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Spectral timbre discrimination

Dear list,

We have recently completed an auditory memory study using the
interpolated tone paradigm (Deutsch, 1970). Our aim was to look at
spectral timbre through varying the position of one of the harmonics in
the two sounds to be compared. We used a same-different task. Listeners
were presented with two tones - A and B - which could be presented in
any combination. This created four trial types: AA, BB, AB and BA. Pitch
was randomly varied across trials. Tone A featured the fundamental and
the fourth, sixth and eighth harmonics, and tone B featured the
fundamental, and the fourth, seventh and eight harmonics. So, the only
difference between the tones was whether it included the sixth or the
seventh harmonic. For example, on an AB trial the sixth harmonic present
in tone A would be shifted upward to the seventh harmonic in tone B. 

Along with uncovering the interference effect we anticipated when
certain intervening tones were placed into the interstimulus interval,
we also found that participants were much worse on the BA trials (i.e.
where the seventh harmonic was shifted down to the sixth harmonic) than
on AB trials (where the sixth harmonic was shifted up to the seventh
harmonic). We found this effect both during initial training and when
the intervening tone was placed into the sequence. 

Whilst this phenomenon is not the main focus of our study, we would be
very interested in trying to understand why this has occurred but so far
we have been unable to come across any papers which have found a similar
effect. Can anyone recommend any studies which have found related
effects to ours? That is, where a certain component change is easier or
more difficult to identify than another, particularly in relation to
'upward' or 'downward' shifted harmonics? 

Any help would be much appreciated.

Thanks in advance,

Tom Mercer
Postgraduate Research Student
Institute of Psychological Sciences
University of Leeds
Email: T.Mercer04@xxxxxxxxxxx