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Figure 1.1. Schematic representation of layers of origin and type of callosal connections. In general, callosal axons originate from pyramidal neurons located above layer IV; fewer axons originate below layer IV. Most callosal axons are excitatory (+ and forked termination), although some of them probably terminate on inhibitory neurons (— and small triangles). A few nonpyramidal inhibitory neurons appear to establish callosal connections. For further explanations, see the text.

Figure 1.2. Schematic representation of the organization of callosal connections. An area (shaded) is strongly callosally connected (thick arrows) to its contralateral corresponding area (homotopic connections) as well as, more weakly (thin arrows), to noncorresponding areas (heterotopic connections). Connections are reciprocal and roughly symmetrical. However, the reciprocal connections between areas at different levels of cortical processing do not necessarily originate from the same layers (see the text). Nonsymmetrical connections can be generated experimentally, and they also must be expected to exist in anatomically or functionally asymmetric brains.

Figure 1.2. Schematic representation of the organization of callosal connections. An area (shaded) is strongly callosally connected (thick arrows) to its contralateral corresponding area (homotopic connections) as well as, more weakly (thin arrows), to noncorresponding areas (heterotopic connections). Connections are reciprocal and roughly symmetrical. However, the reciprocal connections between areas at different levels of cortical processing do not necessarily originate from the same layers (see the text). Nonsymmetrical connections can be generated experimentally, and they also must be expected to exist in anatomically or functionally asymmetric brains.

(Innocenti and Fiore, 1976; Caminiti, Innocenti, and Manzoni, 1979; Innocenti, 1980; Kennedy, Dehay, and Bullier, 1986). This suggested that the maps generated by thalamic afferents in the somatosensory areas are re-scaled in the corpus callosum by virtue of the distribution of its neurons of origin (Innocenti, 1978) (see Figure 1.4). In addition, callosal connections might ''filter out'' other aspects of the cortical maps and distribute them to specific cortical sites. Indeed, both the neurons of origin of callosal projections and the callosal terminal axons were occasionally reported to be distributed in discrete "columnar" patches (reviewed and discussed in Innocenti, 1986; see also Olavarria and Abel, 1996).

The corpus callosum implements neuronal operations that lead to integrate or to lateralize the activity of the

Figure 1.3. Schematized pattern of callosal connectivity among areas, four of which are represented for each hemisphere. As exemplified by the two shaded ones, each area forms callosal connections with a restricted set of contralateral areas; homo-topic callosal connections are between corresponding areas, heterotopic connections between noncorresponding areas. The latter normally include areas that also receive intrahemispheric connections. For further explanations, see the text.

Figure 1.3. Schematized pattern of callosal connectivity among areas, four of which are represented for each hemisphere. As exemplified by the two shaded ones, each area forms callosal connections with a restricted set of contralateral areas; homo-topic callosal connections are between corresponding areas, heterotopic connections between noncorresponding areas. The latter normally include areas that also receive intrahemispheric connections. For further explanations, see the text.

cerebral hemispheres. The structure of callosal connections provides precise indications of what some of the operations might be. In particular, the selective distribution of callosally projecting neurons and of their axons mentioned above suggested that callosal connections perform specific transformations of the cortical maps and therefore, in a broad sense, operations of a computational kind. This was stressed by the recent analysis of the morphology of individual callosal axons based on new anterograde axonal tracers and computer methods for the three-dimensional reconstruction and analysis (Houzel, Milleret, and Innocenti, 1994; Bressoud and Innocenti, 1999) (see Figure 1.5). At this new level of resolution, it became clear that by virtue of their geometry, individual callosal axons perform at least three types of operations; these are in the spatial (mapping), intensity (weighting), and temporal (synchronization or desynchronization) domains (Figure 1.6), as detailed below.

It should be stressed that the morphology of callosal axons is not unique in the cerebral cortex. On the contrary, there is consensus that except for their long trajectories, callosal axons are representative of cortico-cortical inter-areal and possibly intra-areal axons (Hubel and Wiesel, 1967; Innocenti, 1986; Kennedy et al., 1991). Thus, most or all the concepts that are coming into focus in structural or functional studies of the corpus callosum can probably be generalized to other cortical connections.

Figure 1.4. Callosally projecting neurons (solid triangles) are usually unevenly distributed in a cortical area. As shown in the bottom panels, this results in the primary visual areas (17 and/or 18), in differential magnification, in the corpus

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Figure 1.4. Callosally projecting neurons (solid triangles) are usually unevenly distributed in a cortical area. As shown in the bottom panels, this results in the primary visual areas (17 and/or 18), in differential magnification, in the corpus

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