Horseradish peroxidase (HRP) was injected within the proximal limb and trunk representation zones of the first somatosensory area (SI) of 16 cats. The tangential and laminar distributions of retrogradely labelled neurones (callosal neurones) of the contralateral homotopic cortex were studied. This cortex was explored with microelectrodes on the day after HRP delivery to relate the distribution of callosal neurones to the electrophysiological map of the trunk. 2. Callosal neurones were found in the contralateral SI area mainly in layer III, but also many in layer VI, especially following large HRP injections, and very few in the other layers. Callosal neurones of layer III were mostly pyramidal, those of layer VI pyramidal and non-pyramidal. Many neurones were intensely stained by HRP, and cell details, such as fine dendritic branchings, spines, and axon collaterals, could be seen. 3. Callosal neurones are grouped within two regions located, respectively, in the rostral and caudal parts of the exteroceptive trunk map. The rostral region overlaps the representation of the dorsal midline (cytoarchitectonic field 3b) and the second one that of the ventral body midline (cytoarchitectonic field 2). The cortex intermediate between these two fields contains rare callosal cells and receives afferences from the lateral trunk surface. Few or no callosal cells were found within the proximal limb zones. Neurones recorded from the two midline zones have bilateral receptive fields straddling either the back or the ventral surface of the trunk. 4. It is concluded that the interhemispheric fusion between the two hemibody representations in areas SI is brought about by the mutual callosal links which the two midline zones entertain with their contralateral homologues.
