Alpha oscillations of the brain prefigure the saccades by which our eyes move as we read.
Reading is a complicated activity. We scan symbols on a page, focusing on some in turn, while scanning along for the next one. Data goes to the brain not in full images, but in the complex coding of differences from moment to moment. Simultaneously, various levels of processing in the brain decode the dark and light spots, the letter forms, the word chunks, the phrases, and on up to the ideas being conveyed.
While our brain is not rigidly clocked like a computer, (where each step of computation happens in sync with the master clock), it does have dynamic oscillations at several different frequencies and ranging over variable regions and coalitions of neurons that organize its processing. And the eye is really a part of that same central nervous system- an outpost that conveys so much sensitive information, both in and out.
We take in visual scenes by jerks, or saccades, using our peripheral vision to orient generally and detect noteworthy portions, then bringing our high-acuity fovea to focus on them. The eye moves about four times per second, a span that is used to process the current scene and to plan where to shift next. Alpha oscillations (about 10 per second) in the brain, which are inhibitory, are known to (anti-) correlate with motor control of the saccade period. The processing of the visual sensory system resets its oscillations with each shift in scene, so is keyed to saccades in a receiving sense. Since vision only happens in the rest/focal periods between saccades, it is helpful, conceptually, to coordinate the two processes so that the visual processing system is maximally receptive (via its oscillatory phase) at the same time that the eye comes to rest after a saccade and sends it a new scene. Conversely, the visual sensory system would presumably tell the motor system when it was done processing the last unit, to gate a shift to the next scene.
A recent paper extended this work to ask how brain oscillations relate to the specific visual task of reading, including texts that are more or less difficult to comprehend. They used the non-invasive method of magnetic encephalography to visualize electrical activity within the brains of people reading. The duration of saccades were very uniform, (and short), while the times spent paused on each focal point (word) varied slightly with how difficult the word was to parse. It is worth noting that no evidence supports the lexical processing of words out of the peripheral vision- this only happens from foveal/focused images.
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| Subjects spent more time focused on rare/difficult words than on easy words, during a free reading exercise (C). On the other hand, the duration of saccades to such words was unchanged (D). |
In the author's main finding, alpha oscillations were correlated as the person shifted from word to word, pausing to view each one. These oscillations tracked the pausing more closely when shifting towards more difficult words, rather than to simple words. And these peaks of phase locking happened anatomically in the Brodmann area 7, which is a motor area that mediates between the visual system and motor control of the eye. Presumably this results from communication from the visual processing area to the visual motor area, just next door. They also found that the phase locking was strongest for the start of saccades, not their end, when the scene comes back into focus. This may simply be a timing issue, since there are lags at all points in the visual processing system, and since the saccade duration is relatively fixed, this interval may be appropriate to keep the motor and sensory areas in effective synchronization.
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| Alpha oscillation locks to some degree with initiation of saccades, and does so more strongly when heading to difficult words, rather than to easy words. Figure B shows the difference in alpha power between the easy and difficult word target. How can this be? |
So while higher frequency (gamma) oscillations participate in sensory processing of vision, this lower alpha frequency is dominant in the area that controls eye movement, in keeping with muscle control mechanisms more generally. But it does raise the question of why they found a signal (phase locking for the initiation of a saccade) for the difficulty of the upcoming word, before it was actually lexically processed. The peripheral visual system is evidently making some rough guess, perhaps by size or some other property, of the difficulty of words, prior to fully decoding them, and it will be interesting to learn where this analysis is done.
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