Brain Waves: synchronization of tetanically-evoked gamma - revised version

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Introduction

Cortical oscillations at gamma (30-100 Hz) and beta (10-30 Hz) frequencies are implicated in cognitive tasks. Gamma and beta oscillations evoked in the hippocampal slice in vitro by tetanic stimulation can be synchronised with phase lags faster than the conduction delays expected from the distance between the stimulating electrodes. This led Traub to develop an innovative model based on networks of fast synapses using glutamate and GABA as their transmitters. While this theoretical model is feasible on the basis of the known cellular and network properties of the hippocampus, and may well apply under some experimental circumstances, we now have doubts on its application to tetanically evoked gamma rhythms because of: (1) the spatial extent of the gamma focus means that the actual distance between the oscillating populations is much less than the distance between the two stimulating electrodes (typically ~1-2mm), and (2) new evidence and ideas on the mechanism of tetanically-evoked gamma rhythms.

One of our earlier publications illustrates the first problem (spatial extent) with a figure concerning the relationship between gamma rhythms in the CA1 region and those in the subiculum.

 

 

 

The stimulus, 20 pulses at 100Hz delivered to the CA3 end of CA1, evokes population spikes at 31Hz (low gamma range) 0.4mm away. The point of this particular figure was to show that the gamma rhythm is projected to the subiculum, presumably via CA1 pyramidal neuron axons in the alveus. It also shows that, if a second stimulus delivered to the other end of CA1, say at 1.0mm, evoked an oscillation in a similar area of tissue, then there would be no more than 0.2mm separating the oscillating areas, much less than the apparent 1mm. In fact it now looks as though oscillating regions become synchronised if they fuse into one single oscillating focus.

 

 

Recordings not made synchronously. Figure modified from:

S. B. Colling, I. M. Stanford, R. D. Traub, and J. G. R. Jefferys. Limbic gamma rhythms. I. Phase-locked oscillations in hippocampal CA1 and subiculum. J.Neurophysiol. 80:155-161, 1998.

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