[Motor responses produced by microstimulation of the rostral region of the corpus callosum].

In six chronic cats microstimulation experiments were carried out in the rostral portion of the corpus callosum (CC) where fibres run which interconnect 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. Stimulation was delivered at 100 micron steps and, when an effect was present, threshold was determined. 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 slight increase in current gave rise to symmetrical bilateral effects. Motor effects had thresholds higher than 10 microA. Thresholds were the lowest in the middle portion of CC and gradually rose towards it dorsal and ventral ends. Shoulder and eyelid muscles were represented in the rostral and caudal half of effective zone, respectively, whereas the representation of whisker muscles overlapped with the previous two in the rostral third or the middle part of the effective zone.