Survival of early monocular deprivation effects in cortical cells of kittens following prolonged dark rearing.

To study whether early visual experience survives the absence of consequent visual stimulation during development, experimental kittens were reared in the dark for 5-13.5 months following monocular deprivation (MD) periods of 2-11 weeks which were initiated at the time of natural eye opening (MD-dark). For comparison, experimental kittens, normally reared after equivalent MD periods (MD-bino.), were also studied. Cats raised with permanent MD, dark-reared cats and normal cats, served as controls. The proportion of responsive cells was considerably reduced by the dark-rearing. It was especially reduced for the MD-dark kittens following monocular deprivation limited to the first postnatal month (58.3% responsive cells) in comparison to the equivalent group of MD-bino. kittens (80.5%). This is also in keeping with the diminution in cortical responsiveness obtained in the kittens which were dark-reared from birth (55.5%). The responsiveness level found in the normal control cats was 87.3%. It was found that the duration of the MD period prior to the dark-rearing period was directly related to the ocular dominance (OD) distribution of cortical cells. The susceptibility period to MD in both MD-dark and MD-bino. groups ends at approximately 3 months of age; the lower limit for the susceptibility period is at approximately 1-2 weeks after natural eye opening. The main period of sensitivity within this period of time is the first 4 postnatal weeks following natural eye opening. It is concluded that once the effect of monocular deprivation has been established, it will survive for the rest of the cat's life, even under conditions of complete absence of consequent visual stimulation. Furthermore, a certain degree of consolidation of the MD effect takes place in the light (i.e. in MD-bino. cats) despite their return to normal binocular vision. A somewhat opposite occurrence takes place in the dark (in MD-dark cats) with a tendency for masking of the MD effect previously induced in the light to be found.