Hi Dr. Jones,
Your post is Beautiful, and I've no problem with anything that's
in-it except with respect to the final sentence.
While what's being referred to as 'synchrony' does reflect
'cross-correlation' with respect to activation [of course], it
doesn't reflect anything with respect to specific
information-content, so 'synchrony' says nothing about 'percept'
other than the trivial fact that, yeah, 'thought' happens.
In the future, as monitoring techniques continue to become
more-precise, folks'll see that everything is, in fact, asynchronous,
fleetingly-dynamic, energy-flow gradients.
My 'point' is subtle, and I don't expect anyone to agree, but the way
that 'oscillations' have come to be routinely invoked makes me want
to 'throw-up', because of the 'herd-mentality' inherent, and the way
it gives short-shrift to almost all of what's in the functional
Neuroanatomy.
It's been a 'pet-peeve' of mine. I'm Sorry that such has
'rubbed'-against you.
Cheers, k. p. collins
Matt Jones wrote in message ...
>mats_trash at hotmail.com (mat) wrote in message
news:<43525ce3.0207240235.5989ab74 at posting.google.com>...
>> Im reading around the literature on nerual oscillations given the
>> current vogue for explaining many aspects of function (binding
>> problem, consciousness) through them. However, I'm a little
confused
>> as to what is actually referred to by 'oscillation' (i.e. what is
>> oscillating?). Is it the fluctuation of resting membrane
potentials
>> or is it more about the sequential firing of spatially distributed
at
>> certain frequencies. i.e. given neurons A, B Is the oscillation
>> A-B-A-B-A-B
>>>> Cheers for any explanations or refs to that effect.
>>>Hi mats,
>>When people talk about oscillations in the brain, they essentially
>mean rhythmic activity that can be detected at the EEG level. The
EEG
>(electroencephalogram) works by recording synchronous activity
across
>fairly large populations of neurons.
>>As you know, neural electrical activity is comprised of changes in
the
>membrane conductance of individual neurons. When ion channels open
>(e.g., during synaptic transmission or during an action potential),
>current flows across the membrane. This has two related effects: 1)
it
>changes the cells membrane potential, and 2) it causes a small
change
>in the distribution of charges inside and outside the cell. If you
>perform a "single unit" recording in a living animal, what you are
>doing is placing a small wire -outside- a neuron. Since you're
>outside, you can't see small changes in membrane potential the way
you
>could if you were making an intracellular recording. But you -can-
>detect the small change in extracellular charge distribution that
>occurs when the neuron fires a spike. This is because during a
spike,
>a truly huge number of ion channels are doing the same thing all at
>the same time. The tiny local effects of each channel all sum up
>together to give a just-barely-detectable change in the
extracellular
>charge. This change in charge results in a small, quick change in
the
>local voltage, and if your electrode is near enough (i.e., a few
>microns), you can measure it.
>>However, the small local voltage change from a single cell, even
>during a spike, is too small to be seen with the EEG, which is
usually
>an array of electrodes farther away in the tissue, or most often, at
>the surface of the scalp. But if you can get -many- neurons to fire
>spikes at the same time, then the local extracellular potentials all
>add up together, and you can detect it. This change in potential
from
>many synchronous individual neurons goes by many names: local field
>potential (LFP), event-related potential (ERP), or
>electroencephalogram (EEG). The core idea is that you are recording
>simultaneous (but not necesserily rhythmic) activity from many
cells,
>usually hundreds, thousands, or millions in the case of the scalp
EEG.
>>>Now, oscillations: During many different behaviors, it turns out
that
>the EEG does in fact display some rhythmic behavior. That is, if you
>look at it, you can actually see repeating wiggles at certain
>frequencies. In practice, you would take the wiggly signal and pass
it
>through a spectrum analyzer or FFT, and note where the peaks
occurred.
>For example, during exploration in rats, the EEG recorded near the
>hippocampus develops a prominent peak around 4-14 Hz (so called
theta
>rhythm). During certain other behaviors (usually involving that
>"binding" thingy), the prominent rhythm in various parts of cortex
is
>around 20-80 Hz (so called gamma rhythm). There's a bunch of other
>greek letter rhythms too, but I forget when and where they occur.
>>What these rhythms signify is that lots and lots of neurons are
doing
>whatever they're doing in a roughly synchronous concerted manner.
THIS
>DOES NOT MEAN THAT EVERY NEURON IS FIRING AT 20 Hz !!!! In fact,
I'm
>pretty sure that what EEG, ERP and LFP are most often measuring is
>-not- spike firing, but rather the slower subthreshold potentials
>associated with synaptic potentials. In a "single-unit" recording,
you
>can also see these synaptic potentials from large numbers of cells,
>but typically one is interested in the spikes, so one high-pass
>filters out anything slower than spikes. It is possible to get both
>the spike infrmation and the LFP information by passing the same
>signal throu two different filters.
>>OK, so that's what people -really- mean when they talk about
>oscillations. But nowadays, people are always talking about
>synchronous spiking in the same way that they talk about
oscillations,
>so what's up with that? Well it turns out that you can try and
>determine whether two neurons (possibly in different regions of the
>brain) are participating in oscillations with the same frequency and
>phase, by analyzing the crosscorrelation between their spiketrains.
>The idea is that if the two spike trains are crosscorrelated with
each
>other, one way of getting such a crosscorrelation (xcorr) would be
if
>they were both firing spikes at approximately the same times. This
>shows up as a peak near zero milliseconds in the xcorr. If you see
>such a peak, you might be tempted to shout from the rooftops that
>these two cells were spiking together, and since one is in the
>auditory system and the other one is in the visual system (for
>example), you might conclude that they were both encoding part of
the
>same complex "percept" (e.g., the sound of screeching tires and the
>image of a rapidly approaching truck). This is the idea behind the
>whole business of synchrony, oscillations, and the "binding" thingy.
>>And maybe this is indeed what's going on.
>>However, there are also other possibilities. For example, there are
>other ways of getting two spiketrains to be correlated besides
>synchronizing the spikes between two neurons. One way is to simply
>have both neurons -start- firing at the same time, but fire each
spike
>randomly (i.e., correlated latency, without synchrony). Another way
is
>to have both neurons fire at random times but have their mean spike
>rates be correlated. So crosscorrelataion does not imply synchrony
>(which is in fact the title of a very nice paper on the subject by
>Carlos Brody - do a medline search on him for more info on the these
>issues). On the other hand, I believe synchrony -does- strongly
imply
>crosscorrelation.
>>Cheers,
>>Matt
