Occurrence Of Express Saccades Research Articles

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.

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.

Differential effects of non-target stimuli on the occurrence of express saccades in man

The effect of competing non-target stimuli (distractors) presented together with a target for saccades was investigated with special emphasis on saccadic reaction time. Using the gap task (fixation point offset preceding target onset by 200 msec) high numbers of express saccades were obtained from four human subjects. A single non-target stimulus in the visual hemifield contralateral to the target reduced the number of express saccades. The extend of reduction increased with increasing stimulus size leading to a situation where express saccades were replaced entirely with saccades having a latency mode 30 msec later. Non-target stimuli occurring in the hemifield ipsilateral to the saccade target could also reduce the number of express saccades in favour of fast regular saccades. The effect of ipsilateral non-target stimuli depended mainly on their position in the visual field: ipsilateral non-target stimuli outside the dead zone for express saccades caused only a slight reduction of express saccades. Non-target stimuli invading the dead zone, however, suppressed express saccades almost completely, even though the saccade target appeared well outside the dead zone. The onset of a whole field of bars of one orientation (serving as distractors) simultaneously with the target (consisting of a single bar of another orientation = pop-out condition), suppressed express saccades completely. The same result, however, was obtained with only two distractor bars presented ipsilateral and contralateral within the dead zone for express saccades. Express saccades were present, however, when the whole field of distracting bars was continuously visible throughout the whole trial.

Visual attention may not control the occurrence of express saccades

Visual attention may not control the occurrence of express saccades

Occurrence of express saccades under isoluminance and low contrast luminance conditions

Saccadic reaction times (SRTs) of three human subjects were analyzed. The gap paradigm was used (i.e. fixation point offset precedes target onset) to obtain high proportions of express saccades (i.e. saccades of extremely short reaction times) in the SRT distributions. In one set of experiments, the luminance of the (red) saccade target was varied from brighter to darker than the (green) background including an isoluminance condition. Express saccades were obtained in response to pure color contrast stimuli with about the same frequency and reaction time as to stimuli with both color and luminance contrast. In a second experiment, the luminance contrast of a white target on a white background was lowered below 10%. Again the number of express saccades was not reduced. Thus, in contrast to other perceptual phenomena the visual neural mechanisms underlying the generation of express saccades are not affected by isoluminance nor low contrast luminance.

Mechanisms of visual attention revealed by saccadic eye movements

This paper summarizes recent data on the initiation of saccadic eye movement in relation to the mechanisms of visual attention. In particular, the occurrence of express saccades, defined by their extremely short reaction times, is discussed on the basis of the observation that these saccades do not occur when the subjects (man or monkey) are attending to either a fixation point or to any other visual stimulus in the periphery of their field of view including the “future” saccade target location. It is concluded that the system of visual attention can be in two states: engaged or disengaged. In order to generate a saccade or to move attention from one point to another visual attention must be in the disengaged state. The disengagement takes some time which is or is not included in the saccadic reaction time depending on whether or not visual attention is engaged at the time of the onset of the saccade target. During engaged visual attention saccades are inhibited thereby providing steady central fixation or the absence of saccades during directed peripheral attention.

Further observations on the occurrence of express-saccades in the monkey.

Express-saccades, i.e. goal directed eye movements with extremely short saccadic reaction times (SRT) have recently been observed in rhesus monkey (70-80 ms) and human subjects (around 100 ms). In the gap task which has been used so far, a central fixation point (Fp) was turned off a short time before a new target (Tg) in the near periphery was presented. Therefore, express-saccades occurred when the goal of fixation was no longer visible. To determine whether or not the absence of the Fp is a necessary condition for the execution of an express-saccade, we used an overlap task in which the monkeys had to change the direction of gaze in the presence of the Fp. The results for this overlap task were compared to those found in the gap task. Three major observations have emerged from the present study. Even though the Fp remained visible, a suddenly appearing peripheral target could be reached by an express-saccade. Express-saccades persisted if the location as well as the time of the appearance of the target was randomized. It appears that for an express-saccade to occur, the process of interruption of previous active fixation must be completed at the time when a new target becomes visible. The spectrum of the monkey's saccadic reaction times contains at least three different peaks: express-saccades with reaction times below 100 ms, fast regular saccades with reaction times around 130 ms, and slow regular saccades with reaction times around 180 ms.

The role of fixation and visual attention in the occurrence of express saccades in man.

The differential influence of fixation and directed visual attention on reaction times of goal-directed saccades and especially on the occurrence of express saccades was investigated. In all the experiments the subjects were instructed first to keep their direction of gaze at the center of a translucent screen with or without a central fixation point. When a new stimulus appeared, the subjects had to look at it as soon as possible. In some control experiments the subjects had to direct their gaze to the screen center and simultaneously direct their attention to a peripheral light spot before the target for the saccade appeared. Many express saccades occurred when either active fixation of a central fixation point or attention directed to a peripheral visual target (regardless of its position) was interrupted 200 ms before the target for the saccade appeared. Express saccades were almost completely abolished in the presence of fixation and/or directed visual attention at the moment in which the saccade target appeared. We conclude that express saccades occur if visual attention has already been released at the moment when the target for the saccade appears. This disengagement needs some time which adds to the reaction time.