Study with totally blind people shows how light helps activate the brain

November 1, 2013

Photoreceptive ganglion cell (credit: David Berson’s lab/Brown University)

Light stimulates brain activity during a cognitive task even in some people who are totally blind, according to a study conducted by researchers at the University of Montreal and Boston’s Brigham and Women’s Hospital.

“We were stunned to discover  that the brain still responds to light in these rare three completely blind patients  despite having absolutely no conscious vision at all,” said senior co-author Steven Lockley.

“Lght stimulates day-like brain activity, improving alertness and mood, and enhancing performance on many cognitive tasks,” explained senior co-author Julie Carrier.

The results indicate that their brains can still “see,” or detect, light via a novel photoreceptor in the ganglion cell layer of the retina, different from the rods and cones we use to see, called intrinsically photosensitive retinal ganglion cells (ipRGCs).

Scientists believe, however, that these specialized photoreceptors in the retina also contribute to visual function in the brain even when cells in the retina responsible for normal image formation have lost their ability to receive or process light.

A previous study in a single blind patient suggested that this was possible but the research team wanted to confirm this result in different patients. To test this hypothesis, the three participants were asked to say whether a blue light was on or off, even though they could not see the light.

“We found that the participants did indeed have a non-conscious awareness of the light — they were able to determine correctly when the light was on greater than chance without being able to see it,” explained first author Gilles Vandewalle.

The next steps involved looking closely at what happened to brain activation when light was flashed at their eyes at the same time as their attentiveness to a sound was monitored. “The objective of this second test was to determine whether the light affected the brain patterns associated with attentiveness — and it did,” said first author Olivier Collignon.

Finally, the participants underwent a functional MRI brain scan as they performed a simple sound-matching task while lights were flashed in their eyes. “The fMRI further showed that during an auditory working memory task, less than a minute of blue light activated brain regions important to perform the task. These regions are involved in alertness and cognition regulation as well being as key areas of the default mode network,” Vandewalle explained.

Researchers believe that the default network is linked to keeping a minimal amount of resources available for monitoring the environment when we are not actively doing something. “If our understanding of the default network is correct, our results raise the intriguing possibility that light is key to maintaining sustained attention” agreed Lockley and Carrier. “This theory may explain why the brain’s performance is improved when light is present during tasks.”

This study was supported by the Réseau Vision du Québec and Réseau de Bioimagerie du Québec (RBiQ) and the Fonds de recherche du Québec — Santé (FRQ-S).

Abstract of Journal of Cognitive Neuroscience paper

Light regulates multiple non-image-forming (or nonvisual) circadian, neuroendocrine, and neurobehavioral functions, via outputs from intrinsically photosensitive retinal ganglion cells (ipRGCs). Exposure to light directly enhances alertness and performance, so light is an important regulator of wakefulness and cognition. The roles of rods, cones, and ipRGCs in the impact of light on cognitive brain functions remain unclear, however. A small percentage of blind individuals retain non-image-forming photoreception and offer a unique opportunity to investigate light impacts in the absence of conscious vision, presumably through ipRGCs. Here, we show that three such patients were able to choose nonrandomly about the presence of light despite their complete lack of sight. Furthermore, 2 sec of blue light modified EEG activity when administered simultaneously to auditory stimulations. fMRI further showed that, during an auditory working memory task, less than a minute of blue light triggered the recruitment of supplemental prefrontal and thalamic brain regions involved in alertness and cognition regulation as well as key areas of the default mode network. These results, which have to be considered as a proof of concept, show that non-image-forming photoreception triggers some awareness for light and can have a more rapid impact on human cognition than previously understood, if brain processing is actively engaged. Furthermore, light stimulates higher cognitive brain activity, independently of vision, and engages supplemental brain areas to perform an ongoing cognitive process. To our knowledge, our results constitute the first indication that ipRGC signaling may rapidly affect fundamental cerebral organization, so that it could potentially participate to the regulation of numerous aspects of human brain function.