NEURONAL RESPONSES TO VESTIBULAR AND CALLOSAL STIMULATION IN THE ANTERIOR SUPRASYLVIAN GYRUS OF THE CAT

Neuronal responses to electrical stimulation at the horizontal ampulla (HA), vestibular nerve (at the windows) and corpus callosum (CC) were investigated in neurons in the anterior suprasylvian gyrus of the cat. The field potentials to HA stimulation had short latency: 2.9 +/- 0.3 (mean +/- SD) ms from the stimulus to the onset and 5.6 +/- 1.9 ms to the peak. The focus of the evoked potentials was located in the anterior suprasylvian (ASS) gyrus or near the ASS sulcus. HA stimulation activated 6 neurons out of 674 examined, with the mean latency of 4.3 +/- 1.1 ms. Of these 6, four neurons also responded to window stimulation. Fifty-six neurons responded to window stimulation with the mean latency of 6.1 +/- 2.4 ms. The mean latency for CC stimulation was 1.9 +/- 0.9 ms (n = 76). Four neurons responded to CC stimulation antidromically (mean = 0.9 +/- 0.3 ms) and one of them also responded orthodromically. The convergence of CC inputs in relation to HA or window stimulation was examined. One (17%) of the 6 HA-activated cells responded to CC stimulation, compared with 8 (14%) of the 56 neurons activated by window stimulation. The other 612 neurons did not respond to either HA or window stimulation, and 80 (13%) of the 612 responded to CC stimulation. Therefore, it is concluded that neurons in the ASS gyrus received callosal input equally irrespective of the presence or absence of responses to ampulla or window stimulation. WGA-HRP was injected in the ASS gyrus to identify the passing callosal fibers in the CC. Fibers from the ASS area passed at the rostral third of the CC. The present results indicate that the ASS area received vestibular projection with short latency, but responses of this projection did not seem to be very strong, at least from the present unit study, to HA stimulation. Discussion was made on the poor neuronal responses to electrical HA stimulation in comparison with previous studies. Also consideration was made on neuronal activity to CC stimulation.