Early post‐natal development of neuronal function in the kitten's visual cortex: a laminar analysis.

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Abstract
The normal postnatal development of visual cortical functions was studied by recording extracellularly from 612 single neurons in the striate and parastriate cortex of anesthetized and paralyzed kittens (6 to 24 days old). Analyses were made of laminar differences in the developmental trends of receptive field properties such as orientation specificity and spatial organization of on and off zones. At the beginning of the 2nd postnatal wk the majority of neurons (76%) only respond to light off (unimodal off neurons). Only later does the frequency of occurrence of unimodal on neurons and of bimodal or multimodal neurons (with spatially segregated on and off zones arranged side by side) increase so that, by the middle of the 4th wk, about equal numbers of these 3 receptive field types are found. The proportion of on-off neurons (with spatially coincident on and off zones) remains low (between 9% and 12%) during the early psot-natal period. In layers 4 and 6 of areas 17 and 18 the frequency of occurrence of visual neurons is qute normal even in the youngest kittens, whereas the probability of recording neurons in layers 2/3 and 5 in kittens less than 14 days old is remarkably low and only gradually improves up to the middle of the 4th wk. A rudimentary order in the spatial arrangement of orientation-specific neurons and ocular dominance distribution is observed even in very young kittens. This order improves rapidly and reaches adult levels during the 4th postnatal wk. In visually inexperienced kittens, on average 11% of all responsive neurons are selective for the orientation of elongated visual stimuli and 58% are biased. The proportion of orientation-selective cells begins to increase rapidly about 2 days after lid opening and proportions of orientation-selective cells similar to that in the adult are reached by the end of the the 4th postnatal wk. Orientation-selective neurons in kittens less than 10 days old are only found in layers 4 and 6 and the lower part of layer 3. In layers 2/3 and 5 they are first seen in larger proportions by the beginning of the 3rd postnatal wk. During the 1st postnatal mo., the time course of the functional development of visual cortical neurons depends on receptive field type and on intracortical location. Implications for the development of intracortical connectivity and the generation of simple receptive fields are discussed. Possible mechanisms for, and functional implications of the late generation of complex fields are suggested.