Maturation of evoked response of the visual cortex in the postnatal rabbit

Abstract

1. 1. The response of the visual cortex to electrical stimulation of the optic nerve in rabbits is evident at birth (when the visual path to the cortex is still unmyelinated), although at long latency and with marked fatigability. The cortical light response does not appear until the 7th postnatal day. 2. 2. A progressive change in form of the response to electrical stimulation occurs between 1 and 21 days. The response first contains all elements of its adult' from at days. Thereafter the amplitude of the initial surface-positive phase continues to increase and the following negative and later phases appear to diminish until the adult form is achieved at 21 days. After the 10th day cortical light and electrical responses follow a parallel course of maturation. 3. 3. Collicular responses are surface-negative from birth and reverse in polarity as a probe electrode is thrust into the colliculus. Thus they approximate those characteristics of the form of the adult response. Collicular latency shortens much less than cortical latency as maturation progresses. 4. 4. In two relationships myelination may play a significant role in the maturation of the responses. The optic nerve commences to myelinate as the light response a ppear int eh cortex, and the appearance of the intial posituive phase of the response relates chronologically to the period of most rapid myelination in the internal capsule and external sagittal stratum. 5. 5. Even though differentiation of cortical elements appears to commence in the lower layers and to proceed upwards during the period when the response polarity is principally negative, there is insufficient evidence to establish what change in cortical differentiation relates to the appearance at 11 to 12 days of the initial positive phase of the cortical response.