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

1. Callosal neurons in primary auditory cortex project heavily to regi ons of the contralateral homotopic held that are excited by stimulatio n of one or both ears and minimally to regions that are excited by sti mulation of one ear and inhibited by stimulation of the other ear. The intent of this study is to begin assessing the function of this exten sive intercortical projection in determining the response properties o f single units in the recipient auditory cortex.2. Callosal neurons in the right auditory cortex of barbiturate-anesthetized ferrets were st imulated 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 relationshi p to the onset of each of a set of monaural or binaural acoustic stimu li. Pulse amplitude was almost always <350 mu A. Acoustic stimuli cons isted of 100-ms best-frequency tones delivered monaurally or binaurall y at 1-s intervals. The influence exerted by electrical stimulation of the right auditory cortex on responses of single units isolated in th e left auditory cortex was assessed by comparing responses evoked by t he same acoustic stimuli delivered alone and when accompanied by the e lectrical stimulation. In addition, the latencies of discharges evoked by the electrical stimuli delivered alone were assessed. Three catego ries of influence were observed: suppression, excitation, and a mixed class consisting of both suppression and excitation. 3. Suppression wa s the most commonly observed influence of callosal input on acoustical ly evoked responses. Depending on the strength of the acoustic stimuli and electrical pulse and the temporal relationship between the two ki nds of stimulation, responses to acoustic stimuli could be completely suppressed or reduced to a smaller version of the control responses. T he duration of the inhibitory influence was often >100 ms and the supp ression was frequently effective at latencies of 2-4 ms. 4. Although u nit activity was commonly excited by electrical stimulation of the rig ht 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 excit atory 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 b eing unresponsive to acoustic stimulation. This population was the lar gest class of units observed in this study. The mean latency of the in itial evoked discharges was 5.4 ms, a value that is very similar to th e mean latencies of initial discharges evoked in the visual and somato sensory cortices by electrical stimulation of the homotopic contralate ral field. We discuss this population of units in relationship to a po pulation of corticothalamic units previously described in the somatose nsory cortex. 5. The mixed class of interaction consisted of both exci tation and inhibition. Electrical stimulation of the contralateral cor tex both evoked discharges directly and suppressed acoustically driven activity. When an electrical pulse was delivered concurrently with th e acoustic stimulus, discharges evoked at a short latency by the elect rical stimulus were followed by a profound inhibition of the longer-la tency response to the acoustic stimulus.