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Archive for 23/12/2010

The noisy brain

It is generally assumed, currently, that neural synchronization is the method of communication in networks of neurons involved in perception, cognition and action. In a recent paper Ward and others (citation below) have investigated the importance of stochastic resonance in this synchrony. So what is this thing called stochastic resonance?

You will eventually run into stochastic resonance no matter which science you are involved in, from geology to quantum physics. The maths are not easy but the basic idea is simple, at least simple in its simplest form. Suppose you have a surface with two depressions on it and a high area between them. A ball can roll around in the one depression or the other but it has no way to climb out of one once in it. This is a bistable state – two stable states with an unstable state between them. Call the high area the threshold. Now suppose that at a regular interval something gives the ball a pull in the direction of the other depression but not enough of a pull to get it over the barrier. You can visualize this pull as a magnet on a pendulum that swings from directly over the one depression to directly over the other, back and forth. And make the ball an iron one. Call this pull the forcing periodic signal. It can bias the movement of the ball but not enough to get it over the barrier. Now suppose we wiggle the whole affair so that the ball has a fairly large random motion. But this random motion rarely is enough to take the ball over the barrier. This is the stochastic or random component, call it noise. Add the right amount of noise to the signal and presto, the signal added to the noise can often take the ball over the barrier. So a signal that is too weak to be effective is enhanced by the addition of noise. Ordinarily we think of noise as weakening a signal but in this case it is strengthened. Too little noise does not work and too much does not work either. The little extra pull of the signal loses significance when it is drowned in heavy noise. SR only works in a narrow band of noise strength that depends on the nature of the bistable and of the signal. This is a simple, even simplistic, way of visualizing stochastic resonance.

Wikipedia says:

Stochastic resonance (SR) is a phenomenon that occurs in a threshold measurement system (e.g. a man-made instrument or device; a natural cell, organ or organism) when an appropriate measure of information transfer (signal-to-noise ratio, mutual information, coherence, d, etc.) is maximized in the presence of a non-zero level of stochastic input noise thereby lowering the response threshold; the system resonates at a particular noise level.”

It is also assumed by many, on the basis of a number of experiments, that stochastic resonance is part of the environment of neurons in the brain and an ingredient of neural processing of information. But in what way does SR act? Where does the noise originate? What is the signal being brought over a threshold? What is the threshold? Where does the signal go and what does it do? Ward and his co-researchers look that what SR has to do with synchrony.

Here is the abstract:

Neural synchronization is a mechanism whereby functionally specific brain regions establish transient networks for perception, cognition, and action. Direct addition of weak noise (fast random fluctuations) to various neural systems enhances synchronization through the mechanism of stochastic resonance (SR). Moreover, SR also occurs in human perception, cognition, and action. Perception, cognition, and action are closely correlated with, and may depend upon, synchronized oscillations within specialized brain networks. We tested the hypothesis that SR-mediated neural synchronization occurs within and between functionally relevant brain areas and thus could be responsible for behavioral SR. We measured the 40-Hz transient response of the human auditory cortex to brief pure tones. This response arises when the ongoing, random-phase, 40-Hz activity of a group of tuned neurons in the auditory cortex becomes synchronized in response to the onset of an above-threshold sound at its “preferred” frequency. We presented a stream of near-threshold standard sounds in various levels of added broadband noise and measured subjects’ 40-Hz response to the standards in a deviant-detection paradigm using high-density EEG. We used independent component analysis and dipole fitting to locate neural sources of the 40-Hz response in bilateral auditory cortex, left posterior cingulate cortex and left superior frontal gyrus. We found that added noise enhanced the 40-Hz response in all these areas. Moreover, added noise also increased the synchronization between these regions in alpha and gamma frequency bands both during and after the 40-Hz response. Our results demonstrate neural SR in several functionally specific brain regions, including areas not traditionally thought to contribute to the auditory 40-Hz transient response. In addition, we demonstrated SR in the synchronization between these brain regions. Thus, both intra- and inter-regional synchronization of neural activity are facilitated by the addition of moderate amounts of random noise. Because the noise levels in the brain fluctuate with arousal system activity, particularly across sleep-wake cycles, optimal neural noise levels, and thus SR, could be involved in optimizing the formation of task-relevant brain networks at several scales under normal conditions.

Their research seems to indicate that stochastic resonance is contributing to the establishment of synchrony in local sensory areas and also between areas of the brain. As widespread synchrony is one the the hallmarks of the conscious process, we should watch this area of research closely. I noticed two particular results were interesting to me and not included in the abstract.

…it is striking that synchronization in the theta band has a more continuous and general character in this experiment, and is significantly non-zero even in the no-noise condition, whereas that in the alpha and gamma bands is more intermittent and tends to be significantly non-zero only in the added-noise conditions.


…it is apparent that attention did not abolish SR, as SR occurred for both left (attended ear) and right (unattended ear) standards. … The present data reinforce their conclusion that attention does not attenuate noise for near threshold stimuli … Rather, SR operates for weak stimuli in noise whether or not attention is being paid to them.
Ward, L., MacLean, S., & Kirschner, A. (2010). Stochastic Resonance Modulates Neural Synchronization within and between Cortical Sources PLoS ONE, 5 (12) DOI: 10.1371/journal.pone.0014371