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Notes: Abstract  Corticospinal (CS) axon terminations in several species are widespread early in development but are subsequently refined into a spatially more restricted distribution. We studied the role of neural activity in sensorimotor cortex in shaping postnatal development of CS terminations in cats. We continuously infused muscimol unilaterally into sensorimotor cortex to silence neurons during the postnatal CS refinement period (weeks 3–7). Using anterograde transport of WGA-HRP, we examined the laterality of terminations from the muscimol-infused (i.e., silenced) and active sides in the spinal cord, as well as in the cuneate nucleus and red nucleus. We found that CS terminations from the muscimol-infused cortex were very sparse and limited to the contralateral side, while those from the active cortex maintained an immature bilateral topography. Controls (saline infusion, noninfusion) had dense, predominantly contralateral, CS terminations. There was a substantial decrease in the spinal gray matter area occupied by terminations from the side receiving the blockade and a concomitant increase in the area occupied by ipsilateral terminations from the active cortex. Optical density measurements of HRP reaction product from the active cortex in muscimol-infused animals showed substantial increases over controls in the ratio of ipsilateral to contralateral CS terminations for all laminae examined (IV–V, VI, VII). Our findings suggest that ipsilateral dorsal horn terminations reflect new axon growth during the refinement period because they are not present there earlier in development. Those in the ventral horn are present earlier in development and thus could reflect maintenance of transient terminations. Increased ipsilateral terminations from active cortex were due to recrossing of CS axons in lamina X and not to an increase in labeled CS axons in the ipsilateral white matter. Examination of brain stem terminations suggested that, between postnatal weeks 3 and 7, development of corticocuneate terminations also is activity-dependent but that development of corticorubral terminations is not. Activity-dependent CS development is a plausible mechanism by which early motor experiences could shape the anatomical and functional organization of the motor systems during a critical postnatal period.

Notes: Summary (1) Participation of the motor cortex in initiating muscle contraction in an isometric tracking task was assessed in cats trained to make accurate force adjustments using forelimb muscles, in response to a vibrissal/visual display stimulus. Behavior in the task was characterized by short reaction times. While the task was performed, recordings of single cortical units were made in zones within area 4γ defined by the effects of microstimulation in forelimb muscles and by receptive fields on the forelimb. (2) Two types of receptive fields with different regional distributions were observed. Cells with simple receptive fields (superficial or deep) were seen throughout the area sampled, consisting of the lateral half of the anterior and posterior sigmoid gyri. Cells whose receptive fields had complex features (directional specificity, temporal lability, multiple foci, etc.) were preferentially located in the cortex rostral to the cruciate sulcus. (3) The area of motor cortex rostral to the cruciate sulcus also differed from the area caudal to the cruciate sulcus in the timing of task-related activity. Neurons that were active before response onset (lead cells), and could therefore contribute to response initiation, were preferentially located in the rostral cortex, and, in general, had complex receptive fields. (4) Lead cells were active at a constant latency from the stimulus, rather than being timed to response onset. However, the modulation of their activity was related to both the direction and magnitude of the force response. (5) These results suggest that the pericruciate motor cortex of the cat contains two functional subdivisions: a caudal one concerned with ongoing movement, perhaps under the control of specific sensory inputs from the responding limb, and a rostral one involved in initiating movement. Because behaviorally relevant stimuli can rapidly activate a specialized population of cells in the rostral cortex, this area is able to participate in responses with short reaction times.

Notes: Summary In a previous study in the cat, we have reported that motor cortex neurons discharging before the initiation of an aimed forearm response (lead cells) are better timed to movement of a display (stimulus) than to the response. The present study was done to distinguish the coding of stimulus and response features in the discharge patterns of such early activity in motor cortex. Single neurons were recorded in the arm area of motor cortex in three cats performing the same pair of responses (forearm flexion and extension) but to display movements in either of the two directions by changing display polarity. The modulation of lead cell activity was contingent on the occurrence of the learned motor response and timed to the stimulus in all conditions. The majority of lead cells (88%, n = 50) fell into one of two distinct classes. In one class of neurons, force-direction (56%, n = 32), activity was contingent on a single direction of forelimb response (flexion or extension) and was thus independent of the direction of the display stimulus. The only muscles whose patterns matched the activity of this class of response-related neurons were forelimb flexors and extensors. In these neurons, the onset of modulation was timed to one or the other of the two stimuli according to the stimulus direction which elicited the appropriate response. Thus, the display-related input to these neurons varied according to the response required. In the second class of neurons, stimulus-direction (32%, n = 18), modulation was associated with a specific stimulus direction rather than the response direction. The pattern of activity of these neurons was similar to the pattern of EMG signals of shoulder and neck muscles during the different task conditions. The contraction of proximal and axial muscles corresponded to a second response elicited by the stimulus, namely attempts at head rotation towards the moving display and was independent of the conditioned forelimb response in both time of onset and direction. To test the possibility that stimulus-direction neurons participated in the control of head rotation we trained two of the animals to also produce isometric changes in neck torque in the direction of the moving display without making the forelimb response. The activity of stimulus-direction neurons was similarly modulated during performance of the neck task. By contrast, force-direction neurons examined during the neck task were either unmodulated or discharged after the neck response. These data suggest that force-direction neurons participate in response initiation and that their activity is triggered by stimuli specific for the task. The reorganization of the inputs to motor cortex is likely to result from gating mechanisms associated with behavioral set. Such neural gates could provide for the efficient transfer of any member of an array of behaviorally relevant stimuli to restricted sectors of the somatotopically organized motor areas.

Notes: Summary In the present study we recorded the activity of single neurons in the forelimb area of red nucleus (RN) during performance of three step-tracking tasks designed to dissociate the coding of stimulus and response variables in the discharge of recorded neurons. In two of these tasks, the standard and stimulus-reversal arm tasks, elbow flexion and extension were elicited by different stimuli enabling us to distinguish activity correlated with the forelimb response from the stimulus eliciting it. The third task (neck task) allowed us to determine whether neuronal modulation was related to an unconditioned orienting response that occurred concurrently with the forelimb response. We have previously reported that these three tasks separate neurons in MCx whose modulation precedes the response (lead cells) into three distinct classes in which task-related activity either is correlated with the direction of the forelimb response, correlated with the stimulus, or not correlated with either (Martin and Ghez 1985). All lead cells, however, remained timed to the stimulus rather than to the response. The present results show that RN lead cells can be subdivided into the same three classes as those in MCx and their discharge was also contingent on the subsequent production of a behavioral response. (1) Force-direction neurons (35%; n = 16) showed changes in activity correlated with the production of forearm force in a particular direction suggesting that they could participate in selecting the appropriate forelimb response. The onset of task-related modulation of activity was better timed to the response, in contrast to force-direction neurons in MCx, which were better timed to the stimulus. (2) Stimulus-direction neurons (18%; n = 8) modulated their activity in relation to a particular stimulus evoking either flexor or extensor responses and during neck task performance. These neurons could be involved in processing stimulus information or in the production of neck torque. The task-related discharge of these lead cells was better timed to the stimulus than to either the forelimb or the neck response. (3) Nondirectional neurons (47%; n = 21) modulated their activity during all tasks examined. Their discharge did not correlate with any specific feature of the stimulus or response, and as a group, was better timed to the stimulus than to the response. Nondirectional neurons may participate in some aspect of motor preparation. To determine the relative contributions of RN and MCx lead cells to response initiation, we compared the amount of response latency variance that could be explained by variation in the latency of the unit modulation to the stimulus for the present data and the data in the earlier MCx study (Martin and Ghez 1985). Between 38% and 53% of response latency variance (for trials examined during performance of the standard arm and stimulus reversal tasks) was accounted for by the latency variations of RN force direction neurons; in contrast, 8% and 11% for MCx force-direction neurons. Variations in timing of stimulus-direction neurons in both RN and MCx account for less than 10% of response latency variance. Our findings suggest that, in the tasks examined, RN force-direction neurons play a more direct role than MCx force-direction neurons in initiating and selecting responses to stimuli. We hypothesized that this subcortical control reflects the high degree of stereotypy of the motor response examined.

Notes: Abstract This study examined changes in the performance of a single-joint, elbow task produced by reversible inactivation of local regions within the proximal forelimb representation in area 4γ of motor cortex (MCx) and the red nucleus (RN) of the cat. Inactivation was carried out by microinjecting lidocaine, γ-aminobutyric acid, or muscimol into sites where microstimulation evoked contraction of elbow muscles. Reaction time, amplitude, and speed (velocity or dF/dt) of position and force responses elicited during inactivation were compared to control values obtained immediately prior to inactivation. In addition, we assessed qualitatively the effects of inactivation on reaching, placing reactions, and proprioceptive responses to imposed limb displacement. In the single-joint task, injections in MCx did not increase reaction time (simple or choice) and produced modest and inconsistent reductions in response amplitude (mean-8%) and speed (mean -19%). In contrast, injections of the same amounts of inactivating agents in the forelimb representation of RN consistently increased reaction time (34.4%), and increased the reaction time coefficient of variability (32%). There were small reductions in response amplitude (-4%) and speed (-10%) which were less than those produced by MCx inactivation. During reaching, however, these same injections in MCx and RN produced a substantial loss of accuracy. For MCx, this was due, in part, to systematic hypometria: for RN, inaccuracy resulted from increased variability in paw paths. Placing reactions and corrective responses to imposed limb displacements were also depressed by the cortical and rubral injections. Our results suggest that the forelimb representation in RN plays a role in the initiation of the single-joint, elbow tracking response examined here. The RN may mediate cerebellar regulation of response timing, a function that is likely to be important for interjoint coordination. Although neurons in the forelimb representations of MCx may contribute to force generation in single-joint movements, their contribution to multijoint control appears to be more important and is examined in the subsequent report (Martin and Ghez 1993).

Notes: Abstract This study analyzed changes in the performance of a reaching task and its adaptive modification produced by reversible inactivation of three sites within the forelimb representation of the motor cortex (MCx, area 4γ) in five cats by microinjections of muscimol. Two sites were located in the lateral MCx, rostral (RL-MCx) and caudal (CL-MCx) to the end of the cruciate sulcus, where intracortical microstimulation (ICMS) produced contraction of the most distal muscles. The third site was located more medially, in the anterior sigmoid gyrus (RM-MCx) where ICMS primarily produced contraction of more proximal muscles. The task required the animals to reach into a horizontal target well, located in front of them at one of three possible heights, to grasp and retrieve a small piece of food. The height of the reach was primarily achieved by elbow flexion. Grasping consisted primarily of digit flexion, and food retrieval consisted of forearm supination and shoulder extension. In some blocks of trials, an obstacle was placed in the path of the limb to assess the animal's ability to adaptively adjust the kinematic characteristics of their response trajectory. In normal animals, contact with the bar on the first trial triggered a corrective response at short latency that allowed the paw to circumvent the bar. On all subsequent trials, the trajectory was adapted to prevent contact with the obstacle, with a safety margin of about 1 cm. Inactivation at all sites produced a slowing of movement, a protracted and extended forelimb posture, and increased variability of initial limb position. In addition, inactivation of RL-MCx immediately produced systematic reaching errors, consisting of hypermetric movements, as well as impaired grasping and food retrieval. The degree of hypermetria was similar for all target heights and was not associated with alterations in trajectory control. During inactivation, animals did not compensate for the hypermetria by reducing paw path elevation, suggesting a defect in kinematic planning or in adaptive control. This was confirmed by finding that trajectory adaptation to avoid bar contact was impaired during RL-MCx inactivation. The short latency corrective response, triggered by contact of the limb with the obstacle was, however, preserved. Inactivation of CL-MCx did not impair aiming, grasping, or adaptation immediately after injection. However, impairments occurred after about 1 h postinjection, and at that time mimicked the effects of RL-MCx inactivation. This delay suggests that the drug was acting indirectly on the RL-MCx. Inactivation of RM-MCx did not impair the control of distal muscles, but the reaches became hypometric. The hypometria was greater for higher targets, suggesting that it resulted from weakness. Our results suggest that both rostral regions of the forelimb area of MCx play a more important role in the planning and execution of the prehension response than the caudal portion. We hypothesize that (1) the slowing of movement, forelimb postural changes, hypometria, and grasping and food retrieval impairments are due to defective control of muscles represented locally at each site in MCx and that (2) aiming and adaptation defects, which are produced only by RL-MCx inactivation, result from disruption of integrative mechanisms underlying sensorimotor transformations that normally assure movement accuracy.