D. Tollin - Neural sensitivity to interaural level differences determines virtual acoustic space minimum audible angles for single neurons in the lateral superior olive
Because the peripheral receptors of the ear have no mechanism to directly sense sound location on their own (unlike the topographic organization of the retina), location must be computed at more central levels. This makes sound localization a fascinating neuro-computational problem, particularly from a developmental perspective. 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°. 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 acoustical head related transfer functions, the virtual acoustic space technique was used to manipulate source azimuth in the physiological experiments. For each neuron signal detection theory was used to compute the smallest increment in azimuth necessary to discriminate that change based on discharge rate and associated response variability. Minimum neural MAAs were 2.3° for midline sources (median = 4.5°, n = 32 neurons). The good neural acuity for spatial location will be explained in terms of the changes in the frequency-specific acoustic ILD cue with changes in source location along with the underlying sensitivity of the LSO neurons to the acoustical ILD cue itself. The results demonstrate that LSO neurons can signal changes in sound azimuth that match or exceeded behavioral capabilities.