Interaural Time Discrimination of Envelopes Carried on High-Frequency Tones as a Function of Level and Interaural Carrier Mismatch
from Ear and Hearing
Objectives: The present study investigated interaural time discrimination for binaurally mismatched carrier frequencies in listeners with normal hearing. One goal of the investigation was to gain insights into binaural hearing in patients with bilateral cochlear implants, where the coding of interaural time differences (ITDs) may be limited by mismatches in the neural populations receiving stimulation on each side.
Design: Temporal envelopes were manipulated to present low frequency timing cues to high-frequency auditory channels. Carrier frequencies near 4 kHz were amplitude modulated at 128 Hz via multiplication with a half-wave rectified sinusoid, and that modulation was either in-phase across ears or delayed to one ear. Detection thresholds for nonzero ITDs were measured for a range of stimulus levels and a range of carrier frequency mismatches. Data were also collected under conditions designed to limit cues based on stimulus spectral spread, including masking and truncation of sidebands associated with modulation.
Results: Listeners with normal hearing can detect ITDs in the face of substantial mismatches in carrier frequency across ears.
Conclusions: The processing of ITDs in listeners with normal hearing is likely based on spread of excitation into binaurally matched auditory channels. Sensitivity to ITDs in listeners with cochlear implants may depend on spread of current that results in the stimulation of neural populations that share common tonotopic space bilaterally.