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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