Blink, and the brain misses it

We would immediately notice if the outside world suddenly went dark every few seconds. But we rarely become aware of our blinks, even though they cause a similar reduction in the amount of light entering the eye. So why are we not aware of the frequent mini-blackouts caused by blinks? In the 1980s, scientists discovered that visual sensitivity begins decreasing immediately before a blink, but the brain mechanisms underlying this process have until now remained unclear. As they report this week, Davina Bristow and a team of scientists at University College London led by Dr. Geraint Rees may now have found a reason for why blinks go unnoticed.

In their study, the researchers devised a clever way to monitor the brain's activity under conditions in which the amount of light received by the eye was constant, regardless of blinking. The researchers achieved this by placing a light-emitting optical fibre in the mouth of volunteers wearing light-proof goggles. Because the light was bright enough to pass through tissues of the face, the fibre could be used to illuminate the retina through the roof of the mouth. Therefore, the amount of light falling on the retina remained constant, even when the volunteers blinked. The researchers then performed a type of brain scan known as functional magnetic resonance imaging and could thus measure whether the act of blinking--independently of any change in light normally caused by eyelid closure--would influence the level of light-activated brain activity.

The UCL scientists found that when volunteers were blinking, brain activity was suppressed in areas that respond to visual input, even though the light falling onto the retina remained constant throughout the blink. Many of these brain areas are activated when people become conscious of visual events or objects in the outside world. "Transiently suppressing these brain areas involved in visual awareness during blinks may be a neural mechanism for preventing the brain from becoming aware of the eyelid sweeping down over the pupil during a blink and the world going dark," explains Professor Frith. In summing up the study's implications, the authors suggest that when we blink, the brain may just miss it.

The researchers include Davina Bristow, John-Dylan Haynes, Richard Sylvester, Christopher D. Frith, and Geraint Rees of University College London. The Wellcome Trust funded this work.

Bristow et al.: "Blinking suppresses the neural response to unchanging retinal stimulation." Publishing in Current Biology, Vol. 15, 1296–1300, July 26, 2005. DOI 10.1016/j.cub.2005.06.025 www.current-biology.com

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