Dynamics of Motor Cortical Networks
The Complementarity of Spike Synchrony and Firing Rate
The temporal coding hypothesis suggests that not only changes in firing rate but also precise spike timing, especially synchrony, constitute an important part of the representational substrate for perception and action. In this framework, the concept of cell assemblies uses synchrony as an additional dimension to firing rate, as a candidate for information processing. Consequently, the observation of spike synchrony between neurons might be interpreted as an activation of a functional cell assembly. When, in an instructed delay task, prior information is provided about movement parameters, such as movement direction (spatial parameters) or the moment when to move (temporal parameters), movement initiation is faster. Cortical neurons selectively modulate their activity in relation to this information. To indicate the end of an instructed delay, motor cortical neurons synchronize significantly their activity at the moment of signal expectancy, often without any detectable modulation in firing rate. The observed increase of the temporal precision of synchrony toward the end of an instructed delay is interpreted to facilitate the efficiency of the motor output, leading to an increase of performance speed. Finally, the chapter shows that the timing of the task is dynamically represented in the temporal structure of significant spike synchrony at the population level, which is shaped by learning and practice. The emergence of significant synchrony becomes more structured; that, is it becomes stronger and more localized in time with practice, in parallel with a decrease in firing rate and an improvement of the behavioral performance. Performance optimization through practice might therefore be achieved by boosting the computational contribution of spike synchrony, allowing an overall reduction in population activity.
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