RECURRENT EXCITATORY POSTSYNAPTIC POTENTIALS INDUCED BY SYNCHRONIZED FAST CORTICAL OSCILLATIONS

Gamma frequency (about 20-70 Hz) oscillations occur during novel senso ry stimulation, with tight synchrony over distances of at least 7 mm, Synchronization in the visual system has been proposed to reflect coac tivation of different parts of the visual field by a single spatially extended object. We have shown that intracortical mechanisms, includin g spike doublet firing by interneurons, can account for tight long-ran ge synchrony. Here we show that synchronous gamma oscillations in two sites also can cause long-lasting (>1 hr) potentiation of recurrent ex citatory synapses, Synchronous oscillations lasting >400 ms in hippoca mpal area CA1 are associated with an increase in both excitatory posts ynaptic potential (EPSP) amplitude and action potential afterhyperpola rization size, The resulting EPSPs stabilize and synchronize a prolong ed beta frequency (about 10-25 Hz) oscillation, The changes in EPSP si ze are not expressed during non-oscillatory behavior but reappear duri ng subsequent gamma-oscillatory events, We propose that oscillation-in duced EPSPs serve as a substrate for memory, whose expression either e nhances or blocks synchronization of spatially separated sites, This p henomenon thus provides a dynamical mechanism for storage and retrieva l of stimulus-specific neuronal assemblies.