Neuronal activities in the ventral premotor cortex during a visually guided jaw movement in monkeys

Neuronal activities in the ventral part of the premotor cortex (PMv) and the primary motor cortex (MI) were analyzed during a visually guided jaw movement task. Based on the type of neuronal activity observed, when monkeys closed or opened their mouths in response to a visual stimulus, PMv neurons could be classified into three categories: (1) signal-related neurons, which transiently responded to visual stimuli, (2) movement-related neurons which were time-locked to jaw opening and/or jaw closing movements, and (3) set related neurons which exhibited gradually increasing activities while jaw position was maintained. However, all MI neurons exhibited movement-related activities and responded differently between the closing and opening dynamic phases. These results suggest that PMv neurons may be involved in motor preparation, initiation and control of jaw movements and task behavior based on visual information, and that MI neurons may be involved in controlling jaw movements, especially contraction of the masticatory muscles.     

Time-modulated neuronal activity in the premotor cortex of macaque monkeys

A voluntary motor act, executed in response to a stimulus, requires both spatial and temporal computation. Even though electrophysiological and positron emission tomography (PET) investigations on humans suggest that SMA, medial prefrontal cortex and primary motor cortex play a role in temporal mechanisms, we have few data about neuronal time computation in the premotor cortex. The involvement of monkey premotor area (PM) in motor learning and cognitive processes, and the presence of buildup neurons, whose activity is closely related to the motor action, prompted us to investigate the involvement of these set-related neurons in the time domain. To this end we manipulated the duration of a pre-cue in a visuomotor task while recording unit activity. We found that, when the duration of the pre-cue was predictable and long (5 s), delay of the onset of cell activity in consecutive trials gradually increased. On the other hand, when the duration was unpredictable or predictable and short (1 s), this phenomenon could not be detected. The inconsistent discharge correlations with expected reward and attentional processes, and the specific discharge relationship to the time instruction, suggest that these buildup neurons reflect a learning process in the time domain.     

Shift of preferred directions of premotor cortical cells with arm movements performed across the workspace

Summary The activity of 156 neurons was recorded in the premotor cortex (Weinrich and Wise 1982) and in an adjoining rostral region of area 6 (area 6 DR; Barbas and Pandya 1987) while monkeys made visually-guided arm movements of similar direction within different parts of space. The activity of individual neurons varied most for a given preferred direction of movement within each part of space. These neurons (152/156, 97.4%) were labeled as directional. The spatial orientation of their preferred directions shifted in space to follow the rotation of the shoulder joint necessary to bring the arm into the different parts of the work-space. These results suggest that the cortical areas studied represent arm movement direction within a coordinate system rotating with the arm and where signals about the movement direction relate to the motor plan through a simple invariant relationship, that between cell preferred direction and arm orientation in space.     

Premotor cortex of the rhesus monkey: neuronal activity in anticipation of predictable environmental events

Summary The activity of premotor cortex neurons was studied in macaque monkeys that were operantly conditioned to perform a visually guided motor task. The monkeys were given a visuospatial instruction stimulus (IS) that provided the target for a limb movement, but the monkey was not allowed to execute the movement until the later presentation of a trigger stimulus (TS). The IS was sometimes removed or the target changed during the interval between the IS and TS. Certain neurons became active before the IS, appearing to anticipate its location or time of occurrence, some of these and other neurons increased or decreased their discharge prior to a possible change in the IS, and other cells showed neuronal modulation that preceded the TS. The properties of some of these neurons were examined when the timing of the behavior-guiding visual signals or their probability of occurrence was made less predictable. In general, the neuronal activity described here was strongly influenced by changes in event predictability. These findings suggest that neuronal activity within the premotor cortex reflects the anticipation of predictable environmental events.     

State dependent activity in monkey visual cortex

Summary This study examined the extent to which the responses of single cells in the striate cortex (V1) and the extrastriate cortex (V4) of the alert rhesus monkey are modulated by visual stimuli whose relevance in a behavioral task is varied. The animal had to detect the repetition of a visual pattern (i.e. detect similarity), preceded by a randomized number of alternations between two different patterns. The responses produced by the last, reward contingent stimulus were compared with responses obtained to that same stimulus earlier in the sequence. Modulatory effects in V1 were moderate: 31% of the cells (63 of 200) showed response increments of 20% or more to the last, reward contingent stimulus. In V4 the effects were much more pronounced: 72% of the cells (110 of 154) showed modulatory effects of more than 20%. In V4 but not in V1 orientation tuning curves showed a significant narrowing as well as a peak response increment to the behaviorally salient stimulus, suggesting a feature specific mechanism associated with the detection of similarity. Although a response decrement was observed in many cells during the repeated alternations, this effect was significantly smaller than the modulation produced by the detection of similarity. Controls included the presentation of novel stimuli during the presentation sequence which did not produce an enhanced response. It is hypothesized that the feature specific effects reported here are produced by higher order feedback systems.     

Arm-movement-related neurons in the primate superior colliculus and underlying reticular formation: comparison of neuronal activity with EMGs of muscles of the shoulder, arm and trunk during reaching

 Neuronal activity was recorded from the superior colliculus (SC) and the underlying reticular formation in two monkeys during an arm reaching task. Of 744 neurons recorded, 389 (52%) clearly modulated their activity with arm movements. The temporal activity patterns of arm-movement-related neurons often had a time course similar to rectified electromyograms (EMGs) of particular muscles recorded from the shoulder, arm or trunk. These reach cells, as well as the muscles investigated, commonly exhibited mono- or biphasic (less frequently tri- or polyphasic) excitatory bursts of activity, which were related to the (pre-)movement period, the contact phase and/or the return movement. The vast majority of reach cells exhibited a consistent activity pattern from trial to trial as did most of the muscles of the shoulder, arm and trunk. Similarities between the activity patterns of the neurons and the muscles were sometimes very strong and were especially notable with the muscles of the shoulder girdle (e.g. trapezius descendens, supraspinatus, infraspinatus or the anterior and medial deltoids). This high degree of co-activation suggests a functional linkage, though not direct, between the collicular reach cells and these muscles. Neuronal activity onset was compared with that of 25 muscles of the arms, shoulders and trunk. The majority of cells (78.5%) started before movement onset with a mean lead time of 149±90 ms, and 36.5% were active even before the earliest EMG onset. The neurons exhibited the same high degree of correlation (r=0.97, Spearman rank) between activity onset and the beginning of the arm movement as did the muscles (r=0.98) involved in the task. The mean neuronal reach activity (background subtracted) ranged between 7 and 193 impulses/s (mean 40.5±24.2). The mean modulation index calculated [(reach activity −background activity)/reach activity+background activity)] was 0.75±0.23 for neurons (n=358) and 0.87±0.14 for muscles (n=25). As the monkeys fixated the reach target constantly during an arm movement, neuronal activity which was modulated in this period was not related to eye movements. The three neck muscles investigated in the reach task exhibited no reach-related activity modulation comparable to that of either the reach cells or the muscles of the shoulder, arm and trunk. However, tonic neck muscle EMG was monotonically related to horizontal eye position. The clear skeletomotor discharge characteristics of arm-movement-related SC neurons revealed in this study agree with those already known from other sensorimotor regions (for example the primary motor, the premotor and parietal cortex, the basal ganglia or the cerebellum) and are consistent with the possible role of this population of reach cells in the control of arm movements. Received: 17 June 1996 / Accepted: 24 December 1996     

Oscillations in the premotor cortex: single-unit activity from awake, behaving monkeys

We examined single-unit activity in the dorsal premotor cortex for evidence of fast neuronal oscillations. Four rhesus monkeys performed a task in which visuospatial instruction stimuli indicated the direction of forelimb movement to be executed on each trial. After an instructed delay period of 1.5–3 s, movements to either the right or left of a central origin were triggered by a second visuospatial stimulus. From a database of 579 single units, 78 units (13%) contained periodic peaks in their autocorrelation histograms (ACHs), with oscillation frequencies typically 20–30 Hz (mean 27 Hz). An additional 26 units (5%) had oscillatory features that were identified in joint interspike-interval (ISI) plots. Three observations, taken together, suggest entrainment by rhythmic drive extrinsic to these neurons: shuffling ISIs attenuated ACH peaks, indicating a dependency on serial-order effects; oscillation frequency did not change during either increases or decreases in firing rate; and joint ISI plots contained features consistent with a rhythmicity interrupted by intervening discharges. In some cells, oscillations occurred for only one of the two directions of movement. During the delay period, such directional selectivity was observed in 37 units (60% of delay-period oscillators). For at least 17 of these units, we could exclude the possibility that oscillatory directional selectivity resulted from the difficulty in detecting oscillations due to low discharge rates (for one of the two movement directions). Directional selectivity in fast oscillations shows that they can reflect specific aspects of an intended action. Received: 2 March 1999 / Accepted: 26 May 1999     

Differential relation of discharge in primary motor cortex and premotor cortex to movements versus actively maintained postures during a reaching task

The activity of cells in primary motor cortex (MI) and dorsal premotor cortex (PMd) were compared during reaching movements in a reaction-time (RT) task, without prior instructions, which required precise control of limb posture before and after movement. MI neurons typically showed strong, directionally tuned activity prior to and during movement as well as large gradations of tonic activity while holding the limb over different targets. The directionality of their movementand posture-related activity was generally similar. Proximal-arm muscles behaved similarly. This is consistent with a role for MI in the moment-to-moment control of motor output, including both movement and actively maintained postures, and suggests a common functional relation for MI cells to both aspects of motor behavior. In contrast, PMd cells were generally more phasic, frequently emitting only strong bursts of activity confined mainly to the behavioral reaction time before movement onset. PMd tonic activity during different postures was generally weaker than in MI, and showed a much more variable relation with their movement-related directional tuning. These results imply that the major contribution of PMd to this RT task occurred prior to the onset of movement itself, consistent with a role for PMd in the selection and planning of visually guided movements. Furthermore, the nature of the relative contribution of PMd to movement versus actively maintained postures appears to be fundamentally different from that in MI. Finally, there was a continuous gradient of changes in responses across the rostrocaudal extent of the precentral gyrus, with no abrupt transition in response properties between PMd and MI.     

Gaze effects in the cerebral cortex: reference frames for space coding and action

Visual information is mapped with respect to the retina within the early stages of the visual cortex. On the other hand, the brain has to achieve a representation of object location in a coordinate system that matches the reference frame used by the motor cortex to code reaching movement in space. The mechanism of the necessary coordinate transformation between the different frames of reference from the visual to the motor system as well as its localization within the cerebral cortex is still unclear. Coordinate transformation is traditionally described as a series of elementary computations along the visuomotor cortical pathways, and the motor system is thought to receive target information in a body-centered reference frame. However, neurons along these pathways have a number of similar properties and receive common input signals, suggesting that a non-retinocentric representation of object location in space might be available for sensory and motor purposes throughout the visuomotor pathway. This paper reviews recent findings showing that elementary input signals, such as retinal and eye position signals, reach the dorsal premotor cortex. We will also compare eye position effects in the premotor cortex with those described in the posterior parietal cortex. Our main thesis is that appropriate sensory input signals are distributed across the visuomotor continuum, and could potentially allow, in parallel, the emergence of multiple and task-dependent reference frames. Received: 21 September 1998 / Accepted: 19 March 1999     

Primate frontal cortex: neuronal activity following attentional versus intentional cues

We examined neuronal activity in three parts of the primate frontal cortex: the dorsal (PMd) and ventral (PMv) premotor cortex and a ventrolateral part of the dorsolateral prefrontal (PF) cortex. Two monkeys fixated a 0.2° white square in the center of a video display while depressing a switch located between two touch pads. On each trial, a spatial-attentional/mnemonic (SAM) cue was presented first. The SAM cue consisted of one 2° × 2° square, usually red or green, and its location indicated where a conditional motor instruction would appear after a delay period. The stimulus event containing the motor instruction, termed the motor instructional/conditional (MIC) cue, could be of two general types. It might consist of a single 2° × 2° square stimulus identical to one of the SAM cues presented at the same location as the SAM cue on that trial. When the MIC cue was a single square, it instructed the monkey to move its forelimb to one of the two touch pads according to the following conditional rule: a green MIC cue meant that contact with the right touch pad would be rewarded on that trial and a red MIC cue instructed a movement to the left touch pad. Alternatively, the MIC cue might consist of two 2° × 2° squares, only one of which was at the SAM-cue location: in those cases, one square was red and the other was green. The colored square at the SAM cue location for that trial was the instructing stimulus, and the other part of the MIC cue was irrelevant. When, after a variable delay period, the MIC cue disappeared, the monkey had to touch the appropriate target within 1 s to receive a reward and could break visual fixation. The experimental design allowed comparison of frontal cortical activity when one stimulus, identical in retinocentric, craniocentric, and allocentric spatial location as well as all other stimulus parameters, had two different meanings for the animal's behavior. When a stimulus was the SAM cue, it led to either a reorientation of spatial attention to its location, or the storage of its location in spatial memory. By contrast, when it was the MIC cue, the same stimulus instructed a motor act to be executed after a delay period. For the majority of PMd neurons (55%), post-MIC cue activity exceeded post-SAM cue activity. In many instances, no activity followed the SAM cue, although the identical stimulus caused profound modulation when it served as the MIC cue. In PF, by contrast, significantly fewer cells (30%) showed such a property, and PMv was intermediate in this respect (36%). The results support the hypothesis that many PMd cells reflect the motor significance of stimuli, and that a significantly smaller proportion of cells in PF do so.     

Neuronal activity of somatosensory cortex in a cross-modal (visuo-haptic) memory task

Studies have shown that in the monkey′s associative cerebral cortex, cells undergo sustained activation of discharge while the animal retains information for a subsequent action. Recent work has revealed the presence of such ″memory cells″ in the anterior parietal cortex (Brodmann′s areas 3a, 3b, 1, and 2) – the early stage of the cortical somatosensory system. Here we inferred that, in a cross-modal visuo-haptic short-term memory task, somatosensory cells would react to visual stimuli associated with tactile features. Single-unit discharge was recorded from the anterior parietal cortex – including areas of hand representation – of monkeys performing a visuo-haptic delayed matching-to-sample task. Units changed firing frequency during the presentation of a visual cue that the animal had to remember for making a correct tactile choice between two objects at the end of a delay (retention period). Some units showed sustained activation during the delay. In some of them that activation differed depending on the cue. These findings suggest that units in somatosensory cortex react to visual stimuli behaviorally associated with tactile information. Further, the results suggest that some of these neurons are involved in short-term active memory and may, therefore, be part of cross-modal memory networks. Received: 24 March 1997 / Accepted: 8 May 1997     

Corticostriatal projections from the somatic motor areas of the frontal cortex in the macaque monkey: segregation versus overlap of input zones from the primary motor cortex, the supplementary motor area, and the premotor cortex

It is an important issue to address the mode of information processing in the somatic motor circuit linking the frontal cortex and the basal ganglia. In the present study, we investigated the extent to which corticostriatal input zones from the primary motor cortex (MI), the supplementary motor area (SMA), and the premotor cortex (PM) of the macaque monkey might overlap in the putamen. Intracortical microstimulation was performed to map the MI, SMA, and dorsal (PMd) and ventral (PMv) divisions of the PM. Then, two different anterograde tracers were injected separately into somatotopically corresponding regions of two given areas of the MI, SMA, PMd, and PMv. With respect to the PMd and PMv, tracer injections were centered on their forelimb representations. Corticostriatal input zones from hindlimb, forelimb, and orofacial representations of the MI and SMA were, in this order, arranged from dorsal to ventral within the putamen. Dense input zones from the MI were located predominantly in the lateral aspect of the putamen, whereas those from the SMA were in the medial aspect of the putamen. On the other hand, corticostriatal inputs from forelimb representations of the PMd and PMv were distributed mainly in the dorsomedial sector of the putamen. Thus, the corticostriatal input zones from the MI and SMA were considerably segregated though partly overlapped in the mediolateral central aspect of the putamen, while the corticostriatal input zone from the PM largely overlapped that from the SMA, but not from the MI. Received: 30 June 1997 / Accepted: 2 October 1997     

Anatomical distribution of arm-movement-related neurons in the primate superior colliculus and underlying reticular formation in comparison with visual and saccadic cells

 We recorded from 389 ”reach” neurons (two monkeys) in the superior colliculus (SC) and underlying reticular formation (RF) or adjacent periaqueductal grey, whose activity was related to visually guided arm movements. Reach neurons were present from approximately 0.7 mm down to a depth of 6 mm below the surface of the SC (mean 3.7±1.3, n=389). Although this mean distribution was different from that of cells with visual (mean depth 1.7±1.4 mm, n=283) or saccadic responses (mean depth 2.0±1.4 mm, n=232), there was a large amount of overlap. Fifty-five per cent of all reach cells (213/389) were assumed to be located inside the SC. The others were considered to be located in the underlying RF. The characteristics of visual responses and saccadic bursts (e.g. response latencies, discharge rates, burst durations) of arm-movement-related neurons were not different from those of typical visual or saccade cells in the SC. Although reach neurons could be recorded in a large area of the SC, they were found more often in the lateral than in the medial parts (chi-squared=19.3, P<0.001). Possible pathways by which arm-movement-related neuronal activity in and below the SC might gain access to spinal motor structures are discussed. The location of arm-movement-related neurons described in this study is in accordance with the known target areas of skeletomotor-related corticotectal projections and with the sites of origin of tectofugal pathways. It is concluded that this population of reach cells is in a position to relay and transmit limb movement information to the spinal motor system, where it may influence (or interact with) motor commands coming from other motor centres. Received: 17 June 1996 / Accepted: 24 December 1996     

Neuronal activity in dorsomedial frontal cortex and prefrontal cortex reflecting irrelevant stimulus dimensions

Previous studies of the dorsomedial frontal cortex (DMF) and the prefrontal cortex (PF) have shown that, when monkeys respond to nonspatial features of a discriminative stimulus (e.g., color) and the stimulus appears at a place unrelated to the movement target, neurons nevertheless encode stimulus location. This observation could support the idea that these neurons always encode stimulus location, regardless of its relevance to an instrumentally conditioned behavior. Past studies, however, leave open the possibility that activity observed during one operant task might reflect the contingencies of a different task, performed at different times. To test these alternatives, we examined the activity of DMF and PF neurons in two rhesus monkeys conditioned to perform an operant eye-movement task in which only the color and shape of visual stimuli served as salient discriminative features. Each of eight stimuli was associated with a response to a different eye-movement target. The location of these stimuli varied from trial to trial but was of no behavioral relevance, and the monkeys did not perform any operant task in which stimulus location controlled behavior. A substantial minority of neurons in both DMF and PF nevertheless encoded stimulus location, which indicates that this property does not depend on its relevance in an instrumentally conditioned behavior. Electronic Publication     

Distribution of neurons with set- and movement-related activity before hand and foot movements in the premotor cortex of rhesus monkeys

Summary Neuronal activity was studied in the premotor cortex (PM) of two rhesus monkeys, each of which performed both forelimb and hindlimb movements. On each trial, the monkey received a visual instruction stimulus (IS) that indicated whether a foot or a hand movement would be rewarded on that trial. After a delay period, during which the monkey withheld an overt movement, a visual trigger stimulus (TS) was presented to indicate that the monkey should execute a movement. Of 572 task-related neurons recorded in PM, 149 neurons showed set-related activity, defined as a significant increase or decrease in discharge rate throughout most of the instructed delay period, and 299 neurons showed movement-related activity, defined as a significant change in discharge rate between the TS and movement onset. Both setand movement-related activity were subdivided into three patterns: activity modulation 1) before a foot movement only (foot neurons); 2) before a hand movement only (hand neurons); and 3) before both foot and hand movements (mixed neurons). The distribution of set-related neurons mostly overlapped with that of movement-related neurons, although set-related neurons were located in more restricted regions than movement-related neurons. Foot neurons with setand movementrelated activity were distributed near the superior precentral sulcus. Hand neurons were mainly located lateral to the foot neurons with some overlap. The results indicate that most PM setand movement-related neurons contribute, respectively, to the preparation for and execution of specific limb movements, as opposed to movement per se. Further, the differential distribution of neurons with activity related to hindlimb vs. forelimb movement supports previous indications that PM is topographically organized.     

Neuronal activity preceding self-initiated or externally timed arm movements in area 6 of monkey cortex

Summary Several lines of evidence suggest that the supplementary motor area (SMA) and the premotor cortex (PM) may participate in neuronal mechanisms for the initiation of movements. We recorded the impulse activity of single neurons in monkeys that were trained in two behavioral tasks employing, respectively, self-initiated and externally timed movements. Neurons in both areas were activated up to 2.6 s in advance of self-initiated, reward-related arm reaching movements. In the externally timed task, changes occurred during light instructions that preceded movements by 2 s. Neurons also responded to the trigger stimulus for movement. In view of similar premovement activity in the basal ganglia, these cortical regions appear to be parts of a distributed neuronal system for movement initiation.     

Eye-hand coordination in uni- and bimanual goal-oriented tasks

 Two different drawer tasks were investigated with the aim of assessing the role of eye movements in well-coordinated hand movements. In an unimanual step-tracking task, which had a predictive and an unpredictive movement, a two-way repeated-measures ANOVA showed a significant effect of prediction on the onset of grip-force (GF) rate (300±39 ms for the predictive condition versus 394±53 ms for the non-predictive condition, P<0.0001). Correlation coefficients, computed from the eye and the hand movements were low for the right and the left hand. The saccade was more coupled with the visual step change than with the action of the hand per se. In a second bimanual pull-and-pick task, the instruction was to pull a drawer with the left hand from a closed position to a LED-cued open position and then to grasp and reinsert a small peg in the drawer with the right hand. Correlation coefficients, computed from the latencies of saccades and of the leading left hand or of the right hand, were significant in four of five subjects. Intermanual correlations were significant in all five subjects. In conclusion, we found that the initial saccade in the unimanual task was best related with the visual step change, but was poorly correlated with the pulling/pushing hand. In the bimanual task, a moderate, but significant temporal coupling between the eyes and hand events was observed. This coupling was, however, less tight than that between both hands. Received: 24 August 1998 / Accepted: 13 January 1999     

Distribution of neurons related to a hindlimb as opposed to forelimb movement in the monkey premotor cortex

Summary Single cell recordings were made from the premotor cortex (lateral part of area 6) of a monkey trained to perform either a distal hindlimb or forelimb movement separately. Out of 175 movement-related neurons, 59 neurons showed modulation of activity only prior to the hindlimb movement, and the majority of them was distributed in a focal region around the superior precentral sulcus, several mm posteromedial to the genu of the arcuate sulcus. The hindlimb focus was separate from a focal region for forelimb movement-related neurons, which lay immediately posterior to the genu of the arcuate sulcus.Supported in part by Ministry of Education, Science and Culture of Japan (grant 58106001 and 58570046)     

Context-dependent force coding in motor and premotor cortical areas

In three monkeys trained to finely grade grip force in a visuomotor step-tracking task, the effect of the context on neuronal force correlates was quantitatively assessed. Three trial types, which differed in force range, number, and direction of the force steps, were presented pseudo-randomly and cued with the color of the cursor serving as feedback of the exerted force. Quantitative analyses were made on 85 neurons with similar discharge patterns in the three trial types and significant linear positive (54 cells) or negative (31 cells) correlation coefficients between firing rate and force. An analysis of covariance (ANCOVA) showed that the population slopes for 2-step were steeper than for 3-step trials. Another ANCOVA at the population level, computed on the differences in firing rate and force between force steps, persistently disclosed a significant effect of trial type. For the first two force steps, the differences in firing rate were significantly larger in the 2-step than in the 3-step increase trials. Further analyses revealed that neither the force range nor the number of steps was a unique factor. A small group of neurons was tested in an additional trial series with a uniform cue for all three trials, leading to either a loss of context-dependency or to unexpected changes in firing rate. This demonstrates that the cue color was an important instruction for task performance and neuronal activity. The most important findings are that the context-dependent changes were occurring ”on-line”, and that neurons displaying context-dependency were found in all three lateral premotor cortex hand regions and in the primary motor cortex. Finger muscle activity did not show any context dependency. The context-dependent effect leads to a normalization of the cortical activity. The advantage of normalization is discussed and mechanisms for the gain regulation are proposed. Received: 10 November 1998 / Accepted: 13 March 1999     

Visual responses with and without fixation: neurons in premotor cortex encode spatial locations independently of eye position

 The ventral premotor cortex (PMv) of the macaque monkey contains neurons that respond both to visual and to tactile stimuli. For almost all of these “bimodal” cells, the visual receptive field is anchored to the tactile receptive field on the head or the arms, and remains stationary when the eyes fixate different locations. This study compared the responses of bimodal PMv neurons to a visual stimulus when the monkey was required to fixate a spot of light and when no fixation was required. Even when the monkey was not fixating and the eyes were moving, the visual receptive fields remained in the same location, near the associated tactile receptive field. For many of the neurons, the response to the visual stimulus was significantly larger when the monkey was not performing the fixation task. In control tests, the presence or absence of the fixation spot itself had little or no effect on the response to the visual stimulus. These results show that even when the monkey’s eye position is continuously changing, the neurons in PMv have visual receptive fields that are stable and fixed to the relevant body part. The reduction in response during fixation may reflect a shift of attention from the visual stimulus to the demands of the fixation task. Received: 8 April 1997 / Accepted: 16 July 1997