Carolyn R. Mason

Central processes for the multiparametric control of arm movements in primates

Recent single-unit recording studies have clarified how multiple parameters of movement are signaled by individual cortical and cerebellar neurons, and also that multiple coordinate frames are utilized. Cognitive processes also modulate the firing of these neurons. The various signals and coordinate systems vary in time and evolve throughout a behavioral sequence, consistent with the demands of the task and the required sensorimotor transformations.

Signaling of Grasp Dimension and Grasp Force in Dorsal Premotor Cortex and Primary Motor Cortex Neurons During Reach to Grasp in the Monkey

A fundamental question is how the CNS controls the hand with its many degrees of freedom. Several motor cortical areas, including the dorsal premotor cortex (PMd) and primary motor cortex (M1), are involved in reach to grasp. Although neurons in PMd are known to modulate in relation to the type of grasp and neurons in M1 in relation to grasp force and finger movements, whether specific parameters of whole hand shaping are encoded in the discharge of these cells has not been studied. In this study, two monkeys were trained to reach and grasp 16 objects varying in shape, size, and orientation. Grasp force was explicitly controlled, requiring the monkeys to exert either three or five levels of grasp force on each object. The animals were unable to see the objects or their hands. Single PMd and M1 neurons were recorded during the task, and cell firing was examined for modulation with object properties and grasp force. The firing of the vast majority of PMd and M1 neurons varied significantly as a function of the object presented as well as the object grasp dimension. Grasp dimension of the object was an important determinant of the firing of cells in both PMd and M1. A smaller percentage of PMd and M1 neurons were modulated by grasp force. Linear encoding was prominent with grasp force but less so with grasp dimension. The correlations with grasp dimension and grasp force were stronger in the firing of M1 than PMd neurons and across both regions the modulation with these parameters increased as reach to grasp proceeded. All PMd and M1 neurons that signaled grasp force also signaled grasp dimension, yet the two signals showed limited interactions, providing a neural substrate for the independent control of these two parameters at the behavioral level.

Movement kinematics encoded in complex spike discharge of primate cerebellar Purkinje cells

MONKEYS performed a multijoint arm-reaching task that systematically varied movement direction and distance. Purkinje cell activity was recorded from 231 task-related cells, and the complex spike discharge was analyzed in relation to distance and direction. The complex spike activity of 123 Purkinje cells changed significantly relative to the background rate. Of these 123, the activity of 85 cells was related to distance and/or direction. The complex spike activity of 54 of these 85 cells fitted a cosine tuning curve for direction, generally at one distance. Using a simple linear regression model, the complex spike activity of 56 cells was significantly correlated with movement distance, usually in one direction. We conclude that the complex spike discharge of Purkinje cells is spatially tuned and strongly related to movement kinematics.

Finger movements during reach-to-grasp in the monkey: Amplitude scaling of a temporal synergy

To reduce the complexity of controlling hand-shaping, recent evidence suggests that the central nervous system uses synergies. In this study, two Rhesus monkeys reached-to-grasp 15 objects, varying in geometric properties, at five grasp force levels. Hand kinematics were recorded using a video-based tracking system. Individual finger movements were described as vectors varying in length and angle. Inflection points (i.e., stereotypic minima/maxima in the temporal profile of each finger vector) exhibited a temporal synchrony for individual fingers and in the coupling across fingers. Inflection point amplitudes varied significantly across objects grasped, scaling linearly with the object grasp dimension. Thus, differences in the vectors as a function of the objects were in the relative scaling of the vector parameters over time rather than a change in the temporal structure. Mahalanobis distance analysis of the inflection points confirmed that changes in inflection point amplitude as a function of objects were greater than changes in timing. Inflection points were independent of the grasp force, consistent with the observation that reach-to-grasp kinematics and grasp force are controlled independently. In summary, the shaping of the hand during reach-to-grasp involves scaling the amplitude of highly stereotypic temporal movements of the fingers.

Temporal profile of the directional tuning of the discharge of dorsal premotor cortical cells

This study examined the directional modulation of dorsal premotor (PMd) cells as a function of time in an instructed delay, reaching task that systematically varied direction and accuracy constraints. In two monkeys, the activity of 150 PMd cells was recorded and the preferred direction (PD) of the firing as a function of time, the PD trajectory, was calculated. Forty-one cells had nearly continuous significant directional tuning of at least 1 s duration (mean duration 1694 ± 754 ms) that began in the instructed delay period and continued into the movement period. The PD gradually changed in time (mean change of 47.7 ± 40.8°), a change best described as a rotation. The change in the directional tuning as a function of time is consistent with the hypothesis that the PMd plays a role in the non-standard mapping of sensory stimuli into motor commands.