Monday, July 25, 2011

Could Neural Oscillations Correspond to the Cycling of Imagery Between Sensory and Association Areas?

Brain waves are regularly occurring front-to-back cycles of propagating energy, as this graphic depicts. (Credit, Nature Magazine).

Since the 1930s, scientists have known that the brain has fluctuating fields of electric current that cycle back and forth from one area of the brain to another. These fields cycle very fast, several times per second in fact. The location and the speed of cycling, which is measured in hertz (cycles per second), tells us what kind of mental state one is in from intense thought (30+ hz.) to listlessness (around 10 hz.) to sleep (4 hz. and lower). These cycles can be measured by electroencephalography where electrodes placed on the scalp can measure variations in local activity. I think that these cycles may actually correspond to cycles of cortical activation between bottom-up sensory areas and top-down association areas. In other words, these brain waves are created by the process of throwing perceptions forwards and interpretations of those perceptions backward. Perhaps the back and forth reciprocations between bottom-up and top-down cognition map directly on to neural oscillations.
When you first open your eyes and see a new scene, bottom-up visual areas piece together the visual elements in front of you to create an image of the present scenery. But these early areas don’t know what the objects in front of you are or what to do with them they only piece together a picture of them. These early visual processing areas send the image on to higher association areas “downstream” that combine the knowledge about the existence of the objects with other concerns normally related with these types of objects and then send this message back to the bottom-up areas. The message that is sent backwards will subtly influence how you perceive your surroundings and how you understand them. Our whole world is pieced together from this back and forth cross talk. The early bottom-up areas, to some degree, wait for a split second after sending off their report about what they have perceived. This delay allows the top-down areas a little bit of time to process what it was sent and to send it back. Then the top-down area waits its turn, again for a split second. If this waiting didn’t occur to some degree, and if these brain areas just sent randomly timed messages, then the backward to forward cycling could not be measured with electrodes. Also, perhaps adjacent brain areas have to give each other time to finish their modularized processing. For instance, after association areas hand down information to early visual cortex (V1), the visual cortex needs a little time to use these specifications to create imagery which it will then relay back to the association cortices.
Maybe all of these bottom-up to top-down reciprocations are organized into very precise oscillations that propagate in regularly timed intervals across the brain so that they do not interfere with each other. The oscillations reciprocate back and forth at just the right speed so that each area has the time to process its inputs and send an output before the next complement of inputs arrive. Perhaps, messaging would be muddled if areas were to get information while they are processing, or if they didn’t receive all of their inputs at the same time. I wonder if it is possible to determine empirically if these bottom-up to top-down cycles of imagery map on neatly to the synchronized oscillations of neural populations known to give rise to macroscopic oscillatory electric fields, which can be observed in the electroencephalogram. I used to imagine that these oscillations traveled from the very front of the brain to the very back, giving all areas a chance to add their two cents to the imagery that was being created. Many of these reciprocating fields are much more localized though, and it will be interesting to understand how they interact with each other and spread in the brain. Also, there is not enough time for the brain to send information across the entire cortex 40 times every second (a common cycling frequency in the brain during heavy concentration and “cross-modal” processing).
Could it possibly be the case that when our brain is oscillating in the range of 30 hz, we are creating and then analyzing 30 mental images per second? I try to ask myself: “can you see images flitting back and forth several times per second?” It is probably impossible to perceive this “fabric” of thought because the mental act of noticing this and being aware of it would itself necessitate a number of oscillations. If true, this would mean that we are always experiencing life at these oscillatory intervals but we cannot notice it because it takes multiple intervals to notice and analyze a single interval. At any rate, it will take years of further research to determine if brain waves map directly onto patterns of bottom-up to top-down processing.

Read the full article that I wrote on this topic here:

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