Early prenatal development of cortical surface responses to visual stimuli in sheep

Cortical surface responses to electrical stimulation of the optic nerve were recorded in sheep fetuses between 55 and 124 days of gestation (term 145 days). The cortical response obtained in the youngest fetuses was a long-latency surface-positive wave, followed by a small negativity. Later during development the negativity increased in amplitude and dominated the response. This early developmental pattern of the visual response was similar to that of the somesthetic response. On basis of findings on the maturation of structure and function in the somesthetic cortex the genesis of the visual response is discussed. The hypothesis is put forward that, at its earliest appearance, the positive wave of the visual response is predominantly reflecting synchronous depolarization of developing corticopetal nerve terminals and later arises from summation of excitatory postsynaptic potentials of deep-seated cortical neurons. The surface negativity of the response is considered to reflect excitatory postsynaptic activation of neural elements in the superficial neocortex. At about 98 days of fetal age a short-latency component appeared in the response. It is suggested that this component is mediated through the specific optic pathways, whereas the earlier-appearing long-latency response is suggested to be mediated via the unspecific visual projection. During the succeeding development the two components apparently coalesced to build up a positive-negative response containing all elements of the adult form.