Date: Mon, December 11, 15:00-(17:00)
Place: Room Dw601, D Block, IIS, The University of Tokyo

Invited Speaker: Prof. Hugh Robinson (University of Cambridge)

Title: Dynamics of gamma oscillations in local networks of the cortex

Abstract:
The gamma oscillation can be observed as an oscillation of local field 
potential (LFP), which reflects synchronous neuronal activity in the cortex 
at frequencies between 30 and 80 Hz. Gamma (γ) oscillation is one of 
the most prominent types of large-scale synchronization in the conscious 
cortex, and is believed to play a role in various neurocognitive functions, 
including feature binding, selective attention, and consciousness (Buzsaki 
& Draguhn, 2004). Gamma oscillations can be produced in cortical slices, 
indicating that they are generated by local cortical circuitry (Whittington 
et al., 1995). In this talk, I will describe our work on the dynamical 
behaviour of two main cortical cell types, regular-spiking (RS) and 
fast-spiking (FS) cells, during gamma oscillations, using the techniques of 
conductance injection and planar multielectrode array recording. FS cells 
form an electrical syncytium through gap (electrical) junctions, and also 
exert mutual inhibition through GABAergic chemical connections (Galarreta & 
Hestrin, 1999; Gibson et al., 1999). We found that single FS cells have a 
type 2 threshold with regular firing starting abruptly at a critical 
frequency in the gamma range (Tateno et al., 2004). To begin to 
characterize the process of synchronization amongst FS neurons during gamma 
firing, we measured the synaptic interaction function (SIF) which describes 
how synaptic input adjusts the phase of spiking. GABAergic input applied 
early in the interspike period delays the subsequent spike, while 
gap-junctional input, applied later, advances it. Analysing the SIF shows 
that the intrinsic biophysical properties of FS neurons and their compound 
synaptic connections allow them to entrain each other's firing over wide 
frequency bands, and that the upper and lower limits of these bands are 
determined almost independently by electrical and GABAergic synapses, 
respectively. In locally-generated gamma oscillations, RS cells are driven 
by recurrent excitatory inputs from other RS cells, and by inhibition from 
FS cells. Using the known firing phase preferences of RS and FS cells in 
vivo, we used conductance injection to reconstitute gamma oscillations in 
RS cells, by synthesizing the network synaptic conductance to individual 
cells (Morita et al., 2006). In order to reproduce the in vivo firing phase 
preferences of RS cells, we found that it was essential to provide strong 
low-latency inputs from FS cells to RS cells, while strong recurrent 
excitatory inputs within the RS cell population were inconsistent with the 
in vivo phase distribution of RS cell firing, unless they were so dispersed 
in time that EPSPs in RS cells are only very weakly γ-modulated. We 
showed that γ-modulation of recurrent excitation could be weakened to 
this expected extent by distributed, stochastic propagation delays, through 
axonal arborizations. RS pyramidal cells are interconnected in small 
subnetworks, each driven by common layer 4 inputs (Yoshimura et al., 2005). 
It seems possible that the smoothing-out of the gamma rhythm, specifically 
in excitatory recurrent input could allow multiple, distinct phase 
preferences of RS cell subnetworks to coexist locally. The release from 
inhibition by FS cells every cycle appears to be the main drive for the 
γ rhythm, which could provide a time reference, or clock, relative to 
which RS spike timing can encode information. The distinct intrinsic 
dynamical properties of RS and FS cells would be well-suited to these 
different roles.

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