Neuronal synchrony in monkey striate cortex: interocular signal flow and dependency on spike rates

A hypercolumn of monkey striate cortex was studied with an array of 30 closely spaced microelectrodes. Prominent broad peaks appearing in spike train correlograms are considered here. These were not due to shared stimulation, were mostly 30 to 100 ms wide, and were presumably the consequence of intraretinal lateral interactions. The correlogram peak areas were found to be predictable from the products of the spike rates, to which they were proportional. One can conclude that the correlation occurs before the overall reduction of spike rates from retina to cortex takes place. Furthermore, when a neurone dominated by one eye was stimulated via that eye, the correlogram formed with a neurone dominated by the other eye showed a displaced peak, indicating that excitation traveled from the well-responding to the unresponsive neurone in about 10 ms. When a left-eye stimulus was delivered, the same pair of neurones had a correlogram with a reversed peak displacement. This effect was only observed in layers IVb and c, indicating that in these layers the paths from the two eyes to a given cell are of unequal length, whereas in other layers, cells receive input from both eyes via similar connections differing only in strength.