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REPORT
Paris Descartes University and Centre National de la Recherche Scientifique, Laboratoire de Psychologie et Neurosciences Cognitives, Unité Mixte de Recherche 8189, Boulogne-Billancourt, France
Submitted 28 September 2007; accepted in final form 16 March 2008
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ABSTRACT |
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INTRODUCTION |
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for a review). Programming an antisaccade is generally assumed to take two steps: first the inhibition of the saccade toward the visual target (prosaccade) and then the programming of a saccade to the opposite direction (antisaccade). Indeed, antisaccade latency is longer than prosaccades because of these supplementary processes. The process by which the location of the visual stimulus is transformed into a motor command to the opposite side could take two forms. The visual vector—corresponding to the distance between central gaze and the visual target—could be inverted. Visual vector inversion would thus require computing the position of the visual target, reflecting that about the origin, and programming a saccade to it. Alternatively, the motor vector—corresponding to the motor command of the inhibited prosaccade—could be inverted. Motor vector inversion would require computing the metrics of the prosaccade directed toward the visual stimulus and reflecting the spatial attributes of the motor plan about the origin.
Evidence for vector inversion in the antisaccade task was reported in the frontal eye fields (FEF) by Sato and Schall (2003) and Schall (2004). (Because the FEF movement neurons represent the veridical saccade, the command to invert the vector must be at or upstream of the FEF). In prosaccade trials, FEF visual neurons were activated when the target was inside but not when it was outside their receptive field. In antisaccade trials, the same neurons were initially responsive to the target but switched their activity and responded more to the target placed on the opposite side, outside their receptive field, but toward which the upcoming antisaccade would be directed. This suggests that, at some point during the delay between target presentation and movement onset, there is a switch in activity. However, whether this activity was motor or visual remained unanswered. Further evidence for vector inversion was found in the lateral intraparietal area (LIP) by Zhang and Barash (2000, 2004). LIP visual neurons respond to the visual target regardless of the direction of the upcoming saccade. A subset (30%) of these visual neurons also shows paradoxical activity: in addition to the visual response to targets inside the receptive field, when the upcoming antisaccade was directed toward the receptive field, these neurons also had a visual response to the target on the opposite side of their receptive field (see also Gottlieb et al. 2005). Zhang and Barash (2000, 2004) proposed that this paradoxical activity could be related to the inversion of the visual vector (see also Barash 2003; Barash and Zhang 2006). Funahashi et al. (1993) reported paradoxical activity in prefrontal motor neurons that were activated when an antisaccade was made in the direction opposite their movement field. Medendorp et al. (2005) showed that goal-related activity in the putative human homolog of monkey LIP in the posterior parietal cortex (PPC) switched from the contralateral to the ipsilateral PPC, suggesting that the visual stimulus location was remapped to the opposite side. Nyffeler et al. (2007) recently reported a case study of selective deficit in the antisaccade task following PPC damage. Ipsilateral prosaccades were normometric, but both contralateral prosaccades and ipsilateral antisaccades were hypometric. This pattern could result from a deficit in the contralateral visual vector caused by parietal damage and is consistent with the inversion of a (damaged) visual vector during antisaccade programming.
We sought to study vector inversion in antisaccades in humans with a behavioral approach: saccadic adaptation. Saccadic adaptation occurs following targeting errors, such as those observed after extraocular muscle dysfunction (Abel et al. 1978) or in the laboratory with the double-step procedure (McLaughlin 1967): the visual target is surreptitiously displaced during the saccade directed toward it, thus mimicking a targeting error when the eye lands. After several such trials, saccade amplitude adapts to compensate for the artificial error, thus introducing a discrepancy between the visual vector and the motor vector (see Hopp and Fuchs 2004 for a review). Indeed, saccadic adaptation affects the motor vector of the saccade and not the visual vector of the target. This has been supported by several lines of evidence. First, saccades of a given vector directed toward different positions of the visual field were measured before and after the adaptation of a vector of same magnitude and direction but aiming for a different position. If adaptation led to a local modification of the representation of the visual field, the tested saccades should not be affected by the adaptation. Several studies have shown that this is not the case (Albano 1996; Frens and van Opstal 1994; Wallman and Fuchs 1998). A second approach examined the effect of adaptation on nonvisually guided saccades. If adaptation modifies the representation of the visual field, the adaptation of a given saccade vector should not transfer to saccades with the same vector be directed to an auditory stimulus. Frens and van Opstal (1994) showed, on the contrary, that adaptation of visually guided saccades transferred to auditorily guided saccades. A third approach examined the transfer of adaptation to manual movements. If saccadic adaptation modified the representation of visual space, all movements guided in this space should be modified by adaptation transfer. On the contrary, several studies have shown no transfer of saccadic adaptation to movements of the head (Kröller et al. 1999) or the hand (McLauglin et al. 1968; but see de Graaf et al. 1995 for limited transfer).
The goal of our study was to examine how the adaptation of a saccadic motor vector transfers to antisaccades. To test this issue optimally, we adapted volitional rather than reactive prosaccades because adaptation transfer is usually found within a saccade category (i.e., volitional or reactive) but not systematically between saccade categories (i.e., reactive to volitional; we would thus not expect any transfer from reactive prosaccades to antisaccades in any direction; see METHODS). In the first experiment, we adapted a horizontal rightward prosaccade and tested the transfer of this adaptation to leftward and rightward anti- and prosaccades of similar amplitude. The four saccade types examined are shown in Fig. 1.
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). Two hypotheses can be made about the transfer of rightward prosaccade adaptation (V+M+) to antisaccades. We distinguished the motor vector inversion hypothesis from the visual vector inversion hypothesis (Fig. 1). If antisaccades depend on the inversion of the motor vector of the unexecuted prosaccade, adaptation should transfer to antisaccades to the opposite location (V+M–). Prosaccade adaptation should not transfer to antisaccades made in the same direction (V–M+) because these antisaccades would be based on the motor vector of the unexecuted prosaccade in the opposite direction, which was not adapted. This hypothesis assumes that motor vector inversion occurs after saccadic adaptation. Alternatively, if antisaccades depend on the inversion of the visual vector, prosaccade adaptation should transfer to antisaccades in the same direction (V–M+), because the visual vector of the target on the opposite side would be inverted and the saccade would be programmed. This saccade would thereafter be based on the same sensory-motor transformation as the adapted saccade and should be adapted. Rightward prosaccade adaptation should not transfer to antisaccades in the opposite direction (V+M–) because their visual vector is not influenced by the prosaccade adaptation. This hypothesis assumes that visual vector inversion occurs before saccadic adaptation. These alternative hypotheses rest on two related assumptions. First, saccadic adaptation modifies the motor vector. Consequently, if antisaccades depend on the inversion of the visual vector, they would not be affected by adaptation. Second, the motor vector is affected by adaptation before vector inversion. Consequently, if antisaccades depend on the inversion of the motor vector, they would be affected by adaptation. We return to the two assumptions in the DISCUSSION.
In the first experiment, there were only two possible targets (left and right). In the second experiment, the number of saccade targets was increased to six possible saccade targets, only one of which was adapted. Moreover, we tested both rightward and leftward prosaccade adaptation. Both experiments led to similar results.
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METHODS |
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SUBJECTS. Seven subjects with normal vision participated in this experiment. Four were naïve to the goal of the experiment, and three were authors (S1, S3, and S6). All were familiar with the eye movement recording and gave their informed consent. The experimental protocol was approved by the local ethics committee as being within the regulations for human experimentation under the Helsinki protocol.
INSTRUMENTS AND EYE MOVEMENT RECORDING.
The experimental sessions took place in a dimly lit room. Subjects were seated 57 cm away from the screen and their head was kept stable with a submaxillar dental print and forehead rest. The stimuli were presented on an Iiyama HM240DT monitor with a refresh rate of 170 Hz. Eye movements were monitored by a Bouis Oculomotor system (Bach et al. 1983), with an absolute resolution of 6 arc min and a linear output over 12° of visual angle. Viewing was binocular, but only the movements of the right eye were monitored. Signal from the oculometer was sampled every 2 ms. Saccades were detected with an in-house program using Labview 7.1 by velocity (>40°/s), acceleration (>3,000°/s/s), and minimal displacement (0.15°) thresholds.
STIMULI AND PROCEDURE. Stimuli were 1x 1° white crosses on a medium gray background. The fixation cross was positioned in the center of the screen, and the saccade target could appear 6° to the left or right.
Each session began with a full calibration procedure during which subjects had to saccade to five bars presented successively from left to right in steps of 3°. Reference measures were taken for each of the five bars. If the variability of each measure was below a threshold (0.4 V), and if the values for the five bars were linear, the calibration was considered successful and the experiment was started. Each experimental trial started with a calibration check. Subjects were required to fixate a bar that appeared in the center of the screen. If the recorded value was different from full calibration (±0.1°), it was automatically renewed. When successful calibration was detected, the fixation cross and saccade target appeared simultaneously. After 600 ms, the fixation cross disappeared; this was the go-signal for the saccade. After their saccade, subjects had to press on a button to initiate the next trial.
Each session was composed of 320 trials in three successive phases: pretest, adaptation, and posttest. In the pretest phase, subjects performed a block of 40 antisaccade trials and a block of 40 prosaccade trials. The instructions were given to the subject before each block on the screen, but no feedback as to their saccade performance was given. In antisaccade trials, subjects were instructed to look to the mirror position. In each of the pretest blocks, the target appeared equally often on the left or right. During the saccade directed toward it, the target disappeared, and the screen remained blank until the subject pressed a button to indicate she was ready for the next trial. The two pretest blocks were followed by the adaptation phase. Subjects were instructed to make only prosaccades. The saccade target appeared to the right in 100 trials (62.5%). During the rightward saccade, the target stepped back by 2°. In the remaining 60 trials, the saccade target appeared to the left and did not step back during the saccade. All trials were randomly mixed. Finally, the adaptation phase was followed by a posttest phase in which 40 prosaccade and 40 antisaccade trials were tested in separate blocks. The characteristics of these two blocks were identical to the pretest. In particular, the target was extinguished during the saccade directed toward it, such that when the eyes landed, there was no longer a visual reference from which a targeting error signal could be computed (Seeberger et al. 2002). This allowed adaptation to be maintained throughout the 80 posttest trials. Five subjects performed two replications of the 320-trial session (separated by a minimum of 48 h), and two subjects performed one replication. We averaged across the two replications and compared the mean results of the seven subjects.
By using a 600-ms overlap between the fixation cross and the saccade target, we elicited volitional saccades. We tested volitional rather than reactive prosaccades because reactive saccade adaptation does not transfer to volitional saccades (Alahyane et al. 2007; Collins and Doré-Mazars 2006; Deubel 1995). Although transfer of adaptation from visually guided volitional saccades to antisaccades has not been specifically tested, robust transfer from one type of volitional saccade to another has been reported (Deubel 1995; Fujita et al. 2002).
We tested only backward adaptation by stepping the visual target in the direction opposite the saccade during its execution and did not test forward adaptation (during which the target steps in the same direction as the saccade and leads to an increase of amplitude). Backward adaptation has been shown to be faster than forward adaptation (Bahcall and Kowler 2000). Because we wanted to obtain optimal amounts of adaptation to be able to examine transfer, we chose to study backward adaptation only.
DATA ANALYSES. We analyzed primary saccades. The following trials were eliminated from further analyses: prosaccade errors in antisaccade trials (1%), antisaccade errors in prosaccade trials (1%), and trials in which blinks (2%) or anticipatory saccades (3%; saccade onset before the go-signal or with latency <100 ms) occurred.
Percent gain change was calculated as the ratio of the difference between the mean gain (saccade amplitude/target eccentricity) of each saccade type and each direction in the pretest phase (n = 20) and the mean gain of the same saccade type and direction in the posttest phase (n = 20) to the pretest gain
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We ran a 4 (saccade type: V+M+, V+M–, V–M+, V–M–) x 2 (phase: pretest vs. posttest) ANOVA on saccade gain and latency. A 2 (target side: left vs. right) x 2 (saccade: pro- vs. anti-) ANOVA was run on adaptation transfer. P values are indicated in parentheses. When indicated, t-tests were also performed.
Experiment 2
SUBJECTS. Five subjects participated in the experiment; two were authors (S1 and S3) and four had also participated in the previous experiment (S1–S4).
INSTRUMENTS AND EYE MOVEMENT RECORDING. The instruments and recording were identical to experiment 1.
STIMULI AND PROCEDURE. The stimuli and calibration procedure were identical to experiment 1.
Each subject ran two sessions on different days (separated by a minimum of 48 h): a rightward adaptation session and a leftward adaptation session. Each session was composed of three successive phases: pretest (124 trials), adaptation (180 trials), and posttest (124 trials). The pretest was broken into two successive blocks of 62 antisaccade trials and 62 prosaccade trials. In each block, the saccade target was located at one of six locations: –8, –6, –4, +4, +6, and +8° from screen center where the fixation cross was located. Targets at –6 and +6° were tested 15 times; the others were tested 8 times. During the saccade, the target was extinguished, and the screen remained blank until the subject pressed a button to initiate the next trial. In the pretest and posttest phases, the six target positions were mixed. In the adaptation phase (180 trials), subjects were instructed to make only prosaccades. Four saccade targets were mixed. In the rightward adaptation session, we tested one target 6° to the right (120 trials) and three targets to the left (20 trials each; –8, –6, and –4°). During the saccade directed toward the target located at +6°, it was stepped back by 2° and remained there until the end of the trial. The three lefthand targets were not stepped back during the saccades directed to them but remained on until the end of the trial. The leftward adaptation session was similar except that we tested the target 6° to the left (120 trials) and three to the right (20 trials each; +4, +6, and +8°). The intrasaccadic target back-step occurred only during the saccade directed to the target at –6°. For both adaptation sessions, the posttest was identical to the pretest except that the 62 prosaccades were run first.
As in the previous experiment, the go-signal to make the saccade was always the extinction of the central fixation cross, 600 ms after the target presentation: we thus evoked volitional prosaccades. Each subject took one replication of the rightward adaptation session and one replication of the leftward adaptation session. Three of them began with the rightward adaptation session and two with the leftward adaptation session.
DATA ANALYSES. We analyzed primary saccades. The following trials were eliminated from further analyses: prosaccade errors in antisaccade trials (1%), antisaccade errors in prosaccade trials (<1%), and trials in which blinks or anticipatory saccades (10%; saccade onset before the go-signal or with latency <100 ms) occurred.
Percent gain change, amount of adaptation, and % transfer were calculated for each of the six saccade targets as before. We ran a 3 (saccade eccentricity: 8, 6, and 4°) x 2 (phase: pretest vs. posttest) x 2 (saccade direction: leftward vs. rightward saccades) x 2 (adaptation side: left vs. right adaptation) ANOVA on saccade gain and latency; each session (leftward vs. rightward adaptation) was analyzed separately. A 2 (target side: left vs. right) x 3 (eccentricity: 8, 6, and 4°) x 2 (saccade: pro- vs. anti-) x 2 (adaptation side: left vs. right adaptation) ANOVA was run on adaptation transfer. When indicated, t-tests were also performed.
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RESULTS |
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BASELINE CHARACTERISTICS. In the pretest phase, mean gain of all prosaccades (rightward and leftward) was 0.92 ± 0.10, and mean gain of all antisaccades was 0.87 ± 0.24 (SD). Mean saccade latency (time between the go-signal—fixation offset—and saccade onset) was significantly different between pro- and antisaccades: 182 ± 26 versus 259 ± 60 ms, respectively [F(3,9) = 4.2, P < 0.04], but did not depend on phase (F < 1). Figure 2 presents the mean data for gain and latency.
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30 percentage points; as shown in the individual subject data in the bottom panels of Fig. 4, although the values of the adaptation transfer differed between subjects, the pattern of results was quite similar across subjects.
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Experiment 2
BASELINE CHARACTERISTICS. In the pretest phase, the mean gain of all prosaccades was 0.89 ± 0.06, and mean gain of all antisaccades was 0.90 ± 0.19. Antisaccade latency was longer than prosaccade latency [259 ± 41 vs. 198 ± 35 ms; F(1,4) = 22.3, P < 0.01] but depended neither on phase [F(1,4) = 3.4, P > 0.1] nor saccade target position (F < 1). Figure 5 presents the mean gain and latency for all conditions.
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ADAPTATION TRANSFER. Figure 7 presents percent adaptation transfer for the tested saccade types, depending on the direction of adaptation (left or right). As in experiment 1, between-subject variability was relatively large; nevertheless, the pattern of results was similar across subjects, as can be seen in Fig. 7 and as indicated by the statistical analyses below.
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There was no overall effect of target eccentricity [F(2,8) = 2.7, P > 0.12]. Descriptively, it seems that the transfer of adaptation from saccades aiming for a 6° target to saccades aiming for 4 or 8° targets is <100%. However, probably because of the intersubject variability, these differences did not reach significance.
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DISCUSSION |
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We adapted a rightward prosaccade and tested the transfer to pro- and antisaccades in the same or opposite direction. The goal was to distinguish between hypotheses regarding the type of vector inversion that must be made during the programming of an antisaccade.
We did not observe any transfer of rightward prosaccade adaptation to leftward prosaccades, as expected because they are outside the adaptation field, i.e., the spatial zone around the adapted saccade vector inside which adaptation transfers to other vectors (Collins et al. 2007; Noto et al. 1999; Semmlow et al. 1989). Of interest was the transfer of adaptation to antisaccades. We hypothesized that, if antisaccades depended on the inversion of the motor vector of the prosaccade, rightward prosaccade adaptation should have transferred to the leftward antisaccade, because these saccades would depend on the inversion of an adapted vector. Furthermore, because rightward antisaccades would depend on the inversion of the leftward prosaccade vector, which should not be affected by saccadic adaptation because it is outside the adaptation field, the gain of rightward antisaccades should not be modified after adaptation. Neither of these hypotheses was verified, going against the motor vector inversion hypothesis. We observed no transfer of adaptation to leftward antisaccades but a complete transfer of adaptation to rightward antisaccades. Such results are in line with the visual vector inversion hypothesis. Indeed, the visual vector of rightward antisaccades depends on the lefthand visual target. This vector is inverted, and the motor vector would be prepared. Therefore the sensory-motor transformation takes place on the adapted side, which can explain why adaptation transferred to rightward antisaccades.
Our subjects were required to make a prosaccade or antisaccade to one of two possible targets. Because of the relative simplicity of the spatial computations necessary to perform this task, it is possible that subjects simply prepared two saccade programs and executed whichever one was required on a given trial. In this case, a vector inversion could take place only on the first trial or trials. Although the latency of antisaccades was longer than that of prosaccades throughout all phases of the experiment, possibly because of greater spatial computation needs, we wanted to make sure to eliminate this possibility in a task where the need for spatial computation on every trial was more explicit. We did this in experiment 2 by increasing the number of possible saccade targets from two to six. We also repeated the experiment with leftward adaptation.
Experiment 2
The results of the second experiment confirmed and extended those of the first experiment. Rightward prosaccade adaptation transferred to antisaccades in the same direction (evoked by a visual target on the opposite side) but not to antisaccades directed in the opposite direction (evoked by the same visual target). As in the previous experiment, these results are compatible with the idea that the visual vector rather than the motor vector is inverted during antisaccade preparation. This second experiment replicated this result with leftward prosaccade adaptation.
General
The goal of the two experiments presented here was to distinguish between two alternative hypotheses regarding the type of vector inversion made during the programming of an antisaccade. Antisaccade preparation could involve the inversion of the visual vector of the stimulus in response to which a saccade is made, or alternatively, could involve the inversion of the motor vector of the unwanted prosaccade to the visual stimulus. We dissociated these two vectors by means of saccadic adaptation, which modifies the motor vector but does not affect the visual vector. In the first experiment, we adapted a rightward volitional prosaccade and tested the transfer to pro- and antisaccades in the same or opposite direction. The results showed that adaptation transferred to antisaccades in the same but not in the opposite direction.
In the second experiment, we replicated the first experiment with both leftward and rightward volitional prosaccade adaptation. We also increased the number of possible saccade targets to increase the spatial computation load and ensure that a vector inversion was necessary on a trial-by-trial basis. This second experiment confirmed the results of the first experiment: prosaccade adaptation transferred to antisaccades in the same direction (evoked by a visual stimulus in the opposite hemifield) but not to antisaccades in the opposite direction (evoked by a visual stimulus in the adapted hemifield).
Overall, these results are compatible with the predictions of the visual vector inversion hypothesis. Recall that such a hypothesis depends on the assumption that saccadic adaptation is motor rather visual (see Introduction). Our results are also consistent with this view: if adaptation resulted from a modification of the sensory coordinates of the target, the visual vector would have been affected, and when it was inverted during antisaccade preparation, we should have observed transfer. This was clearly not the case, providing evidence against sensory adaptation and in favor of motor adaptation. This result also suggests that the neural site of saccadic adaptation is not in the parietal cortex where the visual vector inversion likely takes place. We propose that, during antisaccade preparation, the visual vector is inverted and the resulting motor vector is adapted by transfer if it is inside the adaptation field of the adapted prosaccade. The results do not fit with the hypothesis that the inverted vector is motor. Indeed, if a motor vector is the final outcome of sensory-motor transformation, it should be adapted before the inversion, and we should have observed a different pattern of results (Fig. 1, motor vector inversion hypothesis).
However, our results are compatible with an alternative interpretation according to which, during antisaccade preparation, the motor vector could be inverted before the level of adaptation and subsequently modified. We call this the "early motor vector" hypothesis as opposed to the "final motor vector" hypothesis. According to the early motor vector hypothesis, after rightward prosaccade adaptation, rightward antisaccades would depend on the inversion of an unadapted leftward motor vector, which would be adapted after having been inverted to the right side. No transfer to the opposite (leftward) antisaccade would be expected. In this case, the absence of transfer we observed would not reflect visual vector inversion but the inversion of an early motor vector.
Our results alone cannot definitively rule out this possibility; however, we support the visual vector inversion hypothesis rather than the early motor vector inversion hypothesis, because the latter presents several limitations. First, if the inverted motor vector was not adapted, the motor vector inverted during antisaccade preparation would not be the final vector actually driving the saccade. If the representation that is inverted does not correspond to the metrics of the saccade but is more closely related to the position of the relevant saccade target, it seems that such an "early motor" representation is actually a visual representation. Second, in addition to evidence showing that adaptation is present in the cerebellum (Barash et al. 1999; Catz et al. 2005; Inaba et al. 2003; Robinson and Noto 2005; Robinson et al. 2002; Scudder and McGee 2003; Soetedjo and Fuchs 2006) and in cerebellar inputs (Collins et al. 2008; Straube and Deubel 1995), recent neurophysiological evidence has shown that adaptation is present in the superior colliculus (Takeichi et al. 2007). This could suggest that adaptation might even be present upstream of the superior colliculus, in brain areas that would code "early motor" representations such as the FEFs (Hopp and Fuchs 2006; Noto et al. 1999).
Independently of this debate, our results inform about the relative timing of the vector inversion and adaptation processes. Our results suggest that the process of inverting a response to a visual target into activity in the contralateral hemisphere to the opposite location (i.e., the programming of the antisaccade) precedes the adaptive modification of the saccade vector in time. The idea that the vector inversion precedes adaptation is compatible with current thinking about the neural substrates of the two processes: vector inversion could take place in the LIP (Barash and Zhang 2006) or prefrontal cortex (Funahashi et al. 1993) and adaptation in the cerebellum.
In conclusion, our results show that the vector inversion made during antisaccade preparation precedes the adaptive modification of the motor vector. They also suggest that the inversion process could be that of transforming a visual representation in one hemifield into a visual representation in the other. Such visual vector inversion could be linked to parietal visual activity in response to a target in the hemifield opposite the receptive field when an antisaccade was to be made (Zhang and Barash 2000, 2004).
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Address for reprint requests and other correspondence: K. Doré-Mazars, Lab. de Psychologie et Neurosciences Cognitives, Paris Descartes Univ. and CNRS, 71 avenue E. Vaillant, 92 774 Boulogne-Billancourt, France (E-mail: karine.dore-mazars{at}univ-paris5.fr)
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