Horizontal interactions between visual cortical neurones studied by cross‐correlation analysis in the cat.

1. To explore the functional significance of horizontal neural connections in the extent of a ‘hypercolumn’ of the cat visual cortex, we carried out cross‐correlation analysis of spike trains recorded simultaneously from a pair of neurones separated horizontally by less than 1 mm. 2. Significantly correlated firings, which were found in sixty‐eight pairs of cells among 327 pairs analysed, were classified into three types on the basis of their functional implications: (1) excitatory interactions, (2) inhibitory interactions and (3) common inputs to both neurones of a pair from other sources. 3. Of these three types, common inputs were encountered most frequently. Excitatory interactions were always accompanied by common inputs. Inhibitory interactions were observed least frequently. 4. The proportion of cell pairs with correlated firings was high in pairs with a horizontal separation of less than 200 microns and decreased markedly with a horizontal separation of more than 400 microns. 5. Regarding laminar locations of cells, common inputs and excitatory interactions were often observed in layers II + III and V, whereas laminar bias was not seen in inhibitory interactions. 6. With respect to difference in orientation preference between two cells, all the three types of correlations were observed, mostly in cell pairs with a difference of less than 45 deg. In particular, common inputs and excitatory interactions were often seen in cell pairs with matched orientation preferences, but inhibitory interactions were found mostly in those with slightly different orientation preferences. In addition, common inputs and excitatory interactions tended to be found between cells with the same eye preference. 7. These results suggest that horizontal functional interactions exist mainly in a range of up to 400 microns as far as the extent of a hypercolumn of the visual cortex is concerned, and these interactions operate effectively between cortical cells with similar receptive field properties except for inhibitory interactions.