Oscillations, Waves, and Interactions - GWDG

Specific signal types in hearing research 63
Figure 18. A scheme for the
generation of transposed stimuli.
The top part shows the generation
of masker and signal waveforms,
in this case for a binaural
condition with the masker
in phase and the signal interaurally
out of phase. In the
lower part, the low-frequency signals
are halfwave rectified, lowpass
filtered and multiplied with
a high-frequency carrier. Reused
with permission from Ref. [52].
Copyright 1997, Acoustical Society
of America.
The question remained whether the differences in BMLDs at low and high frequencies
were primarily a consequence of the loss of fine-structure information, or whether
they reflected a structural difference of binaural processing between low and high frequencies.
In order to distinguish between these explanations, transposed stimuli with
a high carrier were introduced which contained in their envelope the same temporal
information as a related low-frequency stimulus in its fine structure.
4.2.1 Definition
A transposed stimulus is derived from a narrowband low-frequency stimulus by the
following operations. First, the low-frequency signal is halfwave rectified and lowpass
filtered. In Ref. [52], a second-order lowpass with a cut-off frequency of 500 Hz was
used. This signal is then used to modulate a high-frequency carrier, e. g., a sinusoid
at 4 kHz.
In Fig. 18, these steps are shown schematically for stimuli as they are used in a
masking experiment with adaptive adjustment of the signal level. In the top part
of the figure, the generation and mixing of the low-frequency masker and the lowfrequency
signal are shown. The signal is added to the masker with a certain level,
which is controlled by the adaptive procedure and adjusted by the gain stage. Depicted
is the generation of an N0Sπ stimulus, for which the signal is inverted in one
channel. After addition, the stimuli are ready for a standard low-frequency masking
experiment. The bottom part shows the additional steps needed for the generation
of a transposed stimulus.