Re: Cochlea Amplifier models : a new list

[Gestur Björn Christianson <g.christianson@xxxxxxxxx>]

On 11 Oct 2007, at 9:55, Martin Braun wrote:
> A "stochastic model of the fiber response" is not needed. The rate  
> count figures are so high that their significance need not be  
> tested by further mathematics.

This is a claim that requires justification, I think.  As a rule,  
variability of response increases proportionally to the mean rate of  
discharge.  While the proportionality is normally less than that seen  
in cortical areas, it is still significant.  In this case, as Erik  
pointed out, it is very important to demonstrate that the response at  
5.5 kHz is significantly lower that the response to the surrounding  
frequencies, as in the absence of that single point all that is  
evident is that the discharge rate saturates at 100 dB.

In the context of this discussion, there is also a significant  
methodological confound in this study.  Geisler et al. did not use  
pseudo-random stimulus delivery.  Instead, they presented a single  
frequency repeatedly, stepping up the intensity from the minimum to  
the maximum.  This raises concerns about habituation.  Note that at  
both 5.5 and 5.75 kHz, you can see an increase in discharge rate with  
increasing intensity until the peak discharge rate of slightly more  
than 250 spikes/sec is achieved, at which case response begins to  
drop off.  Because of their methodology, this is a progression in  
time, and consistent with known spike-rate adaptation effects; thus,  
even if the response to 5.5 kHz at 100 dB is significantly lower than  
the response to surrounding frequencies, it potentially reflects  
spiking mechanics and not contributions from the cochlea (or  
otherwise).  I'd suggest that caution should be taken when trying to  
interpret the highest stimulus intensity plots in their data.

Bjorn