Summary: New research finds that prolonged exposure to dim light can impair memory and learning. In a controlled study, diurnal rodents exposed to low light lost roughly 30% of hippocampal structural markers and showed worse performance on previously learned spatial tasks.
Source: Michigan State University.
Living and working for long periods in dim indoor light may alter brain structure and reduce memory and learning ability, according to new neuroscience research from Michigan State University.
Neuroscientists examined the effects of ambient light on the brains and behavior of Nile grass rats, a diurnal rodent species that, like humans, is active during the day and sleeps at night. Animals housed for four weeks under dim lighting conditions showed substantial declines in hippocampal markers and in spatial memory performance, while animals kept under bright light conditions showed improved results.
Specifically, rats exposed to dim light (50 lux) for four weeks exhibited about a 30 percent decrease in measures of hippocampal capacity and performed poorly in a spatial navigation task they had previously learned. By contrast, rats maintained under bright light (approximately 1,000 lux) demonstrated significantly better spatial task performance. Importantly, animals that were first exposed to dim light and later switched to bright light for an additional four weeks showed full recovery: hippocampal measures and task performance returned to levels comparable to those of continually bright-exposed animals.
The study, supported by the National Institutes of Health, is the first to show that changes in everyday environmental light intensity—within the range commonly experienced indoors—can produce detectable structural effects in the brain. For context, the Environmental Protection Agency estimates that people in the United States spend the majority of their time indoors, which makes understanding the cognitive consequences of indoor lighting levels a public-health concern.
“When we exposed the rats to dim light, mimicking cloudy days or typical indoor lighting, the animals showed impairments in spatial learning,” said Antonio “Tony” Nunez, a psychology professor and co-investigator. He compared the animals’ difficulty navigating to the common human experience of exiting a darkened movie theater or shopping mall and temporarily losing one’s bearings in a bright, busy parking lot.
The research team included principal investigator Lily Yan, associate professor of psychology, Nunez, and lead author Joel Soler, a doctoral student in psychology. Their peer-reviewed report appears in the journal Hippocampus.
At a molecular and cellular level, sustained exposure to dim light produced notable reductions in brain-derived neurotrophic factor (BDNF), a peptide that supports neuronal health and synaptic connections in the hippocampus. The dim-light group also showed lower dendritic spine density in hippocampal CA1 neurons—reductions concentrated in mushroom and stubby spine types, which are important for strong synaptic communication.
“Fewer dendritic spines mean fewer synaptic connections, and that corresponds with reduced learning and memory functions that depend on the hippocampus,” Soler explained.
The researchers note that light does not act directly on the hippocampus. Instead, visual input first reaches other brain regions that relay information and neuromodulatory signals to the hippocampus. One candidate region under investigation is a cluster of hypothalamic neurons that produce orexin, a peptide involved in arousal and multiple brain functions. The team plans to explore whether stimulating these orexin-producing neurons—or administering orexin—could offset hippocampal deficits caused by dim light without requiring increased ambient illumination.
These findings have practical implications for populations at risk of reduced light exposure, such as older adults, people with glaucoma or retinal degeneration, and individuals with certain cognitive disorders. The results suggest two possible avenues for intervention: optimizing indoor lighting to provide brighter, more beneficial illumination patterns, or targeting the neural systems that relay light information to the hippocampus to mimic the positive effects of bright light.
“For people with eye disease who receive limited light, it may be possible to target brain systems directly to deliver some of the cognitive benefits of bright light exposure,” Yan said. “Similarly, improved lighting or neuromodulatory approaches could help preserve or restore cognitive function in aging or clinical populations.”
Source: Andy Henion – Michigan State University
Publisher: Organized by NeuroscienceNews.com.
Image Source: Public domain image.
Original Research: Abstract published in Hippocampus.
doi: 10.1002/hipo.22822
Michigan State University. “Dim Light May Make Us Dumber.” NeuroscienceNews. 5 February 2018.
Abstract
Light modulates hippocampal function and spatial learning in a diurnal rodent species: A study using male Nile grass rats (Arvicanthis niloticus)
Previous human studies have shown that brighter illumination can enhance cognitive performance in children, healthy adults, and patients with early dementia, but the neural mechanisms were not well defined. This study used diurnal Nile grass rats to examine how ambient light affects hippocampal function and spatial memory. Animals were housed under either a bright light–dark cycle (brLD, ~1,000 lux) or a dim light–dark cycle (dimLD, ~50 lux) for four weeks. DimLD animals displayed impaired spatial memory on the Morris Water Maze task, along with reduced hippocampal BDNF expression—especially in the CA1 subregion—and lower dendritic spine density on CA1 apical dendrites. Transfer of dimLD animals to brLD for four weeks restored both behavioral performance and hippocampal BDNF and spine density. The results indicate that ambient light intensity affects both cognitive performance and hippocampal structural plasticity in this diurnal species and highlight potential paths for understanding and mitigating light-related cognitive decline.