Local receptive fields

[Scott.Fahlman@SEF1.SLISP.CS.CMU.EDU]

      Which (sense) is truer to its physiological origins?
    
    who knows?  (anyone who does please comment)  but receptive field
    properties can be complex--they can be "simple".  Clearly not all neurons
    are connected to all other neurons --so in that trivial sense
    they are "local".  Do they have preference for other cells? No doubt.

It's clear that in the big visual areas that have been mapped, some sort of
mosaic encoding is in use.  That is, the most obvious dimensions of the
space -- the X/Y diemensions of the 2-D image on the retina -- are not
represented by varying levels of activation, but rather are mapped across
sets of cells in a regular (though distorted and interleaved) 2-D mosaic.
Other major systems seem to use the same kind of encoding: hearing, tactile
senses, motor control, etc.  There may or may not be some continuous,
analog encoding (levels or pulse-frequency) at other levels or among some
of the low-level control systems.  So clearly, for most parts of the neural
system in mammals, the idea of "local receptive field" for cells near the
input layer maps cleanly into some sort of range limitation on the map in
question.

I think what you are talking about above, however, is what I would call
"local connectivity" rather than "local receptive fields".  If you allow
signals to take a few hops, a net whose immediate physical connections are
mostly local can give you a very large receptive field.  The wire on my
telephone runs to an office a few blocks away, but my telephone's receptive
field includes just about any place on earth.  And if a net makes
significant use of recurrent links and delays, the receptive field extends
back in time in a complicated way.  The term "receptive field" has mainly
been used in describing combinational logic (with no memory, maybe just
some adaptation and gain control) and should probably be reserved for that
context.

-- Scott

"No, no Igor!  The OTHER brain..."