Oscillations, Waves, and Interactions - GWDG

Specific signal types in hearing research 65
Figure 20. The spectrum of a 25-Hz-wide 125-Hz transposed stimulus. Most of the stimulus
energy is around 4 kHz. The spectrum of the original low-frequency stimulus now occurs
at 4125 Hz and the mirror image of that spectrum is at 3875 Hz. Additional peaks in the
spectrum occur with spacings of 250 Hz. Reused with permission from Ref. [52]. Copyright
1997, Acoustical Society of America.
Spectral sidebands occur at spectral distances of 125, 250, 500, 750 etc. Hz. The
spectrum of the original noise band is contained in the two sidebands directly to the
left and the right of the carrier frequency.
4.2.3 Role in hearing research and perceptual insights
In Ref. [52], a number of experiments using transposed stimuli are described. Experiments
were performed for transposed stimuli derived from 125-Hz and also from
250-Hz low-frequency conditions. Binaural conditions tested were N0Sπ and NπS0.
For comparisons, thresholds were also obtained in the low-frequency condition, from
which the transposed stimuli were derived, and for a standard high-frequency condition
with a narrowband masker. Figure 21 shows experimental data for the condition
N0Sπ for 125 Hz. Squares and circles indicate 125-Hz and 125-Hz transposed results,
while diamonds indicate results for a standard 4-kHz measurement. The mean data
show very clearly that 125-Hz and 125-Hz transposed stimuli give the same results,
not only in the size of the BMLD, but also in the dependence on the masker bandwidth.
In contrast, 4-kHz BMLDs in the standard condition are much lower.
This result is strong support for the idea that the major source of the decrease in
BMLDs at high frequencies lies in the effect of peripheral processing on the available
temporal information in the stimuli entering the binaural processing stage. If we
manage to present very similar amounts of temporal information at a frequency with
phase locking, e. g., 125 Hz and at a frequency without phase locking, e. g. 4 kHz, by
using our modulation technique, then very similar amounts of binaural unmasking
are measured. This observation thus supports an older hypothesis from Colburn and