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The Journal of Neurophysiology Vol. 80 No. 4 October 1998, pp. 1775-1786
Copyright ©1998 by the American Physiological Society
Department of Neurobiology and Brain Research Institute, University of California at Los Angeles School of Medicine, Los Angeles, California 90095-1763
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ABSTRACT |
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 Abstract
 Introduction
Methods
Results
Discussion
References
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Amador, Nelly, Madeleine Schlag-Rey, and John Schlag. Primate antisaccades. I. Behavioral characteristics. J. Neurophysiol. 80: 1775-1786, 1998. The antisaccade task requires a subject to make a saccade to an unmarked location opposite to a flashed stimulus. This task was originally designed to study saccades made to a goal specified by instructions. Interest for this paradigm surged after the discovery that frontal lobe lesions specifically and severely affect human performance of antisaccades while prosaccades (i.e., saccades directed to the visual stimulus) are facilitated. Training monkeys to perform antisaccades was rarely attempted in the past, and this study is the first one that describes in detail the properties of such antisaccades compared with randomly intermingled prosaccades of varying amplitude in all directions. Such randomization was found essential to force the monkeys to use the instruction cue (pro- or anti-) and the location cue (peripheral stimulus) provided within a trial rather than to direct their saccades to the location of past rewards. Each trial began with the onset of a central fixation target that conveyed by its shape the instruction to make a pro- or an antisaccade to a subsequent peripheral stimulus. In one version of the task, the monkey was allowed to make an immediate saccade to the goal; in a second version, the saccade had to wait for a go signal. Analyses of the accuracy, velocity, and latency of antisaccades compared with prosaccades were performed on a sample of 7,430 pro-/antisaccades in the "immediate saccade" task (delayed saccades suffering from known distortions). Error rates fluctuated ~25%. Results were the same for the two monkeys with respect to accuracy and velocity, but they differed in terms of reaction time. Both monkeys generated antisaccades to stimuli in all directions, at various eccentricities, but antisaccades were significantly less accurate and slower than prosaccades elicited by the same stimuli. Interestingly, saccades to the stimulus could be followed by appropriate antisaccades with no intersaccadic interval. Such instances are here referred to as "turnaround saccades." Because they occurred sometimes with a long latency, turnaround saccades did not simply reflect the cancellation of an early foveation reflex. Consistent with human data, latencies of one monkey were longer for antisaccades than for prosaccades, but the reverse was true for the other monkey who was trained differently. In summary, this study demonstrates the feasibility of providing a subhuman primate model of antisaccade performance, but at the same time it suggests some irreducible differences between human and monkey performance.
The antisaccade task requires a subject to generate a saccade of equal eccentricity and opposite direction to a peripheral visual stimulus. This is one of several tasks designed by Hallett (1978) Two adult female macaque monkeys (MkA, nemestrina; MkD, rhesus) were implanted with a scleral search coil according to the method described by Judge et al. (1980) Tasks
The monkeys were trained to perform interleaved prosaccades and antisaccades in two versions of the task, illustrated in Fig. 1. The two versions are referred to as "immediate" and "delayed" saccade tasks to emphasize a difference consisting in withholding (or not) a prepared saccade rather than a difference in memory load.
Training procedure
The two monkeys were trained to the same level of performance but in different ways. After MkA learned the one-step basic prosaccade task, training started on a no-saccade task as a transitional stage. Despite this prior conditioning in suppressing foveation reflexes, MkA appeared unable to suppress them in the immediate antisaccade condition. Antisaccade training was then postponed until she learned to make prosaccades to remembered targets (with delays Data analysis
The time of occurrence and the spatial coordinates of all events pertaining to an experiment were stored in binary files. Such were, for instance, the shape of the fixation point (square or dot), its onset/offset, the location of the visual stimulus, its onset/offset, the timing of reinforcement, the trajectories, and the timing of all saccades made during a trial. Each session was replayed off-line to delete artifactual detection of saccades. Pro- and antisaccade trajectories were then plotted and quantitatively analyzed. The statistical analysis of the results was made by analysis of variance and
Progress during the different phases of training will not be described here, except to note a feature relevant to the interpretation of the results; the learning curves always showed a dramatic regression with every modification of the task. In particular, each novel direction of antisaccade had to be trained anew by successive approximation; there was no evidence of sudden insight in the antisaccade rule, e.g., no immediate transfer of learning from the horizontal to the vertical dimension.
Accuracy
Figure 2 documents the ability of the two monkeys to generate antisaccades in eight general directions (underscored by arrows superimposed on the end points of saccades). In addition, Fig. 2 allows one to appreciate, on anti-trials, the difference
Velocity
For the two monkeys, correct antisaccades had a significantly lower peak velocity than correct prosaccades of equal amplitude (P < 0.001). This difference obtained for all saccade amplitudes, as shown in Fig. 6, which graphs main sequences (peak velocity vs. amplitude) for pro- and antisaccades ranging from 5 to 30° amplitude. The curves of Fig. 6 were derived from the same database as the accuracy plots of Fig. 3. Although MkD's accuracy on prosaccade trials improved after memory training (Fig. 3), her average saccade velocity decreased.
Reaction time
The normalized distributions of saccade latency from stimulus onset are shown in Fig. 7 for correct saccades (upward histograms) and incorrect saccades (downward histograms) made during the same sessions. Surprisingly, when MkA was correct, her reaction time was shorter on antisaccade trials than on prosaccade trials (mean = 244 ms for prosaccades and 209 ms for antisaccades; the difference was significant at the P < 0.001 level). This is the opposite of what could have been expected from our knowledge of all human antisaccades. However, consistent with the human literature, MkD had a shorter reaction time for correct prosaccades (mean = 194 ms for prosaccades and 230 ms for antisaccades; significant difference at P < 0.001). After memory training, MkD's mean latencies of correct pro- and antisaccades dropped to 188 and 195 ms, respectively (the difference was still significant, P < 0.01). It should be remarked that the average reaction time of the two monkeys was not far apart on antisaccade trials (209 ms for MkA vs. 230-195 ms for MkD). Therefore it is the large difference between the prosaccadic reaction times of the monkeys that is responsible for the opposite trend observed when, for each monkey, a comparison is made between reaction times for pro- and antisaccades on correct trials.
The most important finding of this study is that monkeys can be taught to make antisaccades in any direction, in response to unpredictable stimuli, and that their choice between making an antisaccade versus a prosaccade can be guided by a symbolic instruction cue that alone does not specify the direction of the saccade to be made. As the monkeys were making antisaccades in all directions, the probability of securing reinforcement by chance or by repetition of the last rewarded saccade became very low. Thus it may be safely concluded that the monkeys came to understand the abstract rule encoded in the instruction cue and that they learned to use it appropriately. This conclusion is further supported by the higher percentage of correct saccades (71%) made on trials in which the instruction differed from that of the previous trial. High error rates in antisaccade tasks are commonly observed in humans and particularly in children (Fischer et al. 1997 Accuracy
Antisaccades aiming toward the goal, i.e., a virtual target, were found less accurate than prosaccades to the site of a real target. Both amplitude and angular errors contributed to the inaccurate end points of primary antisaccades. Antisaccade trajectories were rarely corrected by secondary saccades before visual feedback, and when secondary antisaccades eventually occurred they sometimes drove the eyes farther away from the goal. This summary account of monkey antisaccades does not differ from that originally given for human antisaccades by Hallett and colleagues (Doma and Hallett 1988 Reaction time
For MkD, the average reaction time of a correct antisaccade was longer than that of a correct prosaccade, whereas for MkA it was shorter. However, in absolute values, the distributions of antisaccade latencies of the two monkeys were similar and did not include express saccades. However, MkD made some express prosaccades in error on anti-trials. This observation is in line with the majority of results from human studies (Fischer and Weber 1992 Velocity
The reduced peak velocity of monkey antisaccades compared with prosaccades was the most consistent result in this study, conspicuous in single trials (e.g., in Fig. 4) as well as in the main sequences of the two monkeys (Fig. 6). Slower trajectories were commonly observed in human antisaccades but not in all subjects. Admittedly, these results were obtained from two monkeys only, but they are unambiguous. Several factors may have contributed to slow down the monkeys' antisaccades. One, already discussed in relation to their relative inaccuracy, is the uncertainty about the exact location of the antisaccade goal. Another one is an adaptive strategy of slowly approaching the invisible reinforcement window. Still another factor is an ever-present degree of uncertainty about the rules of the game. The latter uncertainty is absent from human experiments because subjects are verbally instructed and do not doubt what they have to do. In contrast, the whole training history of monkeys in complex tasks prepared them for changes in the rules of reward.
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
to study the properties of human saccades to goals defined by instruction. In this first study, human antisaccades were found to occur with less accuracy, lower velocity, and longer latencies than saccades directed to a visible stimulus. The explanation of their characteristics seems to lie in the indirect visual information supplied about the antisaccade goal and does not seem to require new assumptions about saccadic mechanisms or oculomotor paths (Doma and Hallett 1988; Hallett 1978; Hallett and Adams 1980; but see Smit et al. 1987).
reported that patients affected by frontal lobe lesions could not make antisaccades, although the same individuals could make normal saccades to visual targets. The main problem for these patients was their inability to suppress reflexive glances to sudden visual stimuli. These results were quite provocative in suggesting that antisaccade performance provides means for testing higher oculomotor functions, especially those depending on the integrity of the fontal lobe or its associated forebrain structures. Indeed, subsequent studies of antisaccades in patients affected by prefrontal (Pierrot-Deseilligny et al. 1991) and basal ganglia disorders (Lasker et al. 1987; Tian et al. 1991) showed similar impairments in antisaccade tasks that are now routinely included in clinical neurological studies.
; Fischer and Ramsperger 1984). Using this paradigm, Fischer and Weber (1992) showed that, in contrast to prosaccades, antisaccades are never made with a latency in the range defining express saccades. They interpreted their finding as supporting the hypothesis of a reflexlike pathway connecting the retina to the oculomotor centers in the brain stem
a pathway mediating express saccadesif it is disinhibited at the time of target appearance. More recent studies by the same authors (Fischer and Weber 1997; Weber 1995) analyzed the experimental conditions that govern the occurrence of disallowed glances to the stimulus.
). The other region implicated by Everling and colleagues is the posterior parietal cortex, over which, during an antisaccade trial, a negative potential shifts from the contralateral to the ipsilateral hemisphere, reflecting the change of planning from a saccade to the stimulus to a saccade to the opposite side. This may be still another type of change of motor planning besides those already associated with neuronal correlates in the lateral intraparietal area (Bracewell et al. 1996; Snyder et al. 1997).
, in agreement with cortical potentials and unit studies, demonstrated a pattern of cortical activation that differentiates antisaccades from prosaccades by a comparatively greater activation of the SEF accompanying antisaccades.
; Funahashi et al. 1993; Schlag-Rey et al. 1997). Yet, how monkeys generate saccades to virtual targets is a field of research wide open, not only because human antisaccade experiments multiplied with diverse paradigms but because in principle any question posed about prosaccades may be posed about antisaccades generated in the same condition. Furthermore, from a neurophysiological viewpoint, the antisaccade task is ideal for determining whether the neuronal discharge of a cell is associated with the presentation of the visual stimulus regardless of the subsequent eye movement or the impending eye movement to be made regardless of where the visual stimulus is presented or else with both events (Schlag-Rey et al. 1997).
could be explored in monkeys if the conceptual and computational aspects of the task were preserved, i.e., if we ensured that the monkeys derived their antisaccade goals from the current cues, not from the locus of past rewards. For instance, when antisaccade trials are run in blocks of trials in two directions, a monkey can ignore the instruction and successfully repeat the same saccade until the reward shifts to the other side. Therefore, in adapting the human task for monkeys, we attempted to preserve the conceptual element, by requiring the monkey to interpret a symbolic instruction cue specifying whether a pro- or an antisaccade should be made on a given trial, and the computational element, by forcing the monkey to use a current, unpredictable stimulus, not a reinforced association, for programming the eye movement. Our departure from the two-choice left-right antisaccade paradigm used in human studies was also motivated by the plan to tailor the vector of antisaccades to the particular response field of a cell in subsequent unit recording experiments. (A preliminary analysis of primate antisaccade behavior was made by Sanchez et al. 1994.)
METHODS
Abstract
Introduction
Methods
Results
Discussion
References
. The leads of the coil were affixed to a pedestal of dental cement that embedded skull screws and nuts to which a head restraint apparatus was secured during experimental sessions. The surgical procedures, training, and care of the animals followed the guidelines of the National Institute of Health's Guide for the Care and Use of Laboratory Animals and the instructions of the UCLA Animal Research Committee. During training sessions, the monkey sat in a primate chair with head fixed and facing a tangent screen 132 cm away from the eyes (to minimize the occurrence of converging eye movements). Visual stimuli, positioned by joysticks, were low-intensity (25 mcd/m2) luminous dots or squares (<1°) produced by a Tektronix 608 oscilloscope and rear projected through a wide angle lens. Eye position recording methods were previously described (Schlag et al. 1998). The monkey's facial movements were continuously monitored by an infrared camera. The tasks were performed in a dim red light environment (to permit rapid dark adaptation) or in complete darkness. Drops of apple juice (diluted 50% and sweetened with aspartame) were used as a reward. All stimuli and behavioral events were controlled by a Macintosh Quadra 840AV and stored for off-line analysis. Eye position was sampled at 1 kHz, and saccades were automatically detected. All computer programs were written in MacProbe software.
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FIG. 1.
Prosaccade and antisaccade tasks. Top and bottom panels: spatial display of stimuli and saccade trajectories in the immediate and delayed saccade tasks, respectively. The traces between the panels indicate the sequence of events. Each trial began with a fixation point stimulus encoding, by its shape, the instruction to make a prosaccade ( 
) or an antisaccade (
, <1°). The disappearance of the fixation point was the go signal for the saccade. An eccentric visual stimulus (always ) was flashed for 50-100 ms when the fixation point disappeared (immediate saccade task) or at variable times during fixation of the instruction cue (delayed saccade task). The required saccade is indicated by an arrow.
) or an antisaccade () to a subsequent peripheral target (always ), appearing at an unpredictable location. Thus, during initial fixation, the monkey knew what type of saccade (pro- vs. anti-) would be appropriate but did not yet know in what specific direction the movement should be made. In the "immediate saccade condition," the offset of the fixation point coincided with the onset of a peripheral stimulus (always ) to which the monkey was required to make a pro- or an antisaccade, depending on the prior instruction cue. In the"delayed saccade condition," the peripheral target was flashed while the monkey was fixating the central target, and the monkey had to withhold a saccade to it until the fixation target disappeared. In both tasks, the disappearance of the fixation point was the go signal for the eye movement, but no constraint was imposed on the monkey's reaction time. Stimuli were positioned with a joystick, in directions spanning 360° and eccentricity varying from 5 to 25°. Different stimulus durations, ranging from 10 to 200 ms, were used to test whether short or long durations would facilitate the inhibition of a foveating reflex on antisaccade trials. A short stimulus duration appeared more conducive to the production of antisaccades when no delay was involved; therefore 50 ms became the standard duration. However, this duration was increased to 100 or 200 ms for long delayed saccades (e.g., 900 ms).
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FIG. 2.
Trajectories of antisaccades made by MkA and MkD to stimuli presented in 8 randomized directions. Arrows were superimposed on the termination points. The location of the stimuli (not shown) was diametrically opposite to the evoked antisaccades. The 2 samples of antisaccades were chosen to illustrate the contrast between antisaccades performed in complete darkness (MkA) and in a dim light environment (MkD).
1 s). She then learned to perform delayed antisaccades and finally immediate antisaccades. Because MkA's reaction times were found unexpectedly shorter for antisaccades than for prosaccades and because this idiosyncratic behavior was perhaps linked to her prolonged training in saccades to remembered targets, MkD's training in pro- and antisaccades proceeded on par, first in the immediate condition, second in the delayed condition. After learning the basic prosaccade task, MkD learned to make antisaccades without any detour through a no-saccade task.
2 tests.
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FIG. 3.
Distribution of prosaccades (in gray) and antisaccades (in black) terminating in a 30° reinforcement window. Abscissa: actual direction disparity with respect to the required direction. Ordinate: amplitude ratio of the actual/required amplitude. The perfectly accurate vector has a direction disparity of 0° on prosaccade trials and 180° on antisaccade trials, and its amplitude ratio is 1.0 on both types of trial. In this and subsequent illustrations, the database consists of saccades made in the immediate saccade task only. For both monkeys, the results focus on asymptotic performance of this task after they learned to perform delayed antisaccades (i.e., memory training). However, for MkD, a comparison is offered with the performance of immediate antisaccades before memory training.
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
65% for both types of trial. We analyzed the trials on which the instruction differed from that of the preceding trial and found that 71% of these trials were correct. Because in our experiments antisaccades were pseudorandomly interleaved with prosaccades and reinforced with the same criterion window, valid comparisons of the two types of eye movement could be made despite fluctuations in the percentage of correct saccades from session to session. The numbers of correct and incorrect pro- and antisaccades analyzed for each monkey appear in Fig. 7.
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FIG. 7.
Distributions of saccade latency from stimulus onset for MkA and MkD (before and after memory training). To facilitate the comparison of the different conditions for MkA and MkD, the latency distributions were normalized separately for correct trials (upward histograms) and incorrect trials (downward histograms). The absolute number of trials included in each histogram appears at the right of each plot.
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FIG. 4.
Six examples of the 5 most accurate trajectories of prosaccades and antisaccades of MkA and MkD, elicited by the same target on randomly alternating trials, while recording from cells having their response fields centered on the target (*). To avoid obscuring the termination points of the saccade trajectories, the target is represented at a position more eccentric (~1°) than it actually was.
).
well known on pro-trials (Gnadt et al. 1991; White et al. 1994)between saccades generated in complete darkness (MkA) and those generated in an illuminated environment (MkD). Clearly, a prominent upward drift occurred at the end of antisaccades made by MkA but was absent from antisaccades made by MkD. The drift depended on the scotopic condition only not on individual differences between these two monkeys.
that the large windows used for reinforcing saccades to visual targets in the same way as saccades to somatosensory targets did not induce the monkeys to relax visuooculomotor accuracy. The difference between pro- and antisaccades visible in the three panels of Fig. 3 was extremely significant in all analyses of direction disparity (P < 0.0001), but it was less or not significant in terms of amplitude ratio. Additionally, Fig. 3 (right panel) shows that immediate prosaccades, but not antisaccades, were more accurate after MkD was trained to make delayed saccades, either because this "memory training" was beneficial or simply because later training sessions were involved. However, despite overtraining, no improvement resulted from reducing the criterion window to <10° for a 20° antisaccade.
).
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FIG. 5.
A: raw record. B: sample of trajectories of "turnaround saccades" occasionally made by the 2 monkeys on antisaccade trials. The 1st saccade was made by mistake to the visual stimulus and was followed, without pause, by a correct antisaccade. The examples in B illustrate the wide range of latencies from stimulus onset with which turnaround saccades may occur.
). However, undershooting was more frequent (Fig. 3). Despite this fact, primary antisaccades were rarely corrected by secondary saccades generated before visual feedback of the antisaccade goal. When secondary antisaccades occurred, they sometimes brought gaze farther from the goal, as was observed for human antisaccades (Hallett 1978). The larger window sizes that were used during training when all directions were mixed may have contributed to a strategy of producing the smallest antisaccades that would terminate within the window. However, as mentioned above, this strategy was definitely not used for prosaccade. Remarkably, an accurate corrective prosaccade invariably followed visual feedback of antisaccades terminating in the criterion window, although the reward had already been delivered.
5° from each other. Although reasonably good, their aim was definitely less precise, and their velocity conspicuously reduced in comparison with the prosaccades elicited by the same stimulus.
; Fischer et al. 1997).
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FIG. 6.
Plots of peak velocity vs. amplitude of correct prosaccades (in gray, circles) and antisaccades (in black, triangles) for MkA and MkD (before and after memory training). Means (dots) and SDs (vertical bars) were computed for 5 levels of average amplitude, ranging from 5 to 10° to 25 to 30°.
2 = 5.91, P < 0.05 before memory training; 2 = 13.18, P < 0.001 after memory training).
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
) with whomallowing for differences in experimental conditionsour monkeys compare favorably.
; Hallett 1978; Hallett and Adams 1980). Indeed, these investigators observed that antisaccades differ from prosaccades by a decreased accuracy of primary saccades, not always improved by secondary saccades. They also found that, apart from eliminating unwanted reflex foveations, antisaccade trajectories could not be improved by further practice.
, confirmed by later parametric studies (Gnadt et al. 1991; Smit et al. 1987). Comparing saccades with visual targets, saccades to remembered targets, and antisaccades, Smit et al. (1987) found the latter to be the most inaccurate. Comparing human and monkey saccades with remembered targets in the dark, Gnadt and colleagues (1991) demonstrated a systematic hypermetria for upward saccades coupled with hypometria for downward saccades, which we also found evident in the antisaccades of MkA and MkD (Fig. 2).
and dramatically more so for monkeysto make accurate saccades to somatosensory targets when the source of the sensory stimulus is invisible.
). Because the gap paradigm favors the occurrence of express saccades thought to be involuntary, Fischer and Weber (1992) used the gap paradigm to show that correct antisaccadesbeing voluntarynever occur with express latency, in contrast to unwanted foveations. They refer to the latter as "direction errors" (equivalent to "mistakes" in Hallett's terminology). They are, in fact, gross direction errors (~180°) rather than minor deviations from the antisaccade goal. Their occurrence depend on decision and attention mechanisms (Fischer and Weber 1992). For example, with pro- and anti-trials intermixed, Weber (1995) found that the error rate is greatest when the instruction cue mandating a pro or an antisaccade is given at the same time (0-ms lead time) as the visual stimulus indicating the goal (directly or indirectly). In that condition, the type of (gross) direction error is strongly dependent on the type of response made on the previous trial. The influence of the previous trial disappears when the instruction cue lead time exceeds 300 ms, which was always the case in our experiments with monkeys.
; but see Fischer et al. 1997). This may also be true of monkeys. Otherwise, why would they generate these remarkable turnaround saccades (Fig. 3) as if two motor programs collided? The first program, directing gaze toward the stimulus, may have been generated through a reflexive pathway. Supporting the reflex hypothesis is the fact that some of the mistaken prosaccades made by MkD on anti-trials had express latencies. However, we observed that turnaround saccades can have very long latencies, suggesting that a reflex may be stored or that over time it becomes a voluntary movement.
) because, after making an initial saccade to the stimulus, they often generated a large antisaccade to the virtual target, although this correct reaching of the goal was not reinforced (to extinguish the tendency to look first at the stimulus). The latter movement may be subserved by quasi-visual cells (Mays and Sparks 1980) and similar types of cells. This suggests that quasivisual cells could be involved in all antisaccades. They would represent a saccade to a virtual stimulus specified by instruction rather than a saccade to a second stimulus impinging on the retina.
). As far as MkA's counterintuitive reaction times are concerned, they might simply attest to individual differences. On the other hand, they may be the product of a prolonged memory training with prosaccades only. If this hypothesis were true, saccade latency appears to depend on different factors than those that affect saccade accuracy and velocity. In any event, it has been shown that, even with a precise determination of the time of target discrimination, the time of saccade initiation cannot be predicted (Thompson et al. 1996).
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ACKNOWLEDGEMENTS |
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We thank H. Sanchez for contributions in training the monkeys, J. Kroger for programming assistance, and A. Karapogosyan and A. Mohempour for general assistance.
This work was supported by National Eye Institute Grants EY-02305 and EY-05879.
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FOOTNOTES |
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Address for reprint requests: M. Schlag-Rey, Dept. of Neurobiology, UCLA School of Medicine, Los Angeles, CA 90095-1763.
Received 11 March 1998; accepted in final form 9 June 1998.
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REFERENCES |
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Abstract
Introduction
Methods
Results
Discussion
References
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