Express saccades in cat: effects of task and target modality

Saccadic eye movements to visual, auditory, and bimodal targets were measured in four adult cats. Bimodal targets were visual and auditory stimuli presented simultaneously at the same location. Three behavioral tasks were used: a fixation task and two saccadic tracking tasks (gap and overlap task). In the fixation task, a sensory stimulus was presented at a randomly selected location, and the saccade to fixate that stimulus was measured. In the gap and overlap tasks, a second target (hereafter called the saccade target) was presented after the cat had fixated the first target. In the gap task, the fixation target was switched off before the saccade target was turned on; in the overlap task, the saccade target was presented before the fixation target was switched off. All tasks required the cats to redirect their gaze toward the target (within a specified degree of accuracy) within 500 ms of target onset, and in all tasks target positions were varied randomly over five possible locations along the horizontal meridian within the cat's oculomotor range. In the gap task, a significantly greater proportion of saccadic reaction times (SRTs) were less than 125 ms, and mean SRTs were significantly shorter than in the fixation task. With visual targets, saccade latencies were significantly shorter in the gap task than in the overlap task, while, with bimodal targets, saccade latencies were similar in the gap and overlap tasks. On the fixation task, SRTs to auditory targets were longer than those to either visual or bimodal targets, but on the gap task, SRTs to auditory targets were shorter than those to visual or bimodal targets. Thus, SRTs reflected an interaction between target modality and task. Because target locations were unpredictable, these results demonstrate that cats, as well as primates, can produce very short latency goal-directed saccades.     

Neural correlates of inter- and intra-individual saccadic reaction time differences in the gap/overlap paradigm

To examine the neural correlates of contextually differing control mechanisms in saccade initiation, we studied 18 subjects who performed two saccade paradigms in a pseudo-random order, while their eye movements were recorded in the MRI scanner (1.5 T). In the gap task the fixation point was extinguished 200 ms before target onset, and in the overlap task the fixation point vanished 500 ms after target onset. Subjects were asked to maintain stable fixation in the fixation period and to quickly saccade to peripherally presented targets. Inter-individual activation differences were assessed using regression analyses at the second level, with mean saccadic reaction time (SRT) of subjects as a covariate. To identify brain regions varying with trial-by-trial changes in SRTs, we included SRTs as a parametric modulation regressor in the general linear model. All analyses were regions of interest based and were performed separately for the gap and overlap conditions. For the gap paradigm, we did not obtain activation in regions previously shown to be involved in preparatory processes with much longer gap periods. Interestingly, both inter- and intra-individual variability analyses revealed a positive correlation of activation in frontal and parietal eye-movement regions with SRTs, indicating that slower saccade performance is possibly associated with higher cortical control. For the overlap paradigm, the trial-by-trial variability analysis revealed a positive correlation of activation in the right opercular inferior frontal gyrus with SRTs, possibly linked to fixation-related processes that have to be overcome to perform a speeded saccade in presence of a fixation point.     

Reaction times of vertical prosaccades and antisaccades in gap and overlap tasks

Horizontal saccadic reaction times (SRTs) have been extensively studied over the past 3 decades, concentrating on such topics as the gap effect, express saccades, training effects, and the role of fixation and attention. This study investigates some of these topics with regard to vertical saccades. The reaction times of vertical saccades of 13 subjects were measured using the gap and the overlap paradigms in the prosaccade task (saccade to the stimulus) and the antisaccade task (saccade in the direction opposite to the stimulus). In the gap paradigm, the initial fixation point (FP) was extinguished 200 ms before stimulus onset, while, in the overlap paradigm, the FP remained on during stimulus presentation. With the prosaccade overlap task, it was found that most subjects (10/13) — whether they were previously trained making horizontal saccades or naive — had significantly faster upward saccades compared with their downward saccades. One subject was faster in the downward direction and two were symmetrical. The introduction of the gap reduced the reaction times of the prosaccades, and express saccades were obtained in some naive and most trained subjects. This gap effect was larger for saccades made to the downward target. The strength of the updown asymmetry was more pronounced in the overlap as compared to the gap paradigm. With the antisaccade task, up-down asymmetries were much reduced. Express antisaccades were absent even with the gap paradigm, but reaction times were reduced as compared to the antisaccade overlap paradigm. There was a slight tendency for a larger gap effect of downward saccades. All subjects produced a certain number of erratic prosaccades in the antitaks, more with the gap than with the overlap paradigm. There was a significantly larger gap effect for the erratic prosaccades made to the downward, as compared to the upward, target, due to increased downward SRTs in the overlap paradigm. Three subjects trained in both the horizontal and the vertical direction showed faster SRTs and more express saccades in the horizontal directions as compared to the vertical. It is concluded that different parts of the visual field are differently organized with both directional and nondirectional components in saccade preparation.     

Influence of stimulus eccentricity and direction on characteristics of pro- and antisaccades in non-human primates

The ability to inhibit reflexes in favor of goal-oriented behaviors is critical for optimal exploration and interaction with our environment. The antisaccade task can be used to investigate the ability of subjects to suppress a reflexive saccade (prosaccade) to a suddenly appearing visual stimulus and instead generate a voluntary saccade (antisaccade) to its mirror location. To understand the neural mechanisms required to perform this task, our lab has developed a non-human primate model. Two monkeys were trained on a task with randomly interleaved pro- and antisaccade trials, with the color of the central fixation point (FP) instructing the monkey to either make a prosaccade (red FP) or an antisaccade (green FP). In half of the trials, the FP disappeared 200 ms before stimulus presentation (gap condition) and in the remaining trials, the FP remained visible (overlap condition) during stimulus presentation. The effect of stimulus eccentricity and direction was examined by presenting the stimulus at one of eight different radial directions (0-360 degrees ) and five eccentricities (2, 4, 8, 10, and 16 degrees ). Antisaccades had longer saccadic reaction times (SRTs), more dysmetria, and lower peak velocities than prosaccades. Direction errors in the antisaccade task were more prevalent in the gap condition. The difference in mean SRT between correct pro- and antisaccades, the anti-effect, was greater in the overlap condition. The difference in mean SRT between the overlap and the gap condition, the gap effect, was larger for antisaccades than for prosaccades. The manipulation of stimulus eccentricity and direction influenced SRT and the proportion of direction errors. These results are comparable to human studies, supporting the use of this animal model for investigating the neural mechanisms subserving the generation of antisaccades.     

Relationship between directed visual attention and saccadic reaction times

Summary Saslow (1967) and Fischer and Ramsperger (1984) found that saccadic reaction time (SRT) depends on the interval between the fixation point offset and the target onset. Using a continuously visible fixation point, we asked whether a similar function would be obtained if subjects attended to a peripherally viewed point extinguished at variable intervals before or after the target onset. The interval was varied between -500ms (i.e., attention stimulus offset after saccade target onset = overlap trials) and 500ms (i.e., attention stimulus offset before saccade target onset = gap trials). The results show a constant mean SRT of about 240 ms for overlap trials, and a U-shaped function with a minimum of 140 ms, at a gap duration of 200 ms, for gap trials. These findings suggest that saccadic latencies do not depend on the cessation of fixation per se, but rather on the disengagement of attention from any location in the visual field. The time required for subjects to disengage their attention is approximately 100 ms. This disengaged state of attention — during which short latency (express) saccades can be made — can be sustained only for a gap duration of 300 ms. At longer gap durations mean SRTs increase again.     

Characteristics of “anti” saccades in man

Summary Four subjects — all made large numbers of Express saccades in the normal gap task — were instructed to make saccades in the direction opposite to the side where a visual stimulus appeared (anti task). Gap and overlap trials were used. Saccadic reaction time (SRT), velocity and amplitude of the corresponding eye movements were analysed and compared to those of saccades made in the normal task. The velocity of anti saccades was found to be slightly (up to 15%) but significantly slower in two subjects. The distributions of SRTs in normal gap tasks show a small group of anticipatory saccades (with SRT below 80 ms and slower velocities) followed by a group of saccades with fast reaction times between 80 ms and 120 ms (Express saccades) followed by another large group ranging up to 180 ms (regular saccades). In the gap anti task there are anticipatory saccades and saccades with SRTs above 100 ms; Express saccades are missing. The distribution of SRTs obtained in the overlap anti task was unimodal with a mean value of 231 ms as compared to 216 ms in the normal task. The introduction of the gap therefore clearly decreases the reaction times of the anti saccades. Control experiments show that the delay of anti saccades is not due to an interhemispheric transfer time but must be attributed to the saccade generating system taking more time to program a saccade to a position where no visual stimulus appears. These data are discussed as providing further evidence for the existence of a reflex-like pathway connecting the retina to the oculomotor nuclei mediating the Express saccade.     

Differential effects of target probability on saccade latencies in gap and warning tasks

Saccade latencies are significantly reduced by extinguishing a foveal fixation stimulus before the appearance of a saccade target. It has been shown recently that this fixation offset effect (FOE) can be modulated by varying target probability. Cortico-collicular top-down effects have been assumed to mediate this strategic FOE modulation. Here, we have investigated strategic FOE modulation in 14 healthy human subjects performing gap and warning tasks. In the former task, the central fixation point was extinguished 200 ms before target onset. In the latter task, the central fixation point changed its colour 200 ms before target onset, but remained illuminated until the target appeared. Target probability was varied block-wise between 25 and 75%. In both tasks, mean latencies decreased with increasing target probability. However, in contrast with what can be expected from preceding studies, we found no differential modulation of mean latencies by target probability between tasks. Instead, we observed differential probability-dependent changes in latency distributions. In the gap task, discrete changes of saccade latencies were found, with a probability-dependent change in frequency of express and regular latencies. By contrast, in the warning task a shift of the entire latency distribution towards longer latencies with low target probability was found. We conclude that strategic modulation of saccade latencies by target probability may be mediated by two distinct neural mechanisms. Selection of either mechanism seems to depend critically on activation of the fixation system.     

Fixation disengagement enhances peripheral perceptual processing: evidence for a perceptual gap effect

Temporal gaps between the offset of a central fixation stimulus and the onset of an eccentric target typically reduce saccade latencies (saccadic gap effect). Here, we test whether temporal gaps also affect perceptual performance in peripheral vision. In Experiment 1, subjects executed saccades to briefly presented peripheral target letters and reported letter identity afterwards. A central fixation stimulus either remained visible throughout the trial (overlap) or disappeared 200 ms before letter onset (gap). Experiment 2 tested perceptual performance without saccade execution, whereas Experiment 3 tested saccade execution without perceptual demands. Peripheral letter perception performance was enhanced in gap as compared to overlap conditions (perceptual gap effect) irrespective of concurrent oculomotor demands. Furthermore, the saccadic gap effect was modulated by concurrent perceptual demands. Experiment 4 ruled out a general warning explanation of the perceptual gap effect. These findings extend recent theories assuming a strong coupling between the preparation of goal-directed saccades and shifts of visual attention from the spatial to the temporal domain.     

The effects of bottom-up target luminance and top-down spatial target predictability on saccadic reaction times

Saccadic reaction times (SRT) are composed of the sum of multiple processes, including bottom-up sensory processing, top-down goal oriented processing, and afferent and efferent conduction delays. In order to determine the timing dependencies and potential interactions between bottom-up and top-down processes on SRTs, we trained monkeys to perform several variants of visually guided saccade tasks. Bottom-up components of SRT were manipulated by varying target luminance from near detection threshold to supra-threshold ranges (i.e., 0.001–42.5 cd/m²). There was a significant reduction of mean SRT with increases in target luminance up to 3.5–17.5 cd/m². Luminance increases above these ranges produced significant increases in SRT when the target was within 6° from the fovea. Top-down components were assessed by manipulating spatial target predictability across blocks of trials using either 1, 2, 4 or 8 possible target locations. Decreasing spatial target predictability increased SRT across target luminances from 1 to 4 targets in the gap task, but then paradoxically decreased SRT again when there were 8 possible targets in both the gap and step tasks. Finally, a gap task (200 ms gap) was used to determine the dependence of target luminance on the magnitude of the gap effect. Decreasing target luminance significantly reduced the magnitude of the gap effect indicating that the gap effect is strongly influenced by bottom-up factors.     

Effects of pre-cues on voluntary and reflexive saccade generation I. Anti-cues for pro-saccades

Experiments on visual attention have employed both physical cues and verbal instructions to enable subjects to allocate attention at a location that becomes relevant within a perceptual or motor task some time later (cue lead time, CLT). In this study we have used valid visual peripheral cues (CLT between 100 and 700 ms) to indicate the direction and location of the next saccade. A cue is considered valid or invalid if its meaning with respect to the next saccade is correct or incorrect. A cue is called an anti- or pro-cue if the side of its presentation is opposite to or the same as the direction of the saccade required on a given trial. Correspondingly, a saccade is called an anti- or pro-saccade if it is directed to the side opposite to or the same as the stimulus presentation. A condition in which the cue and the stimulus are presented on opposite sides provides a simple way of dissociating voluntary attention allocation from automatic orienting. This paper considers the anti-cue pro-saccade task: the subjects were instructed to use the cue to direct attention to the opposite side, i.e. the location, where on valid trials the saccade target would occur. In the companion paper we have used the same physical condition, but we have reversed the instructions as to saccade direction and we have reversed the meaning of the cue, i.e. we designed a pro-cue anti-saccade task. In this first paper, the saccadic reaction times (SRTs) of pro-saccades of five adult subjects were measured in the gap paradigm (fixation point offset precedes target onset by 200 ms). With a CLT of 100 ms, valid anti-cues reduced the number of express saccades (i.e. saccades with SRTs in the range 80–120 ms) significantly compared with the control values (no cues). Valid anti-cues with increasingly long CLTs (100–700 ms) resulted in an increasing incidence of anticipatory saccades and saccades with longer SRTs (more than 120 ms), while the frequency of express saccades remained below the control value. When cue and saccade target were dissociated in location or in both location and direction, the effects of the cueing revealed a much lower spatial selectivity as compared to the effects that have been described for voluntary attention allocation by means of central cues. The results suggest that voluntary allocation of attention and cue-induced automatic orienting not only have different time courses but also have opposite effects on the generation of express saccades, and different spatial selectivities. A possible neuronal basis of these results is discussed considering related findings from electrophysiological studies in monkeys. Received: 27 March 1997 / Accepted: 17 December 1997     

Effects of pre-cues on voluntary and reflexive saccade generation II. Pro-cues for anti-saccades

The reaction times of saccades (SRT) to a suddenly presented visual stimulus (pro-saccade) can be decreased and a separate mode of express saccades can occur when a gap paradigm is used (i.e. fixation-point offset precedes target onset by 200 ms). A valid peripheral cue, presented briefly (100 ms) before target onset, has been found to facilitate the generation of saccades to the target, thereby increasing the frequency of express saccades and decreasing the mean latency. This facilitation occurs only for cues that correctly indicate the direction of the subsequent target presentation (valid cues). The present study investigates the effects of valid cues on SRTs and error rate in the anti-saccade task (saccades in the direction opposite to the stimulus) by systematically varying the cue lead time (CLT) and using the gap and overlap conditions, i.e. fixation point remains on throughout the trial. For a CLT of 100 ms, both reaction times and error rates were significantly increased. With increasing CLT (200–500 ms), both the reaction times of the anti-saccades and the error rates returned to approximately control level, with CLT more than 200 ms in both the gap and the overlap condition. Additional experiments using non-informative cues in the overlap task showed that the reaction times of correct anti-saccades and the error rate were decreased when cue and stimulus appeared at the same side. Analysis of the erratic pro-saccades revealed that almost all of them were corrected, i.e. they were followed by a second saccade towards the required location. It is found that the correction times were usually very short, with intersaccadic intervals between 0 and 150 ms. We suggest that the orienting mechanism, elicited by a transient peripheral cue, relates to the command and the decision to make a pro- rather than an anti-saccade. The cue elicits pro-orienting towards its position when a pro-saccade is required, and anti-orienting when an anti-saccade is required. The orienting effect is transient and decays with CLTs of more than 200 ms; this result holds for both anti-saccades and pro-saccades. Since subjects reported that they could not prevent the erratic pro-saccades or were often not aware of them, we conclude that this orienting mechanism occurs automatically, beyond voluntary control.     

Separate populations of visually guided saccades in humans: reaction times and amplitudes

Summary The saccadic eye movements of 20 naive adults, 7 naive teenagers, 12 naive children, and 4 trained adult subjects were measured using two single target saccade tasks; the gap and the overlap task. In the gap task, the fixation point was switched off before the target occurred; in the overlap task it remained on until the end of each trial. The target position was randomly selected 4° to the left or 4° to the right of the fixation point. The subjects were instructed to look at the target when it appeared, not to react as fast as possible. They were not given any feedback about their performance. The results suggest that, in the gap task, most of the naive subjects exhibit at least two (the teenagers certainly three) clearly separated peaks in the distribution of the saccadic reaction times. The first peak occurs between 100 and 135 ms (express saccades), the second one between 140 and 180 ms (fast regular), and a third peak may follow at about 200 ms (slow regular). Other subjects did not show clear signs of two modes in the range of 100 to 180 ms, and still others did not produce any reaction times below 135 ms. In the overlap task as well three or even more peaks were obtained at about the same positions along the reaction time scale of many, but not all subjects. Group data as well as those of individual subjects were fitted by the superposition of three gaussian functions. Segregating the reaction time data into saccades that over- or undershoot the target indicated that express saccades almost never overshoot. The results are discussed in relation to the different neural processes preceding the initiation of visually-guided saccades.     

Influence of pre-target cortical potentials on saccadic reaction times

 Saccades elicited by suddenly appearing targets show a broad distribution of reaction times. This may depend on variations in the subject’s state of preparation before target onset. To test this hypothesis, we recorded scalp event-related potentials from eight human subjects to investigate whether differences in saccadic reaction times (SRTs) are related to differences in cortical slow potentials prior to target onset. Compared with trials with medium SRTs (180–230 ms), trials with fast SRTs (130–180 ms) were found to be preceded by a more negative slow potential and trials with slow SRTs (230–280 ms) were found to be preceded by a more positive slow potential. These results support the hypothesis that cortical activation prior to target appearance influences SRTs. Received: 21 June 1996 / Accepted: 15 January 1997     

Deficits of cortical oculomotor mechanisms in cerebellar atrophy patients

Commonly, the cerebellum is not associated with cortical components of saccadic eye movement programming. The present study investigates cerebellar effects on visually guided saccades in reflexive tasks (step, gap, overlap) and on internally driven saccades in intentional tasks (anti, memory, short memory sequences of four targets) in five patients with isolated cerebellar atrophy. The cerebellar dysfunction led to impairments in both reflexive and intentional saccades. Cerebellar atrophy patients showed an increase in the gain variability and an increase in the saccade latency. Furthermore, in the memory and anti task, suppression and pro-saccade errors were more frequent in the atrophy group compared to the control group. In the sequence task, patients had difficulties reproducing all four target locations in the order of the displayed sequence. The high variability of the saccade gain is a common observation in cerebellar atrophy patients and can be explained by the general variability present in the saccadic system. The increase in the saccade latency could be due to a cerebellar contribution to cortical processes related to fixation and target selection preceding the initiation of a saccade. Furthermore, the frequent occurrence of saccade errors in the memory and anti task suggests a cerebellar involvement in frontal inhibition of unwanted reflexive saccades. The impaired reproduction of saccade sequences in atrophy patients points to a deficit in short-term memory processes. Thus, this study provides further evidence that the cerebellum is involved in different cortical mechanisms related to the control of saccadic eye movements.     

Contribution of pedunculopontine tegmental nucleus neurons to performance of visually guided saccade tasks in monkeys

The cholinergic pedunculopontine tegmental nucleus (PPTN) is one of the major ascending arousal systems in the brain stem and is linked to motor, limbic, and sensory systems. Based on previous studies, we hypothesized that PPTN would be related to the integrative control of movement, reinforcement, and performance of tasks in behaving animals. To investigate how PPTN contributes to the behavioral control, we analyzed the activity of PPTN neurons during visually guided saccade tasks in three monkeys in relation to saccade preparation, execution, reward, and performance of the task. During visually guided saccades, we observed saccade-related burst (26/70) and pause neurons (19/70), indicating that a subset of PPTN neurons are related to both saccade execution and fixation. Burst neurons exhibited greater selectivity for saccade direction than pause neurons. The preferred directions for both burst and pause neurons were not aligned with either horizontal or vertical axes, nor biased strongly in either the ipsilateral or the contralateral direction. The spatial representation of the saccade-related activity of PPTN neurons is different from other brain stem saccade systems and may therefore relay saccade-related activity from different areas. Increasing discharges were observed around reward onset in a subset of neurons (22/70). These neurons responded to the freely delivered rewards within ~140 ms. However, during the saccade task, the latencies of the responses around reward onset ranged between 100 ms before and 200 ms after the reward onset. These results suggest that the activity observed after appropriate saccade during the task may include response associated with reward. We found that the reaction time to the appearance of the fixation point (FP) was longer when the animal tended to fail in the ensuring task. This reaction time to FP appearance (RTFP) served as an index of motivation. The RTFP could be predicted by the neuronal activity of a subset of PPTN neurons (13/70) that varied their activity levels with task performance, discharging at a higher rate in successful versus error trials. A combination of responses related to saccade execution, reward delivery, and task performance was observed in PPTN neurons. We conclude from the multimodality of responses in PPTN neurons that PPTN may serve as an integrative interface between the various signals required for performing purposive behaviors.     

Effects of stimulus conditions on the performance of antisaccades in man

 We investigated the effect of different spatial and temporal parameters on the saccadic reaction times (SRTs) of the antisaccades and on the frequency and the SRTs of erratic prosaccades in five adult human subjects. The subjects were instructed to aim their saccades to the side opposite to where a visual go-stimulus occurred. Parameters under consideration were: the gap duration (between 0 and 600 ms, and an overlap paradigm); the stimulus size (sizes of 0.1°, 0.2°, and 0.4°, using the gap 200-ms paradigm); and the stimulus eccentricity (1°, 2°,4°, 8°, and 12°, with the gap 200-ms paradigm). A decrease in the anti SRTs and an increase in the error rate were observed with medium gap durations (200 ms, 250 ms), while the anti-SRTs were longer and the error rates lower with the shorter values (0 ms, 100 ms, and with the overlap paradigm) and with the long values (600 ms). A slight decrease in the anti-SRTs and an increase in the error frequency occurred with increasing eccentricity; the SRT distributions of the errors resembled closely those of prosaccades in corresponding prosaccade tasks with the same eccentricities. The stimulus size had only modest or no effects at all. Analysis of the distributions of the correction times of the erratic prosaccades showed that the intersaccadic intervals could be very short: in the range of express saccades, with a peak at 100 ms; or in some subjects even shorter, with a peak at 40–50 ms. It is concluded that the performance of antisaccades is influenced by parameters that interact with the fixation and/or attention system of oculomotor control. Parameters supporting a disengagement of fixation at the time of stimulus onset provoke a reduction of the saccadic reaction times not only of prosaccades but also of antisaccades. Moreover, a certain state of disengagement seems to facilitate the occurrence of reflex-like errors. Received: 3 July 1996 / Accepted: 29 March 1997     

Observing human interaction with physical devices

Triggering of saccades depends on the task: in the gap task, fixation point switches off and target appears after a gap period; in the overlap task, target appears while fixation point is still on. Saccade latencies are shorter in the gap task, due to fixation disengagement and advanced movement preparation during the gap. The two modes of initiation are also hypothesized to be subtended by different cortical-subcortical circuits. This study tested whether interleaving the two tasks modifies latencies, due to switching between different modes of triggering. Two groups of healthy participants (21–29 vs. 39–55 years) made horizontal and vertical saccades in gap, overlap, and mixed tasks; saccades were recorded with the Eyelink. Both groups showed shorter latencies in the gap task, i.e. a robust gap effect and systematic differences between directions. For young adults, interleaving tasks made the latencies shorter or longer depending on direction, while for middle-age adults, latencies became longer for all directions. Our observations can be explained in the context of models such as that of Brown et al. (Neural Netw 17:471–510, 2004), which proposed that different combinations of frontal eye field (FEF) layers, interacting with cortico-subcortical areas, control saccade triggering in gap and overlap trials. Moreover, we suggest that in early adulthood, the FEF is functioning optimally; frequent changes of activity in the FEF can be beneficial, leading to shorter latencies, at least for some directions. However, for middle-age adults, frequent changes of activity of a less optimally functioning FEF can be time consuming. Studying the alternation of gap and overlap tasks provides a fine tool to explore development, aging and disease. M. Vernet and Z. Kapoula contributed equally to this work.     

Visuomotor dependency on an initial fixation target involved in the disorder of visually-guided manual movement in Parkinson's disease

Role of a central fixation target on the latencies of visually guided manual movement was analyzed on young healthy subjects, age-matched control subjects and patients with Parkinson's disease (Hoehn and Yahr stages II, III, and IV). Two paradigms were used: overlap paradigm where a central fixation target was lighted throughout the test, and gap paradigm where a central fixation target was turned off 200 ms before a peripheral target was lighted. The subject was first asked to fixate the central target then instructed to locate a peripheral target with a laser beam spot, operated with wrist flexion or extension as quickly as possible. Latencies of gap paradigm are always shorter than those of overlap task in all the groups. Latencies of both overlap and gap tasks prolonged from young to elder, from elder to PD II, from PD II to PD III and from PD III to PD IV. Also latencies were extremely prolonged in the overlap tasks and correlated with disease severity. Latencies in the gap tasks were less prolonged as compared with those in the overlap tasks. The visual fixation target prolonged the visuo-motor latency in association with severity of Parkinson's disease.     

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.     

Prior information and oculomotor initiation: the effect of cues in gaps

Eye movements reflect not only an important output of various neural control systems, but also often reflect cognitive processing. For example, saccades are frequently used as a behavioural index of attentional processing. A second important eye movement type, smooth pursuit (SP), has received much less attention in this regard. These two types of eye movement were classically thought of as being separate, but recent results have suggested a closer linkage of their control mechanisms and perhaps their interactions with cognitive processes. Prior information, in the form of cues, alters saccade latency leading to characteristic cueing effects. When the period between the appearance of the cue and the appearance of the saccade target is sufficiently long, the latency of saccades to targets appearing at cued locations is increased. This "inhibition of return" is enhanced by a second type of stimulus manipulation, the early removal of the fixation target a few hundred milliseconds before the target appears (the gap paradigm). In the current experiments, the effect of cues, and interactions between cues and long gaps were investigated. In the main pursuit experiment, and in a separate saccade experiment, subjects were presented with interleaved runs of tasks with and without long gaps (gap duration = 1 s), and with and without cues. In tasks without cues, SP latency was reduced by long gaps (mean reduction 8 ms); unexpectedly, saccade latency for non-cue tasks was increased by long gaps (mean increase 41 ms). In a control experiment with only non-cue tasks, in which SP and saccade gap and non-gap tasks were run together, SP latency was again reduced in gap tasks, while saccade latency was increased, but by much less than in the first experiment. Analysis of individual subjects' data showed that while gaps increased saccade latency in two subjects who had participated in the main experiment (in which cues and gaps had been combined), in two naive subjects long gaps did not affect saccade latency. In the main pursuit experiment, cues had both spatially specific and non-spatially specific (warning) effects on pursuit latency. In non-gap conditions, latency was greater when contralateral cues were presented 250 ms prior to the appearance of the pursuit target, compared to ipsilateral cues, a pattern of effect consistent with inhibition of return. However, this was reversed when cues appeared during a gap--contralateral cues increased while ipsilateral cues decreased latency. For saccades, as expected, in both gap and non-gap conditions, cue effects were consistent with inhibition of return (latency was lower with contralateral cues), and the inhibition of return effect was larger in gap, compared to non-gap conditions. The results suggest that, in appropriate contexts (or as a result of appropriate training), there are distinct inhibitory mechanisms that operate on saccades but not pursuit. What appears to be an inhibition of return effect on pursuit latency when static cues are presented in pursuit tasks, may be better understood as the product of a modulation of mechanisms active in pursuit initiation, perhaps related to motion processing. In contrast to some recent evidence suggesting a close anatomical and functional linkage between pursuit and saccade initiation, the results are consistent with the involvement of a wider range of mechanisms, or a greater degree of flexibility, in programming the initiation of these two oculomotor behaviours.