THE RESPONSES OF NEURONS IN SUBDIVISIONS OF THE INFERIOR COLLICULUS OF CATS TO TONAL, NOISE AND VOCAL STIMULI

The aim of this study was to gain information from anesthetized cats a bout the differential coding properties of neurons in the three major subdivisions of the inferior colliculus: the central (CNIC) and extern al (EN) nuclei and dorsal cortex (DC). Stimuli were presented in the f ree field from a speaker facing the contralateral pinna. For each unit , the characteristic frequency (CF, where threshold was lowest) was de termined, and impulse rates to CF tone bursts, noise bursts and four f eline vocal stimuli were measured as a function of increasing sound pr essure level (rate/level functions). Peristimulus-time histograms were computed for responses to all stimuli. Sustained firing patterns to C F stimuli were observed for 81% of units in CNIC, for 50% of units in EN and 27% of units in DC. Sustained discharges were evoked by noise i n 78-100% of units in all regions, and by at least one vocal stimulus in 86% of units in CNIC, 82% in EN and 55% in DC. In the CNIC, non-mon otonic rate/level functions to CF stimuli were more common (41%) than either monotonic or plateau functions, whereas the reverse was the cas e with noise and vocal stimuli. Non-monotonic functions were uncommon to any stimulus in EN and DC (21-24%). Vocal stimuli were more effecti ve in terms of higher firing rates than noise or CF stimuli in 27% of units in CNIC, 82% in EN and 72% in DC. There were no units that respo nded exclusively to one vocal stimulus, but a high proportion of units in EN responded strongly to broad band stimuli, and some of these sho wed clear preferences for one vocal stimulus over others.