Wednesday, July 27, 2005

Blink

Fascinating study on what happens in your brain when you blink:
BBC: Parts of the brain are temporarily "switched off" when we blink, scientists have found. The team from University College London found the brain shut down parts of the visual system for each blink.

Writing in Current Biology, they said this was the case even if light was still entering the eyes. The researchers said this could explain why people do not notice their own blinking, as it gave us an "uninterrupted view of the world". ...

They found that blinking suppressed brain activity in the visual cortex and other areas of the brain - known as parietal and prefrontal - which are usually activated when people become conscious of visual events or objects in the outside world.*
Something phenomologically similar happens during saccades**: when you dart your eyes around the visual field, you don't perceive the blurred image that you would see if a video camera did the same thing, because your brain just ignores visual input during saccades. In fact, there have been experiments where an eye tracker tells a computer when a subject is making a saccade while looking at an image on the computer screen, and the computer changes a major aspect of the image during the saccade. Invariably, the subjects don't notice the change.
The changes can be quite dramatic and can even occur with objects that a subject has just fixated upon. Still they remain unnoticed. For example, a prominent building in a city skyline became 25% larger and 100% of the subjects failed to detect any change. One hundred percent also failed to notice that two men exchanged hats of different colors and styles.
This is a special case of change blindness, where people fail to notice extremely obvious changes in the visual scene that occur across a visual discontinuity (e.g., by a blank screen, a blink, or a saccade) or they are paying attention to something else. Hence the usual failure to notice continuity errors in movies.

Daniel Dennett has elaborated on the implications of these related phenomena for visual consciousness: we never take in a visual scene all at once, because we can only really clearly see the focus of our vision. Instead, our brains build up a coherent image of the world as a composite of all the tiny snapshots taken by our eyes constantly darting around. Visual consciousness is a fictional narrative constructed by the brain to make sense of visual input. Perturbing the narrative by switching the visual scene in the middle of a saccade or a blink reveals just how much it is constructed out of bits and pieces.

Actually, Dennett takes this further and argues against the common intuition that after the brain puts together this narrative, it goes "on display" in a sort of "Cartesian theater" where the "real you" sits there watching the processed movie of what's coming in through your eyes. But this is just dualism - in fact, this pieced-together narrative is consciousness. Pretty cool.

*Footnote for those interested in how the study was done. The way they designed the study was ingenious: in order to get around the problem that when you close your eyes, your visual input and hence brain activity changes drastically, they designed a special light that subjects put in their mouths that shone light through the bone into the retina, while they wore goggles to block light coming in through the pupil (see slightly creepy picture). Meanwhile, to get around the problem that a blink lasts a fraction of a second while fMRI has a temporal resolution on the order of 2 seconds, they had subjects blink "at a fast regular rate" during "blink" periods, and just blink at a spontaneous resting rate during "no blinking" periods.

**Interestingly, though, while blinking suppresses activity in the parietal and prefrontal cortex, the neural effect of saccades apparently reaches only as far up as the visual cortex. This is somewhat odd as blinks and saccades seem quite similar qualitatively, but these fMRI studies are both quite new, so hopefully more studies will be forthcoming.

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