readings> saccades and consciousness

Set the kids loose with the camcorder and the result is always the same: a mad dancing picture as if the scenes had been shot from a particularly hairy fair ground ride. Why can’t they let the camera linger?

As any neurologist knows, the lurching lens is just imitating the child’s saccading eye. Kids naturally expect to be able to shift their attention in eyeball-swivelling leaps three or four times a second and still see a perfectly stable world. So no mystery. Yet in fact the business of saccades remains an especially troublesome one for consciousness theorists.

In the 1990s, the big issue was the binding problem. If the cortex is a hierarchy of processing modules, then how do the various computations get glued together to make a seamless conscious whole? This question has been reasonably well answered with the discovery that coherent firing rhythms help knit widely scattered neurons into fleeting global ensembles. Now mind scientists are asking the same kind of question but concerning the unity of consciousness over seconds rather than milliseconds.

The naive view of the brain is that it merely reflects what the sense organs report. Yet if the eyes are skipping from fixation to fixation, then the resulting cortex mappings must leap about like the pictures from the kids’ camcorder. So how can a steady stream of experience be constructed from such violently wrenching foundations?

The need for saccades is clear enough. The cone-dense fovea covers only 1 degree of the visual field. So it’s reckoned that to have foveal level acuity over the whole retina, the brain would need to be hundreds of thousands times bigger and weigh ten tons!

Saccading itself is a complex affair. Even when the eyes are fixated, there are constant tremors and microsaccades that keep the retina refreshed. Saccades then come at two rates depending on how habitual or routine the situation happens to be. With highly predictable events, our eyes can flick to the spot in as little as a tenth of a second in an express saccade. For more exploratory looking, it takes a fifth of a second to find a new fixation point.
Of course, during a saccade, retinal output is suppressed to cut down the blur of motion - though recent experiments suggest it is may be rather that the brain immediately forgets any visual input captured while the eye is in transit. But either way, once we count in the 20,000 eyeblinks we make each day, we must be effectively blind about a third of the time! 

Actual fixations last around a third of a second. Another recent surprise is that the time the eye lingers is mostly to do with the processing needs of the brain. A visual image can be “snatched” in a tenth of second. But it seems the brain then holds the eye in place largely to give itself grace to complete its processing. So fixation prevents fresh input before the brain is ready.

Change blindness experiments, in which background visual features are sneakily altered while a subject’s eyes are in mid-saccade, raise yet further questions. It used to be thought that vision was stabilised by a simple anticipatory routine. Each planned eye movement generated an efference copy – an exact forward motor projection – that recalibrated the visual cortex, cancelling every apparent jump in the visual scene.

But change blindness studies demonstrate that the brain can just gloss over some quite abrupt changes in sensory input. It is only if the background features have been highlighted by prior acts of attention that any changes become detectable – apparently a required part of the forward model.

So how does the brain create a coherent stream of experience out of such restless visual machinery? To cut a long story short, the stability of our mental representations seem the result of a clever mix of rapid forgetting and unconscious prediction. Raw sensory information seems to flow through to the cortex more freely than some older theories suggest. But it is just as freely ignored if it does not fit into a general, expectancy-led, running model of the world.

The lurching shifts in visual input are not gated or cancelled out but instead downplayed, rendered vague, as the brain gets on with tracking its own gist-based view of what is happening. So in a neat turnaround, as with the child holding the camcorder, it is the inner narrative which comes first.

The conscious stream – the so-called output – is the foundation for the mental experience. And the wrenching changes in visual input then only get cortically represented to the degree to which they play into the inner tale being told.

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