A long-lasting change in ocular dominance of kitten striate neurons induced by reversible unilateral blockade of tonic retinal discharges.

The effect of reversible blockade of tonic retinal discharges upon the excitability of binocular visual cortical neurons was studied in kittens during the "critical period". Following the direct application of a small amount of blocking agent to the unilateral optic nerve, the responsiveness of single cortical neurons to ipsilateral eye (non-blocked side) stimulation was enhanced while that to contralateral stimulation was suppressed. Changes started soon after blocking and were long lasting, over 1 hour, compared with the duration blocking (7 min) as measured at the level of the later geniculate nucleus. This effect was found to be age-dependent: in older kittens which were out of the critical period, results were ambiguous and in young adult cats the same treatment induced no obvious changes. The results favor the idea of binocular competition at postsynaptic sites of the geniculo-cortical projection during the critical period. Furthermore, tonic afferent activity in the visual system is proposed as one of the primary carriers of effects of the environmental manipulation of visual inputs in the developing visual cortex. This is consistent with the previous notion that tonic afferent activity is indispensable for maintenance of existing synaptic contacts in the matured brain.