PATTERN OF DEVELOPMENT OF THE CALLOSAL TRANSFER OF VISUAL INFORMATIONTO CORTICAL AREAS 17 AND 18 IN THE CAT

The aim of this study was to investigate the development of visual cal losal transfer in the normally reared cat. Two- to nine-week-old kitte ns and adults (used as controls) underwent section of the optic chiasm . Three days later, the animals were placed under anaesthesia and para lysed; unit activities were recorded from visual cortical areas 17 and 18 and from the white matter in one hemisphere. The units were tested for their responses to visual stimulation of each eye successively. O ut of 1036 recorded neurons, 185 could be activated through the eye co ntralateral to the explored cortex via callosal transfer. Most of them could also be driven through the ipsilateral eye via the 'direct' gen iculo-cortical pathway. For animals aged greater than or equal to 2 we eks, virtually all of these units were located at the 17/18 border zon e, with a majority in the supragranular layers. When activated through the corpus callosum, they displayed receptive fields located either o n the central vertical meridian of the visual field or in the hemifiel d ipsilateral to the explored cortex. Such extension into the ipsilate ral hemifield as well as receptive field disparities of binocular unit s decreased with age, while spontaneous activity, strength of response , orientation selectivity and ability to respond to slits moving at mi ddle-range velocity increased. The main conclusion is that the transie nt callosal projections described by anatomists, which are present unt il 3 months of age, do not achieve supraliminar synaptic contacts with parts of areas 17 and 18 other than the 17/18 border zone, at least f rom 12 days after birth. However the visual callosal transfer in young animals displays some characteristics which disappear with age.