Discrimination thresholds for interaural-time differences and interaural-level differences in naïve listeners: Sex differences and learning.

The two primary cues to sound-source location on the horizontal plane are interaural time differences (ITDs) and interaural level differences (ILDs). Here we asked whether the ability to discriminate small changes in each of these interaural cues differs between the sexes. We tested one group of males (n = 43) and females (n = 94) on ITD discrimination at 0.5 kHz and a separate group of males (n = 80) and females (n = 166) on ILD discrimination at 4 kHz. None of the participants had any prior experience with psychoacoustic tasks. Testing of each participant was completed in a single testing session of 4-5 blocks of 60 trials. For ILD discrimination, the overall mean threshold, as well as the mean threshold for each block, was statistically significantly lower for males than for females. Despite that, males and females learned at an equal rate over the course of testing. For ITD discrimination, in contrast, thresholds did not differ significantly between the sexes for the overall mean or for any block. There also was no statistically significant learning across blocks for either sex. For both tasks and both sexes, the individual thresholds spanned a wide range. The presence of a statistically significant sex difference and learning for ILD but not for ITD discrimination, along with a larger effect size for ILD than for ITD discrimination, suggests that the factors responsible for these outcomes acted upon an ILD-specific neural pathway, and not upon an ITD-specific pathway, nor any pathway common to the two cues. Because the ILD and ITD specific pathways are most separable initially, the factors associated with sex and learning may have acted upon the ILD-specific pathway at an early stage.