Volume 128, Issue 1, July 2010
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
128(2010); http://dx.doi.org/10.1121/1.3397640View Description Hide Description
Auditory brainstem responses (ABRs) are recorded in ten normal-hearing adults (20 ears) in response to a standard click and five chirps having different durations (sweeping rates). The chirps are constructed from five versions of a power function model of the cochlear-neural delay that is based on derived-band ABR latencies from normal-hearing adults [Elberling, C., and Don, M. (2008). J. Acoust. Soc. Am.124, 3022–3037]. The click and the chirps have identical amplitude spectra and, in general, for each of the three stimulus levels 60, 40, and 20 dB nHL, the ABRs to the chirps are significantly larger than the ABRs to the click. However, the shorter chirps are the most efficient at higher levels of stimulation whereas the longer chirps are the most efficient at lower levels. It is suggested that two different mechanisms are responsible for these observed changes with stimulus level—(1) upward spread of excitation at higher levels, and (2) an increased change of the cochlear-neural delay with frequency at lower levels.
128(2010); http://dx.doi.org/10.1121/1.3397622View Description Hide Description
Cortical sensitivity to the periodicity of speechsounds has been evidenced by larger, more anterior responses to periodic than to aperiodic vowels in several non-invasive studies of the human brain. The current study investigated the temporal integration underlying the cortical sensitivity to speech periodicity by studying the increase in periodicity-specific cortical activation with growing stimulus duration. Periodicity-specific activation was estimated from magnetoencephalography as the differences between the N1m responses elicited by periodic and aperiodic vowel stimuli. The duration of the vowel stimuli with a fundamental frequency representative of typical male speech was varied in units corresponding to the vowel fundamental period (9.4 ms) and ranged from one to ten units. Cortical sensitivity to speech periodicity, as reflected by larger and more anterior responses to periodic than to aperiodic stimuli, was observed when stimulus duration was 3 cycles or more. Further, for stimulus durations of 5 cycles and above, response latency was shorter for the periodic than for the aperiodic stimuli. Together the current results define a temporal window of integration for the periodicity of speechsounds in the F0 range of typical male speech. The length of this window is 3–5 cycles, or 30–50 ms.
128(2010); http://dx.doi.org/10.1121/1.3436527View Description Hide Description
The chirp-evoked ABR has been regarded as a more synchronous response than the click-evoked ABR, referring to the belief that the chirp stimulates lower-, mid-, and higher-frequency regions of the cochlea simultaneously. In this study a variety of tools were used to analyze the synchronicity of ABRs evoked by chirp- and click-stimuli at 40 dB HL in 32 normal hearing subjects aged 18 to 55 years (, ). Compared to the click-evoked ABRs, the chirp-evoked ABRs showed larger wave V amplitudes, but an absence of earlier waves in the grand averages, larger wave V latency variance, smaller FFT magnitudes at the higher component frequencies, and larger phase variance at the higher component frequencies. These results strongly suggest that the chirp-evoked ABRs exhibited less synchrony than the click-evoked ABRs in this study. It is proposed that the temporal compensation offered by chirp stimuli is sufficient to increase neural recruitment (as measured by wave V amplitude), but that destructive phase interactions still exist along the cochlea partition, particularly in the low frequency portions of the cochlea where more latency jitter is expected. The clinical implications of these findings are discussed.