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.     

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     

Development of prosaccade and antisaccade task performance in participants aged 6 to 26 years

There are few studies on the development of oculomotor functions during childhood. B. Fischer, M. Biscaldi, and S. Gezeck (1997) reported improvement of antisaccade task performance between ages 6 and 16 years. The present study is a replication and extension of those results. In three age groups (6-7, 10-11, 18-26 years), saccades during pro- and antisaccade tasks with 200-ms gap and overlap and during a fixation task were measured. Adults exhibited faster saccades and less prosaccades during the antisaccade tasks than 10-11-year-old children; these two groups had faster saccades during all tasks and less prosaccades during the anti- and the fixation task than 6-7-year-old subjects. Both children groups made more express saccades than adults. Results suggest different degrees of age-related improvement for different saccadic parameters, the effects being greatest for prosaccade inhibition during the antisaccade task and in line with the assumed protracted development of prefrontal functions.     

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.     

The influence of stimulus direction and eccentricity on pro- and anti-saccades in humans

We examined the sensory and motor influences of stimulus eccentricity and direction on saccadic reaction times (SRTs), direction-of-movement errors, and saccade amplitude for stimulus-driven (prosaccade) and volitional (antisaccade) oculomotor responses in humans. Stimuli were presented at five eccentricities, ranging from 0.5° to 8°, and in eight radial directions around a central fixation point. At 0.5° eccentricity, participants showed delayed SRT and increased direction-of-movement errors consistent with misidentification of the target and fixation points. For the remaining eccentricities, horizontal saccades had shorter mean SRT than vertical saccades. Stimuli in the upper visual field trigger overt shifts in gaze more easily and faster than in the lower visual field: prosaccades to the upper hemifield had shorter SRT than to the lower hemifield, and more anti-saccade direction-of-movement errors were made into the upper hemifield. With the exception of the 0.5° stimuli, SRT was independent of eccentricity. Saccade amplitude was dependent on target eccentricity for prosaccades, but not for antisaccades within the range we tested. Performance matched behavioral measures described previously for monkeys performing the same tasks, confirming that the monkey is a good model for the human oculomotor function. We conclude that an upper hemifield bias lead to a decrease in SRT and an increase in direction errors.     

Effects of procues on error rate and reaction times of antisaccades in human subjects

In a gap paradigm, where the saccadic reaction times are usually short, the number of express saccades can be further increased and their latency decreased when a valid transient peripheral cue is given 100 ms before target occurrence. In the present study we measured the saccadic reaction times of seven human subjects who had been instructed to make antisaccades (saccades to the side opposite to stimulus presentation) in the gap paradigm. In the first experiment, we presented a 100% valid cue with 100 ms cue lead time. To explore whether the cue reduced the reaction times of the antisaccades, the cue was always presented on the opposite side to where the stimulus occurred (stimulus direction was randomized between 4° to the left and right), and it was thus indicated in each trial to which side the antisaccade was required (procue). In the second set of experiments the cue was consistently presented on either the left or the right side in two different blocks; it was thus noninformative with respect to the direction of the antisaccade. In the first experiment, a significant increase in mean reaction times of correct antisaccades and a considerable increase in erratic prosaccades to the stimulus were obtained compared with a control session with no cue. In the two experimental blocks with noninformative cues, the reaction times of correct antisaccades were decreased when cue and stimulus were on at the same side, while large numbers of erratic prosaccades were again obtained when cue and stimulus were presented on opposite sides. These results suggest that the orienting mechanism elicited by a transient peripheral cue relates to the command and to the decision to make a proversus an antisaccade. Since the subjects reported that they could not prevent, or, moreover, in some cases did not even realize that they were making erratic prosaccades, we conclude that this orienting mechanism occurs automatically, being largely beyond voluntary control.     

Event-related potentials associated with correct and incorrect responses in a cued antisaccade task

In an antisaccade task, subjects are instructed to inhibit a reflexive saccade towards a peripheral stimulus flash and to generate a saccade in the opposite direction. It has been shown recently that normal subjects will generate a high number of incorrect prosaccades in an antisaccade task if the fixation point is extinguished 200 ms before the stimulus appears and if a valid cue for the subsequent antisaccade is given during this gap period. In the present study we recorded cerebral event-related potentials from 19 scalp electrodes from normal subjects prior to correct and incorrect responses in a cued antisaccade task to investigate the neural processes associated with correct antisaccades and incorrect prosaccades in this task. Correct antisaccades and incorrect prosaccades were associated with a negative potential with a maximal amplitude around stimulus onset over the dorsomedial frontal cortex. This potential was higher prior to correct antisaccades than prior to incorrect prosaccades. The execution of a correct antisaccade was preceded by a shift of a negative potential from the parietal hemisphere contralateral to the visual stimulus towards the parietal hemisphere ipsilateral to the stimulus. These results support the view that the supplementary eye fields participate in the inhibition of incorrect saccades in a cued antisaccade task and show that the parietal cortex participates in generating a neural representation of the visual stimulus in the hemifield ipsilateral to the stimulus before generating a motor response. Received: 20 December 1996 / Accepted: 18 June 1997     

Differing effects of prosaccades and antisaccades on postural stability

The goal of the study was to examine the effect of different types of eye movements on postural stability. Ten healthy young adults (25 ± 3 years) participated in the study. Postural control was measured by the TechnoConcept© platform and recorded in Standard Romberg and Tandem Romberg conditions while participants performed five oculomotor tasks: two fixation tasks (central fixation cross, without and with distractors), two prosaccade tasks toward peripheral targets displayed 4° to the left or to the right of the fixation cross (reactive saccades induced by a gap 0 ms paradigm and voluntary saccades induced by an overlap 600 ms paradigm) and one antisaccade task (voluntary saccade made in the opposite direction of the visual target). The surface, the length, and the mean speed of the center of pressure were analyzed. We found that saccadic eye movements improved postural stability with respect to the fixation tasks. Furthermore, antisaccades were found to decrease postural stability compared to prosaccades (reactive as well as voluntary saccades). This result is in line with the U-shaped nonlinear model described by Lacour et al. (Neurophysiol Clin 38:411–421, 2008), showing that a secondary task performed during a postural task could increase (prosaccade task) or decrease (antisacade task) postural stability depending on its complexity. We suggest that the different degree of attentional resources needed for performing prosaccade or antisaccade tasks are, most likely, responsible for the different effect on postural control.     

The recognition and correction of involuntary prosaccades in an antisaccade task

 This paper reports a striking misperception associated with involuntary saccadic eye movements: when subjects are instructed to look to the opposite side of a suddenly presented stimulus (antisaccade), they produce a certain number of involuntary prosaccades to the stimulus before they move their eyes to the other side by a corrective saccade of approximately twice the size. When asked to indicate at the end of each trial whether they believed that they made such a detour sequence of two saccades, one finds that, on average, 50±25% of these involuntary movements are not recognized. The average size and correction time for recognized prosaccades is larger than unrecognized prosaccades, while their mean reaction times are the same. The corrective saccades compensate for the size of both the recognized and unrecognized errors. When similar sequences of saccades are made voluntarily, the time spent at the stimulus side was 222 ms compared with 95 ms for unrecognized and 145 ms for recognized errors. The distributions of the corresponding correction times differ in their multimodal composition. Whether voluntary and involuntary saccades and their corrections are associated with different effects on the updating of the perceptual spatial frame and attention allocation is discussed. Received: 16 October 1998 / Accepted: 2 December 1998     

Antisaccades generate two types of saccadic inhibition

To make an antisaccade away from a stimulus, one must also suppress the more reflexive prosaccade to the stimulus. Whether this inhibition is diffuse or specific for saccade direction is not known. We used a paradigm examining inter-trial carry-over effects. Twelve subjects performed sequences of four identical antisaccades followed by sequences of four prosaccades randomly directed at the location of the antisaccade stimulus, the location of the antisaccade goal, or neutral locations. We found two types of persistent antisaccade-related inhibition. First, prosaccades in any direction were delayed only in the first trial after the antisaccades. Second, prosaccades to the location of the antisaccade stimulus were delayed more than all other prosaccades, and this persisted from the first to the fourth subsequent trial. These findings are consistent with both a transient global inhibition and a more sustained focal inhibition specific for the location of the antisaccade stimulus.     

Response selection in prosaccades, antisaccades, and other volitional saccades

Saccades made to the opposite side of a visual stimulus (antisaccades) and to central cues (simple volitional saccades) both require active response selection but whether the mechanisms of response selection differ between these tasks is unclear. Response selection can be assessed by increasing the number of response alternatives: this leads to increased reaction times when response selection is more demanding. We compared the reaction times of prosaccades, antisaccades, saccades cued by a central arrow, and saccades cued by a central number, in blocks of either two or six possible responses. In the two-response blocks, reaction times were fastest for prosaccades and antisaccades, and slowest for arrow-cued and number-cued saccades. Increasing response alternatives from two to six caused a paradoxical reduction in reaction times of prosaccades, had no effect on arrow-cued saccades, and led to a large increase in reaction times of number-cued saccades. For antisaccade reaction times, the effect of increasing response alternatives was intermediate, greater than that for arrow-cued saccades but less than that for number-cued saccades. We suggest that this pattern of results may reflect two components of saccadic processing: (a) response triggering, which is more rapid with a peripheral stimulus as in the prosaccade and antisaccade tasks and (b) response selection, which is more demanding for the antisaccade and number-cued saccade tasks, and more automatic when there is direct stimulus–response mapping as with prosaccades, or over-learned symbols as with arrow-cued saccades.     

Less attention is more in the preparation of antisaccades, but not prosaccades

To make a saccadic eye movement to a target we must first attend to it. It is therefore not surprising that diverting attention increases saccade latency, but is latency increased in all cases? We show that attending to a peripheral discrimination task has a paradoxical effect. If the stimulus to be attended appears shortly (100 to 300 ms) before an eye movement is made in a direction opposite to that of a presented stimulus (an antisaccade), its latency is reduced to well below baseline performance. In contrast, latencies for saccades toward the stimulus (prosaccades) are increased under similar conditions. This paradoxical effect may arise from competition between the processes mediating prosaccades and antisaccades. When the discrimination task is presented at the critical moment, it interferes with a reflexive prosaccade, allowing faster antisaccades. The results suggest that the suppression of sensorimotor reflexes can facilitate volitional motor acts.     

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     

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.     

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.     

The inter-trial effects of stimulus and saccadic direction on prosaccades and antisaccades, in controls and schizophrenia patients

We investigated the influence of the direction of preceding saccadic trials on the latency of current prosaccades and antisaccades, in healthy subjects and patients with schizophrenia. When prosaccades and antisaccades were performed in separate, single-task blocks, we found that only prosaccades were delayed if the saccade in the prior trial was in the same direction, consistent with the expected directional effect from an ‘inhibition of return’-like alternation advantage. However, both types of saccades were executed more quickly when the saccade in the penultimate trial was in the same direction, consistent with previous demonstrations of directional plasticity in monkeys. In blocks of randomly mixed prosaccades and antisaccades, the directional effects in healthy subjects were greatest when a prosaccade was preceded by an antisaccade, consistent with a summation of effects of alternation advantage (from the prior stimulus) and directional plasticity (from the prior saccade). Schizophrenic patients showed an additional phenomenon, a directionally specific inhibition of upcoming saccades by preceding antisaccades. These results suggest that saccades in humans are modulated by inter-trial effects attributable to both an ‘inhibition of return’-like alternation advantage and directional plasticity.     

Cortical activation associated with midtrial change of instruction in a saccade task

The appearance of a visual stimulus in the peripheral visual field can elicit different saccade responses depending on prior instruction. This flexibility is commonly attributed to differences in motor set. Little is known about how the brain switches between one saccade response and another. To investigate the neural processes associated with switches between saccade motor sets, we recorded event-related potentials (ERPs) in 13 subjects, in three tasks that required subjects to generate prosaccades to a visual stimulus on 75% of the trials. On 25% of the trials, the color of the fixation point (FP) changed 300 ms prior to stimulus presentation. In the "ANTI" task, the change of the FP was the instruction to generate an antisaccade; in the "NOGO" task, subjects were instructed to maintain fixation; and in the "PRO" task, subjects were instructed to generate a prosaccade. The switch in motor set from prosaccades to antisaccades in the ANTI task and the cancellation of the prosaccade motor set in the NOGO task modulated frontal and frontocentral channels. Futhermore, the ANTI task but not the NOGO task was associated with differences at central and parietal channels compared with the PRO task. We hypothesize that the frontal activation in the ANTI and NOGO task reflects inhibition and task-switching processes, whereas the parietal activation reflects the preparation of this area for the sensorimotor transformation process that is necessary for the generation of an antisaccade.     

Fixation and saccade control in an express-saccade maker

In express-saccade makers a large incidence of express saccades (latencies around 100 ms) is paralleled by a reduced ability to suppress saccade generation when required. Such a behavior occurs frequently in dyslexies. We studied the latencies and the metrical properties of saccades in the very rare case of an adult, nondyslexic express-saccade maker (male, age 29 years). The subject produced 65–95% express saccades in the gap (fixation point removed 200 ms before target onset) as well as in the overlap (fixation point not removed) paradigm, which qualified the subject as the most clear case of an express-saccade maker found so far. The number of express saccades increased rather than decreased when fixation foreperiod, gap duration, and target location were randomized from trial to trial as compared to when they remained constant. In the memory-guided saccade and in the antisaccade paradigms in which immediate saccade execution to a visual target had to be suppressed, the subject often reacted to the target with express saccades in an involuntary way. The amplitudes of express saccades were — in some conditions — found to progressively decrease with increasing latency, giving rise to amplitude transition functions. The present findings disprove the notion that express saccades are generated based on the prediction of the time and location of target appearance and support the notion that they are the result of an optomotor reflex. It is argued that the operation of the reflex is gated by a separate fixation system. Express-saccade makers are described as subjects with a dysfunction of the fixation system. Recent neurophysiological findings suggest that the subject studied in the present study has a selective dysfunction of the fixation system at the level of the superior colliculus.     

The initiation of smooth pursuit eye movements and saccades in normal subjects and in “express-saccade makers”

A vast knowledge exists about saccadic reaction times (RT) and their bi- or multimodal distributions with very fast (express) and regular RT. Recently, there has been some evidence that the smooth pursuit system may show a similar RT behavior. Since moving targets usually evoke a combined pursuit/saccade response, we asked which processes influence the initiation of pursuit and saccadic eye movements. Furthermore, we investigated whether and how the pursuit and saccadic system interact during the initiation of eye movements to moving targets. We measured the RT of the initial smooth pursuit (iSP) response and of the first corrective saccade and compared the RT behavior of both. Furthermore we compared the behavior of the corrective saccades to moving targets to that of saccades to stationary targets, known from the literature. The stimulus consisted of a target that moved suddenly at constant velocity (ramp). In addition, prior to the movement, a temporal gap, a position step or a combination of both could occur (gap-ramp, step-ramp, gap-step-ramp, respectively). Differently from most previous studies, we chose step and ramp with the same direction to provoke competition between the pursuit and saccade system. For the first time we investigated pursuit initiation in "express-saccade makers" (ES makers), a subject group known to produce an abnormally high percentage of short-latency saccades in saccade tasks. We compared their results with subject groups who were either naive or trained with respect to saccade tasks. The iSP started at approximately 100 ms, which corresponds to express saccade latencies. These short iSP-RT occurred reflex-like and almost independent of the experimental task. A bimodal frequency distribution of RT with a second peak of longer iSP-RT occurred exclusively in the ramp paradigm. The RT of the first corrective saccades in a pursuit task were comparable with that in a saccade task and depended on the stimulus. The ability of ES makers to produce a high number of express saccades was transferred to corrective saccades in the pursuit task, but not to pursuit initiation. In summary, short-latency pursuit responses differ from express saccades with respect to their independence of experiment and subject group. Therefore, a simple analogy to express saccades cannot be drawn, although some mechanisms seem to act similarly on both the pursuit and the saccade system (such as disengagement of attention with the gap effect). Furthermore, we found evidence that the initial pursuit response and the first corrective saccade are processed independently of each other. The first corrective saccades to moving targets behave like saccades to stationary targets. Normal pursuit but abnormal saccade RT of ES makers can be explained by recent theories of superior colliculus (SC) function in terms of retinal error handling.     

Primate antisaccade. II. Supplementary eye field neuronal activity predicts correct performance

Neuronal activities were recorded in the supplementary eye field (SEF) of 3 macaque monkeys trained to perform antisaccades pseudorandomly interleaved with prosaccades, as instructed by the shape of a central fixation point. The prosaccade goal was indicated by a peripheral stimulus flashed anywhere on the screen, whereas the antisaccade goal was an unmarked site diametrically opposite the flashed stimulus. The visual cue was given immediately after the instruction cue disappeared in the immediate-saccade task, or during the instruction period in the delayed-saccade task. The instruction cue offset was the saccade gosignal. Here we focus on 92 task-related neurons: visual, eye-movement, and instruction/fixation neurons. We found that 73% of SEF eye-movement-related neurons fired significantly more before anti-saccades than prosaccades. This finding was analyzed at 3 levels: population, single neuron, and individual trial. On individual antisaccade trials, 40 ms before saccade, the firing rate of eye-movement-related neurons was highly predictive of successful performance. A similar analysis of visual responses (40 ms astride the peak) gave less-coherent results. Fixation neurons, activated during the initial instruction period (i.e., after the instruction cue but before the stimulus) always fired more on antisaccade than on prosaccade trials. This trend, however, was statistically significant for only half of these neurons. We conclude that the SEF is critically involved in the production of antisaccades.