Re: Cross-correlations DO NOT imply synchrony

Peter Cariani (peter@epl.meei.harvard.edu)
Tue, 23 Mar 1999 10:56:31 -0500

Comp-Neuro Mailing List wrote:
>
> >From carlos@sonnabend.ifisiol.unam.mx Mon Mar 22 12:44:26 1999
> Date: Fri, 12 Mar 1999 20:38:19 -0600
> From: Carlos Brody <carlos@sonnabend.ifisiol.unam.mx>
> To: cneuro@bbb.caltech.edu, connectionists@cs.cmu.edu
> Subject: Cross-correlations DO NOT imply synchrony
>
> Cross-correlations DO NOT imply synchrony: announcing 3 papers
> --------------------------------------------------------------
>
> Suppose that you record from two stimulus-driven cells simultaneously,
> over many trials. Interested in whether they are synchronized, you
> compute the average cross-correlogram of their spike trains. (For the
> initiated, you compute their shuffle-corrected cross-correlogram, so
> as to get rid of direct stimulus influences.) You find, in the
> resulting correlogram, that there is a narrow peak, centered at zero,
> with a width of say 15 ms. "Ah! The cells are synchronized on a 15-ms
> timescale!" you conclude.
>
> In concluding this you will be doing what most people do, and what
> most papers in the literature do.
>
> THIS CONCLUSION DOES NOT NECESSARILY FOLLOW.
>
> How and why? If the PSTHs of the cells have narrow peaks, by which I
> mean as narrow as the peak in the xcorrelogram itself, then even if
> the mechanism synchronizing the cells has a very very slow timescale
> (e.g. tens of seconds), the xcorrelogram will have a narrow peak.
>
> Such a peak would NOT be an artifact. It arises ONLY if there *IS* an
> interaction -- synchrony, if you will -- between the two cells. What
> is wrong is the conclusion regarding the timescale of the interaction.
> A narrow peak (tens of ms) does NOT necessarily mean a fast
> interaction or a fast timescale of synchronization. Wrong
> interpretations of this sort can make nonsense of the arguments one is
> making with respect to the data. An example in point is Sillito et
> al. "Feature-linked synchroni`zation of thalamic relay cell firing
> induced by feedback from the visual cortex", Nature 369: 479-482
> (1994). A paper recently published in J. Neurophysiol (see pointer
> below) uses a simple biophysical model to go through that example in
> detail. It shows how one can get exactly the same xcorrelograms
> Sillito et al. got, but without any binding-related (i.e. fast)
> synchrony at all. Instead, in the model the only interaction between
> the cells is that their resting potential slowly covaries over the
> trials of the experiment. That slow (tens of seconds) covariation
> reproduces Sillito et al.'s data in remarkable detail.
>
> Two other papers, in press in Neural Computation, go through these
> kind of issues in a more abstract manner. The first describes the
> problem, and tries to provide rules of thumb for being alert to when
> interpretation problems may arise. The second paper suggests a couple
> of methods to try to disambiguate interpretations.
>
> Comments welcome.
>
> Carlos Brody
> carlos@sonnebend.ifisiol.unam.mx http://www.cns.caltech.edu/~carlos

My understanding is that these cross-correlograms have mostly
been used to uncover underlying connections between neurons rather
than for examining stimulus-dependent response patterns. This is
the reason that the shuffling is done; to weed out those nasty
stimulus-driven synchronies. But what if the stimulus-driven
synchronies are what the system uses to encode edges and
spatial intervals between edges? One has thrown out the most
important aspects of the neural response.

I have never understood why synchronies that are not related
to the stimulus are so highly prized, but those related to the
stimulus are routinely discarded. Could you shed some light on
this?

I think we desperately need to find a cure for myopia.

Peter Cariani

Eaton Peabody Laboratory of Auditory Physiology
Massachusetts Eye & Ear Infirmary
243 Charles St., Boston, MA 02114

(617) 573-4243
peter@epl.meei.harvard.edu