Differential involvement of neurons in the dorsal and ventral premotor cortex during processing of visual signals for action planning

We examined neuronal activity in the dorsal and ventral premotor cortex (PMd and PMv, respectively) to explore the role of each motor area in processing visual signals for action planning. We recorded neuronal activity while monkeys performed a behavioral task during which two visual instruction cues were given successively with an intervening delay. One cue instructed the location of the target to be reached, and the other indicated which arm was to be used. We found that the properties of neuronal activity in the PMd and PMv differed in many respects. After the first cue was given, PMv neuron response mostly reflected the spatial position of the visual cue. In contrast, PMd neuron response also reflected what the visual cue instructed, such as which arm to be used or which target to be reached. After the second cue was given, PMv neurons initially responded to the cue's visuospatial features and later reflected what the two visual cues instructed, progressively increasing information about the target location. In contrast, the activity of the majority of PMd neurons responded to the second cue with activity reflecting a combination of information supplied by the first and second cues. Such activity, already reflecting a forthcoming action, appeared with short latencies (<400 ms) and persisted throughout the delay period. In addition, both the PMv and PMd showed bilateral representation on visuospatial information and motor-target or effector information. These results further elucidate the functional specialization of the PMd and PMv during the processing of visual information for action planning.     

Differential roles of neuronal activity in the supplementary and presupplementary motor areas: from information retrieval to motor planning and execution

We explored functional differences between the supplementary and presupplementary motor areas (SMA and pre-SMA, respectively) systematically with respect to multiple behavioral factors, ranging from the retrieval and processing of associative visual signals to the planning and execution of target-reaching movement. We analyzed neuronal activity while monkeys performed a behavioral task in which two visual instruction cues were given successively with a delay: one cue instructed the location of the reach target, and the other instructed arm use (right or left). After a second delay, the monkey received a motor-set cue to be prepared to make the reaching movement as instructed. Finally, after a GO signal, it reached for the instructed target with the instructed arm. We found the following apparent differences in activity: 1) neuronal activity preceding the appearance of visual cues was more frequent in the pre-SMA; 2) a majority of pre-SMA neurons, but many fewer SMA neurons, responded to the first or second cue, reflecting what was shown or instructed; 3) in addition, pre-SMA neurons often reflected information combining the instructions in the first and second cues; 4) during the motor-set period, pre-SMA neurons preferentially reflected the location of the target, while SMA neurons mainly reflected which arm to use; and 5) when executing the movement, a majority of SMA neurons increased their activity and were largely selective for the use of either the ipsilateral or contralateral arm. In contrast, the activity of pre-SMA neurons tended to be suppressed. These findings point to the functional specialization of the two areas, with respect to receiving associative cues, information processing, motor behavior planning, and movement execution.     

Neurons in the rostral cingulate motor area monitor multiple phases of visuomotor behavior with modest parametric selectivity

We examined the cellular activity in the rostral cingulate motor area (CMAr) with respect to multiple behavioral factors that ranged from the retrieval and processing of associative visual signals to the planning and execution of instructed actions. We analyzed the neuronal activity in monkeys while they performed a behavioral task in which 2 visual instruction cues were given successively with an intervening delay. One cue instructed the location of the target to be reached; the other cue instructed which arm was to be used. After a second delay, the monkey received a motor-set cue to be prepared to initiate the motor task in accordance with instructions. Finally, after a GO signal, the monkey reached for the instructed target with the instructed arm. We found that the activity of neurons in the CMAr changed profoundly throughout the behavioral task, which suggested that the CMAr participated in each of the behavioral processing steps. However, the neuronal activity was only modestly selective for the spatial location of the visual signal. We also found that selectivity for the instructional information delivered with the signals (target location and arm use) was modest. Furthermore, during the motor-set and movement periods, few CMAr neurons exhibited selectivity for such motor parameters as the location of the target or the arm to be used. The abundance and robustness of the neuronal activity within the CMAr that reflected each step of the behavioral task and the modest selectivity of the same cells for sensorimotor parameters are strikingly different from the preponderance of selectivity that we have observed in other frontal areas. Based on these results, we propose that the CMAr participates in monitoring individual behavioral events to keep track of the progress of required behavioral tasks. On the other hand, CMAr activity during motor planning may reflect the emergence of a general intention for action.     

Movement-related neuronal activity reflecting the transformation of coordinates in the ventral premotor cortex of monkeys

We examined how the transformation of coordinates from visual to motor space is reflected by neuronal activity in the ventral premotor cortex (PMv) of monkeys. Three monkeys were trained to reach with their right hand for a target that appeared on a screen. While performing the task, the monkeys wore prisms that shifted the image of the target 10 degrees, left or right, or wore no prisms, for a block of 200 trials. The nine targets were located in the same positions in visual space regardless of whether the prisms were present. Wearing the prisms required the monkeys to initiate a movement in a direction that was different from the apparent target location. Thus using the prisms, we could dissociate visual space from motor space. While the monkey performed the behavioral task, we recorded neuronal activity in the left PMv and primary motor cortex (MI), and various kinds of task-related neuronal activity were found in the motor areas. These included neurons that changed their activity during a reaction time (RT) period (the period between target presentation and movement onset), which were called "movement-related neurons" and selected for analysis. In these neurons, activity during a movement time (MT) period was also compared. Using general linear models for our statistical analysis, the neurons were then classified into four types: those whose activity was consistently dependent on location of targets in the visual coordinates regardless of whether the prisms were present or absent (V type); those that were consistently dependent on target location in the motor coordinates only; those that had different activity for both of the motor and visual coordinates; and those that had nondifferential activity for the two types of coordinates. The proportion of the four types of the neurons differed significantly between the PMv and MI. Most remarkably, neurons with V-type activity were almost exclusively recorded in the PMv and were almost exclusively found during the RT period. Such activity was never observed in an electromyogram of the working forelimb. Based on these observations, we postulate that the V and other types may represent the various intermediate stages of the transformation of coordinates and that the PMv plays a crucial role in transforming coordinates from visual to motor space.     

Neuronal correlates of movement dynamics in the dorsal and ventral premotor area in the monkey

We investigated how neurons in the different motor areas of the frontal lobe reflect the movement dynamics, and how their neuronal activity undergoes plastic changes when monkeys adapt to perturbing forces (they learn new dynamics). Here we describe the results obtained in the dorsal premotor area (PMd) and ventral premotor area (PMv). Monkeys performed visually instructed, delayed reaching movements before, during and after exposure and adaptation to a viscous, curl force field. During movement planning (i.e., during an instructed delay that followed the cue and preceded the go signal), we found dynamics-related activity in PMd but not in PMv. A closer analysis revealed that the population of PMd reflected the dynamics of the upcoming movement increasingly over the course of the delay, starting from a kinematics-related signal. During movement execution, dynamics-related activity was present in both PMd and PMv. In this respect, the results for PMd were similar to that previously found for the supplementary motor area (SMA) whereas the results for PMv were more similar to that previously found for the primary motor cortex (M1). Plastic changes associated with the acquisition of new dynamics found in PMd and PMv were qualitatively similar to those previously observed in M1 and SMA. The ensemble of our experiments suggest a broader picture of the cortical control of movements, whereby multiple areas all contribute to the various sensorimotor processes, including “low” computations such as the movement dynamics, but also express a degree of specialization.     

Primate frontal cortex: effects of stimulus and movement

We compared neuronal activity in the dorsal premotor cortex (PMd), ventral premotor cortex (PMv), and prefrontal (PF) cortex of two rhesus monkeys. The behavioral design was a variant of the instructed delay task which established that: (1) a given visual stimulus could, on different trials, instruct different limb movements and (2) several different visual stimuli could instruct the same movement. Neurons in all frontal areas displayed the often replicated activity patterns that occur during instructed delay tasks, including phasic increases after instruction stimuli (signal-related activity), tonic discharge during an instructed delay period (set-related activity), and phasic premovement discharge (movementrelated activity). For signal-, set-, and movement-related activity, the majority of neurons in PMd (51–64%), but only a minority in PF (16–18%) and PMv (32–40%), showed activity levels that significantly depended on the action instructed by that stimulus rather than simply the characteristics of the stimulus per se. Thus, most PMd activity, including the aspects that most resembled a sensory response, reflected factors in addition to the signal. Taken together with the results of related studies, it seems most likely that these other factors are dominated by the motor instructional significance of the stimulus. In addition, many neurons (17–37%) in all examined areas showed activity that significantly depended on which of various stimuli guided the same movement. This finding shows that, in those frontal areas, neuronal activity can be affected by both the action to be taken and the events guiding that action.     

Laterality of movement-related activity reflects transformation of coordinates in ventral premotor cortex and primary motor cortex of monkeys

The ventral premotor cortex (PMv) and the primary motor cortex (MI) of monkeys participate in various sensorimotor integrations, such as the transformation of coordinates from visual to motor space, because the areas contain movement-related neuronal activity reflecting either visual or motor space. In addition to relationship to visual and motor space, laterality of the activity could indicate stages in the visuomotor transformation. Thus we examined laterality and relationship to visual and motor space of movement-related neuronal activity in the PMv and MI of monkeys performing a fast-reaching task with the left or right arm, toward targets with visual and motor coordinates that had been dissociated by shift prisms. We determined laterality of each activity quantitatively and classified it into four types: activity that consistently depended on target locations in either head-centered visual coordinates (V-type) or motor coordinates (M-type) and those that had either differential or nondifferential activity for both coordinates (B- and N-types). A majority of M-type neurons in the areas had preferences for reaching movements with the arm contralateral to the hemisphere where neuronal activity was recorded. In contrast, most of the V-type neurons were recorded in the PMv and exhibited less laterality than the M-type. The B- and N-types were recorded in the PMv and MI and exhibited intermediate properties between the V- and M-types when laterality and correlations to visual and motor space of them were jointly examined. These results suggest that the cortical motor areas contribute to the transformation of coordinates to generate final motor commands.     

Activity properties and location of neurons in the motor thalamus that project to the cortical motor areas in monkeys

The activity of neurons in the motor nuclei of the thalamus that project to the cortical motor areas (the primary motor cortex, the ventral and dorsal premotor cortex, and the supplementary motor area) was investigated in monkeys that were performing a task in which wrist extension and flexion movements were instructed by visuospatial cues before the onset of movement. Movement was triggered by a visual, auditory, or somatosensory stimulus. Thalamocortical neurons were identified by a spike collision, and exhibited 2 distinct types of task-related activity: 1) a sustained change in activity during the instructed preparation period in response to the instruction cues (set-related activity); and 2) phasic changes in activity during the reaction and movement time periods (movement-related activity). A number of set- and moment-related neurons exhibited direction selectivity. Most movement-related neurons were similarly active, irrespective of the different sensory modalities of the cue for movement. These properties of neuronal activity were similar, regardless of their target cortical motor areas. There were no significant differences in the antidromic latencies of neurons that projected to the primary and nonprimary motor areas. These results suggest that the thalamocortical neurons play an important role in the preparation for, and initiation and execution of, the movements, but are less important than neurons of the nonprimary cortical motor areas in modality-selective sensorimotor transformation. It is likely that such transformations take place within the nonprimary cortical motor areas, but not through thalamocortical information channels.     

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.     

Effects of hand movement path on motor cortical activity in awake,behaving rhesus monkeys

Summary Neuronal activity was studied in the primary (M1), supplementary (M2), dorsal premotor (PMd), and ventral premotor (PMv) cortex of awake, behaving rhesus monkeys. The animals performed forelimb movements to three targets, each approached by three different types of trajectories. With one trajectory type, the monkey moved its hand straight to the target, with another, the path curved in a clockwise direction, and with a third, the path curved in a counter-clockwise direction. We examined whether neuronal activity in these areas exclusively reflects a hand movement's net distance and direction or, alternatively, whether other factors also influence cortical activity. It was found that neuronal activity during all phases of a trial reflects aspects of movement in addition to target location. Among these aspects may be selection of an integrated motor act from memory, perhaps specifying the entirety of a path by which the hand moves to a target.     

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.     

Role of primate basal ganglia and frontal cortex in the internal generation of movements

Summary This study is a part of a project investigating neuronal activity in the basal ganglia and frontal cortex and describes externally and internally induced preparatory activity in the supplementary motor area (SMA), which forms a closed neuronal loop with the striatum. Monkeys made self-initiated arm reaching movements toward a constant target in the absence of phasic external stimuli. In separate blocks of trials, animals performed in a delayed go no-go task in which an instruction cue prepared for subsequent movement or no-movement to a trigger stimulus. A total of 328 neurons were tested in the delay task. Of these, 91 responded transiently to the instruction light with a median latency of 262 ms. Three quarters of these responses were restricted to the instruction preparing for arm movement, as opposed to with-holding it, and thus may be involved in movement preparation processes. Sustained activation during the instruction-trigger interval was found for 67 neurons and occurred nearly exclusively in movement trials. Activation usually increased gradually after the cue and ended abruptly upon movement onset and thus could be related to the setting and maintenance of processes underlying the preparation of movement. Time-locked responses to the trigger stimulus were found in 38 neurons and were usually restricted to movement trials (median latency 80 ms). Activity time-locked to movement execution occurred in 67 neurons, beginning up to 252 ms before movement onset. A total of 266 neurons were tested with self-initiated arm movements. Of these, 43 showed premovement activity beginning 610–3030 ms before movement onset (median 1430 ms). The activity increased slowly and reached its peak at 370 ms before movement onset. It ended before movement onset or continued until the arm began to move or reached the target. This activity appears to reflect neuronal processes related to the internal generation of movements. Two thirds of activations preceding self-initiated movements occurred in neurons not activated before externally instructed movements, suggesting a selectivity for the internal generation process. Activity related to the execution of self-initiated movements occurred in 67 neurons: it began during and up to 420 ms before movement onset and was usually not associated with pre-movement activity. Most of these neurons were also activated with stimulus-triggered movements, suggesting a lack of selectivity for the execution of self-initiated movements. In comparison with the striatum, more SMA neurons showed preparatory activity preceding externally instructed movements (transient 27% vs 16%, sustained 20% vs 12%) and self-initiated movements (16% vs 11%). Whereas transient responses showed similar latencies and durations in the two structures, sustained preparatory activity preceding externally instructed or self-initiated movements began and reached its peak earlier in SMA compared to striatal neurons. However, due to the long durations, sustained activation largely overlapped in the two structures, and thus essentially occurred simultaneously. Instruction-induced or internally generated preparatory activity may originate outside of the SMA and striatum or may derive from activity reverberating in cortico-basal ganglia loops, possibly in conjunction with other, closely associated cortical and subcortical structures. These data would favor a conjoint role for SMA and striatum in the internal generation of individual behavioral acts and the preparation of behavioral reactions.     

Reacquisition deficits in prism adaptation after muscimol microinjection into the ventral premotor cortex of monkeys

A small amount of muscimol (1 microl; concentration, 5 microg/microl) was injected into the ventral and dorsal premotor cortex areas (PMv and PMd, respectively) of monkeys, which then were required to perform a visually guided reaching task. For the task, the monkeys were required to reach for a target soon after it was presented on a screen. While performing the task, the monkeys' eyes were covered with left 10 degrees, right 10 degrees, or no wedge prisms, for a block of 50-100 trials. Without the prisms, the monkeys reached the targets accurately. When the prisms were placed, the monkeys initially misreached the targets because the prisms displaced the visual field. Before the muscimol injection, the monkeys adapted to the prisms in 10-20 trials, judging from the horizontal distance between the target location and the point where the monkey touched the screen. After muscimol injection into the PMv, the monkeys lost the ability to readapt and touched the screen closer to the location of the targets as seen through the prisms. This deficit was observed at selective target locations, only when the targets were shifted contralaterally to the injected hemisphere. When muscimol was injected into the PMd, no such deficits were observed. There were no changes in the reaction and movement times induced by muscimol injections in either area. The results suggest that the PMv plays an important role in motor learning, specifically in recalibrating visual and motor coordinates.     

Activity in ventral and dorsal premotor cortex in response to predictable force-pulse perturbations in a precision grip task

This study compared the responses of ventral and dorsal premotor cortex (PMv and PMd) neurons to predictable force-pulse perturbations applied during a precision grip. Three monkeys were trained to grasp an unseen instrumented object between the thumb and index finger and to lift and hold it stationary within a position window for 2-2.5 s. The grip and load forces and the object displacement were measured on each trial. Single-unit activity was recorded from the hand regions in the PMv and PMd. In some conditions a predictable perturbation was applied to the object after 1,500 ms of static holding, whereas in other conditions different random combinations of perturbed and unperturbed trials were given. In the perturbed conditions, some were randomly and intermittently presented with a warning flash, whereas some were unsignaled. The activities of 198 cells were modulated during the task performance. Of these cells, 151 were located in the PMv, and 47 were located in the PMd. Although both PMv and PMd neurons had similar discharge patterns, more PMd neurons (84 vs. 43%) showed early pregrip activity. Forty of 106 PMv and 10/30 PMd cells responded to the perturbation with reflexlike triggered reactions. The latency of this response was always <100 ms with a mean of about 55 ms in both the PMv and the PMd. In contrast, 106 PMv and 30 PMd cells tested with the perturbations, only 9 and 10%, respectively, showed significant but nonspecific adaptations to the perturbation. The warning stimulus did not increase the occurrence of specific responses to the perturbation even though 21 of 42 cells related to the grip task also responded to moving visual stimuli. The responses were retinal and frequently involved limited portions of both foveal and peripheral visual fields. When tested with a 75 x 5.5-cm dark bar on a light background, these cells were sensitive to the direction of movement. In summary, the periarcuate premotor area activity to related to predictable force-pulse perturbations seems to reflect a general increase in excitability in contrast to a more specific anticipatory activity such as recorded in the cerebellum. In spite of the strong cerebello-thalamo-cortical projections, the results of the present study suggest that the cortical premotor areas are not involved in the elaboration of adaptive internal models of hand-object dynamics.     

Parietal inputs to dorsal versus ventral premotor areas in the macaque monkey: evidence for largely segregated visuomotor pathways

The lateral premotor cortex plays a crucial role in visually guided limb movements. Visual information may reach this cortical region from the parietal cortex, the highest stage in the dorsal visual stream. Anatomical studies indicate that the parietal projections to the dorsal (PMd) and ventral (PMv) premotor areas arise from separate parietal regions, supporting the notion of parallel visuomotor pathways. We tested the degree of segregation of these pathways by injecting retrograde tracers into PMd and PMv in the same monkeys, under physiological control. Eleven injections were made in four animals, and the analysis of retrograde labelling revealed that parietal cells projecting to PMd and those projecting to PMv are largely segregated. The strongest projections to PMd arise from the superior parietal lobule, including the medial intraparietal area (MIP), PEc and PGm, and the parieto-occipital area. These areas were devoid of labelling following injections into PMv, which receives its major projections from the anterior intraparietal area (AIP), area PEip, the anterior portion of the inferior parietal gyrus (area 7b), and the somatosensory areas. In addition to their strong projections to PMv, areas 7b and PEip send minor projections to PMd as well. Additional projections to PMd arise from the ventral intraparietal area and the inferior parietal lobule. The present findings are direct anatomical evidence for largely segregated visuomotor pathways linking parietal cortex with PMd and PMv.     

Rostrocaudal distinction of the dorsal premotor area based on oculomotor involvement

To investigate functional differences between the rostral and caudal parts of the dorsal premotor cortex (PMd), we first examined the effects of intracortical microstimulation (ICMS) while monkeys were performing oculomotor and limb motor tasks or while they were at rest. We found that saccades were evoked from the rostral part (PMdr) whereas ICMS in the caudal part (PMdc) predominantly produced forelimb or body movements. Subsequently, we examined neuronal activity in relation to the performance of visually cued and memorized saccades while monkeys reached an arm toward a visual target. We found that roughly equal numbers of PMdr neurons were active during performance of the oculomotor and limb motor tasks. In contrast, the majority of PMdc neurons were related preferentially to arm movements and not to saccades. In the subsequent analysis, we found that the oculomotor effects evoked in the PMdr differ from the effects evoked in either the frontal eye field (FEF) or supplementary eye field (SEF). These findings suggest that the PMdr is involved in oculomotor as well as limb motor behavior. However, the oculomotor involvement of the PMdr seems to have a functional aspect different from that operating in the FEF and SEF.     

Macaque SEF neurons encode object-centered directions of eye movements regardless of the visual attributes of instructional cues.

Macaque SEF neurons encode object-centered directions of eye movements regardless of the visual attributes of instructional cues. Neurons in the supplementary eye field (SEF) of the macaque monkey exhibit object-centered direction selectivity in the context of a task in which a spot flashed on the right or left end of a sample bar instructs a monkey to make an eye movement to the right or left end of a target bar. To determine whether SEF neurons are selective for the location of the cue, as defined relative to the sample bar, or, alternatively, for the location of the target, as defined relative to the target bar, we carried out recording while monkeys performed a new task. In this task, the color of a cue-spot instructed the monkey to which end of the target bar an eye movement should be made (blue for the left end and yellow for the right end). Object-centered direction selectivity persisted under this condition, indicating that neurons are selective for the location of the target relative to the target bar. However, object-centered signals developed at a longer latency (by approximately 200 ms) when the instruction was conveyed by color than when it was conveyed by the location of a spot on a sample bar.     

Neural correlates of cognitive control of reaching movements in the dorsal premotor cortex of rhesus monkeys

Canceling a pending movement is a hallmark of voluntary behavioral control because it allows us to quickly adapt to unattended changes either in the external environment or in our thoughts. The countermanding paradigm allows the study of inhibitory processes of motor acts by requiring the subject to withhold planned movements in response to an infrequent stop-signal. At present the neural processes underlying the inhibitory control of arm movements are mostly unknown. We recorded the activity of single units in the rostral and caudal portion of the dorsal premotor cortex (PMd) of monkeys trained in a countermanding reaching task. We found that among neurons with a movement-preparatory activity, about one-third exhibit a modulation before the behavioral estimate of the time it takes to cancel a planned movement. Hence these neurons exhibit a pattern of activity suggesting that PMd plays a critical role in the brain networks involved in the control of arm movement initiation and suppression.     

Comparison of population activity in the dorsal premotor cortex and putamen during the learning of arbitrary visuomotor mappings

A previous study found that as monkeys learned novel mappings between visual cues and responses, neuronal activity patterns evolved at approximately the same time in both the dorsal premotor cortex (PMd) and the putamen. Here we report that, in both regions, the population activity for novel mappings came to resemble that for familiar ones as learning progressed. Both regions showed activity differences on trials with correct responses versus those with incorrect ones. In addition to these common features, we observed two noteworthy differences between PMd and putamen activity during learning. After a response choice had been made, but prior to feedback about the correctness of that choice (reward or nonreward), the putamen showed a sustained activity increase in activity, whereas PMd did not. Also in the putamen, this prereward activity was highly selective for the specific visuomotor mapping that had just been performed, and this selectivity was maintained until the time of the reward. After performance reached an asymptote, the degree of this selectivity decreased markedly to the level typical for familiar visuomotor mappings. These findings support the hypothesis that neurons in the striatum play a pivotal role in associative learning. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Covariation of primate dorsal premotor cell activity with direction and amplitude during a memorized-delay reaching task

Extensive behavioral evidence suggests that the direction and amplitude of reaching movements are planned as two independent parameters by the motor system. However, whereas direction-related activity has been well documented by neurophysiological studies in many motor structures including the dorsal premotor cortex (PMd), there is much less concensus about the prominence and timing of amplitude-related premotor activity. We studied this issue using an instructed-delay task in which prior information about target location (direction and distance) must be memorized before movement initiation. The results show that prior information about distance is reflected in PMd activity during the delay period well before movement initiation, and begins to be expressed as early as 150 ms after presentation of target location. The prominence of neural correlates with direction is relatively constant throughout the trial, but distance correlates become gradually more prominent with time, both during and after the delay period. A small majority of cells were modulated only by direction during the delay period, but very few were modulated only by distance, and most of the rest were modulated by both. Therefore PMd neurons usually process information about distance only in conjunction with directional information. These results do not support a separate neuronal substrate for distance in PMd, but do not preclude its existence elsewhere. The results also support a progressive change in the nature of the movement-related representation in PMd with time in an instructed-delay paradigm.