NEURAL RESPONSES TO FREE-FIELD AUDITORY-STIMULATION IN THE SUPERIOR COLLICULUS OF THE WALLABY (MACROPUS-EUGENII)

Auditory responses to free-field broad band stimulation from different directions were recorded from clusters of neurones in the superior co lliculus (SC) of the anaesthetized tammar wallaby. The auditory respon ses were found approximately 2 mm beneath the first recording of visua lly evoked responses in the superficial layers, the vast majority bein g solely auditory in nature; only one recording responded to both audi tory and visual stimulation. Responses to suprathreshold intensities d isplayed sharp spatial tuning to sound in the contralateral hemifield. Those from the rostral pole of the SC disclosed a preference for audi tory stimuli in the azimuthal anterior field, whereas those in the cau dal SC preferentially responded to sounds in the posterior field. A co ntinuum of directionally tuned responses was seen along the rostrocaud al axis of the SC so that the entire azimuthal contralateral auditory hemifield was represented in the SC. Furthermore, tight Spatial alignm ent was evident between the best position of the visual responses in t he superficial layers in azimuth and the peak angle of the auditory re sponse in the deeper layers.