Why the world doesn't go black when your eyelids flicker.
Why doesn't the world go dark when we blink? Because a critical part of the brain switches off and fails to detect the blackness behind closed eyes, says a team of neuroscientists.
We blink about ten times a minute without noticing any change in what we see. Researchers had suspected that this is because the visual system is inactivated during blinking, but were not able to prove this.
“It's not that the visual gap is filled in. It's that you're not aware of it. Davina Bristow , University College London”
A team at University College London have cracked the problem by inserting an optical fibre into the mouths of people wearing black-out goggles. The fibre illuminated the back of the subjects' retinas, so that they saw a light at all times, even when they blinked. This allowed the researchers, who publish their results in Current Biology1, to distinguish between the effects of the act of blinking and the darkness that it causes.
Using magnetic resonance imaging (fMRI) brains scans, Davina Bristow and colleagues revealed that activity in a part of the visual system known as V3 was suppressed in subjects when they blinked. V3 is one of a series of brain areas that handle signals sent from the eyes. With it out of action, the blink goes unnoticed.
"It's not that the visual gap is filled in," says Bristow. "It's that you're not aware of it."
Blinded by the light
Similar inactivation is known to allow us to see a smooth image even when our eyes jump between two parts of a scene. But working out what happens in the brain during blinking has proven very difficult.
Blinks normally cause an abrupt change in the amount of light reaching the retina, which in turn causes a massive change in brain activity in the visual region. Seeing any effect in the V3 area has been obscured by this in the past.
"Blinks are hard to study," says Tim Gawne, a vision researcher at the University of Alabama in Birmingham.
Blinking is not the only process that causes areas of the brain to be suppressed. Bristow points out that fMRI scans have shown that tactile areas of the brain are suppressed when we tickle ourselves, but not when someone else does it2.
Bristow says studies of tickling and blinking add to our understanding of how our brains deal with different types of events.
"How do we distinguish between what is caused by you and what is caused by the outside world?" she asks. "It's more important, for all animals, to pay attention to outside causes. Blinking is a way of studying how they do so."
BristowD., HaynesJ. D., SylvesterR., FrithC. G. & ReesG.Current Biology, 15. 1296 - 1300 (2005).
BlakemoreS. J., WolpertD. M. & FrithC. D. Nat. Neurosci., 1. 635 - 640 (1998).
University College London