If you were seated in a dark room and the lights flickered off every few seconds, you'd definitely notice. Yet when your blinks make the world go momentarily dark – and bear in mind most of us perform around 12 to 15 of these every minute – you are mostly oblivious. It certainly doesn't feel like someone is flicking the lights on and off. How can this be?A new study in Journal of Experimental Psychology: Human Perception and Performance has tested two possibilities – one is that after each blink your brain "backdates" the visual world by the duration of the blink (just as it does for saccadic eye movements, giving rise to the stopped clock illusion); the other is that it "fills in" the blanks created by blinks using a kind of perceptual memory of the visual scene. Neither explanation was supported by the findings, which means that the illusion of visual continuity that we experience through our blinks remains a mystery.One experiment involved students making several judgments about how long a letter 'A' was presented on a computer screen (the actual durations were between 200ms to 1600ms; 1000ms equals 1 second). Sometimes the 'A' appeared at the beginning or end of a voluntary eye blink, other times it appeared during a period when the participant did not blink. If we backdate visual events that occur during blinks, then the 'A's that appeared at the beginning or end of a blink should have been backdated to the onset of the blink, giving the illusion that they'd been presented longer than they actually had, as compared with 'A's that appeared when there was no blink. In fact, the researchers found no evidence that the students overestimated the duration of 'A's that appeared during blinks.
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Another experiment involved students making a voluntary blink while a letter 'A' was already onscreen and making a judgment of how long the 'A' was visible, and also making judgments about the duration of other 'A's that were onscreen during non-blink periods. If backdating or perceptual "filling in" occurs during blinks, then the students should have judged the time onscreen of an 'A' of a given duration as the same whether they blinked during its appearance or they didn't. But this isn't want the researchers found – rather, the students consistently underestimated the duration of 'A's if they blinked during their appearance.
We do know from past research that the brain to some extent shuts down visual processing during blinks – a study from the 80s shone a light up through people's mouths and found their ability to detect changes in its brightness was reduced during blinks, even though the blinks obviously didn't impede the light source. But what the new research shows is still unclear is how the brain weaves the loss of visual input during blinks into a seamless perceptual experience.
Summing up, the University of Illinois researchers David Irwin and Maria Robinson said the brain seems to ignore the perceptual consequences of blinks, but they're not sure how this is done. "Having ruled out the temporal antedating and perceptual maintenance hypotheses," they said, "the question still remains: Why does the visual world appear continuous across eye blinks?".
