Neural activity was recorded with two independent electrodes separated by 0.5-2 mm, aligned in parallel, and advanced perpendicular to the surface of the cat auditory cortex. Because the experiments were part of a study into laminar interaction the difference in recording depths for the two independently movable electrodes was never > 100 microns. Multi-unit activity on each electrode was separated on-line into single-unit spike-trains with a maximum variance spike sorting algorithm. Off-line controls on the quality of the spike-train separation were routinely performed. The first aim of this study was to describe the age dependence of spontaneous burst firing and to explore if and how it could be explained by age dependent changes in firing rate. The second aim was to investigate a potential layer dependence on burst firing. The third aim was to describe the effect of burst-removal procedures on the shape, strength, and width of the cross-correlogram and to investigate whether an age dependence in burst firing might account for the previously reported age dependence in correlation strengths. 2. Recordings were made from 237 single units from primary auditory cortex in nine adult cats and from 67 units in seven kittens age 10-52 days. The incidence of burst firing as a function of firing rate, age and depth of recording and unit characteristic frequency was investigated. In addition the effect of burst firing on the strength and width of the central peak in 471 neural pair correlograms was analyzed. 3. Burst firing could be distinguished at many different time scales; bursts lasting of the order of 10 s contained bursts with durations of the order of 1 s, which in turn contained bursts of 30-50-ms duration. The analysis in this paper was restricted to the short-duration bursts. 4. Burst firing on the short-time scale of 50 ms was characterized by relatively well defined intervals between the first two spikes (3-15 ms) followed by higher-order intervals with large spread (range 4-50 ms) but with increasing modal interval value. The typical adult five-spike burst template featured spikes at 0, 3.3, 14.6, 27.2, and 34.8 ms. Burst with fewer spikes showed larger intervals between the first three spikes. 5. The probability of occurrence of isolated spikes, pairs, triplets, etc. showed a power-law dependence on firing rate with a coefficient that was significantly lower than expected under Poisson firing conditions.(ABSTRACT TRUNCATED AT 400 WORDS)
