Experiments have been carried out in order to study in the first somato-sensory area (SI) the relationships between the topography of callosal neurones and the physiological map of the trunk. Horseradish peroxidase (HRP) was injected into the posterior sygmoid gyrus of one hemisphere of 20 adult cats. On 11 of them, the contralateral homotopic cortex was explored with tungsten microelectrodes on the day after HRP delivery. Neurones retrogradely labelled by HRP (callosal neurones) were grouped within two regions located, respectively, in the rostral and caudal zones of the exteroceptive trunk map. In both regions, callosal neurones were found mainly in layer III and many also in layer VI, especially following large HRP delivery in the contralateral area SI. Callosal neurones located in layer III were mostly pyramidal, those of layer VI, pyramidal and non-pyramidal. The rostral group of callosal neurones overlaps the representation of the dorsal body midline and the caudal group that of the ventral body midline. The cortex intervening between these two zones has few or no callosal cells and receives the projections from the lateral surface of the trunk. According to the electrophysiological data, neurones located within the two midline zones have a bilateral receptive field, straddling either the back or the ventral surface of the trunk. In the other zones of the trunk and proximal limb map receptive fields were contralateral. It is concluded that callosal connectivity is strictly related to the bilaterality of receptive fields.
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Con il contributo del Ministero per i Beni e le Attività Culturali