Synaptic interactions mediating synchrony and oscillations in primate sensorimotor cortex

The appearance of oscillatory modes of 'gamma' activity in many cortical ar eas of different species has generated interest in understanding their unde rlying 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 osc illatory episodes synchronize widely spread neurons in sensorimotor cortex bilaterally, including descending corticospinal neurons, as evidenced by co rrelated 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 obv ious interactions involved in coordinating movements. Intracellular recordi ngs have elucidated the strength and types of synaptic interactions between motor cortical neurons that are involved in both normal and oscillatory ac tivity. Spike-triggered averages (STAs) of intracellular membrane potential s have revealed serial connections in the form of unitary excitatory and in hibitory post-synaptic potentials (EPSPs and IPSPs). More commonly, STAs sh owed large synchronous excitatory or inhibitory potentials (ASEPs and ASIPs ) beginning before the trigger spike and composed of multiple unitary event s. ASEPs involved synchronous activity in a larger and more widespread grou p of presynaptic neurons than ASIPs. During oscillatory episodes synchroniz ed excitatory and inhibitory synaptic potentials occurred in varying propor tions. EPSPs evoked by stimulating neighboring cortical sites during the de polarizing phase of spontaneous oscillations showed evidence of transient p otentiation. These observations are consistent with several functional hypo theses, but fit best with a possible role in attention or arousal. (C) 2000 Elsevier Science Ltd. Published by Editions scientifiques et medicales Els evier SAS.