Re: perceptual segregation of sound

[Brian Gygi <bgygi@xxxxxxxxx>]

Tania,

If you think about what the auditory system is supposed to do, one of 
the fundamental tasks is to determine the nature, number and location of 
sources in the environment.  This fundamental task is accomplished quite 
handily by much "less intelligent" organisms than ourselves, so I think 
one should avoid the temptation of ascribing too much to "top down" 
processing. Although it is hard to say where auditory objects (to use a 
controversial term) are formed (if indeed they are formed at any one 
place), certainly at the more peripheral levels of auditory processing 
there are mechanisms for grouping frequencies that are correlated, which 
may be automatic, as in Bregman's preattentive stage.  The effects of 
auditory attention also seem to implemented at the cochlear level, even 
though it generally considered a more top down process.  So it is not as 
clear cut as conventional models would have it.  If you want to appeal 
to introspection, don't use hypothetical cases that may never occur 
(such as hearing two lions roaring simultaneously) but do a little 
listening in your everyday environment and ask yourself if the imemdiate 
impression is of one mass of sound that eventually resolves itself into 
individual components or of separate sources.  Obviously, this is 
dependent on the how different the sourses are acoustically, how offset 
they are in time, and how spatially separated they are, and top down 
processes are useful for resolving ambiguous cases. 

In any case, I would strongly recommended you look at the archives of 
the Auditory list for our previous discussion on this topic, which was 
quite interesting.

Brian Gygi