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3 2 10 1 Eccentricity lower figure shows the combined receptive fields in relation to the vertical meridian. (Adapted from Lepore et al., 1986.) For the ectosylvian cortex the receptive fields are located at the same position in regards to the vertical meridian (not shown; see Ptito et al., 1987).

corpus callosum is the unification of both hemifields, thus ensuring the continuity of images through the midline. The cortical areas receiving (afferent) or sending (efferent) callosal inputs are those where the vertical meridian is represented. Electrophysiological studies have continuously supported the ''midline fusion hypothesis" (reviewed by Lepore et al., 1986). By using the split-chiasm preparation, it is possible to record single units in the cortical areas where there is a representation of the vertical midline such as the 17/18 border (the suprasylvian cortex and the ectosylvian cortex) (see Figure 6.10). With this preparation, any unit which could still be activated through each eye (binocular cell) receives an ipsilateral geniculocortical input and a contralateral callosal input. In our laboratory we have previously shown that about 33% of the cells situated in the border region of areas 17/18 were binocularly activated in a split-chiasm preparation (Lepore and Guillemot, 1982). The receptive properties (RF) of these binocular units (such as orientation and directional selectivity, preferred velocity, size, and elevation in visual space) did not differ for each eye. The position of the median edge of the RF determined for each eye was close to the midline abutting or straddling the vertical meridian (VM). Figure 6.10a illustrates the relationship of callosal recipient neurons recorded at the 17/18 border with the vertical meridian. All binocular units have RFs whose medial borders for each eye either abut or cross the VM. The midline fusion hypothesis seems therefore to be confirmed for the primary visual cortex. This hypothesis also seems to hold true for higher-order areas, as demonstrated in the lateral suprasylvian cortex (Antonini et al., 1983). Single-unit recordings performed in this area in split-chiasm cats showed that although many units had very large RFs, the medial border of the RFs for each eye touched or straddled the VM (Figure 6.10b). In the ectosylvian cortex (EVA), which is very remote from the classically defined visual areas and has heavy reciprocal connections with the suprasylvian cortex, the same midline rule can be applied. We have found (Ptito, Tas-sinari, and Antonini, 1987) that visual cells recorded in EVA of split-chiasm cats had large RFs whose median borders also touched or straddled the VM. These results indicate that all cortical visual areas that are callosally connected have their RFs close to the vertical midline (touching or crossing it) and offer evidence that one of the principal functions of the corpus callosum is to unite the two visual hemifields to ensure continuity across the midline. Directly recording the activity of callosal axons also confirmed this hypothesis, since most of the RFs obtained were situated in the vicinity of the VM (Lepore et al., 1986). Studies carried out on other sensory systems (somatosensory and auditory) indicated that both the somatosensory and auditory callosal systems are closely associated with midline functions (reviewed by Lepore et al., 1994).

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