Evidence for plasticity of intrinsic horizontal connections in area 17 of the rat.

The potential for peripheral modification of intrinsic horizontal connections was studied in the primary visual cortex (area 17) of adult, hooded rats that underwent monocular enucleation (ME) or binocular enucleation (BE) at the day of birth. The intrinsic connections were labelled by localized tracer injections at various sites in area 17. The pattern of interhemispheric callosal connections was revealed in the same animals by extensive injections of a distinguishable tracer in the contralateral hemisphere. The results show marked and consistent changes in area 17 intrinsic connections as a result of the neonatal enucleations. Specifically, in contrast to normally reared rats in which intrinsic connections are short ranged and are organized fairly uniformly around the injection site, in ME cases intrinsic connections extend over half the width of area 17, usually targeting a second field within area 17. In BE cases, the injection sites result in the appearance of elongated tongues and patches of label that protrude in different directions from the injection site. The observed modifications of intrinsic connections are clearly related to the abnormalities induced by enucleations in the pattern of callosal connections, such that intrinsic connections tend to form either between callosally connected sites or between a-callosal sites but avoid intermixing the two. The results reveal a high level of plasticity in the developing intrinsic connections. A Hebbian rule of connectivity is proposed as the underlying process guiding the formation of horizontal intrinsic connections in area 17.