162,000 saccades and fixations recorded during a search experiment in which the location of the target was the only unknown. Whether target conspicuity was constant or varied appeared to have minor influence on the time courses of fixation duration and saccade amplitude. We hypothesize an intrinsic coarse-to-fine strategy for visual search that is even used when such a strategy is not optimal.

Saccadic search performance: the effect of element spacing

In a saccadic search task, we investigated whether spacing between elements affects search performance. Since it has been suggested in the literature that element spacing can affect the eye movement strategy in several ways, its effects on search time per element are hard to predict. In the first experiment, we varied the element spacing (3.4°–7.1° distance between elements) and target–distracter similarity. As expected, search time per element increased with target–distracter similarity. Decreasing element spacing decreased the search time per element. However, this effect was surprisingly small in comparison to the effect of varying target–distracter similarity. In a second experiment, we elaborated on this finding and decreased element spacing even further (between 0.8° and 3.2°). Here, we did not find an effect on search time per element for element spacings from 3.2° to spacings as small as 1.5°. It was only at distances smaller than 1.5° that search time per element increased with decreasing element spacing. In order to explain the remarkable finding that search time per element was not affected for such a wide range of element spacings, we propose that irrespective of the spacing crowding kept the number of elements processed per fixation more or less constant.