Two major properties of neurons in the kitten's visual cortex, binocularity and orientation selectivity, are present when the eyes first open, and therefore can be established by genetic instructions alone. However, both of these attributes require visual experience for their maintenance or strengthening; and both can be rapidly modified by unusual kinds of experience. Alternating sequences of cells dominated by one eye, then the other, can be recorded during penetrations through the cortex in binocularly deprived kittens, typical of the 'ocular dominance columns' of the normal adult cat. However, if one eye is deprived by lid-suture, the entire visual cortex becomes strongly dominated by the open eye. Experiments in which each eye saw separately through a transparent neutral density filter or a translucent diffuser showed that this phenomenon is caused not by the reduction in retinal illumination, but by the abolition of contrast in the deprived eye. A study of the retrograde transport of horseradish peroxidase from the visual cortex to the principal laminae of the lateral geniculate nucleus suggested that monocular deprivation from early in life may lead to a gross reduction in the distribution of afferent fibres from the deprived laminae. Previous experiments have found that if a kitten is exposed only to contours of one orientation, its cortical neurons become modified in their distribution of preferred orientations. This phenomenon was re-confirmed in a new study using a rigorously objective method of analysis.