Brief monocular deprivation leaves subthreshold synaptic input on neurones of the cat's visual cortex.

1. We have examined neurones in area 17 of monocularly deprived kittens for subthreshold input in response to visual stimulation through the deprived eye during reversible abolition of activity from the non-deprived eye and during increased excitability of cortical neurones induced by ionophoresis of DL-homocysteic acid (DL-H).2. After two or three days of monocular deprivation, beginning five weeks post-natally, most cortical neurones were dominated by the non-deprived eye. From a sample of forty-three neurones, from three kittens, driven exclusively by the non-deprived eye, only 16% developed responses to stimulation through the deprived eye as a result of reversible pressure blinding of the non-deprived eye. The responses through the deprived eye during pressure blinding usually developed over a period of several minutes and were always transitory: in no cases did the response persist for more than a few minutes after the return of the normal response through the non-deprived eye. Occasionally cells became responsive through the deprived eye during a short period of heightened general excitability after the release of pressure.3. Ionophoretic application of DL-H usually increased the spontaneous activity of cortical neurones. From two kittens monocularly deprived for three days during the fourth or fifth week of life, out of a sample of sixteen cortical neurones initially responsive only through the non-deprived eye, 63% showed responses to visual stimulation through the deprived eye during DL-H application. From a kitten monocularly deprived for 12 d from post-natal day 38, 28% of eighteen initially monocular neurones developed responses through the deprived eye during DL-H application.4. Preliminary results from intracellular recording showed apparently monocular neurones with excitatory input from the deprived eye producing subthreshold synaptic activity in response to moving bars of the same orientation that gave a suprathreshold response through the non-deprived eye.