Saccade target selection in visual search: accuracy improves when more distractors are present

We report four experiments with search displays of Gabor patches. Our aim was to study the accuracy of gaze control in search tasks. In Experiment 1, a target was presented with a single distractor Gabor of a different spatial frequency on the same axis. Subjects could locate the target with the first saccade if the distractor was more distant, but when the distractor was between the fixation point and the target, the first saccade landed much closer to the distractor. In Experiment 2, the number of display items was increased to 16 in a double ring configuration. With this configuration, first saccades were accurately directed to the target, even when there was an intervening distractor in exactly the same configuration as in Experiment 1. Experiment 3 suggested that the improvement in accuracy was not due to distractor homogeneity but rather may be attributable to the increased first saccade latency with the ring configuration. In the final experiment, latency was shown to covary with saccade accuracy. The results are related to a general framework whereby the presence of distractors operates to hold fixation for a longer period of time, thus allowing a greater period of visual processing and more accurate eye movements.     

Inhibition of saccade initiation improves saccade accuracy: The role of local and remote visual distractors in the control of saccadic eye movements

When a distractor appears close to the target location, saccades are less accurate. However, the presence of a further distractor, remote from those stimuli, increases the saccade response latency and improves accuracy. Explanations for this are either that the second, remote distractor impacts directly on target selection processes or that the remote distractor merely impairs the ability to initiate a saccade and changes the time at which unaffected target selection processes are accessed. In order to tease these two explanations apart, here we examine the relationship between latency and accuracy of saccades to a target and close distractor pair while a remote distractor appears at variable distance. Accuracy improvements are found to follow a similar pattern, regardless of the presence of the remote distractor, which suggests that the effect of the remote distractor is not the result of a direct impact on the target selection process. Our findings support the proposal that a remote distractor impairs the ability to initiate a saccade, meaning the competition between target and close distractor is accessed at a later time, thus resulting in more accurate saccades.     

Differential effect of a distractor on primary saccades and perceptual localization

A distractor presented nearby the target of a goal-directed short latency saccade leads to spatial averaging, that is, the saccade lands between the target and the distractor. This so-called global effect is a characteristic feature of the spatial processing underlying the programming of saccadic eye movements. To determine whether this effect of near distractors on saccade metrics is also reflected in perceptual localization, subjects performed a saccade task and a perceptual localization task using identical, briefly flashed visual stimuli. To make the available visual processing time for saccades and perception more similar, we followed the target with a mask.Without the mask, primary saccades with short latency landed between target and distractor. The distractor had less effect on primary saccades with longer latencies (>200 ms) and did not affect the final eye position after late secondary saccades in the dark. This indicates that the oculomotor system can correctly use information about the target location 200 ms after the target flash even if no visual stimulus is present during this period. Likewise the presence of a distractor did not affect perceptual localization.Under the masking condition a similar global effect occurred for primary saccades with short latencies, but the latency dependence of the global effect was weakened. Secondary saccades and perceptual localization still did not show a global effect. The results suggest that the primary saccade is based on a specific target acquisition process that differs from that used for spatial perception and for the programming of memory-guided corrective saccades.     

Spatial and temporal aspects of oculomotor inhibition as revealed by saccade trajectories

The spatial and temporal effect of distractor related inhibition on stimulus elicited (reflexive) and goal driven (voluntary) saccades, was examined using saccade trajectory deviations as a measure. Subjects made voluntary and reflexive saccades to a target location on the vertical midline, while the distance of a distractor from the target was systematically manipulated. The trajectory curvature of both voluntary and reflexive saccades was found to be subject to individual differences. Saccade curvature was found to decrease monotonically with increasing distractor distance from target for some subjects while for others no reduction in curvature or even an increase was found. These results could not be explained by latency differences or landing position effects. The different patterns of distractor effects on saccade trajectories suggest the additional influence of a non-spatial inhibitory mechanism.     

The spatial and temporal shape of oculomotor inhibition

Selecting a stimulus as the target for a goal-directed movement involves inhibiting other competing possible responses. Inhibition has generally proved hard to study behaviorally, because it results in no measurable output. The effect of distractors on the shape of oculomotor and manual trajectories provide evidence of such inhibition. Individual saccades may deviate initially either towards, or away from, a competing distractor - the direction and extent of this deviation depends upon saccade latency, target predictability and the target to distractor separation. The experiment reported here used these effects to show how inhibition of distractor locations develops over time. Distractors could be presented at various distances from unpredictable and predictable targets in two separate experiments. The deviation of saccade trajectories was compared between trials with and without distractors. Inhibition was measured by saccade trajectory deviation. Inhibition was found to increase as the distractor distance from target decreased but was found to increase with saccade latency at all distractor distances (albeit to different peaks). Surprisingly, no differences were found between unpredictable and predictable targets perhaps because our saccade latencies were generally long (∼260-280 ms.). We conclude that oculomotor inhibition of saccades to possible target objects involves the same mechanisms for all distractor distances and target types.     

The spatial coding of the inhibition evoked by distractors

It is generally agreed that saccade deviations away from a distractor location represent inhibition in the oculomotor system. By systematically manipulating the location of a distractor we tested whether the inhibition of the distractor is coded coarsely or fine-grained. Results showed that the location of a distractor had an effect on the saccade trajectories, suggesting that the amount of inhibition observed depends on the location of the distractor. More specifically, the vertical distance of the distractor from fixation seems to be a determining factor. These findings have important implications for models that account for inhibition in the target selection process and the areas that could underlie inhibitory influences on the superior colliculus (SC), like the frontal eye fields (FEF) and the dorsolateral prefrontal cortex (dlPFC). Finally, the initial direction and the endpoint of a saccade were found to be strongly correlated, which contradicts recent models proposing that the initial saccade direction and saccade endpoint are unrelated.     

Saccade adaptation is unhampered by distractors

Saccade adaptation has been extensively studied using a paradigm in which a target is displaced during the saccade, inducing an adjustment in saccade amplitude or direction. These changes in saccade amplitude are widely considered to be controlled by the post-saccadic position of the target relative to the fovea. However, because such experiments generally employ only a single target on an otherwise blank screen, the question remains whether the same adaptation could occur if both the target and a similar distractor were present when the saccade landed. To investigate this issue, three experiments were conducted, in which the post-saccadic locations of the target and distractor were varied. Results showed that decreased amplitude adaptation, increased amplitude adaptation, and recovery from adaptation were controlled by the post-saccadic position of the target rather than the distractor. These results imply that target selection is critical to saccade adaptation.     

Inhibition-of-return and oculomotor interference

The present study shows that inhibition-of-return reduces competition for selection within the oculomotor system. We examined the effect of a distractor when it was presented at an inhibited location (IOR). The results show that due to IOR distractors cause less interference. This was evident in all three measures. First, there was less oculomotor capture when a distractor was presented at an inhibited location. Second, the saccade latency to the target was shorter when a distractor appeared at an inhibited location than when it appeared at a non-inhibited location. Third, there was less curvature towards the distractor when it was presented at inhibited location relative to a non-inhibited location. The observation that there is less interference for a distractor presented at an inhibited location suggests that IOR reduces the exogenous activation of the distractor within the saccade map.     

How global is the global effect? The spatial characteristics of saccade averaging

When a target and a distractor are presented in close proximity, an eye movement will generally land in between these two elements. This is known as the ‘global effect’ and has been claimed to be a reflection of the averaged saccade programs towards both locations. The aim of the present study was to systematically investigate whether there is only a limited area in the saccade map in which saccade averaging occurs. To this end, we examined various distances between target and distractor in two experiments and investigated whether the majority of eye movements landed in between the target and the distractor. Results indicated that the endpoint distribution was unimodal for distances up to 35° (in polar coordinates), with saccades generally landing in between the target and the distractor. When the distance was higher than 45°, the saccade endpoint distribution was predominantly bimodal, with saccades landing either on the target or on the distractor. The decrease in saccade averaging was linear until almost no averaging saccades were observed for the longest distances. As saccades landing in between target and distractor reflect a weak, or absent, top-down signal, the present study indicated that top-down information is unable to strongly influence the oculomotor system when target and distractor are presented in close proximity. In this situation, the resulting eye movement is determined by the weighted average of saccade vectors present in a restricted region in the motor map.     

Saccade target selection: Do distractors affect saccade accuracy?

A study is reported in which eye movements were recorded when observers attempted to make a saccade to a target in the presence of a nearby and visually identical distractor. It was found that saccade targeting accuracy was completely unaffected by the presence of the distractor, except in the cases where the distractor was on the same axis as that of the saccadic movement. In this condition, some saccades landed between target and distractor, thus showing the global effect finding, known to occur when saccades are made to stimuli with sudden onset. The result demonstrates that a perceptual selection process, operating with higher resolution than that often associated with covert visual attention, can be used in the selection of saccadic targets.     

The influence of bright background flicker during different saccade periods on saccadic performance

We investigated saccades from central fixation to targets at 5 degrees to the left or right. These targets were red laser points of light with an intensity unmodulated in time (referred to as steady), while a bright background (76 cd/m(2)) was illuminated by a special fluorescent lamp, the output of which were series of light pulses (at frequencies of 50 or 100 Hz) that were presented only during certain periods, in synchrony with the saccade: e.g. during fixation of the central target, or during the latency (i.e. the period from target onset to saccade onset), or during the execution of the saccade; otherwise, the background luminance was steady. We observed a mean increase in latency of about 23 ms when 50 Hz flicker pulses occurred during the latency alone. This result is interpreted in terms of saccadic inhibition [Reingold & Stampe, (2000) In: Kennedy, Radach, Heller, & Pynte (Eds.) Reading as a perceptual process. Elsevier, Amsterdam]: our bright background flicker during the latency may have produced longer latencies, similar to the remote distractors in the model of Findlay and Walker [Behav. Brain Sci. 22 (1999) 661].     

The influence of object-relative visuomotor set on express saccades

Express saccades are considered to have the shortest latency (70-110 ms) of all saccadic eye movements. The influence of visuomotor set, preparatory processes that spatially affect a sensorimotor response, on express saccades was examined by instructing human subjects to make a saccade to one of two simultaneously appearing spots defined by its position relative to the other. A temporal gap between fixation point disappearance and target appearance was used to facilitate the production of express saccades. For all subjects, the instruction influenced the vector of express saccades without increasing saccade latency. The effect on express saccades was only slightly weaker than that for longer latency saccades. Saccade curvature was minimal and did not depend strongly on task. Further experiments demonstrated that the effect of instruction on express saccade vector was much weaker when saccades were instructed to be made to one side of a single small spot, that the effect of instruction was equally strong when directing saccades to the less salient of two stimuli, and that an instruction could not only determine the direction of the effect but also modulate the effect's magnitude. The effect of instruction on saccade vector was no higher when blocked than when varied across trials. These results suggest that express saccades are influenced by object-relative spatial preparatory processes without increasing their reaction time and, thus, that high-level cognitive processes can influence the most reflexive of saccadic eye movements.     

Recent advances in the study of saccade trajectory deviations

In recent years, the number of studies that have used deviations of saccade trajectories as a measure has rapidly increased. This review discusses these recent studies and summarizes advances in this field. A division can be made into studies that have used saccade deviations to measure the amount of attention allocated in space and studies that have measured the strength of the activity of a distractor. Saccade deviations have also been used to measure target selection in special populations. Most importantly, recent studies have revealed novel knowledge concerning the spatial tuning and temporal dynamics of target selection in the oculomotor system. Deviations in saccade trajectories have shown to constitute a valuable measure of various processes that control and influence our behavior which can be applied to multiple domains.     

Fixation stability and saccadic latency in élite shooters

This study tested the hypothesis that elementary visuo-motor functions involved in visual scanning, as measured by fixation and saccadic tasks, are better in a group of high-level clay target shooters (N=7) than in a control group (N=8). In the fixation task, subject were told to keep fixation as still as possible on a target for 1 min, both in the presence and absence of distracters. For shooters, time did not have an effect on fixation stability, and they had more stable fixation than controls in the distracters condition. Results indicate a difference between groups on both the temporal span of attention and selective attention. In the saccadic task, subjects were asked to saccade, as fast as possible, towards a peripherally displayed target. Two conditions were used: simple reaction to target onset and discrimination between targets and distracters. Shooters had faster saccadic latency to targets than controls in both conditions. Finally, to evaluate the effect of exercise on saccadic latency, we trained one control subject to saccade to a target displayed at a constant spatial position. At the end of the training, saccadic latency reached a value comparable to that recorded in shooters. Learning was largely retinotopic, not showing transfer to untrained spatial positions.     

On the limited role of target onset in the gap task: support for the motor-preparation hypothesis

Saccade latency is reduced when the fixation stimulus is removed shortly before a saccade target appears (gap task) as compared to when the fixation stimulus remains present (overlap task). To test the assumption that this gap effect benefits from advanced motor preparation (M. Paré & D. P. Munoz, 1996), we manipulated target onset independently of the signal to launch a saccade (peripheral offset at the mirror location). In Experiment 1, we showed that, when the target appears at one of only two possible locations, target onset strongly improves performance (lower latency, higher accuracy) in the overlap task but not in the gap task. In Experiment 2, we found that the lack of an effect of target onset in the gap task was not due to inhibition of a reflexive response to the transient associated with the offset (go signal) in our task. In Experiment 3, we manipulated target onset and target uncertainty (two, four, or eight potential target locations) in gap and overlap tasks. As target uncertainty increased, the gap effect decreased, and the effect of target onset on saccade latency in the gap condition became greater. Overall, our results suggest, in line with the motor-preparation hypothesis, that saccade metrics in a gap task are computed before the target is actually displayed and that advanced motor preparation is enhanced when the location of the target is predictable. Analyses of anticipations and regular-latency errors corroborated this view.     

Memory for the position of stationary objects: disentangling foveal bias and memory averaging.

The perceived and remembered position of stationary target objects is subject to a large number of distortions. Objects are localized toward the fovea, and when an additional object (distractor) is presented, a tendency to average target and distractor position was observed. These distortions in visual short-term memory have been referred to as foveal bias and memory averaging, respectively. Because most studies on memory averaging did not monitor eye fixation, foveal bias and memory averaging may have been confounded. That is, observers may have fixated the distractor. To disentangle these factors, target and distractor were presented in the periphery, and fixation was monitored. Memory averaging was not observed. Rather a bias away from the distractor occurred when the distractor was briefly presented during the retention interval, or when it was visible throughout the trial. In contrast, a foveal bias was observed regardless of whether an additional object was present.     

Eye scanning of multi-element displays: II. Saccade planning

The properties of saccadic eye movements were studied in a task that required observers to scan through a display consisting of a set of discrete objects. The saccades forming the scanpath showed very high accuracy with almost no undershoot provided no distractor item was located within a critical region around the saccade target. When a distractor item was located in the critical region, saccade accuracy was impaired. This result suggests that the intrinsic spatial selection process for saccadic scanning movements is of low resolution. The initial saccade following display onset showed different properties. The instructions required the observer to look first at a clearly distinct target at the top left of the display. However, the first saccade frequently showed 'oculomotor capture' and was directed either to a different item in the display, or to a midway location between items. The fixation following such erroneous first saccades was generally very short and showed a substantial drift towards the instructed location.     

Attentional capture by a perceptually salient non-target facilitates target processing through inhibition and rapid rejection

Perceptually salient distractors typically interfere with target processing in visual search situations. Here we demonstrate that a perceptually salient distractor that captures attention can nevertheless facilitate task performance if the observer knows that it cannot be the target. Eye-position data indicate that facilitation is achieved by two strategies: inhibition when the first saccade was directed to the target, and rapid rejection when the first saccade was captured by the salient distractor. Both mechanisms relied on the distractor being perceptually salient and not just perceptually different. The results demonstrate how bottom-up attentional capture can play a critical role in constraining top-down attentional selection at multiple stages of processing throughout a single trial.     

Dynamics of saccade target selection: race model analysis of double step and search step saccade production in human and macaque

We investigated how saccade target selection by humans and macaque monkeys reacts to unexpected changes of the image. This was explored using double step and search step tasks in which a target, presented alone or as a singleton in a visual search array, steps to a different location on infrequent, random trials. We report that human and macaque monkey performance are qualitatively indistinguishable. Performance is stochastic with the probability of producing a compensated saccade to the final target location decreasing with the delay of the step. Compensated saccades to the final target location are produced with latencies relative to the step that are comparable to or less than the average latency of saccades on trials with no target step. Noncompensated errors to the initial target location are produced with latencies less than the average latency of saccades on trials with no target step. Noncompensated saccades to the initial target location are followed by corrective saccades to the final target location following an intersaccade interval that decreases with the interval between the target step and the initiation of the noncompensated saccade. We show that this pattern of results cannot be accounted for by a race between two stochastically independent processes producing the saccade to the initial target location and another process producing the saccade to the final target location. However, performance can be accounted for by a race between three stochastically independent processes--a GO process producing the saccade to the initial target location, a STOP process interrupting that GO process, and another GO process producing the saccade to the final target location. Furthermore, if the STOP process and second GO process start at the same time, then the model can account for the incidence and latency of mid-flight corrections and rapid corrective saccades. This model provides a computational account of saccade production when the image changes unexpectedly.