1. 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 > 1 00 mum. 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. A subgroup of neurons showed firing behavior close to Poisson. The deviation from Poisson behavior was largest for units recorded in kittens younger than 30 days and in adult cats for units recorded in supragranular layers. 6. A significant effect of age on the incidence of bursts with specific spike lengths was found; the number of bursts in a 900 s time window increased with age and so did firing rate. For mean firing rates below 1 spike / s there were more bursts for neurons in kittens < 30 days old than for those recorded in adult cats. 7. There was a significant effect of recording depth on the incidence of burst firing in adult cats; more superficial layers showed a higher incidence of bursting than deeper layer neurons especially for the lower firing rates. 8. Deconvolution of the cross-correlogram with the auto-correlogram of the trigger-unit resulted for 30% of the pairs in a reduction of the half-amplitude width of the cross-correlation peak. In the remaining 70% the halfwidth remained the same. In 75% of the cases that showed a reduction in half-amplitude width, the reduction was on average equal to 30%. Deconvolution had no effect on the half-amplitude width of the narrow, unilateral excitation, correlograms. Thus only some of the variance in the width of cross-correlograms can be attributed to the autocorrelation structure of the unit's firing behavior. This can be interpreted as strong support for a cortical origin of the broad cross-correlograms. 9. For unilateral excitation pairs the amplitude of the synaptic impulse response, estimated by the deconvolution procedure, was linearly related to the effectiveness of the synaptic connection as calculated from the number of spikes in the correlation peak. 10. Auto- and cross-correlograms, calculated separately for isolated-spike sections and for burst-sections of the spike trains, showed that rhythmic or oscillatory correlograms could be found both for the isolated-spike sections and for the burst sections. This suggests that the mechanisms that determine rhythmic firing and bursting in neocortical neurons of the ketamine anesthetized cat are largely independent. 11. In 46.6% of the cross-correlograms the halfwidth of the central peak for the isolated spike correlograms was smaller than for the all-spike correlograms. In 43.6% no change was detected between isolated-spike and all-spike correlograms, and in 9.8% a slight increase was found which was most likely due to statistical fluctuations as a result of the smaller number of spikes in the isolated-spike sections. Secondary effects and common input effects disappeared in some isolated-spike cross-correlograms but the unilateral excitation effects always remained present. For common-input pairs the halfwidth in isolated-spike correlograms was smaller [+/- 27 (SD) ms] than for all-spike correlograms (+/- 35 ms) and about equal to that for nonbursting pair correlograms (+/- 25 ms). 12. Bursting had no specific effect on the value of the correlation coefficient rho; no significant differences were found for rho in isolated-spike correlograms and burst-section correlograms. The age dependence of rho for single electrode pairs was the same for isolated spike correlations and burst-section correlations; both decreased with age.
