1. We recorded extracellularly from 1045 neurones in area 17 of seven monocularly deprived kittens and we employed careful sampling techniques to examine the effects of removing the signals from the non‐deprived eye on the proportion of cells responding to stimulation of the deprived eye. 2. Monocular deprivation itself produced a pronounced over‐all change in the ocular dominance of neurones in favour of the experienced eye, but both between animals and even between different samples of cells in individual animals there were marked variations in the magnitude of the effect. 3. Monocular deprivation starting immediately at the time of natural eye opening and lasting for several weeks reduced to about 10% the proportion of cortical neurones influenced through the deprived eye. Enucleation of the experienced eye did not then produce a significant increase in the proportion of cells responsive to the deprived eye. 4. Monocular deprivation lasting 3 days or more and beginning at 5 weeks of age, after normal binocular vision, also shifted ocular dominance substantially: 11‐27% of neurones responded through the deprived eye. Enucleation of the experienced eye or topical anaesthesia of the optic nerve resulted in a substantial recovery of input from the deprived eye: up to 78% of the cells responded. 5. In such animals the majority of neurones that recovered input from the deprived eye had receptive field properties qualitatively similar to those of normal cortical cells. Recording in a single penetration both before and after enucleation (or optic nerve block) suggested that the orientation preferences of cells with recovered input followed the same sequence as was originally present for the non‐deprived eye. 6. Recovery of input occurred in all cortical laminae in which cells were recorded, even in layer IV, and mainly took the form of an expansion of already existing clusters of cells driven by the deprived eye. 7. Spontaneous activity tended to increase after enucleation. 8. The results indicate that monocular deprivation after a period of normal binocular vision leaves subthreshold but functionally organized synaptic input from the deprived eye on cortical cells, which is revealed when activity arising in the retina of the non‐deprived eye is abolished.
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