Effects of dark‐rearing on the development of area 18 of the cat's visual cortex.

1. We recorded extracellularly from 420 single units in area 18 in visually inexperienced kittens aged 7 days and dark‐reared kittens aged between 3 and 12 weeks and from 60 single units in area 17 in dark‐reared kittens aged 5 and 11 weeks. 2. Visual deprivation generally depressed the maturation of area 18, although some features were affected more than others and certain developmental improvements still occurred. The percentage of visually responsive units in area 18 increased from 40% in 7‐day‐old kittens to about 75% in dark‐reared animals 10‐12 weeks. At each age a proportion of cells was orientation biased (between 15 and 45%) and these neurones appeared to be arranged in a crude columnar fashion. However, dark‐rearing, from birth, prevented the development of a significant proportion of orientation‐selective cells in area 18; no more than 5% of neurones were orientation‐selective at any age. We found no major bias in the over‐all distribution of preferred orientations of cells in area 18 in dark‐reared kittens. 3. Simple cells, which are found in area 18 even in very young, visually inexperienced kittens, persisted after dark‐rearing, although most retained immature properties. Relatively few complex cells were found in area 18 in visually deprived animals. 4. The majority of neurones in area 18 of dark‐reared kittens were binocularly driven, many equally well by either eye; evidence for regional variation in ocular dominance (indicative of a columnar pattern) was found in these deprived animals. 5. A laminar analysis in area 18 showed that percentages of non‐oriented and orientation‐biased cells changed little, if at all, in lower laminae (IV, V, and VI) but increased substantially in upper layers (above layer IV) in the absence of visual stimulation, over the first 12 post‐natal weeks. 6. A comparison of the effects of dark‐rearing on areas 17 and 18 indicates that the normal development of visual responsiveness and specific receptive field properties is suppressed in both areas during the first 12 post‐natal weeks. It is possible that area 17 has a greater degree of orientation selectivity than area 18 in young visually deprived kittens and this may reflect a difference in the type of afferent inputs.