Postnatal development of corticotectal neurons in the kitten striate cortex: an electrophysiological study.

In cats ranging in age from the second week to adult, the postnatal functional development of striate cortical neurons projecting to the superior colliculus (SC) was studied by observing their visual responses, axonal conduction velocities and spontaneous activities. One-hundred-and-fifty-seven cells were identified as projecting to the SC (C-T cells) on the basis of antidromic activation from SC. At the second week of age, 57% of the C-T cells had receptive fields of the complex type and the others were visually unresponsive. C-T cells with 'special complex' properties, which are characteristic of the adult C-T cells, first appeared at the third week and made up 19% of the total. The proportion of such C-T cells increased very rapidly and attained the adult-like value at the fourth week. The spontaneous activity of the C-T cells was very sparse or absent at the second week and increased thereafter with age. The proportion of cells with the high level of spontaneous activity found in the adult increased with the same time-course as did the special complex cells. The axonal conduction velocity of C-T cells was slower than 2 m/s until the third week. C-T cells with the faster velocities, suggestive of axonal myelination, appeared at the fourth week and thereafter the average value increased very rapidly until the weeks 8-9. These results indicate that the most remarkable functional development of C-T cells may occur around the fourth week of age. By comparison with previous data on postnatal development of neurons in the cortex and SC, we suggest that C-T cells may be one of the early-maturing groups of cells in the striate cortex and their maturation may exert a crucial influence on the development of visual response properties of SC neurons.