Oscillations, Waves, and Interactions - GWDG

Specific signal types in hearing research 61
Figure 16. The vector representation
of an N0Sπ stimulus. The
masker M and signal S are added in
the left ear with +90 degree phase
difference, resulting in the vector
L, and added in the right ear with
-90 degree phase difference, resulting
in the vector R. Only interaural
time differences (ITDs) are
present in the resulting binaural
stimulus. Reused with permission
from Ref. [40]. Copyright 1998,
Acoustical Society of America.
is interaurally out of phase, in the right panel, the phase difference between masker
and signal has to be π. The addition of the masker and signal in the left and the
right panels results in two vectors, L and R, which together represent the binaural
stimulus. The vectors L and R have in nearly all cases the same orientation. Only
when the masker envelope M is smaller than the one of the signal S, L and R point in
opposite directions. Basically, for this value of the masker-signal phase, the binaural
stimulus contains only IIDs which vary in time at a rate proportional to the masker
bandwidth.
In Fig. 16 a similar picture is shown as in Fig. 15, but now the masker M and
the signal S are added with phase differences of +90 and -90 degrees. The resulting
vectors L and R have the same length in all cases, only their orientation is different.
This vector diagram describes a condition with no interaural intensity differences,
and only time-varying interaural time differences.
These stimuli were used by van de Par and Kohlrausch [40] in a number of binaural
experiments. We will here only discuss one of the measurements, which reveals nicely,
how much ITDs and IIDs contribute to binaural unmasking at different frequencies.
Figure 17 shows the experimental results for conditions with either only interaural
intensity differences (circles) or only interaural time differences (squares). The difference
between open and close symbols indicates the amount of binaural unmasking
(BMLD). Two results become clear from this graph: when the binaural stimulus
contains IIDs, the amount of binaural unmasking remains constant at all frequencies.
When the stimulus contains interaural time differences, the binaural detection
advantage is limited to low frequencies up to 1 kHz, and disappears completely at
frequencies of 2 kHz and above. The details of the transition between 1 and 2 kHz
seem to vary between the individual listeners. These data clearly demonstrate that
BMLDs do not generally decrease at high frequencies, as is sometimes stated, but
that the detection advantage is strongly linked to the cues that are available in the
stimulus. This aspect will be further elaborated in the next section about transposed
stimuli.
These results from binaural masking experiments agree very well with observations
about the detection of static IIDs and ITDs using sinusoidal signals. The sensitivity