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From: brp@sim.uucp (bruce raoul parnas)
Newsgroups: comp.ai.neural-nets
Subject: Re: Contrast sensitivity of spatiotemporal models models
Keywords: spatiotemporal filtering, motion detection
Message-ID: <20480@pasteur.Berkeley.EDU>
Date: 8 Dec 89 17:37:09 GMT
References: <8977@discus.technion.ac.il> <14700@boulder.Colorado.EDU>
Sender: news@pasteur.Berkeley.EDU
Reply-To: brp@sim.UUCP (bruce raoul parnas)
Organization: University of California, Berkeley
Lines: 23

In article <14700@boulder.Colorado.EDU> bill@synapse.Colorado.EDU () writes:

>performs an operation more like a division than a subtraction (though
>it's really a sort of combination of the two).  Neurophysiologists often
>refer to this as "shunting inhibition".
>
>				  Bill Skaggs

Actually, i believe that "shunting inhibition" is more akin to multiplication
than either of the two operations you mention.  the effect *is* to reduce the
main path signal, as a division by a quantity greater than one would do, but
the action is to divert "shunt" some of the signal through a multiplicative
process.  The shunting inhibition used by Grossberg in his contour enhancement
and ART models takes the form:

(B - xsubi)*f(xsubi)

where B represents the total number of units, or populations, xsubi is the unit
activity level and f(w) is a nonlinear feedback function from unit i to unit i.
the effect of (B-xsubi) is to multiplicatively reduce the effect of the self-
feedback when xsubi gets large to prevent saturation.

bruce