Interhemispheric influences on area 19 of the cat

Abstract

Anatomical studies have shown an extensive network of homotopic and heterotopic interhemispheric connections in area 19 of the cat visual cortex (Segraves and Rosenquist 1982a; 1982b). We have investigated their functional organization by recording visual responses in area 19 of cats following a midsagittal section of the optic chiasm. This operation interrupts all crossed optic fibers coming both from the nasal and the temporal retinae; as a result, each hemisphere receives optic fibers only from the lateral hemiretina of the ipsilateral eye which conveys information from the contralateral visual field. Visual information transmitted to the same hemisphere from the contralateral retina and the ipsilateral visual field must be attributed to an indirect, interhemispheric pathway. We found that a rather high proportion of neurons (31.8%) in area 19 of seven split-chiasm cats responded to visual stimuli presented to the contralateral eye. All neurons receiving this interhemispheric activation were also driven by the ipsilateral eye via an intrahemispheric pathway. The property of binocularity was significantly related to the visuotopic map in that both receptive fields of each binocular neuron adjoined or were in the immediate vicinity of the vertical meridian. Due to the small size of receptive fields in area 19, the contribution of the interhemispheric pathway to the representation of the visual field is rather limited and it is certainly less extensive than that predicted by anatomical studies. The representation of the ipsilateral visual field in area 19 of intact cats, as assessed electrophysiologically, was comparable to that found in split-chiasm cats. Recordings in areas 17-18 of split-chiasm cats showed that the visual field represented through the corpus callosum in these visual areas is certainly not less and probably more, extensive than that found in area 19. The results support the conclusion that the relation to the vertical meridian and the receptive field size can explain the organization of the interhemispheric connections in the visual areas studied so far.

Non-mirror-symmetric patterns of callosal linkages in areas 17 and 18 in cat visual cortex

“…This idea is consistent with previous studies showing that callosal fibers are capable of activating cortical neurons (Choudhury et al, 1965;Berlucchi and Rizzolatti, 1968;Lepore and Guillemot, 1982;Antonini et al, 1985). Moreover, callosal fibers mediate input from both eyes to the representation of ipsilateral fields in suprasylvian cortex in both normal and Siamese cats (Zeki and Fries, 1980;Marzi et al, 1980).…”

“…Although it thus remains unclear how much of the input from the ipsilateral eye to the 17/18 representation of ipsilateral visual fields courses through the callosal commissure in the cat, evidence from experiments using either the split chiasm preparation (Berlucchi and Rizzolatti, 1968;Lepore and Guillemot, 1982;Antonini et al, 1985) or the split callosum preparation (Payne, 1990) indicate that callosal connections in this species do convey input from the contralateral eye to the cortical representation of ipsilateral visual fields. The existence of this callosal input, which largely duplicates the input relayed directly via crossed projections from the temporal retina (see Fig.…”

Function of the corpus callosum in the representation of the visual field in cat visual cortex

“…Following midsagittal section of the optic chiasm, which eliminates crossed inputs from each hemisphere while maintaining ipsilateral projections from temporal retina, some neurons in area 19 and lateral suprasylvian cortex have two components to their receptive field, and the medial borders of the components abut at the visual midline (Antonini et al, 1983;Antonini et al, 1985). Following midsagittal section of the optic chiasm, which eliminates crossed inputs from each hemisphere while maintaining ipsilateral projections from temporal retina, some neurons in area 19 and lateral suprasylvian cortex have two components to their receptive field, and the medial borders of the components abut at the visual midline (Antonini et al, 1983;Antonini et al, 1985).…”

Electrophysiological evidence for interhemispheric transmission of visual information in man.

“…For most cortical areas they are restricted to the projection of the vertical meridian. The receptive field of callosal recipient neurones is made of two areas juxtaposed at the vertical meridian, one provided by the direct LGN input and the other by the callosal input (Berlucchi & Rizzolatti, 1968;Blakemore, Diao, Pu, Wang & Xiao, 1983;Antonini, Berlucchi & Lepore, 1983;Antonini, Di Stefano, Minciacchi & Tassinari, 1985). In addition non-linear interactions between the direct and the callosal input have been described in single cortical neurones of split-chiasm cats (Berardi, Bisti & Maffei, 1984).…”