Re: Sound localization standard model question and confusion

[Mark Riggle <marksriggle@xxxxxxxxx>]

I have a confusion about the evidence for elevation sound localization.

The current theory for sound localization in mammals (which I will refer 
to as the standard model) posits that azimuth localization is done by 
binaural cues (interaural time and level differences) and the elevation 
localization is performed by the monaural spectral cue.  Further, the 
standard model has the elevation localization via that spectral analysis 
occurring in the dorsal cochlear nucleus (DCN).  While I have some 
problems overall with the standard model, I want to address a confusion 
I have about the elevation localization part of the model and its 
supporting data.

The data that supports the DCN as the site of elevation localization is 
in two parts.  The first, which is clear, is that the DCN is sensitive 
to spectral notches. The spectral notches are what carries the 
information imparted by the HRTF for elevations and azimuths.  Then, 
because the DCN can perform that spectral notch analysis, and because 
the DCN only gets an auditory signal from the ipsilateral ear (thus 
monaural), the conclusion is the DCN could perform the elevation 
localization using the monaural signal.  So far this is good.  Further, 
since the DCN only has output to the inferior colliculus (IC), it is 
assumed the IC takes that DCN signal and uses it to determine the 
elevation of the sound.

This DCN to IC connection lead to the experiments that lesioned the 
connection from the DCN to the IC (the DAS); the expectation was, if the 
DCN is the processing unit for elevation localization, then elevation 
localization should be severely impaired.  The experiment and data for 
this is from:
May, B. (2000) "Role of the dorsal cochlear nucleus in the sound 
localization behavior of cats."
 The conclusion -- as used in all current textbooks --  based on this 
experiment (and one earlier one) is that the DCN does do the elevation 
localization.

See my data comparison at:
https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B48PX6LiyabwZWQ2ZGVkZDgtMWVmYy00ZDkwLWE3NzEtY2U4NmRmZWZhMGFm&hl=en_US 

(sorry for the very long URL, blame Google Docs)

And this finally leads to my confusion: I do not see that conclusion in 
the data.  I am baffled as to how that conclusion is reached.  I have 
reproduced the data in a way to easily compare the before and after 
effects of the DAS severing.  In that comparison; I have put the data 
only from the -30, 0, and +30 degree elevations (omitting the +60 degree 
line and two other points).  If someone showed you that data and claimed 
it showed a treatment that destroyed elevation localization, what would 
you say?  While clearly there is an effect on elevation localization 
(certainly for elevations above 30 deg), the hypothesis is that 
elevation localization is exclusively done in the DCN.  To me, the 
a-priori expectation for the results seems not met.

Am I the only one with this confusion?  This result is 11 years old; can 
anyone explain what is going on?

Thank you for your patience.
Mark Riggle