EFFECT OF STIMULATION ON BURST FIRING IN CAT PRIMARY AUDITORY-CORTEX

1. Neural activity was recorded extracellularly with two independent m icroelectrodes aligned in parallel and advanced perpendicular to isofr equency sheets in cat primary auditory cortex. Multiunit activity was separated into single-unit spike trains using a maximum variance spike sorting algorithm. Only units that demonstrated a high quality of sor ting and a minimum spontaneous firing rate of 0.2 spikes/s were consid ered for analysis. The primary aim of this study was to describe the e ffect of periodic click train and broadband noise stimulation on short -time-scale (less than or equal to 50 ms) bursts in the spike trains o f single auditory cortical units and to determine whether stimulation influenced the occurrence, spike count, and/or temporal structure of b urst firing relative to a spontaneous baseline. 2. Extracellular recor dings were made in 20 juvenile and adult cats from 69 single auditory cortical units during click train stimulation and silence, and from 30 single units during noise stimulation and in silence. In an additiona l 15 single units the effect of both click train and noise stimulation was investigated. The incidence, spike count, and temporal structure of short-time-scale burst firing in the first 100 ms following stimulu s presentation was compared with burst firing in the period starting 5 00 ms after stimulus presentation and with spontaneous burst firing. I n addition, the serial dependence of interspike intervals within a bur st was tested during periods of stimulation. 3. Burst firing was prese nt in the stimulation, poststimulation, and spontaneous conditions. Lo nger bursts (consisting of greater than or equal to 3 spikes) were mor e commonly observed in the poststimulation and spontaneous conditions than in the stimulation condition. This effect was most pronounced dur ing click stimulation. A period of elevated firing activity was presen t in a subset of units 0.5-1.5 s after stimulus presentation, indicati ng prolonged effects of stimulation on single-unit firing behavior. 4. For both stimuli, the proportion of single-unit responses composed of bursts was significantly greater in poststimulation and spontaneous p eriods than during stimulation. Burst rate was higher in post-click-tr ain stimulation and spontaneous periods than during periods of click s timulation. The isolated spike rate was significantly higher during pe riods of noise and click stimulation than in the poststimulation and s pontaneous periods. 5. An examination of the autocorrelograms and high er-order interspike interval histograms of single-unit responses durin g click train stimulation indicated that 25% of single-unit spike trai ns contained an excess of brief first-order intervals and 14% of spike trains contained a shortage of long higher-order interspike intervals relative to a spontaneous baseline. During noise stimulation, 10% of single-unit responses contained an excess of short intervals relative to baseline. Interspike intervals of short-duration bursts were not se rially dependent during periods of stimulation. 6. A comparison of the autocorrelograms and higher-order interval histograms of single-unit responses in the poststimulation and spontaneous conditions indicated that 20% of single-unit spike trains contained an excess of short firs t-, second-, and third-order intervals following stimulation. This sub group of single units could not be distinguished on the basis of the a ge of the animal or the depth at which the recording was made. 7. The low incidence of burst filing during stimulation opposes the view that bursts serve as a mechanism to emphasize or amplify particular stimul us-related responses in the presence of ongoing spontaneous activity i n the primary auditory cortex. Moreover, there is little evidence to s upport the notion that brief bursts represent neural codes, because in traburst intervals are not serially dependent. It is suggested that py ramidal burst firing may be an effective way to evoke postsynaptic fir ing in inhibitory interneurons and subsequently reduce or inhibit firi ng activity in neocortex in a self-stabilizing process.