Precise spike synchronization in monkey motor cortex involved in preparation for movement

It is commonly accepted that perceptually and behaviorally relevant events are reflected in changes of activity in largely distributed neuronal populations. However, it is much less clear how these populations organize dynamically to cope with momentary computational demands. In order to decipher the dynamic organization of cortical ensembles, the activities of up to seven neurons of the primary motor cortex were recorded simultaneously. A monkey was trained to perform a pointing task in six directions. During each trial, two signals were presented consecutively. The first signal provided prior information about the movement direction, whereas the second called for the execution of that movement. Dynamic interactions between the activity of simultaneously recorded neurons were studied by analyzing individual epochs of synchronized firing ("unitary events"). Unitary events were defined as synchronizations which occur significantly more often than expected by chance on the basis of the neurons' firing rates. The aim of the study was to describe the relationships between synchronization dynamics and changes in activity of the same neurons during the preparation and execution of voluntary movements. The data show that even neurons which were classified, on the basis of the change in their firing rate, to be functionally involved in different processes (e.g., preparation or execution related, different directional tuning) synchronized their spiking activity significantly. These findings indicate that the synchronization of individual action potentials and the modulation of the firing rate may serve different and complementary functions underlying the cortical organization of cognitive motor processes.