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1Laboratoire de Neurophysiologie and 2Center for Systems Engineering and Applied Mechanics, Université catholique de Louvain, Brussels, Belgium
Submitted 4 January 2006; accepted in final form 7 March 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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; Krauzlis and Stone 1999; Lisberger et al. 1987). Although the principal stimulus for pursuit eye movements is retinal motion, it has been shown that humans can generate robust anticipatory smooth eye movements in the absence of visual stimulation if characteristics such as direction and/or velocity of the upcoming target are predictable (Barnes and Asselman 1991; Blohm et al. 2003a,b; Heinen et al. 2005; Kowler 1989; Kowler and Steinman 1979a,b). In experimental settings, the trajectory of an upcoming target can be made predictable to subjects by repeating it in a series of pursuit trials. For instance, after a few pursuit trials where a target always appears at the center of the visual display and then starts to move toward the left, primates often initiate anticipatory pursuit movements in that direction before target motion onset (see Missal and Heinen 2004).
In the oculomotor literature, this tendency to repeat movements that were previously executed is referred to as a motor habit (Jarrett and Barnes 2001; Kowler 1989). Motor habits could cause inappropriate actions if the actual path of a target is not always the same, a very likely situation in a complex environment. A better way for subjects to predict the trajectory of an object is to use the available prior information to increase their readiness to act to the upcoming event. This readiness to act based on prior information is often referred to as cognitive expectation and is experimentally manipulated using cues carrying information about upcoming events (Posner 1980). In a smooth pursuit task in human subjects, it has been shown that cognitive expectation is a powerful source of anticipatory responses when subjects are informed with a visual or audio cue about the characteristics of upcoming target motion such as direction or timing (Barnes and Asselman 1991; Chakraborti et al. 2002; Jarrett and Barnes 2001; Kowler 1989; Kowler and Steinman 1979a,b). However, it has never been shown that monkeys can voluntarily initiate a smooth movement in the direction of expected target motion on the basis of prior information given by a central cognitive cue. Moreover, the interaction between spatial and temporal cueing in anticipatory pursuit generation in the monkey is unknown.
The initiation of an anticipatory pursuit movement on the basis of prior information requires a high level of control. Experimental subjects have to recall the learned association between color and direction and must voluntarily initiate an eye movement in the appropriate direction. In this paper, we tested whether rhesus monkeys could use a color cue presented at the fixation position to guide their anticipatory responses. Monkeys were trained in a variant of the Rashbass (1961) smooth pursuit task where the direction of upcoming target motion, to the left or to the right, was cued during the fixation period. In addition, information about the timing of target motion onset was provided by varying the probability of occurrence of a gap period before target motion onset in two different experiments. In a first experiment, gap occurrence was randomized. Consequently, target motion onset could occur at the end of the fixation period or at the end of the gap period, effectively giving subjects two possible choices of timing to guide their anticipatory movements. In a second experiment, the fixation period was followed by a gap period in all trials. Consequently, target motion onset always occurred at the same time and subjects had only one possible choice of timing to guide their anticipatory responses. With these experimental paradigms, the influence of cognitive expectation in the directional and temporal domains as well as their interaction could be investigated.
We found that monkeys can use a central nonspatial cue to voluntarily initiate robust anticipatory responses in the direction of future target motion. This prior directional information increased the probability of occurrence of anticipatory movements, shortened their latency, and increased their velocity. It also altered the characteristics of the initiation of visually guided smooth pursuit. We found that according to their latency, anticipatory movements could be classified in two distinct populations: early and late movements. Early movements were most frequent when prior directional information was provided and when two choices of timing of target motion onset were given.
These results show that, in the monkey, anticipatory smooth pursuit is not solely observed after presentation of the same stimulus repeatedly but can be voluntarily guided, as it is expected to occur in a complex visual environment, when both directional and temporal sources of information have to be used.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Rhesus monkeys (Macaca mulatta), one male and one female, referred to as P and T, were used in this study. All procedures were approved by the Institutional Animal Care and Use Committee and were in compliance with the guidelines set forth in the U.S. Public Health Service Guide for the Care and Use of Laboratory Animals. To allow head-fixed eye movement recordings, a scleral eye coil and a head restraint system (Crist Instruments) were implanted in each animal using dental cement and titanium orthopedic bone screws under isofluorane anesthesia and aseptic surgical conditions. Anesthesia was induced with an intramuscular injection of zooletil. Heart rate, blood pressure, respiratory rate, and body temperature were monitored for the duration of the surgery. A coil made of three turns of Teflon-coated stainless steel wire was implanted under the conjunctiva of one eye using the procedure described by Fuchs and Robinson (1966), as modified by Judge et al. (1980). At the completion of the surgery, animals were returned to their home cages. Antibiotics (Cefazolin) and analgesics (Buprenex) were administered as needed during the recovery period under the direction of a veterinarian.
Animal training
The monkeys were seated in a primate chair with head restrained for the duration of the testing sessions. They were trained to execute behavioral tasks for liquid reward and were allowed to work to satiation. Records of each animal’s weight and health status were kept and supplemental water was given as necessary.
The animals typically worked for 5 days and were allowed free access to water on weekends.
Experimental setup
The targets were presented by the computer controlled, analog oscilloscope, which back-projected light spots on a 90 x 90° translucent screen placed 54 cm in front of the monkey. The targets were 1 ° in diameter and 2 cd/m2 in intensity against a diffusely illuminated dim homogeneous background (0.05 cd/m2). The eye movement signals were obtained by placing the head-restrained animal with an implanted scleral coil in a pair of orthogonally aligned magnetic fields maintained electronically in temporal quadrature (Primelec, Regensdorf, Switzerland). Horizontal and vertical eye position measurements were sampled by a 12-bit data acquisition board at 1 kHz and stored on a computer disk.
Behavioral paradigms, visual displays, and data storage were under the control of a real-time program (TEMPO, Reflective Computing, St. Louis, MO) running on a laboratory PC with the DOS operating system.
Behavioral paradigms
Target motion direction was always either to the left or to the right (see Fig. 1). Each trial was initiated by the appearance of a target that the monkey had to fixate for 500 ms. During that fixation period, animals had to maintain gaze within an electronic window of 4 x 4 ° centered on the fixation target. In the condition without cueing about upcoming target motion direction (designated the "no-cue condition"; Fig. 1, A and B; abbreviated as C0.0), the fixation point was green. In the condition with cueing about upcoming target motion direction (designated the "cue condition"; Fig. 1, C and D; abbreviated as C1.0), the fixation point was either yellow (before leftward target motion) or red (before rightward target motion). At the end of the fixation period, two different situations could occur: 1) the fixation target was extinguished and the pursuit target simultaneously appeared at a 6° eccentric position in the direction opposite to future target motion and started to move at a constant velocity (always 65°/s) for 500 ms without any additional delay (Rashbass paradigm; Rashbass 1961; Fig. 1A), and 2) after the fixation target was extinguished, a "gap" period without any stimulus on the screen was introduced before target motion onset. Gap duration was 400 ms (Fig. 1B). In a first experiment, the probability of gap occurrence was set to 0.5 (abbreviated G0.5). In a second experiment, the probability of gap occurrence was set to 1.0 (abbreviated G1.0). During the gap period, the electronic window was expanded to 8 ° horizontal and 4 ° vertical to allow the monkey to freely decide to make an anticipatory movement to the left or to the right. During target motion, the electronic window was also 8 x 4°. At the end of each trial, subjects had to maintain fixation of the target in its final eccentric position for 500 ms. In both experiments, each block contained 200 trials. In monkey P, 25 blocks of each gap condition were collected. In monkey T, 12 blocks of the G0.5 condition and 12 blocks of the G1.0 condition were collected. The four possible combinations of gap and cue conditions (see Table 1) were presented in different randomly interleaved blocks of trials.
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Eye-movement measures
Position signals were low-pass filtered using a zero-phase digital filter (autoregressive forward–backward filter; cutoff frequency: 50 Hz). Velocity and acceleration were derived from filtered position signals using a two-point central difference algorithm. We defined anticipatory pursuit as a nonzero eye velocity signal observed between the onset of the fixation period until 50 ms after target motion onset (period of interest; see Fig. 2). However, eye velocity had to exceed the average velocity of the eye during fixation by at least 2 SDs for 
50 ms. Eye velocity during fixation was estimated by measuring the value of eye velocity 50 ms before gap onset in trials where the eye was immobile. Trials containing saccades or smooth eye movements during a period of 100 ms before gap onset were not used in the procedure to determine eye velocity during fixation. Afterward, the average value and SD were computed with all the selected values of eye velocity measured at 50 ms before gap onset. This average velocity was usually about 0.1°/s. The threshold (average value ±2 SD) was usually around 1.2°/s (–1.2°/s in the leftward direction), and did not significantly vary across monkeys and experimental sessions. The criterion of 50-ms duration was necessary to eliminate trials with an accidental threshold crossing arising from noise. Anticipatory pursuit latency was defined as the time when eye velocity first exceeded the criterion of 2 SDs for 50 ms (see "1" in Fig. 2). Latencies were measured with respect to target motion onset (time 0). With this convention, anticipatory movements have negative latencies. Anticipatory eye velocity was always obtained by computing the average velocity of the eye during a 20-ms interval. The 20-ms interval was centered on two different events: 1) the time of maximum anticipatory pursuit eye velocity (see "2" in Fig. 2), detected between anticipatory pursuit onset and 50 ms after the time of target motion onset and 2) the onset of target motion (see "3" in Fig. 2). After analysis, similar results were obtained independently of the event selected to compute the average anticipatory pursuit eye velocity. However, average eye velocity measured at the time of maximum anticipatory pursuit velocity appeared to be a more reliable experimental variable to characterize the oculomotor behavior of the subjects. Therefore eye velocity averaged around the time of maximum anticipatory pursuit velocity was used as the dependent variable in the results presented. For simplicity, we will refer to this measure as maximum anticipatory pursuit velocity.
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Pursuit latency was defined as the moment when eye acceleration crossed a threshold fixed at 150°/s2 for 50 ms (see "4" in Fig. 2). Eye velocity during pursuit initiation was measured 50 ms after pursuit onset (see "5" in Fig. 2), before the occurrence of "catch-up" saccades.
The significance of all observed effects was tested with a nonparametric (Kruskal–Wallis) ANOVA when multiple comparisons were needed. The statistical significance level (P level) was always 0.05, unless otherwise specified. All results in the text and shown on the figures are expressed as means ± SE.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Influence of directional expectation on anticipatory pursuit
Robust anticipatory movements were observed in both experimental subjects. Figure 3 shows characteristic examples of the different types of anticipatory movements observed. According to our definition (see METHODS), compatible anticipatory pursuit movements were always in the direction of upcoming target motion (see Fig. 3, A and B). Two different types of compatible anticipatory pursuit movements were observed. The first type was characterized by a strong initial increase of eye velocity during the first half of the gap period, between 400 and 200 ms before target motion onset (example presented in Fig. 3A). This will be referred to as an "early" anticipatory pursuit movement. The second type was characterized by an increase of eye velocity between 200 ms before target motion onset (second half of the gap period; example presented in Fig. 3B) and 50 ms after target motion onset. This will be referred to as a "late" anticipatory pursuit movement. According to our definition, incompatible movements were always in the direction opposite to that of the upcoming target motion (see Fig. 3C). Incompatible movements could also be classified as "early" or "late" (only a "late" example is shown in Fig. 3C). Finally, Fig. 3D shows a trial without anticipatory pursuit for comparison.
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The kinematics of anticipatory pursuit was also profoundly influenced by the directional cue. Figure 5A shows a set of eye velocity traces when the subject was not informed about the direction of upcoming target motion (monkey P). In this condition and for this particular block of trials, anticipatory movements occurred in 92% of all trials and were equally likely to be of the early (54%) or late (46%) type and were incompatible in 54% of trials. It can be observed that eye velocity varied in a systematic way during the gap period. During the first half of the gap period (time < –200 ms), eye velocity increased and reached a maximum. Eye velocity decreased during the second part of the gap period and increased again roughly 100 ms before target motion onset. This stereotyped pattern of eye velocity variations was a characteristic of the anticipatory pursuit movements in the two subjects of this study. For the movements presented in Fig. 5A, average latency of anticipatory pursuit was –335 ± 10.4 ms (early responses only; n = 20) and average maximum velocity was –3.0 ± 1.1°/s (n = 20). Figure 5B shows a set of eye velocity traces after the subject was instructed that the target was going to move to the left (red traces) or to the right (blue traces) (monkey P). In this condition, anticipatory pursuit movements were more frequent in a block of trials (97% of all trials), occurred mostly during the early part of the gap period (83% early responses), and were always appropriately oriented (no incompatible responses). Average latency was –355 ± 12.0 ms (early responses only; n = 17) to the right and –385 ± 6.6 ms (n = 22) to the left. Average maximum velocity was 6.2 ± 0.6°/s (n = 17) to the right and –7.8 ± 0.6°/s (n = 22) to the left.
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The quantitative analysis of the influence of directional expectation on anticipatory pursuit latency and maximum velocity is described in Fig. 6. Figure 6, A and B shows the distributions of anticipatory pursuit latencies in monkeys P and T, respectively. The abscissa shows anticipatory pursuit latencies with respect to the time when the fixation point was extinguished (–400 ms) and when target motion started (0 ms on the abscissa). The horizontal black bar represents the end of the fixation period and the horizontal white bar represents the early period of target motion. In both monkeys, the distributions of latencies were bimodal, with an early peak during the beginning of the gap period and a late peak around the time of target motion onset. In the absence of prior information about the direction of future target motion, latencies were more often in the late peak of the bimodal distribution in monkey P (75%; see open bars in Fig. 6A) or were equally distributed between the early and late peaks in monkey T (50%; see open bars in Fig. 6B). When a directional cue was presented during the fixation period, the distribution of anticipatory pursuit latencies changed. In both monkeys, the latencies of most movements were grouped in the early peak of the bimodal distribution (57% in monkey P; 71% in monkey T). In both monkeys, the influence of the directional cue on the shift of latencies between the early and the late peak was statistically significant (test of proportions; P < 0.01). Figure 6, C and D shows the distributions of maximum anticipatory pursuit velocity in both monkeys. Positive values represent the velocity of movements in the direction that was cued during the fixation period; negative values represent incompatible responses (errors). In monkey P, average maximum anticipatory pursuit velocity was 7.5 ± 0.1°/s (n = 1359) in the cue condition and 4.1 ± 0.1°/s (n = 336) in the no-cue condition for compatible movements. In monkey T, average maximum anticipatory pursuit velocity was 5.1 ± 0.3°/s (n = 389) in the cue condition and 5.1 ± 0.2°/s (n = 158) in the no-cue condition for compatible movements.
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The presence of a gap period is known to play an important role as a factor facilitating anticipatory pursuit (Boman and Hotson 1988). We have shown that cueing the direction of future target motion also plays a prominent role and that the percentage of anticipatory movements was the largest in the gap/cue condition (see Fig. 4). What is the interaction of these two factors on anticipatory pursuit latency and velocity? To answer this question, a nonparametric ANOVA was performed (Kruskal–Wallis ANOVA; 
= 0.05). The dependent variable was either anticipatory pursuit latency (Fig. 7, A and B) or maximum velocity (Fig. 7, C and D). The independent variable represented the experimental conditions used (no gap/no-cue; no-gap/cue; gap/no-cue; gap/cue). For the analysis of this effect, all movements collected in each monkey were pooled together. In the no-gap condition, the cue had no statistically significant effect on anticipatory pursuit latency in both monkeys. A significant shortening of the latency was induced by the occurrence of a gap period in both monkeys in the gap/no-cue condition. However, the gap/cue condition induced the largest change in the latency of anticipatory movements.
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Influence of directional expectation on visual pursuit characteristics
The influence of giving prior information about upcoming target motion was not limited to anticipatory pursuit. Pursuit initiation after target motion onset was also profoundly affected by the gap period and/or the directional cue. Figure 8 shows the influence of directional cueing in the gap conditions on pursuit latency (Fig. 8, A and B) and velocity (Fig. 8, C and D). In both monkeys, the latency of pursuit initiation was significantly reduced by the directional cue (compare filled bars with open bars). In monkey P, pursuit latency was 79.3 ± 0.6 ms (n = 976) in the no-cue condition and 64 ± 0.4 ms (n = 1,436) in the cue condition (significant difference; t-test; P < 0.001). In monkey T, the average latency was 85.2 ± 0.8 ms (n = 449) in the no-cue condition and 78.8 ± 0.7 ms (n = 565) in the cue condition (significant difference; t-test; P < 0.001). Eye velocity during pursuit initiation was also compared between conditions (see Fig. 8, C and D). In the gap/cue condition, eye velocity 50 ms after pursuit onset was larger than that in the gap/no-cue condition in both monkeys (in monkey P: 29.0 ± 0.2°/s, n = 1,436 vs. 24.0 ± 0.3°/s, n = 976; t-test; P < 0.001; in monkey T: 29 ± 0.4°/s, n = 565 vs. 26.5 ± 0.5°/s, n = 449; t-test; P < 0.001).
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The occurrence of a gap period was an important factor facilitating anticipatory pursuit. However, if a gap period started after the fixation period was completed, it instructed subjects that the target was going to move after a fixed period of time without a stimulus on the screen (gap duration = 400 ms). Therefore gap occurrence could be considered as a timing cue, potentially causing subjects to build up a temporal expectation of target motion onset. As already mentioned, the best condition to reliably evoke anticipatory pursuit responses in the cued direction was to randomize gap occurrence in blocks of trials where either the left or the right cue was presented during each trial (G0.5/C1.0 condition; see Table 1). In these conditions, because target motion onset could occur either at the end of the fixation period or at the end of the gap period, a temporal expectation of target motion onset could build up around these two possible timing choices, resulting in the pattern of eye velocity observed in this study (see Figs. 2 and 5). We speculated that the first increase in eye velocity (referred to as the "early peak") should not be present or be strongly reduced in blocks of trials where gap occurrence was certain and target motion always occurred 400 ms after fixation point offset. However, the later increase of eye velocity during the second half of the gap period should not be affected. Indeed, in this condition, only one possible choice of timing of target motion was available.
Additionally, we hypothesized that this modulation of expectation in the time domain would be superimposed with the modulation of eye velocity resulting from directional expectation. Therefore we compared anticipatory pursuit in blocks of cue trials where the occurrence of the gap was randomized (G0.5/C1.0 condition) with anticipatory pursuit in blocks of cue trials where the gap always occurred (G1.0/C1.0 condition; see Fig. 11A). In both monkeys, it was found that the time course of eye velocity changed when gap occurrence probability increased from 0.5 to 1.0. The early peak in eye velocity was significantly more pronounced when gap occurrence was randomized (Wilcoxon rank-sum test; P < 2 x 10–5 with Bonferroni correction for multiple comparisons) than when it was not, suggesting that the monkey first anticipated target motion onset at the end of the fixation period. After a deceleration of the eye, a second increase of eye velocity was observed before target motion onset. However, as hypothesized, that second increase of eye velocity was not consistently larger in the G0.5/C1.0 condition than in the G1.0/C1.0 condition (Wilcoxon rank-sum test; P > 2 x 10–5 with Bonferroni correction for multiple comparisons).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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It was previously shown that expectation of an upcoming event can alter perceptual processes and the preparation of motor responses (Posner 1980). In particular, expectation about upcoming target motion leads to anticipatory smooth pursuit eye movements in human subjects (Barnes and Asselman 1991; Kowler 1984, 1989; Kowler and Steinman 1979a,b). To our knowledge anticipatory pursuit in the monkey has always been investigated in tasks where the same target motion characteristics such as direction, velocity, and timing were kept constant during several trials. This paradigm produces stereotyped anticipatory pursuit responses in the direction of the previously experienced target motion. In the present study, it was clearly shown that the direction of anticipatory pursuit can be voluntarily chosen by monkeys, independently of previously executed pursuit movements, even in the absence of a gap period that is usually considered as a determining factor to observe anticipatory pursuit. However, the gap period played a significant role in gating the influence of the directional cue on movement latency. Indeed, the directional cue caused a significant reduction of anticipatory pursuit latency with respect to controls only in the gap condition. The occurrence of a gap period is known to reduce saccadic (Fischer 1986; Fischer and Boch 1983) and visual smooth pursuit latencies (Krauzlis and Miles 1996; Merrison and Carpenter 1995). This effect is usually interpreted as resulting from a disengagement of attention from the fixation point induced by the gap and its reorientation toward potential targets (Mackeben and Nakayama 1993). In the present study, in the gap condition, prior information provided by the directional cue probably caused an early orientation of attention in the direction of future target motion. This orientation of attention might be sufficient to induce a smooth anticipatory movement in the direction of expected target motion, resulting in a shorter movement latency when compared with the gap/no-cue condition.
During the gap period, the elapsed time could be used as a cue to determine when target motion is likely to occur. We have shown that early anticipatory movements in the direction of future target motion were more frequent and faster in the randomized gap paradigm, when target motion could occur 500 or 900 ms after fixation point onset. In this condition, at the end of the fixation period, the probability of target motion onset was 0.5 and eye velocity increased in expectation of target motion onset. Eye velocity reached a maximum and decreased because the target had not appeared. However, as time elapsed during the gap period, the tendency that target motion onset was going to occur—given that it had not yet occurred—increased. This tendency is often referred to as the hazard function (see DISCUSSION in Luce 1986). The sense that target motion was going to occur increased with time during the gap period. This perception of elapsed time is known to be a major factor influencing saccadic reaction time and neurons in the parietal cortex seem to encode the subjective estimate of elapsed time (Janssen and Shadlen 2005). Timing information is used by the pursuit system to predict continuous periodic target motion and to generate anticipatory eye movements during discrete target movements (Barnes and Asselman 1991, 1992; Barnes and Donelan 1999). Here, we suggest that the perception of elapsed time during the gap period is likely to explain the variations of eye velocity during the initiation of anticipatory smooth pursuit movements when gap occurrence was randomized.
Visually initiated pursuit was still profoundly influenced by the prior information provided during the fixation period. The influence of the directional cue was not limited to anticipatory pursuit but continued after target motion onset. However, the initial acceleration of the eye that is commonly used to determine visual pursuit onset could itself be anticipatorily generated. In agreement with this hypothesis, it should be noted that the early peak of eye velocity during the gap period has kinematics similar to that of initiation of visual pursuit. In humans, it has been shown that anticipatory pursuit was not limited to a pretarget period, but could significantly alter eye velocity during visually guided smooth pursuit initiation (Heinen et al. 2005; Kao and Morrow 1994). Independently of this continuation of anticipatory pursuit after target appearance, it has been shown in humans that directional expectation could change the initiation of visual pursuit (Krauzlis and Adler 2001).
Directional expectation alters perceptual judgments about target motion direction and visually guided pursuit eye movements in a similar way, suggesting an influence of directional expectation at an early stage of visual motion information processing (Krauzlis and Adler 2001). In the condition with prior directional cueing, we found that the latency of visual pursuit movements (latency > transition point) was significantly shorter and visual pursuit eye velocity was significantly larger than in the absence of prior directional information. We deliberately selected movements that were not preceded by anticipatory pursuit to rule out the hypothesis that an effect of the cue on visual pursuit initiation could result from addition of an ongoing anticipatory movement that is cue dependent with a visually initiated movement that could be cue independent. Although it is still possible that anticipatory movements could start after the time of target motion onset and alter visual pursuit initiation, this hypothesis is unlikely to explain the observed results. Indeed, because late anticipatory movements were more frequent in the no-cue condition than in the cue condition, the fraction of anticipatory movements starting after target motion onset should be larger in the no-cue than in the cue condition, resulting in a higher visual pursuit velocity in the no-cue condition. However, the opposite effect was observed. Therefore we suggest that because the pursuit target was identical in the cue and no-cue conditions, the effect of the cue on visual pursuit could be attributed to an influence only of prior information on target motion processing for smooth pursuit. We suggest that the directional cue could indirectly facilitate motion detectors with the appropriate preferred direction and therefore reduce the latency of pursuit when the moving target appears. This bias could be interpreted as resulting from the orientation of attentional ressources in the direction of expected target motion (Posner 1980).
The neuronal substrate of anticipatory pursuit is still largely unknown. In the oculomotor system, the resultant hypothetical expectation signal sent to oculomotor structures is likely to be a combination of expectation signals in the directional and time domains. It was previously shown that neurons in the supplementary eye field (SEF) region are active before pursuit (Berman et al. 1999; Heinen 1995; O’Driscoll et al. 2000; Petit and Haxby 1999; Petit et al. 1997), particularly in conditions where target motion timing was predictable (Heinen and Liu 1997). In addition, electrical stimulation in the region of the SEFs facilitated anticipatory pursuit initiation (Missal and Heinen 2004) and visual pursuit initiation (Missal and Heinen 2001). In this study, we have shown that a directional cue presented to the subjects during the fixation period before target motion onset also affects anticipatory and visual pursuit initiations. Therefore we suggest that a neuronal correlate of the behavioral changes observed should be found in that part of the dorsomedial frontal cortex involved in oculomotor control (Missal et al. 2005). Moreover, the interaction between directional cueing and temporal expectation suggests that the neural encoding of these two experimental variables should at least partially overlap.
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FOOTNOTES |
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Address for reprint requests and other correspondence: M. Missal, Laboratoire de Neurophysiologie (NEFY), Université catholique de Louvain, Av. Hippocrate 54 49, 1200 Brussels, Belgium (E-mail: Marcus.Missal{at}nefy.ucl.ac.be)
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