Perceptual Learning and Binaural ProcessingPublic Deposited
Humans determine the horizontal position of sound sources based on two cues: interaural time differences (ITDs) and interaural level differences (ILDs). Consistent with other evidence that these two cues are processed by separate neural mechanisms, Wright and Fitzgerald (2001) observed markedly different learning patterns in listeners who were trained on ITD discrimination with a low-frequency tone compared to those trained on ILD discrimination with a high-frequency tone. Employing the same training regimen, I examined the influence of training on three other binaural discrimination conditions: ITD with a high-frequency sinusoidly amplitude modulated (SAM) tone, ILD with the same SAM tone, and ILD with a low-frequency tone. Training on each condition produced a different learning pattern. The present results, in combination with the previous ones, reveal interesting characteristics of the human binaural system, at least in terms of how that system is influenced by the current multi-hour training paradigm. First, multi-hour practice led to improvements on ILD, but not ITD, discrimination regardless of stimulus frequency or stimulus type, suggesting that in terms of modifiability, the neural mechanisms underlying performance with each cue are similar across frequency regions and tone types, but are different between the two cues even with the same stimulus. Second, training ITD discrimination with a low-frequency, but not with a high-frequency, stimulus induced learning on ILD discrimination with the trained stimulus, suggesting that ITD and ILD sensitivity involve partially shared neural processes at low but not high frequencies. Third, although ILD-discrimination training consistently led to learning, the pattern of the improvements differed between low and high frequencies, as well as between pure and SAM tones at the same frequency, suggesting that ILD processing differs at least partially across frequency regions and between simple and complex stimuli. The characteristics of the binaural system revealed by the present training paradigm confirm as well as extend our current understanding of the mechanisms underlying human sound localization.