Pattern of Development of the Callosal Transfer of Visual Information to Cortical Areas 17 and 18 in the Cat

Abstract

The aim of this study was to investigate the development of visual callosal transfer in the normally reared cat. Two- to nine-week-old kittens and adults (used as controls) underwent section of the optic chiasm. Three days later, the animals were placed under anesthesia and paralysed; 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. Out of 1036 recorded neurons, 185 could be activated through the eye contralateral to the explored cortex via callosal transfer. Most of them could also be driven through the ipsilateral eye via the 'direct' geniculo-cortical pathway. For animals aged > or = 2 weeks, virtually all of these units were located at the 17/18 border zone, with a majority in the supragranular layers. When activated through the corpus callosum, they displayed receptive fields located either on the central vertical meridian of the visual field or in the hemifield ipsilateral to the explored cortex. Such extension into the ipsilateral hemifield as well as receptive field disparities of binocular units decreased with age, while spontaneous activity, strength of response, orientation selectivity and ability to respond to slits moving at middle-range velocity increased. The main conclusion is that the transient callosal projections described by anatomists, which are present until 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 from 12 days after birth. However the visual callosal transfer in young animals displays some characteristics which disappear with age.

Spatial Resolution and Contrast Sensitivity of Single Neurons in Area 19 of Split‐chiasm Cats: A Comparison With Primary Visual Cortex

“…This conclusion is comparable to conclusions reached in similar studies on areas 17 and 18 (Berlucchi and Rizzolatti, 1968;Lepore and Guillemot, 1982;Lepore etal., 1992;Milleret, 1994), 19 (Antonini et at., 1985;Guillemot et al, 1993) and the suprasylvian regions (Antonini et al, 1983), attesting to the apparent similarity of receptive field properties for gross spatial features (receptive field size, orientation and directional selectivities, position, etc.) derived from these two routes of activation.…”

Topographic organisation of extrastriate areas in the flying fox: Implications for the evolution of mammalian visual cortex

“…the midline, but that this may not happen to the same degree in areas that are involved primarily with pattern recognition and high-acuity vision. It is unclear whether the biased topographies appear gradually, as a result of postnatal development (e.g., by a process of ''pruning'' of afferents similar to that described for callosal connections; Milleret et al, 1994), or are genetically ''preprogrammed'' by natural selection. Future studies aimed at defining the timetable of postnatal development of the flying fox visual system may be relevant to this question, given that these animals do not start to fly until the second or third postnatal month.…”

Interhemispheric connections between primary visual areas: beyond the midline rule

“…Recordings from split-chiasm cats indicated, in addition to matched orientation between thalamic and callosal afferents, a precise correspondence of retinotopic position, as evidenced by the high degree of receptive field overlap, which increases progressively with normal binocular experience (10). This was compatible with the classic view that callosal connections perform a point-to-point topographic mapping between corresponding loci located in symmetric regions of the two hemispheres, and thereby contribute to the construction of receptive fields in the vicinity of the vertical midline.…”

“…b) Extensive receptive field mapping of the 17/18 border, confirming earlier recordings from the fibers running within the posterior part of the corpus callosum (8), indicates that the callosally connected parts of visual areas include, on each side, the representation of the vertical midline together with a narrow portion of the ipsilateral hemifield (7). c) Recordings from split-chiasm cats, in which the geniculocortical and transcallosal pathways can be differentially activated by visual stimulation of one or the other eye, reveal that ipsi-and contralateral inputs converging onto callosal recipient neurons are closely matched in terms of retinotopic location, size, and orientation selectivity (9,10). d) Taken as a whole, these and other observations suggest that, indeed, callosal projections are functionally equivalent to corticocortical projections, being only longer-reaching (1,6).…”