Re: Interesting dichotic effect

[Al Bregman <bregman@xxxxxxxxxxxxxxxxxxxx>]

Mitch,

Do you think that this phenomenon is related to "diplacusis", the
fact that the same stimulus can have a different pitch (typically
a small difference) at the two ears?

Were your pitch differences large or small?  Can you estimate it
in semitones?

You say some subjects heard an increase, and others a decrease in
pitch.  Did the same subjects hear the same differences in
Stimuli 3 and 4, as they did in stimuli 1 and 2?  In other words,
was the difference in subjects a stable difference.

Al

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----- Original Message -----
From: Mitchell Sommers <msommers@ARTSCI.WUSTL.EDU>
To: <AUDITORY@LISTS.MCGILL.CA>
Sent: Wednesday, January 24, 2001 2:41 PM
Subject: Interesting dichotic effect


I wonder if anyone has any thoughts or knows of relevant research
for the following phenomenon.  In the course of calibrating
headphones for a dichotic listening task, we generated stimuli
consisting of two pure tones.  The stimuli were presented as
follows.  Stimulus 1 presented to the right ear had
equal-amplitude tones at 100 and 150 Hz.  Stimulus 2 presented to
the left ear had equal-amplitude tones (same amplitude as in
stimulus 1) of 200 and 250 Hz.    Stimuli 3 and 4 were simply the
reverse of these two ( left ear gets 100 and 150; right ear gets
200 and 250).  The rationale behind playing with these stimuli
was that "perfect" integration across the two ears should produce
the identical percept for both pairs of tones.  That is, stimuli
1 and 2 presented dichotically, should sound identical to stimuli
3 and 4 presented dichotically because the spectral content is
the same, we just switched the ear to which each was presented.
Instead, the perceptual experience was a change in pitch.
Interestingly of 4 listeners 2 perceived a decrease in pitch and
2 perceived an increase (i.e., when 3 and 4 were presented
dichotically after hearing 1 and 2, there was either a decrease
or increase in pitch).  We then tried simply reversing the
headphones (left transducer on right ear and vice-versa) and the
perception was the same.  We also tried a different combination
such that stimulus 1 had equal-amplitude components at 100 and
400 Hz and stimulus 2 had equal amplitude components at 200 and
300 Hz.  Again, the perception was a pitch change when we
switched channels for stimulus 1 and 2.   I'd be interested in
any ideas as to why we would get a clear pitch change using this
stimulus configuration.  Many thanks

Mitch Sommers



Mitchell S. Sommers, Ph.D.
Associate Professor
Department of Psychology
Washington University
Campus Box 1125
St. Louis, MO 63130

E-mail: msommers@artsci.wustl.edu
Phone: 314-935-6561
Fax::  314-935-7588