Motor representation in the rostral portion of the cat corpus callosum as evidenced by microstimulation

In six chronic cats experiments were carried out to examine the motor effects elicited by microstimulation of the rostral portion of the corpus callosum (CC) which contains fibres interconnecting the motor cortices of the two hemispheres. Serial dorso-ventral penetrations were stereotaxically performed along the rostro-caudal extent of the CC at 0.25-0.5 mm intervals. Motor responses, consisting of very discrete contractions of shoulder, whisker and eyelid muscles, were obtained upon stimulation of about the most rostral 4 mm at intensities lower than 50 microA. At threshold the responses appeared in only one body region and were often unilateral. A light increase in current gave rise to symmetrical bilateral effects. Thresholds were the lowest in the middle of CC and gradually rose towards its dorsal and ventral surfaces. In the course of a penetration an effect once elicited persisted until either the threshold became higher than 50 microA or the ventral edge was reached. All motor effects had thresholds higher than 10 microA. Contractions of shoulder, whisker and eyelid muscles at a threshold lower than 20 microA were obtained in 62.7%, 25.7% and 54.3% of penetrations, respectively. Shoulder and eyelid muscles were represented in the rostral and caudal half of the effective zone, respectively, whereas the representation of whisker muscles overlapped with the other two in the rostral third or the middle part of the effective zone. Single fibres driven by palpation of muscles responding to microstimulation or by movements of the joint mobilized by microstimulation were found at the most rostral sites, whereas units responding to hair displacement over or around the affected muscles were isolated mainly at the most caudal sites. It was inferred that functionally homogeneous callosal fibres are clustered in bundles arranged in dorso-ventrally oriented laminae. The somatotopic representation of the motor effects elicited by microstimulation along the rostro-caudal extent of the effective zone should be the consequence of the serial arrangement of these partially overlapping laminae.