Separate mechanisms control spike numbers and inter-spike intervals in transient responses of cat auditory cortex neurons

In the anesthetized cat, some cortical auditory neurons discharge a train of up to 5 spikes in response to the onset of a characteristic frequency tone pulse. This report provides the first description of the inter-spike intervals (ISIs) in these responses. The ISIs were typically close to 2.0 ms in length, and, as indexed by the standard deviation of the interval length, were very regular. Except at threshold levels of stimulation, mean ISIs were relatively insensitive to both tone amplitude and repetition rate. This was true even over ranges of those variables that exerted dramatic effects on spike numbers and first spike latency. These data suggest that the relative timing of discharges within the spike burst is controlled by a mechanism which is separable from that which determines the number of spikes in them. The brevity of the ISIs suggest that they may be a means of enhancing the salience of the transient response against a background of spontaneous discharges.