Increment versus decrement detection

[Bruno Repp <repp@xxxxxxxxxxxxxxxx>]

Dear colleagues:

I recently conducted some experiments in which participants had to 
detect a single changed tone in an isochronous melody consisting of 12 
successive complex tones (piano tones) of different pitch but equal 
loudness. The change to be detected was either in intensity or in 
duration, and it was either an increment or a decrement. These were four 
separate tasks, not intermixed. The melody containing the change was 
immediately preceded by the same melody not containing any change. I 
would be grateful for comments or references that would help me 
understand the following trends in my data:
(1) Decrements are harder to detect than increments. Is this well known 
from previous psychoacoustic research? What is the reason?
(2) The pattern of variation in detection scores (hits and false alarms) 
across the 12 tones in the melody, which reflects influences of pitch 
contour and other factors, is uncorrelated between increment and 
decrement detection. It seems like these two tasks have little in 
common. Why?
(3) In increment detection, both hits and false alarms tend to increase 
across the 12 tones in a melody, but decrement detection shows the 
opposite trend. It is as if tones were expected to become softer as the 
melody progresses. Why?
Many thanks in advance for any helpful replies!

Best,
Bruno

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Bruno H. Repp
Haskins Laboratories 
300 George Street
New Haven, CT 06511-6624
Tel. (203) 865-6163, ext. 236
Fax (203) 865-8963
http://www.haskins.yale.edu/staff/repp.html