Synchronization Binding? Freq. Locking? Bursting?

[David C. Somers]

This is in reply to a recent message from the connectionist news group

>======================================================================
>From: connectionists at c.cs.cmu.edu Newsgroups:   bu.mail.connectionists
>Subject: Synchronization  Binding? Freq. Locking? Bursting?   Date: 22
>Aug 91 06:03:07 GMT
>
>>From: Thomas VLSI Edwards <tedwards at wam.umd.edu>

>I have just read "Synchronized Oscillations During Cooperative
>Feature Linking in a Cortical Model of Visual Perception"
>(Grossberg, Somers, Neural Networks Vol. 4 pp 453-466).
>It describes some models of phase-locking (supposedly neuromorphic)
>relaxation oscillators, including a cooperative bipole coupling which
>appears similar to the Kammen comparator model, and fits into BCS
>theory.  

Cooperative Bipole Coupling is significantly different from the comparator
model used by Kammen, Holmes and Koch.  The Bipole mechanism is a sort
of "statistical AND-gate" which becomes active (and thus provides feedback)
only when both of its spatially independent receptive flanks are sufficiently
active.  Feedback then passes to a cell or cells which lie intermediate
to the two flanking regions.  In the full Boundary Contour System, this
feedback is also passed only to cells whose receptive field properties (e.g., 
orientation) are similar to those cells which activate the particular bipole
cell.  This mechanism was proposed (By Grossberg and Mingolla) to handle the 
process of emergent segmentation in the visual system such as occurs in the 
perception of occluding contours, textural boundaries, and illusory or broken 
contours.  As Grossberg and I noted in our paper this mechanism has received
both neuroanatomical and neurophysiological support.

In the context of synchronized oscillations, we used the bipole mechanism
to not only synchronize activity along and over two regions of oscillatory
activity, but also to induce and synchronize oscillatory activity within
a slit region in between the two oscillating regions.  The bipole mechanism
accomplished this "perceptual boundary completion" without inducing a spreading
of oscillatory activity to the outlying regions.  The comparator model 
cannot robustly achieve this effect since it does not distinguish between the
completion of a boundary over a break between two regions and the outward
spreading of activity from the end of a line segment.  That is, the comparator
is not sensitive to the spatial distribution of its inputs but rather only to
the total input.  

The Adaptive Filter mechanism that we use in our simulations reduces to the
comparator mechanism when the fan-in of the Adaptive Filter equals its Fan-out.
The Adaptive Filter achieved synchronization without achieving boundary 
completion.  These results taken together suggest that different architectures
be used to achieve different results.  We interpret the Bipole cell results
as a pre-attentive boundary completion, while the adaptive filter results
correspond to an attentive resonant state as may occur during recall or
learning (cf. Adaptive Resonance Theory).



>I am curious at this date what readers of connectionists think about
>the theory that syncrhonous oscillations reflect the binding of local
>feature detectors to form coherent groups.  I am also curious as to
>whether or not phase-locking of oscillators is a reasonable model
>of the phenomena going on, or whether synchronized bursting, yet
>not frequency-locked oscillation, is a more biologically acceptable
>answer.

Charlie Gray's data seems to indicate that it is actually bursting, not
single spikes that are being synchronized.  This is consistent with our
oscillations of average firing rate.  Note that bursting can still be viewed
as an oscillatory phenomena.  Also note that Gray's data indicates that
synchrony occurs very rapidly--within one or two cycles. Our simulations 
demonstrate this effect, although many other researchers have had great
difficulty in achieving rapid synchrony.  Although the architecture of 
connections between the oscillators is important, so is the form of the
individual oscillators.  Nancy Kopell and I have a series of results that
show the advantages of using neural relaxation oscillators rather than
boring old sinusoids (papers in preparation).

As far as the meaning of the synchronized oscillations, I think we really
need a lot more data to really be able to tell.  Right now we've got a lot
of modellers chasing a little bit of data.  Having said that, there is still
something compelling about synchronized oscillations.  My hunch is that
the oscillations represent a form of multiplexing where the average firing rate
over several burst cycles indicates the information local to the classical
receptive field while the synchronization of the bursts represents some form
of global grouping of information.  Anything less than this multiplexing
would not seem to serve any purpose (at least in vision) -- Why give up
coding by average firing rate in order to code by phase relationship?
This is just trading one dimension for another (with a seemingly small
dynamic range) I suggest that the visual system may be making use of
both coding dimensions. This kind of multiplexing would also allow for
very rapid computations, since global extraction would be performed while local
information accumulates rather than performing these operations
sequentially.


David Somers (dsomers at park.bu.edu)
Center for Adaptive Systems
111 Cummington St.
Boston, MA 02215