Electrical brain oscillations (waves) have been observed for over a hundred years, but it has been hard to pin down what they do. It has been extremely attractive to hypothesize that they knit together various areas of the brain in cognitive coalitions. The brain hosts a great deal of resting activity, and by this kind of theory, it is typically disorganized. The long-range rhythmic harmonization of various areas could form integrated cognition out of all this noise, both conducting information and linking it together, thus solving the binding problem in an elegant way that accords with the speed and spontenaity thought.
"Recently, it has become evident that these brain rhythms are not just a generic sign of the brain-at-work, but actually reflect a highly flexible mechanism for information encoding and transfer. In particular, it has been suggested that oscillatory synchronization between different areas of the cortex underlies the establishment of task-relevant networks."
But how can we tell whether all this is actually going on? Brain scanning can say what is active and when, to a rough degree, so we can trace a long train of activities that follow, say, the presentation of a face to someone's vision. But we can not see what is contained in those oscillations- the code remains rather secret. There are also many different oscillations, doing quite different things. Sleep involves some very heavy-duty slow waves, muscle coordination seems to involve medium frequency waves, as does restful but inattentive wakefulness, while the cognition-related hypotheses above generally invoke the higher frequency gamma waves.
Experimenters have started doing intervention studies that try to get beyond the correlation conundrum by actively manipulating electrical activity in the brain. Obviously, this is quite difficult to do. In rare instances, people are getting electrodes implanted in their brains for other reasons like treatment for epilepsy or Parkinson's, and allow limited research as a side project. The other option is to use transcranial magnetic fields or electrical stimulation, (shades of Frankenstein!), which are obviously rather gross interventions with little ability to focus effects to defined volumes inside the brain. Thankfully, however, some of the interesting activity of the brain happens close to the surface / skull.
The current researchers (review) use "transcranial alternating current stimulation", or tACS, which is pretty much putting current directly through the head with electrodes, presumably at low levels. They ask whether such stimulation, with its alternating current timed either in synchrony with the endogenous gamma rhythm (40 Hz), or against it, can alter a subject's perception according to the theory that brain oscillations constitute the cognitive binding of disparate brain regions.
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| Schematic of experiments. The visual area is in the rear of the brain. Subjects were shown ambiguous dot designs that are interpreted as horizontal motion half the time. Then they were given direct electrical stimulation with electrodes at the back of the head, either in phase or out of phase with the endogenous gamma rhythm. |
The perception they decide to use is visual motion, which has been correlated with gamma oscillation coherence between the right and left visual areas of the brain. An ambiguous motion on a screen can be interpreted as either vertical or horizontal motion, roughly half the time each. The subject is asked which one it is, and this reflects more about the state of their brain than it does about the visual stimulus. Some increased amount of coherence of gamma oscillations is known to correlate especially with (subjective) horizontal motion, intriguingly enough, and the researchers track that through their own subjects.
Then they apply the jumper cables. "A sinusoidally alternating current of 1,000 µA (peak-to-peak) was applied at 40 Hz continuously for 20 minutes during each session." What they found was that the perceived motion could be slightly, but significantly, shifted in the expected direction if brain oscillations are causally important to cognition. When applied in phase with the subject's endogenous cross-brain rhythm, subjective horizontal motion increased, while when it was applied out of phase, thus decreasing the cross-brain coherence, subjective horizontal motion decreased.
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| The result, that perception of motion is affected by the phase of the applied current. |
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| Incidentally, the applied current causes slight but measurable change to the gamma coherence between the rear visual areas. |
A couple of other papers use open-brain studies to reach the same conclusion, for other aspects of cognition:
"We found increases in high gamma (HG) power (70–250 Hz) time-locked to trial onset that remained elevated throughout the attentional allocation period over frontal, parietal, and visual areas. These HG power increases were modulated by the phase of the ongoing delta/theta (2–5 Hz) oscillation during attentional allocation. Critically, we found that the strength of this delta/theta phase-HG amplitude coupling predicted reaction times to detected targets on a trial-by-trial basis. These results highlight the role of delta/theta phase-HG amplitude coupling as a mechanism for sub-second facilitation and coordination within human fronto-parietal cortex that is guided by momentary attentional demands."
"Neocortical-ATN theta oscillatory phase synchrony of local field potentials and neocortical-theta-to-ATN-gamma cross-frequency coupling during presentation of complex photographic scenes predicted later memory for the scenes, demonstrating a key role for the ATN in human memory encoding."
So the role of high-frequency brain oscillations looks increasingly secure as a mode of information transfer, binding and management within the brain. Whether this phenomenon also constitutes conscious perception, forming the thoughts whose contents and sources are so disparate and wide-spread through the brain and body will be the next enormous question to tackle.
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