Development of local horizontal interactions in cat visual cortex studied by cross-correlation analysis.

1. To explore the functional development of local horizontal interactions in the primary visual cortex, we carried out cross-correlation analysis of spike trains recorded simultaneously from a pair of neurons separated horizontally by < 1 mm, in kittens ranging in age postnatally from the second to ninth week. 2. Significantly correlated firings were found in 87 pairs of cells among 423 pairs analyzed, and 77 pairs of them were classified into three types on the basis of their functional implications: 1) excitatory interactions, 2) inhibitory interactions, and 3) common inputs to both neurons of the pair from other sources. 3. Common inputs and excitatory linkage were observed even in animals at the second postnatal week, whereas inhibitory linkage was not seen before the fourth week of age. The probability of observing common inputs and inhibitory linkage increased during development, whereas that of excitatory linkage tended to decrease after the sixth week of age. 4. Significant correlation was rarely seen in pairs with horizontal separation > 600 microns in the seventh to ninth week. In the fourth to sixth week of age, however, approximately 30% of the pairs with horizontal separation between 600 and 800 microns were significantly correlated. 5. Cells that were not sensitive for orientation or that lacked a visual response were observed mainly before the sixth week of age. These cells tended to receive excitatory effects from and share common inputs with other orientation-sensitive cells that were located within the horizontal distance of 400 microns. 6. All three types of correlations were observed mostly in cell pairs with preferred orientations that differed < 45 degrees at all ages studied. In the fourth to sixth week, however, the similarity of orientation preference was not strict, and correlated firings were observed even in a pair with orthogonal orientation preferences; whereas in the seventh to ninth week the tuning became sharper. 7. These results suggest that functional interactions between cortical neurons exist but are much less specific with respect to horizontal separation and orientation preference before the sixth week of age, and these interactions become more specific so as to operate between neurons with similar orientation preferences in more restricted region after the seventh to ninth week of age and thus in adulthood.