MRI measures neural correlates of the alerting effect of light
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
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
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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