The contribution of population activity in motor cortex to the control of skilled hand movements in the primate

In recent years, our understanding of cortical function has been dominated by the view that neurons operate as part of larger assemblies, with each assembly responsible for an aspect of information processing. Advances in multiple neuron electrode recording have enabled the activity of several cells to be sampled simultaneously in vivo in the awake behaving monkey, thereby allowing, in theory at least, the function of assemblies to be monitored. This method can provide important insights into the role of single cells as potential members of a neuronal assembly. The aim of this thesis is to review our current understanding of the cortical control of voluntary hand movements, and to describe experimental work which has explored the simultaneous activity of multiple cortical neurons, and the interaction between them as a basis for that control. The experimental paradigm used was the fine control of forearm and intrinsic hand muscles during performance of the precision grip task by the macaque monkey. Multiple single electrode recordings were made in the motor cortex of two purpose-bred macaque monkeys trained to perform the precision grip task. Up to 16 cells were simultaneously recorded and identified as pyramidal tract neurons or cortico-motoneuronal cells by antidromic invasion and spike-triggered averaging of hand muscle EMG, respectively. Individual cells were characterised according to their antidromic latency and the modulation of their firing with the task. The interactions between cells in a population were analysed by correlation techniques in the time domain. The strength of correlation between pairs of cells was investigated in relation to the cell type involved, the similarity of their task relationship and the distance between them. The contribution of individual cells to oscillatory synchrony between the cortex and hand muscles was investigated by regression-based analysis in the frequency domain. The resulting data were modelled using computer simulations of parts of the motor system. This thesis provides strong evidence for the existence of a cortical network which could be responsible for oscillatory activity across a population of neurons. Synchrony was observed between local field potentials in the cortex and hand muscle EMG. The results suggest a significant contribution of population synchrony to the cortical control of distal muscles during certain phases of movement. The proposed function for synchrony between cortical output cells is in the efficient activation of motoneurons by temporal summation of their excitatory post-synaptic potentials in the target motoneurons