2/16/2006

Entangled Oscillations

One common way of thinking about brain activity is that networks of brain areas activate in sequences corresponding to different cognitive processes. However, as pointed out by Lawrence Ward in his article "Synchronous Neural Oscillations and Cognitive Processes", this way of thinking actually conceals the importance of oscillatory computation, or as Ward puts it, "reverberations of reentrant activity in complex neural networks."

One way of illustrating the importance of these oscillations is that different frequency bands are correlated with different types of cognitive change. For example, spectral power in alpha bands increases while theta and delta frequencies decrease in maturing children; the opposite trend is observed in the elderly. While alpha frequencies were thought to primarily reflect search and retrieval processes, and theta oscillations were correlated with encoding processes in working memory, some new evidence refines these hypothesized roles.

Global theta oscillations are more prominent when subjects navigate mazes without memory cues, and these frequencies are more prevalent the longer the decision time at each turn. Such evidence suggests that the theta band has a role in retrieval processes. Likewise, gamma (40 Hz) oscillations briefly interface the rhinal cortex with the hippocampus during successful memory formation, are prevalent in successful target detection, and have been proposed more generally to be responsible for the effective transmission of information over long cortical distances. Ward reviews compatible evidence, that during successful recollection gamma-band activity is actually modulated by theta waves between frontal and parietal cortex.

So, do gamma- and theta waves reverberate through the network responsible for short-term memory? Ward reviews one such hypothesis, in the form of Lisman and Idiart's computational model of working memory, in which synchronous firing through recurrent pyramidal connections is able only to preserve information transiently, and must therefore be briefly "refreshed" at roughly 40 Hz (gamma) once every 100-200 ms (theta). Such waves have indeed been recorded in cortex.

Some quick math provides a tantalizing, if provocative (and admittedly inexact) insight: if working memory span is related to some interaction betweent theta and gamma waves, it is a coincidence that 40/6 is roughly equal to Miller's number 7, plus or minus two? Further, the range of possible spans based on gamma and theta variability (30 to 70 Hz, and 3.5 to 7 Hz respectively) falls within the range of working memory capacities observed in humans (roughly 3 to 20). This is compatible with earlier suggestions that if oscillations of "perceptual sampling" are responsible for the wagon wheel illusion, they may be closely related to visuo-spatial working memory and/or processing speed.

As Ward admits, these numbers are not universally accepted and the hypothesis is still an empirical question. But some converging evidence comes in the form of a kind of gamma "frequency following response," based on experiments in which the rate of auditory clicks (presented at near-gamma frequencies) was seen to influence WM span. According to Ward's analysis, this "confirms the importance of a gamma-clocked process." One wonders whether other frequency bands might be subject to the same clocking or frequency following response process, perhaps as illustrated in popular depictions of hypnosis.

Along those lines, alpha-band power appears to correlate with some aspects of attention, particularly suppression processes and behaviors such as infant habituation. Alpha waves are seen to increase during memory load, external task load, and cued anticipation of an auditory stimulus. Alpha-band oscillations can even be localized to those exact regions of retinotopic visual cortex in which distractors are expected to appear, as though alpha oscillations are somehow responsible for (or the result of?) supression processes. Finally, alpha-band oscillations are also thought to be phase locked with external stimuli, which may allow peaks in attentional dynamics (such as capacity or switching) to be synchronized with the time course of environmental changes. This suggestion is compatible with new EEG data from attentional blink paradigms, which appears to be a gamma rhythm modulated by an alpha or theta wave.

Koch, Tononi, and others, have even gone so far as to propose that a global, dynamic core of intermixed oscillations may somehow provide a foundation for consciousness. According to this framework, local oscillations only enter conscious awareness when they become integrated with the dynamic core. On its surface, this view of consciousness is compatible with some theories of attention, although clearly it does not specify in detail how consciousness might arise from these oscillations.

Related Posts:
Neural Oscillations and the Mozart Effect
Perceptual Sampling: The Wagon Wheel Illusion
The Mind's Eye: Models of the Attentional Blink
Neural Network Visualization
Anticipation and Synchronization
Gamma Synchrony
Synchrony vs. Polychrony

7 Comments:

Blogger Bob Mottram said...

That's an interesting article. Something caught my attention towards the end under questions for future research. If the centres of EEG frequency bands are powers of two this clearly suggests to me an analogue implementation of a 5 bit binary code. The magical 7 +/- 2 then just becomes one code out of a possible 32 permissable within synchronous states. Under this regime brain waves would become phase locked with sensory stimuli in order to maintain a reliable encoding of their information content.

2/17/2006 01:42:00 PM  
Blogger Chris Chatham said...

Hi Bob - I don't personally think that's the way working memory emerges from neural computation, but a large part of the field seems to think that the brain must be doing some kind of abstract symbolic computation. Your idea might be compatible with the way they view what's going on here, but I wouldn't really know since they rarely mention exactly where or how these symbols emerge.

2/17/2006 02:38:00 PM  
Blogger Bob Mottram said...

Well it is an interesting observation. Why should evolution have selected powers of two, and not some arbitrary set of frequencies? If frequently distribution was not important for brain function surely a linear separation would give optimal performance with minimal interference.

Using powers of two does mean that frequencies could actually encode some kind of information with a number of channels greater than just the basic 5.

2/17/2006 02:57:00 PM  
Blogger Chris Chatham said...

Unless increasingly high frequencies are less reliably produced by neurons, in which case you'd need exactly this kind of exponential difference between bands.

Still, I admit that you could be right, and the idea is indeed very interesting. And I certainly agree with the part about waves becoming phase locked with sensory stimuli.

2/17/2006 03:27:00 PM  
Blogger Bob Mottram said...

There might be another explanation. When you look at simulations such as those done by Eugene Izhikevich the frequencies which are observed are really a consequence of conductance delays between neurons, which in turn are mainly governed by axon length. It might be that the different brain wave frequencies coincide with the activation of certain classes of neural circuitry within the brain with specific spatial architectures. Spatial arrangements which include longer axons, such as thalamo-cortical ones, would be more likely to give rise to lower frequency waves, whereas inter-cortical communications where shorter distances are involved could give rise to higher frequencies.

2/17/2006 03:59:00 PM  
Blogger Chris Chatham said...

Yes, now this is an idea that I am much more inclined to agree with. I think the axonal conduction delays are indeed a significant but often overlooked feature of neural networks, and you bring up a really neat point which is that the suppression activity recorded as alpha waves could actually be a result of the kinds of architectures produced by inhibitory interneurons. This point also illustrates that these EEG recordings could simply be artifacts of the connectivity patterns, and not be explicitly coding anything in their own right. Unfortunately I don't know enough about this side of things to go much further with this line of logic... but it's very fun to brainstorm about.

2/17/2006 04:10:00 PM  
Blogger Chris Chatham said...

Some interesting synchronicity here... I'm just now reading one of the minireviews on oscillation in the newest issue of J Neurosci, and what do I find but this:

"Experimental
studies on rodent cortex implicate inhibitory interneuron networks
as sources of the oscillations and potential targets of topdown
inputs. In particular, excitatory input can rapidly synchronize
a subset of the inhibitory neurons that are in competition
with other inhibitory networks (Tiesinga and Sejnowski, 2004)."


Sounds to me like we're on the right track...

2/17/2006 04:19:00 PM  

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