Prediction of human's ability in sound localization based on the statistical properties of spike trains along the brainstem auditory pathway

Abstract

The minimum audible angle test which is commonly used for evaluating human localization ability depends on interaural time delay, interaural level differences, and spectral information about the acoustic stimulus. These physical properties are estimated at different stages along the brainstem auditory pathway. The interaural time delay is ambiguous at certain frequencies, thus confusion arises as to the source of these frequencies. It is assumed that in a typical minimum audible angle experiment, the brain acts as an unbiased optimal estimator and thus the human performance can be obtained by deriving optimal lower bounds. Two types of lower bounds are tested: the Cramer-Rao and the Barankin. The Cramer-Rao bound only takes into account the approximation of the true direction of the stimulus; the Barankin bound considers other possible directions that arise from the ambiguous phase information. These lower bounds are derived at the output of the auditory nerve and of the superior olivary complex where binaural cues are estimated. An agreement between human experimental data was obtained only when the superior olivary complex was considered and the Barankin lower bound was used. This result suggests that sound localization is estimated by the auditory nuclei using ambiguous binaural information.

Figure 1

MAA experimental results as a function of frequency for a reference azimuth of 0°, (redrawn from [1]).

Figure 2

A sample HRTF gain in dB (a) and phase in degrees (b) as a function of azimuth and frequency for 0° elevation.

Figure 3

Prediction of MAA according to BLB derivation with a single ambiguity direction for all frequencies and directions based on the AN response (color online).

Figure 4

Normalized MAA according to CRLB and BLB as a function of frequency for a reference azimuth of 0° at AN.

Figure 5

A schematic diagram of the binaural processing in the brainstem auditory pathway.

Figure 6

Predicted MAA according to CRLB (a) and BLB (b) as a function of frequency for a reference azimuth of 0° at the SOC level with either single EE cell or single EI cell.

Figure 7

Predicted MAA according to BLB as a function of frequency for reference azimuth of 0° at the SOC level along with Mills' experimental data.