In order to study the callosal connections between the first and the second somatosensory areas (SI and SII) of primates, experiments were performed in 4 macaque monkeys in which horseradish peroxidase (HRP) was injected in area SI. Two animals received multiple HRP injections (12 injections of $0.5 \, \mu l$ each) either in the trunk or in the hand representation zone of area SI. The other two animals received two injections of HRP, one in the trunk and one in the digit zone of the same area. Two series of alternate sections of the brains were prepared. The first was processed for HRP reaction with a cobalt-enhanced method and the second according to the tetramethylbenzidine (TMB) procedure. In the series of brain sections processed according to the TMB procedure, terminals of callosal fibres and callosal projecting neurones labelled with HRP were found in the contralateral area SII. In the series of brain sections processed for HRP reaction according to the cobalt-enhanced method, labelled callosal neurones in area SII were very rare. The topography of callosal terminals and neurones in area SII depended on the zone of the body map of area SI injected with HRP. Callosal terminals occurred predominantly in layer IV. Few terminals were also present in supragranular layers and in layer VI but were absent in layer V. About 90% of the labelled callosal neurones were distributed in layer III and 10% in layers V and VI. The results confirm previous experiments showing that the hand zone of area SI of primates sends callosal fibres to the homofunctional zone of contralateral area SII and show that both hand and trunk zones of the latter area send back callosal fibres to contralateral area SI.
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Con il contributo del Ministero per i Beni e le Attività Culturali