Age-dependent emergence of oscillatory signal flow between the primary and secondary visual cortices in rat brain...

Developmental changes in dynamics of signal propagation between the primary (Oc1) and secondary visual cortex (Oc2) were investigated by using optical recording methods with voltage-sensitive dyes. Propagating oscillatory optical responses were evoked by our previously reported procedure, and were recorded on stimulation to white matter of Oc1 in rat visual cortex slices. In immature slices, evoked signals spread out from the stimulation site by way of deep layers, but were restricted largely to Oc1. In mature slices, however, evoked signals spread upward from the stimulation site at first, and then spread out along layer II/III, finally to penetrate Oc2. More remarkably, after this initial signal was attenuated, oscillatory responses emerged and spread back from Oc2 to Oc1 by way of layer II/III, suggesting that the origin of backpropagating oscillation is located in Oc2. The initial forward component was dependent on both N-methyl-D-aspartate (NMDA) and non-NMDA receptors, and the subsequent backward components were dependent only on NMDA receptors. These results suggest that the extent of corticocoritcal signal propagation within the visual cortex grows wider horizontally during maturation, so that information interchange may become easier between the Oc1 and Oc2.