BURST FIRING AND MODULATION OF FUNCTIONAL CONNECTIVITY IN CAT STRIATECORTEX

We studied the influences of the temporal firing patterns of presynapt ic cat visual cortical cells on spike generation by postsynaptic cells . Multiunit recordings were dissected into the activity of individual neurons within the recorded group. Cross-correlation analysis was then used to identify directly coupled neuron pairs. The 22 multiunit grou ps recorded typically showed activity from two to six neurons, each co ntaining between 1 and 15 neuron pairs. From a total of 241 neuron pai rs, 91 (38%) had a shifted cross-correlation peak, which indicated a p ossible direct connection. Only two multiunit groups contained no shif ted peaks. Burst activity, defined by groups of two or more spikes wit h intervals of less than or equal to 8 ms from any single neuron, was analyzed in terms of its effectiveness in eliciting a spike from a sec ond, driven neuron. We defined effectiveness as the percentage of spik es from the driving neuron that are time related to spikes of the driv en neuron. The effectiveness of bursts (of any length) in eliciting a time-related response spike averaged 18.53% across all measurements as compared with the effectiveness of single spikes, which averaged 9.53 %. Longer bursts were more effective than shorter ones. Effectiveness was reduced with spatially nonoptimal, as opposed to optimal, stimuli. The effectiveness of both bursts and single spikes decreased by the s ame amount across measurements with nonoptimal orientations, spatial f requencies and contrasts. At similar firing rates and burst lengths, t he decrease was more pronounced for nonoptimal orientations than for l ower contrasts, suggesting the existence of a mechanism that reduces e ffectiveness at nonoptimal orientations. These results support the hyp othesis that neural information can be emphasized via instantaneous ra te coding chat is not preserved over long intervals or over trials. Th is is consistent with the integrate and fire model, where bursts parti cipate in temporal integration.