Dynamics of striate cortical activity in the alert macaque: II. Fast time scale synchronization.

Synchronous neuronal activity with millisecond precision has been postulated to contribute to the process of visual perceptual grouping. We have performed multineuron recordings in striate cortex of two alert macaque monkeys to determine if the occurrence and properties of this form of activity are consistent with the minimal requirements of this theory. We find that neuronal synchronization with millisecond precision is a prevalent and robust feature of stimulus-evoked activity in striate cortex. It occurs among adjacent cells recorded by the same electrode (<120 microm), among cells recorded at separate but nearby sites (300-400 microm) and between cells recorded at locations separated by 3-4 mm. The magnitude and probability of synchronous firing is inversely related to the spatial separation between the cells and it occurs within and between groups of cells that are both tuned and untuned for stimulus orientation and direction. Among those tuned for orientation, cell pairs separated by <400 microm showed no clear dependence of correlated firing on orientation preference. The occurrence of gamma-band (20-70 Hz) oscillations in the cellular firing patterns was a strong predictor of synchronous firing at each of the spatial scales. Nearly 90% of the cell pairs showing significant correlation also showed oscillatory firing in one or both cells of the pair. These results are consistent with some, but not all, of the previous reports of synchronous activity in striate cortex of both cat and primates. The similarities in the properties of synchronous oscillations in the monkey and cat suggest that this form of neuronal activity is a general property of mammalian striate cortex. The relation between correlation and oscillation suggests that neuronal rhythmicity is an important mechanism contributing to synchronization.