Neural sensitivity to interaural level differences determines virtual acoustic space minimum audible angles for single neurons in the lateral superior olive.

Abstract

The minimum audible angle (MAA), the smallest angle separating two sound sources that can be reliably discriminated, is a psychophysical measure of spatial acuity. In humans and cats, MAAs for tone and noise stimuli range from 1–5 deg. For high-frequency (>1.5 kHz) tones the predominant cue for azimuth is the interaural level difference (ILD). Neurophysiologically, ILDs are first encoded in the lateral superior olive (LSO). Here, we examined the ability of LSO neurons in cats to signal changes in the azimuth of noise sources. Using measurements of head related transfer functions, the virtual acoustic space technique was used to manipulate source azimuth in the physiological experiments. For each neuron detection theory was used to compute the smallest increment in azimuth necessary to discriminate that change based on discharge rate and response variability. Minimum neural MAAs were 2.3 deg for midline sources (median = 4.5 deg, n = 32 neurons). The good neural acuity for spatial location will be explained in terms of the changes in the acoustic ILD cue with changes in source location along with the underlying sensitivity of the LSO neurons to the ILD cue. LSO neurons can signal changes in sound azimuth that match or exceeded behavioral capabilities. [NIH DC006865.]