Jim Bower's posting

[Carlos Brody]

I will be happy to sound like Ning Qian responding to Geoff Hinton
sounding like Jim Bower, and I will say that

My recent papers on cross-correlation go far beyond simply pointing
out the possibility of an interpretation problem. (That possibility
was pointed out by Aertsen, Gerstein, Habib and Palm in their 1989
paper introducing the JPSTH, J. Neurophysiol. 61:900-917. I was not
until now aware that the Wilson and Bower papers also made a similar
point-- thanks to Jim Bower for pointing it out.)

Of the 3 papers I recently announced:

The "Correlations without synchrony" paper studies what kinds of
xcorrelogram shapes are generated by slow interactions (ACROSS trials)
as compared to fast interactions (WITHIN trials). The specific point
is to know *what to watch out for*. The most important rule of thumb
for being alert to interpretation problems confusing slow and fast
interactions is: if the PSTHs have peak widths of the same order of
magnitude as the xcorr peak width, be careful with your
interpretations! 

   [Related to this, it is also important to know when you DON'T have
    to worry. Examples of perfectly o.k. interpretations (as far as I
    can tell) are Alonso and Reid's 1995 work on connections from LGN
    to simple cells in area 17 of cat Nature 378:281--284; or Ts'o,
    Gilbert, and Wiesel's 1986 work on connections within area 17,
    J. Neurosci. 6:1160--1170. In both of these, the PSTHs were MUCH
    broader than the peaks in the xcorrelograms.]

The "Disambiguating different covariation types" paper tries to
propose a couple of quantitative methods for disambiguating
interpretations when it is not clear how much of the xcorrelogram came
from slow or fast interactions.

Finally, the "Slow covariations in neuronal resting potentials can
lead to artefactually fast cross-correlations in their spike trains"
paper shows how awareness of these issues is still very far from
seeping into our collective consciousness: the paper goes through an
example suggesting that a well-known paper, published in a well-known
journal (Nature) suffered greatly from such interpretation
problems. But that paper is not the only paper with signs of trouble:
it just happened to be one where I was intimately familiar with the
data and thus could go through it in detail and be confident of my
conclusions.


In sum, it is important not only to be aware that there CAN be
trouble; but also to know WHEN there can be trouble. And,
concomitantly, when there won't be. The three papers I announced try
to go in this direction. Cross-correlation is a most useful tool, and
should not be thrown out with the bathwater.

Carlos.

carlos at sonnabend.ifisol.unam.mx      http://www.cns.caltech.edu/~carlos

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   Date: Mon, 15 Mar 1999 09:44:14 -0800
   From: "James M. Bower" <jbower at bbb.caltech.edu>
   Errors-to: owner-connectionists at nntp-server.caltech.edu

   With respect to the recent posting by Carlos Brody, I would point to two
   papers published almost ten years ago by Matt Wilson and Myself:


   Wilson, M.A. and Bower, J.M.  1990  Computer simulation of oscillatory
   behavior in cerebral cortical networks. In:  Advances in Neural information
   processing systems. Vol. 2, D. Touretzky, editor. Morgan Kaufmann, San
   Mateo, CA., pp. 84-91.


   Wilson, M.A. and  Bower, J.M. 1991  A computer simulation of oscillatory
   behavior in primary visual cerebral cortex.  Neural Computation  3: 498-509.


   Quoting from the first:


   "Interpreting phase coherence from correlation functions produced from the
   average of many simulation trials pointed out the need to distinguish
   average phase effects from instantaneous phase effects.  Instantaneous
   phase implies that the statistics of the correlation function taken at any
   trial are consistent with the statistics of the combined data.  Average
   phase allows for systematic within-trial and between-trial variability and
   is, therefore, a weaker assertion of actual coherence.  This distinction is
   particularly important for theories which rely on phase encoding of
   stimulus information.  Analysis of our model results indicates that the
   observed phase relationships are an average, rather than an instantaneous
   effect."

   Remarkable how slowly things change, or are accepted (eh Steve??).


   Jim Bower