Lateralization and frequency selectivity in normal and impaired hearing

Abstract

The onset-time difference delta T required to lateralize a 30-ms bifrequency tone burst toward the leading ear was measured as a function of the frequency difference delta F between the tone in the left ear and the tone in the right ear. At center frequencies of 0.5 and 4 kHz, four normal listeners tested at 80 and 100 dB SPL had delta Ts that were relatively constant at subcritical delta Fs, but increased at delta Fs wider than a critical band. At 1 kHz, delta T increased with delta F even at subcritical delta Fs. Ten listeners with cochlear impairments were tested at 100 dB SPL. Seven had normal delta Ts at 4 kHz, despite hearing losses between 50 and 70 dB. At 0.5 and 1 kHz, mildly impaired listeners had nearly normal lateralization functions, whereas more severely imparied listeners had very large delta Ts and no frequency selectivity. These and other findings indicate that listeners even with moderate to severe hearing losses can lateralize normally on the basis of interaural differences in onset envelope, but not on the basis of temporal differences in the fine structure.

Estimation of cochlear response times using lateralization of frequency-mismatched tones

“…This is consistent with the hypothesis that the lateralization was, at least to some extent, based on a temporal mechanism. This hypothesis is corroborated by the finding that tone-onset phase influences the lateralization of mismatched tones for frequencies below about 2 kHz ͑Scharf et Buus et al, 1984͒. With respect to the estimation of CRT disparities, the observed invariance of the results to loudness balancing is crucial.…”

“…Zerlin ͑1969͒ used 2.5-ms ramps, whereas 10-ms ramps were used in the present study as well as in Scharf et al ͑1976͒ andBuus et al ͑1984͒. However, even with such short ramp durations ͑tested in pilot measurements͒, it was not possible to obtain consistent ITD data at high frequencies.…”

“…The frequency mismatches for all tone pairs used by Zerlin ͑1969͒ exceeded the critical bandwidths given by Scharf et al ͑1976͒ andBuus et al ͑1984͒. For the 3200͉ 4000-Hz and 5000͉ 6300-Hz tone pairs, the reported centering ITDs clearly fall below the corresponding lateralization thresholds in those studies.…”

“…Each tone burst had a total duration of 40 ms, including an exponential onset with a rise time of 10 ms and a 10-ms raised-cosine shaped offset-ramp. In contrast to Scharf et al ͑1976͒ andBuus et al ͑1984͒, who used exponential ramps at onset and offset, a cosine offset-ramp was used here in order to minimize spectral splatter. The tones were presented in sine phase, i.e., the onset-ramp started with a positive-going zero crossing of the sinusoid.…”

“…However, it is not clear if the lateralization in Zerlin's paradigm is based on interaural level differences ͑in the envelope at onset/ offset͒, interaural time differences ͑in the fine structure͒, or a combination of both. Buus et al ͑1984͒ suggested that temporal-fine-structure information during the first tone cycles might play a role in the lateralization of mismatched tones at low frequencies. This was supported by Magezi and Krumbholz ͑2008͒, who provided evidence that the binaural system can extract fine-structure information from interaurally mismatched frequency channels.…”