Analysis of bursting activity in phasic and tonic components of dLGN relay cell responses

Abstract

The temporal structure of the cat’s dLGN relay cell responses to a stationary circular spot of different diameter was investigated. Each response to a visual stimulus was divided into phasic and tonic parts and the temporal structure was analyzed in those components of response independently. We were interested in bursts – clusters of high frequency action potentials, defined by the distribution of interspike intervals. Interspike interval distribution in the phasic part of response was less variable than in the tonic part. Moreover, there was a much stronger dependence of variability on the firing rate in the tonic part of response, showing that the higher frequency (and therefore more clustered response) reduces this variability, whereas there was no such effect in the phasic part of response. According to our “critical interval” criteria, there were more within-burst spikes in the response, and the frequency of bursts was higher in the phasic than in the tonic part of response (median 61.6% and 21.3 bursts/s vs. 52.9% and 8.4 bursts/s). The number of within-burst spikes and the burst frequency highly depended on the firing rate showing a higher number of bursts at higher frequencies instead of burst elongation. More than 70% of bursts were short (composed of 2 or 3 spikes) in both parts of response, although bursts in the tonic part were somewhat shorter than in the phasic. There was no clear within-burst interval dependence on burst length or interval number within a burst. Keywords: LGN, burst, phasic response, tonic response