Relationship between directed visual attention and saccadic reaction times

Saccadic initiation biased by fixational activity

Both the gap and overlap paradigm may reveal the interaction between fixating and moving the eyes, but the effects of the overlap paradigm have not been fully characterized yet. Here we present a series of experiments probing how an overlap paradigm, combined with the manipulation of stimuli durations, saliency and transient changes might modulate saccadic reaction time distributions. We recorded saccadic reaction time in four participants in six experiments in which a saccade-target appeared at a pseudo-random amplitude after a fixation period. First, we parametrically manipulated the duration of the overlap using a range of intervals (from 0 to 200 ms). In a second experiment we probed the interaction of various foreperiod intervals (i.e. the duration of the fixation period prior to saccade-target onset) and overlap using two overlap intervals (20 or 140 ms). In two additional experiments we manipulated either the stimuli sizes or their contrast ratio in overlap paradigms (20 or 140 ms). Lastly, we introduced a visual transient during the overlap interval via two manipulations (both with a range of SOA): either a distractor ring appeared around the fixation-target, or a dynamic random noise patch replaced the fixation-target. Results show reliable modifications in the latency distributions depending on the overlap interval as well as idiosyncratic differences. Additional experimental manipulations also affected the latency distributions revealing strong interacting inhibitory processes. We conclude that the effects of overlap intervals may combine with the influence of other stimuli properties affecting decision process.

Influence of sports experience on distribution of pro-saccade reaction time under gap condition

Background

Previous studies indicated that substantial individual variation exists in the distribution of pro-saccade reaction times under gap condition. To investigate the influence of sports experience on the distribution, we examined distribution of the pro-saccade reaction time under overlap and gap conditions, for the basketball club, table tennis club, and non-sporting control groups.

Methods

Subjects performed pro-saccade tasks under the overlap and gap conditions, in which the intentional and reflexive disengagement of fixation are important, respectively. Under the overlap condition, the central fixation point was illuminated for a random duration of 1–3 s, then the fixation point was turned off. Just after the switch-off of the fixation point, one of the peripheral targets was illuminated for a duration of 1 s. The visual stimulus under the gap condition was almost the same as that under the overlap condition. However, only the temporal gap between the switch-off of the fixation point and the onset of the target differed between those conditions. The gap duration in the gap condition was set at 200 ms. The mean of median value of the bandwidth showing the earliest peak in the histogram was calculated for each group. Thereafter, for each subject, the bandwidth showing the earliest peak under the gap condition was defined as the criterion bandwidth (0 ms bandwidth). Based on this criterion bandwidth, the mean of the relative frequency was calculated for every 10 ms of bandwidth, for the overlap and gap conditions, in each group.

Results

Under the overlap condition, for all subjects, the pro-saccade reaction times showed unimodal distribution. The means of the median value of the bandwidth showing the earliest peak for the basketball and table tennis groups (approximate 170 ms) were significantly earlier than that for the control group (approximate 190 ms). Under the gap condition, the distribution was bimodal for 11 of 15 subjects in the basketball group and for 5 of 15 subjects in the control group. In the table tennis group, the distribution was not bimodal but unimodal for all 15 subjects. For the basketball group, mean of the relative frequency showed bimodal distribution with approximate 120 ms and 170 ms peaks. For the table tennis and control groups, the mean of the relative frequency showed unimodal distribution with approximate 130 ms and 140ms peak, respectively.

Conclusions

The present study indicated that under the gap condition, the sports experience influenced on the distribution of the pro-saccade reaction time. The pro-saccade reaction time under the condition would show a distinct bimodal distribution for the basketball group and show a distinct and early unimodal distribution for the table tennis group. It was suggested that the physiological factor leading the group difference in the distribution was the effect of sports experience on the disengagement function of fixation.

Saccadic eye movement metrics reflect surprise and mental model updating

Two experiments investigated what eye movements can reveal about how we process surprising information and how we update mental models in dynamic and unstructured environments. Participants made saccades to visual targets presented one at a time, radially, around an invisible perimeter. Target locations were normally distributed and shifted at an unannounced point during the task. Trials following the shift were considered surprising and unexpected. These unexpected and surprising events prompted the need to update. Slower saccadic latencies were observed for surprising/unexpected events, perhaps indicative of the need to reorient attention to the unexpected target location. Longer dwell times were observed for events that signaled a change in the distribution. These data show that eye movement metrics provide a reliable indicator of mental model updating when contingencies change even in the absence of explicit change signals.

Distinct Sensory and Goal Related Signals Underlie the Gap Effect in the Superior Colliculus

The removal of a fixation point (FP) prior to the appearance of a saccade target (gap effect) influences pre-motor circuits and reduces saccadic reaction time (SRT). Saccade preparation signals underlying the gap effect have been observed within the intermediate layers of the superior colliculus (SCi). Neurons in the caudal SCi, coding a target location, increase their activity during the gap, while neurons in the rostral SCi, with tonic activity related to visual fixation, decrease activity. However, the gap effect confounds two factors: 1) a goal-driven temporal warning component (upcoming saccade target appearance); and 2) a stimulus-driven sensory component (FP disappearance). These factors combine to reduce SRT and elicit pre-target responses in the SCi. To dissociate warning and sensory effects, we altered the luminance of the FP during the gap period (renamed warning period) such that it could increase, decrease, or stay the same. Faster SRTs resulted with larger decrements in FP luminance. Different categories of SCi warning period activity were evaluated: 1) always increasing or decreasing; or 2) sensory-linked responses to changes in FP luminance. In the caudal SCi (at the location coding the target), all activity correlated negatively with SRT (i.e. saccade facilitation) and two categories of activity were observed (always increasing or opposing FP luminance changes). In the rostral SCi, four categories of activity were observed: Activity that increased or followed the change in FP luminance correlated positively with SRT (i.e. saccade inhibition), while activity that decreased or opposed FP luminance changes correlated negatively with SRT. Such SCi activity reflected both goal-driven saccade preparation signals and FP sensory properties.

Saccadic Eye Movements in Elderly Depressed Patients With Suicidal Behaviors: An Exploratory Eye-Tracking Study

Suicidal behaviors (SBs) are often associated with impaired performance on neuropsychological executive functioning (EF) measures that encourage the development of more specific and reliable tools. Recent evidence could suggest that saccadic movement using eye tracking can provide reliable information on EF in depressive elderly. The aim of this study was to describe oculomotor performances in elderly depressed patients with SB. To achieve this aim, we compared saccadic eye movement (SEM) performances in elderly depressed patients (N = 24) with SB and with no SB in prosaccade (PS) and antisaccade (AS) tasks under the gap, step, and overlap conditions. All participants also underwent a complete neuropsychological battery. Performances were impaired in patients with SB who exhibited less corrected AS errors and longer time to correct them than patients with no SB. Moreover, both groups had a similar performance for PS latencies and correct AS. These preliminary results suggested higher cognitive inflexibility in suicidal patients compared to non-suicidal. This inflexibility may explain the difficulty of the depressed elderly in generating solutions to the resurgence of suicidal ideation (SI) to respond adequately to stressful environments. The assessment of eye movement parameters in depressed elderly patients may be a first step in identifying high-risk patients for suicide.

Variability of visually-induced center of pressure displacements is reduced while young adults perform unpredictable saccadic eye movements inside a moving room

This study investigated the modulation of center of pressure (CoP) displacements of young adults as they performed predictable and unpredictable saccadic eye movements in stationary and moving visual scenarios. We also examined whether the relationship between CoP displacements and visual stimulus, provided by the moving scenario, and gaze control itself, are affected by the demands of the saccadic tasks. Fifteen young adults (20.53 ± 2.1 years old) stood upright on a force plate, inside a moving room, wearing an eye tracker while performing three tasks: gaze fixation (fixating on a target in the center of the screen), predictable task (saccades while following a target which continuously appeared and disappeared on the right and left sides), and unpredictable task (similar to the previous task, but the participants did not know which side the target would appear on). For saccadic tasks, the target appeared at a frequency of 1.1 Hz and with eccentricity of 11.5 degrees of visual angle. Two blocks of six trials were performed: in the first block, the room remained stationary and in the second, it oscillated (0.6 cm amplitude; 0.2 Hz frequency). Mean amplitude of CoP displacements was lower in the saccadic tasks compared to the gaze fixation, in both conditions; and higher in the moving scenario than in the stationary condition. Variability of CoP displacements was lower in the unpredictable saccades than gaze fixation task. Saccade reaction time was longer in the unpredictable than predictable task. We conclude that CoP displacements are reduced to facilitate performance of the saccadic tasks regardless of conditions and task complexity. Furthermore, lower variability suggests modulation of CoP displacements to deal with the increased attentional demands associated with the performance of the unpredictable saccades, indicating the important role of visual task constraints in postural control.

Gap effect and express saccades generation in amblyopia

Amblyopia is a neurodevelopmental vision disorder that is associated with abnormal visual stimulation during early childhood. Although our knowledge regarding spatial vision deficits in amblyopic subjects is well established, the neural control of eye movements in amblyopia is yet to be explored. In the present study we have evaluated the gap effect, and for the first time (to our best knowledge), express saccades generation in amblyopic (strabismic as well as anisometropic) and age-matched control subjects. We have compared the saccadic latency under different gap conditions ("no gap," 50 ms gap, and 200 ms gap), between the amblyopic and control groups. Our results have shown that saccadic latency was reduced during the gap paradigms both for amblyopic and control groups for all viewing conditions. Furthermore, the size of the gap effect was comparable for all groups and viewing conditions (both for short and long gap durations). In addition, consistent with previous results, the amblyopic eye has manifested an increased saccadic latency as compared to the nondominant eye in the control group. Regarding the occurrence of express saccades, the 200 ms gap condition was associated with an increased number of express saccades as compared to 50 ms gap and "no gap" conditions, both for amblyopic and control subjects. We did not observe any significant difference in terms of express saccades production between the control and amblyopic subjects. Our findings may suggest that amblyopia does not alter physiological mechanisms related to the efficiency of visual attention/fixation disengagement as supported by the observation that the gap effect and express saccades production was comparable between the normal and amblyopic subjects.

Both saccadic and manual responses in the amblyopic eye of strabismics are irreducibly delayed

Abnormal early visual development can result in a constellation of neural and visual deficits collectively known as amblyopia. Among the many deficits, a common finding is that both saccadic and manual reaction times to targets presented to the amblyopic eye are substantially delayed when compared to the fellow eye or to normal eyes. Given the well-known deficits in contrast sensitivity in the amblyopic eye, a natural question is whether the prolonged reaction times are simply a consequence of reduced stimulus visibility. To address this question, in Experiment 1 we measure saccadic reaction times (RT) to perifoveal stimuli as a function of effective stimulus contrast (i.e., contrast scaled by the amblyopic eye's contrast threshold). We find that when sensory differences between the eyes are minimized, the asymptotic RTs of our anisometropic amblyopes were similar in the two eyes. However, our results suggest that some strabismic amblyopes have an irreducible delay at the asymptote. That is, even when the sensory differences of the stimulus were accounted for, these observers still had large interocular differences (on average, 77 ms) in saccadic reaction time. In Experiment 2, to assess the role of fixation on saccadic reaction time we compared reaction time with and without a foveal target (the "gap effect"). Our results suggest that, while removing the fixation target does indeed speed up reaction time in the amblyopic eye, the gap effect is similar in the two eyes. Therefore, the gap effect does not eliminate the irreducible delay in the amblyopic eye. Finally, in Experiment 3 we compared the interocular differences in saccadic and manual reaction times in the same observers. This allowed us to determine the relationship between the latencies in the two modalities. We found a strong correlation between the differences in saccadic and manual reaction times; however, the manual RT difference is about half that of saccadic RT, suggesting that there may be two separable effects on saccadic reaction time: (a) a central problem with directing actions to a target, related to disengagement of attention at the fovea, which results in delays in both saccadic and manual reaction times, and (b) a further delay in saccadic reaction times because of the motor refractory period from a previous saccade or microsaccade, made in an attempt to stabilize the amblyopic eye of strabismics.

Transcranial Direct Current Stimulation Facilitates Associative Learning and Alters Functional Connectivity in the Primate Brain

There has been growing interest in transcranial direct current stimulation (tDCS), a non-invasive technique purported to modulate neural activity via weak, externally applied electric fields. Although some promising preliminary data have been reported for applications ranging from stroke rehabilitation to cognitive enhancement, little is known about how tDCS affects the human brain, and some studies have concluded that it may have no effect at all. Here, we describe a macaque model of tDCS that allows us to simultaneously examine the effects of tDCS on brain activity and behavior. We find that applying tDCS to right prefrontal cortex improves monkeys' performance on an associative learning task. While firing rates do not change within the targeted area, tDCS does induce large low-frequency oscillations in the underlying tissue. These oscillations alter functional connectivity, both locally and between distant brain areas, and these long-range changes correlate with tDCS's effects on behavior. Together, these results are consistent with the idea that tDCS leads to widespread changes in brain activity and suggest that it may be a valuable method for cheaply and non-invasively altering functional connectivity in humans.

The disengagement of visual attention in Alzheimer's disease: a longitudinal eye-tracking study

INTRODUCTION

Eye tracking provides a convenient and promising biological marker of cognitive impairment in patients with neurodegenerative disease. Here we report a longitudinal study of saccadic eye movements in a sample of patients with Alzheimer's disease and elderly control participants who were assessed at the start of the study and followed up 12-months later.

METHODS

Eye movements were measured in the standard gap and overlap paradigms, to examine the longitudinal trends in the ability to disengage attention from a visual target.

RESULTS

Overall patients with Alzheimer's disease had slower reaction times than the control group. However, after 12-months, both groups showed faster and comparable reductions in reaction times to the gap, compared to the overlap stimulus. Interestingly, there was a general improvement for both groups with more accurately directed saccades and speeding of reaction times after 12-months.

CONCLUSIONS

These findings point to the value of longer-term studies and follow-up assessment to ascertain the effects of dementia on oculomotor control.

Saccade learning with concurrent cortical and subcortical basal ganglia loops

The Basal Ganglia (BG) is a central structure involved in multiple cortical and subcortical loops. Some of these loops are believed to be responsible for saccade target selection. We study here how the very specific structural relationships of these saccadic loops can affect the ability of learning spatial and feature-based tasks. We propose a model of saccade generation with reinforcement learning capabilities based on our previous BG and superior colliculus models. It is structured around the interactions of two parallel cortico-basal loops and one tecto-basal loop. The two cortical loops separately deal with spatial and non-spatial information to select targets in a concurrent way. The subcortical loop is used to make the final target selection leading to the production of the saccade. These different loops may work in concert or disturb each other regarding reward maximization. Interactions between these loops and their learning capabilities are tested on different saccade tasks. The results show the ability of this model to correctly learn basic target selection based on different criteria (spatial or not). Moreover the model reproduces and explains training dependent express saccades toward targets based on a spatial criterion. Finally, the model predicts that in absence of prefrontal control, the spatial loop should dominate.

Attentional disengagement is modulated by the offset of unpleasant pictures: a saccadic reaction time study

We studied the influence of passively viewing a picture on saccade latencies to peripheral targets. Thirty-two volunteers were instructed to look at a central picture, wait for the onset of a peripheral target, and execute a saccade toward it as quickly as possible - saccadic reaction time (SRT). The central picture (neutral or unpleasant) could be turned off simultaneously with target onset (the no-gap condition) or 200ms prior to target onset (the gap-200 condition). We found that saccade latencies were influenced by emotional valence and condition. In the no-gap condition, SRTs were longer after viewing unpleasant pictures. In the gap-200 condition, the pattern was reversed, and unpleasant pictures induced shorter SRTs in relation to neutral pictures. Furthermore, the influence of unpleasant pictures gradually decreased when images were re-exposed to the participants - affective habituation. The results are discussed in terms of attentional avoidance and disengagement from unpleasant emotional pictures.

Modulation of Beta oscillations in the subthalamic nucleus with prosaccades and antisaccades in Parkinson's disease

Increased oscillations in the beta band are thought to be related to motor symptoms of Parkinson's disease (PD). Previous studies have shown that beta-band desynchronization in the subthalamic nucleus (STN) is reduced just before and during limb movements. While the STN is part of the basal ganglia (BG)-thalamocortical circuit controlling limb movements, it is also part of the BG-brainstem projection controlling saccadic eye movements. Late-stage PD patients have deficits in saccades in addition to difficulties with limb movements arising from impaired functions of the BG. We investigated saccade-related changes in beta-band (15-30 Hz) oscillatory activities in the human STN while PD patients performed visually guided prosaccades and antisaccades, the latter requiring suppression of reflexive responses and volitional initiation of saccades. We recorded local field potentials from deep brain stimulation electrodes implanted in the STN in human PD patients 1-5 d after surgery and compared prosaccades and antisaccades with these and with limb movements. Saccade-related beta-band desynchronizations were observed just before and during saccades in all subjects, suggesting that reduction of beta-band oscillatory activity in the STN is related to preparation and execution of saccades. Furthermore, beta-band desynchronizations for antisaccades started earlier, were sustained for longer periods, were of greater magnitude, and were observed more often than prosaccades. Beta-band desynchronization in the STN may reflect the additional processes associated with suppression of reflexive responses and volitional execution of saccades in the opposite direction.

Abnormal mechanisms of antisaccade generation in schizophrenia patients and unaffected biological relatives of schizophrenia patients

Although errant saccadic eye movements may mark genetic factors in schizophrenia, little is known about abnormal brain activity that precedes saccades in individuals with genetic liability for schizophrenia. We investigated electrophysiological activity preceding prosaccades and antisaccades in schizophrenia patients, first-degree biological relatives of schizophrenia patients, and control subjects. Prior to antisaccades, patients had reduced potentials over lateral prefrontal cortex. Smaller potentials were associated with worse antisaccade performance. Relatives also exhibited reduced pre-saccadic potentials over lateral frontal cortex but additionally had reduced potentials over parietal cortex. Both patients and relatives tended toward increased activity over orbital frontal cortex prior to saccades. Results are consistent with lateral prefrontal dysfunction marking genetic liability for schizophrenia and underlying deficient saccadic control.

The influence of onsets and offsets on saccade programming

When making a saccadic eye movement to a peripheral target, a simultaneous stimulus onset at central fixation generally increases saccadic latency, while offsets reduce latency ('gap effect'). Visual onsets remote from fixation also increase latency ('remote distractor effect'); however, the influence of remote visual offsets is less clear. Previous studies, which used a search task, found that remote offsets either facilitated, inhibited, or did nothing to saccade latencies towards a peripheral target. It cannot be excluded, however, that the target selection process in such search tasks influenced the results. We therefore simplified the task and asked participants to make eye movements to a predictable target. Simultaneously with target onset, either one or multiple remote stimulus onsets and offsets were presented. It was found that peripheral onsets increased saccade latencies, but offsets did not influence the initiation of a saccade to the target. Moreover, the number of onsets and offsets did not affect the results. These results suggest that earlier effects of remote stimulus offsets and of the number of remote distractor onsets reside in the target identification process of the visual search task rather than the competition between possible saccade goals. The results are discussed in the context of models of saccade target selection.

Express saccades and visual attention

One of the most intriguing and controversial observations in oculomotor research in recent years is the phenomenon of express saccades in monkeys and man. These are saccades with such short reaction times (100 msec in man, 70 msec in monkeys) that some experts on eye movements still regard them as artifacts or as anticipatory reactions that do not need any further explanation. On the other hand, some research groups consider them not only authentic but also a valuable means of investigating the mechanisms of saccade generation, the coordination of vision and eye movements, and the mechanisms of visual attention.

This target article puts together pieces of experimental evidence in oculomotor and related research – with special emphasis on the express saccade – to enhance our present understanding of the coordination of vision, visual attention, and the eye movements subserving visual perception and cognition.

We hypothesize that an optomotor reflex is responsible for the occurrence of express saccades, one that is controlled by higher brain functions involved in disengaged visual attention and decision making. We propose a neural network as the basis for more elaborate mathematical models or computer simulations of the optomotor system in primates.

Neuronal encoding of the distance traversed by covert shifts of spatial attention

Neurons in monkey medial superior temporal cortex selectively respond to the patterned visual motion in optic flow that simulates observer self-movement. We trained monkeys in a task that required behavioral responses indicating the location of a precue or the simulated heading direction in a subsequent optic flow stimulus. Medial superior temporal neuronal responses contained transient peaks at latencies proportionate to the distance from the precue to the heading direction represented by the subsequent optic flow. We conclude that these response transients reveal neural mechanisms underlying covert shifts of spatial attention and that the varying latency of these transients reflect the time required for reorientation between attentional targets.

Asymmetry of the amplitude-time properties of directed saccades in monkeys depending on the complexity of the spatial scheme of visual stimulation

Three Macaca rhesus monkeys were used for studies of the performance of visually evoked saccades in single-step changes in the position of a stimulus using standard schemes for presentation of GAP-OVERLAP stimuli. Two spatial schemes were used: presentation of stimuli along the horizontal meridian (one-dimensional) and presentation of stimuli within a rectangular area of the visual field (two-dimensional). Asymmetrical foci of short-and long-latency saccades were found in the visual field. Dispersion factor analysis demonstrated that the dimensionality factor (one-dimensional versus two-dimensional stimulation schemes) had greater effects on the latent period of saccades than the lateralization factor (presentation on the left or right sides of the gaze point). The precision of the performance of visually evoked saccades decreased with increases in its eccentricity in both spatial stimulation schemes.

Effects of uncertainty and target displacement on the latency of express saccades in man

Saccadic eye movements generated in response to a gap paradigm in which the fixation light spot was extinguished 200 ms prior to presentation of the target light spot showed appreciably shorter latencies than for the overlap paradigm in which the target light spot was presented 200 ms prior to extinction of the fixation light spot. When there was unpredictability in the direction of target presentation, i.e., to the left or right of the fixation light spot, the gap paradigm evoked mainly fast regular saccades of peak latency of 155 ms with relatively few express saccades which were defined as having latencies of less than 120 ms. By contrast, when the target always appeared to the right, a substantial population of express saccades with peak latency 95 ms was now generated. There was also a change in the relationship between saccadic latency and target angular displacement which covered the range 5-35 degrees . With the overlap paradigm and unpredictability of target direction, the latencies of the slow regular saccades increased markedly with target angular displacement. This was not the case with the same target direction when the latency of slow regular, fast regular, and express saccades remained constant with increasing target angular displacement. This indicates for targets appearing in the same hemifield that the ocular motor system operates with shortest latency irrespective of target angular displacement.

Behavioral plasticity of antisaccade performance following daily practice

The ability to change behavior to adapt to the environment, known as behavioral plasticity, is an important part of daily life. In the present study subjects' performances on antisaccade tasks were manipulated by training them on one of three different eye movement tasks (antisaccade, prosaccade, and fixation). Thirty subjects were tested at three time points over a 2-week period and practiced their assigned task every day between test sessions. Subjects who trained on antisaccades significantly decreased their error rates, while maintaining their reaction time, suggesting that accuracy did not improve at the expense of speed. Subjects who practiced the prosaccade task made more errors on the antisaccade task on subsequent test sessions, while those who practiced the fixation task showed no change across test sessions. These results suggest that deliberate practice of eye movement tasks can alter antisaccade performance, and that the direction of the effect is dependent upon the type of practice in which the subject engages.

How do target predictability and precueing affect the production of express saccades in monkeys?

The extent to which target predictability and precueing affect express saccade generation was determined in Rhesus monkeys. Target predictability, as manipulated by the probability with which targets appeared at various locations, had a strong influence on express saccade generation. Pre-cueing the location of the appearance of an impending single target with an identical stimulus was effective in increasing express saccade generation when there was a gap of 50-150 ms between fixation spot termination and target onset. However, precueing was not effective when the gap time was set to 0 ms in the single target task, when several simultaneous targets appeared requiring a visual discrimination to be made using an oddity task, or when the precue was not identical to the target. These findings indicate that express saccades are facilitated by a restricted set of conditions that increase the predictability of target location and identity.

Amplitude transition function of human express saccades

The gap paradigm often promotes the occurrence of express saccades, which are supposed to be short latency, visually guided saccades, often forming a separate peak in saccadic latency distribution. We designed six experiments in which we compared the amplitudes of anticipatory, express and regular saccades, for various conditions of target eccentricities, target direction, and predictability. Then, saccadic amplitude was expressed as a continuous function of latency, for the various target eccentricities. From the obtained results, it is proposed that a saccade of a given amplitude is prepared during the gap period, on the basis of internal cues. The latency range of express saccades is a transition zone when the target begins to influence the already prepared saccade. The resulting amplitude will be a weighted average of the value determined during the gap and of the value defined by the target, the weighting being determined by the latency of the saccade. If the preprogrammed saccade is wrongly directed, the target will not be able to correct the saccadic amplitude and the express saccade will have the same amplitude as anticipatory saccades. Regular saccades are delayed sufficiently so that a wrongly directed preprogrammed saccade can be canceled or the amplitude of a rightly directed saccade can be adjusted according to the exact position of the visual target.

Spatial anisotropy of saccadic latency in normal subjects and brain-damaged patients

In the present study, reaction time of oblique and orthogonal saccades was investigated in normal subjects and in two groups of patients with right (RBD) and left (LBD) vascular cerebral lesions and no signs of spatial neglect. Clear altitudinal effects were present in each group of subjects: saccadic latencies were longer in the lower than in the upper part of the visual field for both orthogonal and oblique saccades. Asymmetry along the horizontal meridian was present only in case of right hemisphere damage. This supports the view that a lesion in the right hemisphere causes a greater deficit of visual-spatial processing than a left hemisphere lesion. A cerebral lesion in the right and/or left hemisphere produces a general slowing in the saccadic latency and a general reduction in the accuracy of saccades with respect to normal subjects performance. Further, it seems that making saccades in oblique direction reduces the general saccade efficiency.

Express averaging saccades in monkeys

When monkeys are presented simultaneously with multiple stimuli, they can make one of two types of response. Either they make averaging saccades, that land at intermediate locations between the targets, or target-directed saccades, that land close to one of the targets. The two types of saccades occur at different latencies and are thought to reflect different processes; fast reflexive averaging and slower target selection. We investigated the latency of averaging saccades in five monkeys, with particular emphasis on 'express' latency saccades, which are thought to be inhibited by target selection. Express averaging saccades were made prolifically by the two monkeys that made both express and regular latency saccades, but only when no specific instruction was given regarding the saccade target. When these monkeys had to choose one of the targets, on the basis of its color, they still made averaging saccades. However, the endpoints formed two distributions close to the targets as opposed to one single distribution centered between the targets, as was the case when targets were identical; also, express saccades were almost entirely absent. We conclude that express averaging saccades are a form of spatial and temporal optimization of gaze shifting.

The role of the superior colliculus in saccade initiation: a study of express saccades and the gap effect

Neural mechanisms underlying the initiation of saccadic eye movements were studied by recording the activity of neurons in the superior colliculus of rhesus monkeys that had extensive experience on the gap task using targets restricted to one visual field. The superposition of visual activation upon the increased excitability occurring on gap trials facilitates the occurrence of a motor burst with extremely short latency; the motor burst is tightly coupled to saccade onset for the full range of saccadic reactions times, both regular and express. We found no evidence that express saccades are a special class of saccades triggered directly by visual responses. The low frequency activity, necessary for the occurrence of express saccades, neither initiates express saccades nor serves as an accurate predictor of the direction or latency of saccades. Based upon these findings, the hypothesis that the motor burst of collicular neurons serves as a signal for triggering saccade onset can now be extended to express saccades.

The role of fixation point and subjects' readiness in the occurrence of express saccades as revealed by the self-initiation paradigm

The role of fixation and the subjects' response preparedness in producing express saccades were explored in seven human subjects. The occurrence frequencies of the express saccades were compared in the overlap (continuous presentation of fixation point), gap (fixation point offset 0-400 ms prior to target onset) and no-fixation tasks under the conventional and self-initiation paradigms. In the latter paradigm, the subjects, when ready, touched a sensor in order to ignite the target lamp with a delay time of 0-400 ms (target onset delay time). Therefore, the subjects' response preparedness might be expected to be higher than that in the normal paradigm and equated in each subject at the time when the subjects touched a sensor regardless of the paradigms. Although express saccades were produced neither in the normal overlap nor in the normal no-fixation tasks, they could be produced at the rate of 24 and 48% in the overlap and no-fixation tasks under the self-initiation paradigm, respectively. The highest occurrence frequency of express saccades was obtained when the gap paradigm was combined with the self-initiation paradigm with a delay time of 100 ms (62%). The value was higher by 20% than in the normal gap task. At a target onset delay time of 0 ms under the self-initiation paradigm, the occurrence frequency of express saccades was higher in the overlap task than in the gap task. These results suggest that the subjects' response preparedness has a potentiality to produce express saccades without fixation point offset and that fixation point offset at the same time of the target stimulus onset has an interference, rather than facilitatory, influence on the generation of express saccades.

Control' of reflexive and voluntary saccades in the gap effect

In two experiments, we examined whether voluntary and reflexive saccades shared a common fixation disengagement mechanism. Participants were required to perform a variety of tasks, each requiring a different level of information processing of the display prior to execution of the saccade. In Experiment 1, participants executed either a prosaccade or an antisaccade upon detecting a stimulus array. In Experiment 2, participants executed a prosaccade to a stimulus array only if the array contained a target item. The target could be a line (easy search) or a digit (difficult search). The critical manipulation in both experiments was the relative timing between the removal of the fixation stimulus and the onset of the stimulus array. In both experiments, it was found that saccadic latencies were shortest when the fixation stimulus was removed before the onset of the stimulus array--a gap effect. It was concluded that reflexive and voluntary saccades share a common fixation disengagement mechanism that is largely independent of higher level cognitive processes.

Gap effect and reaction time distribution: simple vs choice manual responses

It is well known that saccadic reaction times (SRT) are reduced when the target is preceded by the offset of the fixation point (FP)--the gap effect. Some authors have proposed that the FP offset also allows the saccadic system to generate a separate population of SRT, the express saccades. Nevertheless, there is no agreement as to whether the gap effect and express responses are also present for manual reaction times (MRT). We tested the gap effect and the MRT distribution in two different conditions, i.e., simple and choice MRT. In the choice MRT condition, subjects need to identify the side of the stimulus and to select the appropriate response, while in the simple MRT these stages are not necessary. We report that the gap effect was present in both conditions (22 ms for choice MRT condition; 15 ms for simple MRT condition), but, when analyzing the MRT distributions, we did not find any clear evidence for express manual responses. The main difference in MRT distribution between simple and choice conditions was a shift towards shorter values for simple MRT.

Response times and eye movements in feature and conjunction search as a function of target eccentricity

In four experiments, saccadic eye movements, reaction times (RTs), and accuracy were measured as observers searched for feature or conjunction targets presented at several eccentricities. A conjunction search deficit, evidenced by a large eccentricity effect on RTs, accuracy, and number of saccades, was seen in Experiments 1A and 1B. Experiment 2 indicated that, when saccades were precluded, there was an even larger eccentricity effect for conjunction search targets. In Experiment 3, practice in a conjunction search task allowed both RT and number of saccades to become independent of eccentricity. Additionally, there was evidence of feature-based selectivity in that observers were more likely to fixate distractors that had the same contrast as the target. Results are consistent with the view that the oculomotor and attentional systems are functionally linked and provide constraints for models of visual attention and search.

The analysis of saccadic eye movements from gap and overlap paradigms

This protocol describes the acquisition and evaluation of saccadic eye movement data for use in basic neuroscience research and clinical application. The experimental protocol requires the subject to make saccadic eye movements in response to visual stimuli presented, in random order, on consecutive trials. The gap and overlap paradigms are described together with the instruction to generate pro- or antisaccades. The protocol includes the description of saccade detection, the determination of the beginning, the end, the size, and the velocity of a saccade, the exact way of calculating the proportion of different kinds of trials, and the treatment of erratic or artifact trials. Relevant variables are defined. The results obtained from a large number (300) of subjects of different ages (8-65 years) are described and analysed with respect to their development with age. The protocol allows to test a subject's saccadic status in many different circumstances in particular with respect to diagnostic help in neurology, psychiatry and psychology.

The spatial relationship between scanning saccades and express saccades

When monkeys interrupt their saccadic scanning of a visual scene to look at a suddenly appearing target, saccades to the target are made after an "express" latency or after a longer "regular" latency. The purpose of this study was to analyze the spatial patterns of scanning, express, and regular saccades. Scanning patterns were spatially biased. Express saccade patterns were biased, too, and were directly correlated with scanning patterns. Regular saccade patterns were more uniform and were not directly correlated with scanning patterns. Express saccades, but not regular saccades, seemed to be facilitated by preparation to scan. This study contributes to a general understanding of how monkeys examine scenes containing both unchanging and suddenly appearing stimuli.