INFLUENCE OF CALLOSAL ACTIVITY ON UNITS IN THE AUDITORY-CORTEX OF FERRET (MUSTELA-PUTORIUS)

1. Callosal neurons in primary auditory cortex project heavily to regions of the contralateral homotopic held that are excited by stimulation of one or both ears and minimally to regions that are excited by stimulation of one ear and inhibited by stimulation of the other ear. The intent of this study is to begin assessing the function of this extensive intercortical projection in determining the response properties of single units in the recipient auditory cortex. 2. Callosal neurons in the right auditory cortex of barbiturate-anesthetized ferrets were stimulated electrically through a pair of monopolar microelectrodes that were separated by 2-3 mm and whose uninsulated tips were located 500 mu m below the pial surface. Electrical stimuli consisted of a single 100-mu s pulse delivered at a fixed intensity and temporal relationship to the onset of each of a set of monaural or binaural acoustic stimuli. Pulse amplitude was almost always <350 mu A. Acoustic stimuli consisted of 100-ms best-frequency tones delivered monaurally or binaurally at 1-s intervals. The influence exerted by electrical stimulation of the right auditory cortex on responses of single units isolated in the left auditory cortex was assessed by comparing responses evoked by the same acoustic stimuli delivered alone and when accompanied by the electrical stimulation. In addition, the latencies of discharges evoked by the electrical stimuli delivered alone were assessed. Three categories of influence were observed: suppression, excitation, and a mixed class consisting of both suppression and excitation. 3. Suppression was the most commonly observed influence of callosal input on acoustically evoked responses. Depending on the strength of the acoustic stimuli and electrical pulse and the temporal relationship between the two kinds of stimulation, responses to acoustic stimuli could be completely suppressed or reduced to a smaller version of the control responses. The duration of the inhibitory influence was often >100 ms and the suppression was frequently effective at latencies of 2-4 ms. 4. Although unit activity was commonly excited by electrical stimulation of the right auditory cortex, facilitation of acoustically evoked responses was infrequently observed. Apparent facilitation usually was the result of the summation of acoustically and electrically evoked discharges. We present evidence that the summation of monaurally evoked excitation in some predominantly binaural cells might involve the transfer of excitatory influences via the corpus callosum. We describe a population of units that is characterized by being driven by electrical stimulation of the right cortex, having essentially no spontaneous activity, and being unresponsive to acoustic stimulation. This population was the largest class of units observed in this study. The mean latency of the initial evoked discharges was 5.4 ms, a value that is very similar to the mean latencies of initial discharges evoked in the visual and somatosensory cortices by electrical stimulation of the homotopic contralateral field. We discuss this population of units in relationship to a population of corticothalamic units previously described in the somatosensory cortex. 5. The mixed class of interaction consisted of both excitation and inhibition. Electrical stimulation of the contralateral cortex both evoked discharges directly and suppressed acoustically driven activity. When an electrical pulse was delivered concurrently with the acoustic stimulus, discharges evoked at a short latency by the electrical stimulus were followed by a profound inhibition of the longer-latency response to the acoustic stimulus.