1. Neural activity was arecorde with two independent electrodes separted by 0.5-2 mm, aligned in parallel, and advanced perpendicular to the surface of the cat auditory cortex. For smaller separations a solid-state multielectrode array with interelectrode distances of 125 mum was used. The difference in recording depths for the two independently movable electrodes was never more than 100 mum; the electrode contacts of the multielctrode array were at the same depth. Thus the correlation studies dominantly explored horizontal interactions. 2. Out of 995 neuron pairs recorded, 478 represented pairs of single units, whereas the other pairs were contaminated with 5-10% misclassified spikes. Only the single-unit pairs were further analyzed. Of those piars, 338 showed a clear correlation peak, and in 329 of these the peak heights were exceeding the Z > 4 significance lwcwl (P < 0.0001). Two hundred fifty-two of the significant correlograms (53% of total) could be attributed to common input; the remaining (16% of total) were indicative of unilateral excitation. For the 181 single-electrode pairs the precentage of unilateral excitation pairs (42%) was about the same as teh percentage of common input paire (38%). For the 297 dual-electrode pairs all but one of the 184 significant correlations were indicative of common input. No correlograms indicative of inhibition were found. 3. The correlograms with clear peaks were classified into three types: narrow (n = 40), mixed (n = 77), and broad (n = 221). Narros and mixed types were with two exceptions found only for single-electrode pairs; broad types were found for singel-and dual-electrode pairs. Narrow-type correlograms were in majority of the unilateral excitation type. Correlograms were calculated both for 1-ms binwidths (50-ms lead/lag time) and for 10-ms binwidth (500-ms lead/lag time). The correlograms were characterized by four parameters; the half width of the central peak, the peak correlation coefficient, the association index, and for unilateral excitation cases also by the effectiveness. 4. Across all three correlogram types the half width of the correlation peaks was significantly smaller for single-electrode pairs (mean, 27 ms) than for dual-electrode pairs (mean, 42 ms). For broad-type correlograms only, the mean half widths were not significantly different between single- and dual-electrode pairs. 5. The correlation coefficients (1-ms bin correlograms) were significantly larger for single-electrode pairs (mean, 0.038) than for dual-electrode pairs (mean, 0.011). The same was found for the 10-ms binwidth correlograms. 6. The association index was not significantly different for single-and dual-electrode pairs. In combination with the significant difference in correlation coefficients, this suggests that firings of single-electrode pairs are better synchronized but that the number of associated spiked remains the same. 7. For the 77 unilateral excitation pairs a presynatic spike produced on average 0.4 postsynaptic spikes, with 61 values < 0.5 and only 16 above that value. The values > 0.5 were consistently found in cases where the postsynaptic neuron was bursting. 8. The parameters of the cross correlogram were studied as a function of recording depth below the dura surface, as a function of electrode separation in characteristic frequency (CF) difference in octaves above the lower CF of the pair, and as a function of age. 9. The correlation coefficient for single-electrode pairs, in both the unilateral excitation and common input cases, decreased with recording depth. In contrast, for dual-electrode pairs, only the association index decreased with depth. 10. There was no change in teh value of the correlation coefficient, rho, nor in the percentage of significant correlations with CF difference in ocatve (range, from 0 to 1.75). The mean rho for common input single-electrode pairs (broad type only, CF difference always 0) was equal to 0.021 +/- 0.019 (mean +/- SD) and was significantly higher than for dual-electrode pairs with zero CF difference (0.01 +/- 0.012). 11. During the middle of the second postnatal week, and continuing up to 50 postnatal dys, the correlation coefficients for unilateral excitation cases were quite high. The mean value was 0.27 (with individual values up to 0.4) and thereafter decreased steadily with age. In fact, correlation coefficient values >0.3 were not observed in kittens of age 90 days and up. For dual-electrode piars both the correlation coefficient and the association index increased with age. This could be explained by an increase in the amount of uncorrelated spontaneous activity in both units as a result of an increas in the number of synaptic contacts. These observations also suggest that once a cortical synapse is formed it is essentailly mature a very short time thereafter, the decrease in correlation coefficients being only the result from the added "noise" from the newly formed synaptic connections.
