MONAURAL SPECTRAL CONTRAST MECHANISM FOR NEURAL SENSITIVITY TO SOUND DIRECTION IN THE MEDIAL GENICULATE-BODY OF THE CAT

Central auditory neurons vary in sound direction sensitivity. Insensit ive cells discharge well to all sound source directions, whereas sensi tive cells discharge well to certain directions and poorly to others. High-frequency neurons in the latter group are differentially sensitiv e to binaural and monaural directional cues present in broadband noise (BBN). Binaural directional (ED) cells require binaural stimulation f or directional sensitivity; monaural directional(MD) cells are sensiti ve to the direction of monaural stimuli. A model of MD sensitivity was tested using single-unit responses. The model assumes that MD cells d erive directional sensitivity from pinna-derived spectral cues (head r elated transfer function, HRTF). This assumption was supported by the similarity of effects that pinna orientation produces on locations of HRTF patterns and on locations of MD cell azimuth function peaks and n ulls. According to the model, MD neurons derive directional sensitivit y by use of excitatory/inhibitory antagonism to compare sound pressure in excitatory and inhibitory frequency domains, and a variety of obse rvations are consistent with this idea. I) Frequency response areas of MD cells consist of excitatory and inhibitory domains. MD cells exhib ited a higher proportion of multiple excitatory domains and narrower e xcitatory frequency domains than ED cells, features that may reflect s pecialization for spectral-dependent directional sensitivity. 2) MD se nsitivity requires sound pressure in excitatory and inhibitory frequen cy domains. Directional sensitivity was evaluated using stimuli with f requency components confined exclusively to excitatory domains (E-only stimuli) or distributed in both excitatory and inhibitory domains (E/ I stimuli). Each of 13 MD cells that were tested exhibited higher dire ctional sensitivity to E/I than to E-only stimuli; most MD cells exhib ited relatively low directional sensitivity when frequency components were confined exclusively to excitatory domains. 3) MD sensitivity der ives from excitatory/inhibitory antagonism (spectral inhibition). Comp arison of responses to best frequency and E/I stimuli provided strong support for spectral inhibition. Although spectral facilitation concei vably could contribute to directional sensitivity with direction-depen dent increases in response, the results did not show this to be a sign ificant factor. 4) Direction-dependent decreases in responsiveness to BBN reflect increased sound pressure in inhibitory relative to excitat ory frequency domains. This idea was tested using the strength of two- tone inhibition, which is a function of stimulus levels in inhibitory relative to excitatory frequency domains. The finding that two-tone in hibition was stronger at directions where BBN responses were minimal t han at directions where they were maximal supports the model.