A little more research about how attention affects visual computation.
Brain waves are of enormous interest, and their significance has gradually resolved over recent decades. They appear to represent synchronous firing of relatively large populations of neurons, and thus the transfer of information from place to place in the brain. They also induce other neurons to entrain with them. The brain is an unstable apparatus, never entraining fully with any one particular signal (that way lies epilepsy). Rather, the default mode of the brain is humming along with a variety of transient signals and thus brain waves as our thoughts, both conscious and unconscious, wander over space and time.
A recent paper developed this growing insight a bit further, by analyzing forward and backward brainwave relations in visual perception. Perception takes place in a progressive way at the back of the brain in the visual cortex, which develops the raw elements of a visual scene (already extensively pre-processed by the retina) into more abstract, useful representations, until we ... see a car, or recognize a face. At the same time, we perceive very selectively, only attending to very small parts of the visual scene, always on the go to other parts and things of interest. There is a feedback process, once things in a scene are recognized, to either attend to them more, or go on to other things. The "spotlight of attention" can direct visual processing, not just by filtering what comes out of the sausage grinder, but actually reaching into the visual cortex to direct processing to specific things. And this goes for all aspects of our cognition, which are likewise a cycle of search, perceive, evaluate, and search some more.
Visual processing generates gamma waves of information in an EEG, directed to, among other areas, the frontal cortex that does more general evaluation of visual information. Gamma waves are the highest frequency brain oscillations, (about 50-100 Hz), and thus are the most information rich, per unit time. This paper also confirmed that top-down oscillations, in contrast, are in the alpha / beta frequencies, (about 5-20 Hz). What they attempted was to link these to show that the top-down beta oscillations entrain and control the bottom-up gamma oscillations. The idea was to literally close the loop on attentional control over visual processing. This was all done in humans, using EEG to measure oscillations all over the brain, and TMS (transcranial magnetic stimulation) to experimentally induce top-down currents from the frontal cortex as their subjects looked at visual fields.
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| Correlation of frontal beta frequencies onto gamma frequencies from the visual cortex, while visual stimulus and TMS stimulation are both present. At top left is the overall data, showing how gamma cycles from the hind brain fall into various portions of a single beta wave, (bottom), after TMS induction on the forebrain. There is strong entrainment, a bit like AM radio amplitude modulation, where the higher frequency signal (one example top right) sits within the lower-frequency beta signal (bottom right). |
I can not really speak to the technical details and quality of this data, but it is clear that the field is settling into this model of what brain waves are and how they reflect what is going on under the hood. Since we are doing all sorts of thinking all the time, it takes a great deal of sifting and analysis to come up with the kind of data shown here, out of raw EEG from electrodes merely placed all over the surface of the skull. But it also makes a great deal of sense, first that the far richer information of visual bottom-up data comes in higher frequencies, while the controlling information takes lower frequencies. And second, that brain waves are not just a passive reflection of passing reflections, but are used actively in the brain to entrain some thoughts, accentuating them and bringing them to attention, while de-emphasizing others, shunting them to unconsciousness, or to oblivion.
- Similar processes occur in working memory.
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