The rapid decline in interaural-time-difference sensitivity for pure tones can be explained by peripheral filtering

Area of Science:

• Auditory Neuroscience
• Psychoacoustics

Background:

• Interaural time difference (ITD) is crucial for horizontal sound localization.
• ITD sensitivity is limited to frequencies below approximately 1400 Hz, with a sharp decline above 700 Hz.
• The physiological basis for this rapid decline in ITD sensitivity remains unclear.

Approach:

• Investigated ITD sensitivity in 16 normal-hearing listeners across frequencies from 900-1500 Hz and sensation levels of 10-50 dB.
• Measured ITD sensitivity as a function of frequency and sound level.

Key Points:

• ITD sensitivity decreased significantly with increasing frequency and decreasing sound level.
• The observed slope of performance decline (90 dB/octave) aligns with the low-frequency slope of the cochlear excitation pattern.
• This suggests peripheral encoding, not central limitations, dictates the rapid decline in ITD sensitivity.

Conclusions:

• High-frequency ITD sensitivity may rely on off-frequency neuronal activation tuned to lower frequencies.
• A simplified model proposes that neurons sensitive to fine-structure ITDs might be limited to around 700 Hz.
• This challenges traditional models of binaural processing that assume a wide range of frequency-tuned neurons.