Synaptic interactions mediating synchrony and oscillations in primate sensorimotor cortex

The appearance of oscillatory modes of 'gamma' activity in many cortical areas of different species has generated interest in understanding their underlying mechanisms and possible functions. This paper reviews evidence from studies on primate motor cortex showing that oscillatory activity entrains many neurons during periods of exploratory manipulative behavior. These oscillatory episodes synchronize widely spread neurons in sensorimotor cortex bilaterally, including descending corticospinal neurons, as evidenced by correlated modulations in EMG activity. The resulting neural synchronization involves task-related and -unrelated neurons similarly, suggesting that it is more likely to play some global role in attention than mediating any obvious interactions involved in coordinating movements. Intracellular recordings have elucidated the strength and types of synaptic interactions between motor cortical neurons that are involved in both normal and oscillatory activity. Spike-triggered averages (STAs) of intracellular membrane potentials have revealed serial connections in the form of unitary excitatory and inhibitory post-synaptic potentials (EPSPs and IPSPs). More commonly, STAs showed large synchronous excitatory or inhibitory potentials (ASEPs and ASIPs) beginning before the trigger spike and composed of multiple unitary events. ASEPs involved synchronous activity in a larger and more widespread group of presynaptic neurons than ASIPs. During oscillatory episodes synchronized excitatory and inhibitory synaptic potentials occurred in varying proportions. EPSPs evoked by stimulating neighboring cortical sites during the depolarizing phase of spontaneous oscillations showed evidence of transient potentiation. These observations are consistent with several functional hypotheses, but fit best with a possible role in attention or arousal.