MONAURAL AND BINAURAL SPECTRUM LEVEL CUES IN THE FERRET - ACOUSTICS AND THE NEURAL REPRESENTATION OF AUDITORY SPACE

1. The role of the structures of the outer ear in producing monaural and binaural spectral cues to sound location was examined acoustically in the ferret. A probe microphone was introduced across the wall of the external auditory canal and its responses to digitally constructed wideband signals were recorded for a large number of free field locations. 2. In the intact animal the patterns of both monaural and binaural cues were asymmetrical for horizontal locations about the interaural axis. For anterior sound locations the monaural transformations demonstrated relative gains at middle and high frequencies and a location-dependent frequency notch. Changing elevation resulted in variations in the corner frequencies of these spectral features. Additionally, there was greater front-back asymmetry in the binaural spectral cues for locations in lateral space when compared with locations near the midline. 3. Surgical removal of the pinna and concha (pinnectomy) eliminated all the major front-back asymmetrical features in the horizon monaural and binaural spectral transformations as well as the elevation-dependent variations in the monaural spectra. Thus the residual transformations were ambiguous for sound locations in lateral space, resulting in ''cones of confusion'' centered on the interaural axis. 4. These cues were reflected in the topographic representation of auditory space in the deeper layers of the superior colliculus (SC). Previous studies have shown that spatial tuning at near threshold sound levers is based on monaural pinna cues, whereas binaural inputs are utilized at higher levels that stimulate both ears. In the intact ferret we examined statistically the topography of the representation of sound azimuth for near-threshold and suprathreshold stimuli and the alignment of the auditory and visual representations in the SC. The distributions of auditory best positions within the SC for near- and suprathreshold stimulus levels were statistically indistinguishable, suggesting that both monaural and binaural cues are integrated in this neural representation of space. 5. Pinnectomy resulted in a large increase in the number of auditory units that responded best to two distinct locations in space. One lobe of the response was tuned appropriately in terms of the position of the unit within the SC, demonstrating that the residual acoustical cues are sufficient for the construction of a topographic representation of auditory space. However, the second lobe of the response was generally directed toward a posterior region of space, thereby producing an ambiguous representation. 6. The best positions of both lobes of the directional responses could be predicted from the topographic arrangement of interaural level difference (ILD) sensitivity previously demonstrated within the SC and from the azimuthal variation in residual cue values. In the intact animal any ambiguity in a single-frequency or white noise ILD cue is probably reserved by integrating the monaural and binaural cues across frequency, thus resulting in an unambiguous representation of auditory space.