Spatially organized spike correlation in cat visual cortex

Inspired by optical recordings from visual cortex which show maps of orientation selectivity, and the finding that very similar patterns of population activity occur when the neurons fire spontaneously [T. Kenet, D. Bibitchkov, M. Tsodyks, A. Grinvald, A. Arieli, Spontaneously emerging cortical representations of visual attributes, Nature 425 (2003) 954–956], we approach the question of how the concept of cortical maps may be related to the concept of temporal coding. To this end we analyzed parallel spike recordings performed using a 10×1010×10 electrode grid covering an area of 3.6mm×3.6mm of cat visual cortex for occurrence of spike correlation.We calculated all possible pairwise correlations between multi-unit activities (MUA) by cross-correlation and extracted significantly correlated pairs using a boot-strap procedure. The MUAs involved in correlated pairs were typically involved in more than a single correlated pair. Using methods of graph theory we found that the whole set of correlated MUAs decomposes into a small number of groups of MUAs that have a high degree of the overlap of mutually correlated pairs.Mapping these groups back onto the spatial arrangement of the recording electrodes revealed that these also correspond to spatially segregated clusters. The spatial scale of this correlation map is in agreement with the scale of orientation tuning maps found by optical imaging.