Oscillations, Waves, and Interactions - GWDG

64 A. Kohlrausch and S. van de Par
Figure 19. An example of three different N0Sπ stimuli before and after peripheral processing.
Panel A shows a 125-Hz stimulus, panel B shows a transposed stimulus at 4 kHz,
and panel C shows a standard 4-kHz stimulus. The intervals 0.0–0.1 s show the N0 masker
alone, the intervals 0.1–0.2 s show the N0 masker plus the Sπ signal at a signal-to-noise
ratio of -10 dB, the intervals 0.2–0.3 s show the masker after peripheral processing, and the
intervals 0.3–0.4 s show the combined masker and signal after peripheral processing. Reused
with permission from Ref. [52]. Copyright 1997, Acoustical Society of America.
4.2.2 Acoustic properties
The construction of transposed stimuli followed the idea to build high-frequency
signals that, after the initial stages of processing on the basilar membrane and the
hair cells, have the same amount of temporal information as they are present in a
standard low-frequency experiment. This property of transposed stimuli is depicted
in Fig. 19. In each of the three panels, the left half of each panel shows acoustic
waveforms as they are presented to subjects, and the right half, starting at 200 ms,
shows these stimuli after the first stages of peripheral processing. The first 100 ms
show the waveform of a noise masker alone, the section 100 to 200 ms contains the
noise masker plus an Sπ signal with a level 10 dB below the masker. The section 200
to 300 ms shows the masker after peripheral processing, and the last 100 ms show
masker plus signal after peripheral processing. Panel A contains signals centered at
125 Hz, and panel B contains transposed signals with a center frequency of 4 kHz,
derived from the low-frequency signal in panel A. Panel C finally shows standard
high-frequency stimuli centered at 4 kHz. In this case, the masker is a narrowband
noise centered at 4 kHz and the signal is a 4-kHz sinusoid.
By comparing panels A and B, we see that, indeed, after peripheral processing, the
low-frequency channel in panel A and the high-frequency channel in panel B contain
the same temporal information. In contrast, this information is strongly reduced in
a standard high-frequency condition as shown in panel C.
Figure 20 shows the spectrum of a transposed stimulus, derived from a narrowband
noise at 125 Hz. The spectral level is highest around the carrier frequency of 4 kHz.