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MRI measures neural correlates of the alerting effect of light

Oct 2006
David Shenkenberg

Decreased alertness causes workers to make more errors, increasing costs and resulting in additional damage. If a doctor or a fireman makes a mistake, for example, it could end someone’s life.

Light enhances alertness by eliciting a response from the brain’s nonimage-forming system rather than from the classical visual system. Many studies have assessed the brain’s response to light exposure at night, but few have examined the effect of increased light exposure during the day.

Gilles Vandewalle and colleagues at the University of Liège in Belgium and at the University of Surrey in Guildford, UK, used functional MRI to monitor the brain activity of human subjects who were either exposed to a bright light or left in constant darkness while performing an auditory task. Vandewalle said they chose functional MRI because it exhibits much better temporal resolution than other neuroimaging techniques.

Subjects rested completely before imaging began, which was five hours after they awoke. Imaging occurred before, during and after they were exposed to light with an intensity of more than 7000 lux for 21 minutes. For comparison, external daylight can reach 50,000 lux or more. For the entire imaging period, they performed an auditory task in which they heard a series of low-pitch sounds interrupted by high-pitch sounds, and then counted the number of odd tones. By commanding the subjects’ attention without involving vision, this task was designed to elicit the nonimage-forming system and eliminate classical visual responses to the light pulse. On another day, subjects were imaged while performing the auditory task in constant darkness.

This functional magnetic resonance image shows the nonimage-forming response to light in the thalamus, a brain center for alertness. Image reprinted with permission of Current Biology.

Light exposure increased brain activity in several regions and prevented the progressive decline in responses seen in constant darkness. However, the researchers discovered that those effects rapidly dissipate at different speeds, depending on the brain region. Some people showed no alerting response to light. After analyzing the study results, Vandewalle said that those subjects probably were already maximally alert and, thus, further light exposure had no effect. Activity occurred in the thalamus, a center of alertness in the brain, whether or not people reported an alerting effect of light. Therefore, this brain region likely modulates the nonimage-forming response to light.

In mice, both classical and melanopsin photoreceptors contribute to the non-image-forming response, but this study could not determine the relative contribution of each. The researchers said that they could differentiate the contribution of each light-detecting system by using blue light to trigger melanopsin and green light to stimulate classical photoreceptors.

Current Biology, Aug. 22, 2006, pp. 1616-1621.


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