Interaural Time Discrimination of Envelopes Carried on High-Frequency Tones as a Function of Level and Interaural Carrier Mismatch

Abstract

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 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 non-zero interaural time differences 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 interaural time differences in the face of substantial mismatches in carrier frequency across ears.Conclusions-The processing of interaural time differences in listeners with normal hearing is likely based on spread of excitation into binaurally matched auditory channels. Sensitivity to interaural time differences in listeners with cochlear implants may depend upon spread of current that results in the stimulation of neural populations that share common tonotopic space bilaterally. suggested keywords binaural hearing; cochlear implant; localization; ITD

Effect of mismatched place-of-stimulation on binaural fusion and lateralization in bilateral cochlear-implant users

“…In NH subjects, ITD thresholds as a function of mismatch have been measured using amplitude-modulated (AM) signals (Henning, 1974;Nuetzel and Hafter, 1981;Blanks et al, 2007;Blanks et al, 2008). The consistent trend in the results from these studies has been that with increasing mismatch, ITD JNDs increased.…”

“…The reduced ability to perceive ITDs and ILDs as changes in the lateral position of a sound source may partially explain the increase in ITD and ILD discrimination thresholds observed in bilateral CI users when measured with a non-pitch-matched pair of electrodes (e.g., van Hoesel, 2004;Francart and Wouters, 2007;Blanks et al, 2008;Poon et al, 2009;Goupell et al, 2013b). Specifically, if the JND is a measure of the smallest perceptible change in the lateral position of an auditory image, then the smallest JND would be at the steepest part of the lateralization curve, which can be assumed to pass through an ITD or ILD of zero.…”

“…However, the results from Poon et al also suggest that there is a degree of tolerance in the amount of mismatch for ITD sensitivity. The spread of current from a stimulating electrode should have a role in ameliorating the effects of mismatch since a wide population of auditory fibers are responding to the stimulation and thus there is an overlap in the area of excitation across the ears (van Hoesel, 2004;Blanks et al, 2008;Goupell et al, 2013b).…”

Effects of envelope shape on interaural envelope delay sensitivity in acoustic and electric hearing

“…Because of the very small peak level differences of the conditions with short off times the interpretation of the peak level effect is restricted to long off times. Previous studies on the effect of the stimulus level either co-varied the slope steepness (e.g., Dye and Hafter, 1984;Klein-Hennig et al, 2011;Blanks et al, 2008;Dreyer and Oxenham, 2008) or the modulation depth (e.g., Klein-Hennig et al). Klein-Hennig et al concluded that their peak-level effect was due to the co-variation of the slope steepness with the peak level.…”

Sensitivity of Bimodal Listeners to Interaural Time Differences with Modulated Single- and Multiple-Channel Stimuli

“…Matching the place of excitation in the cochleas is not strictly required for sensitivity to ITD in NH subjects [Blanks et al, 2008;Nuetzel and Hafter, 1981]. In bilateral CI listeners, the sensitivity to a mismatch in place of excitation is even smaller [Poon et al, 2009].…”