"Going into the study, we knew that ageing changes the way people see the world," says Allison Sekuler, one of the senior authors and a Canada Research Chair at McMaster. "But these results are an unusual twist on the standard 'ageing makes you worse' story, and they provide clear insight into what is changing in the ageing brain."
Using computer-generated stimuli, the researchers monitored how much time subjects needed to process information about the direction in which a set of bars moved. When the bars were small, or when the bars were low in contrast (light gray vs. dark gray), younger subjects took less time to see the direction of motion. But when the bars were large, and high in contrast (black vs. white), older subjects outperformed the younger subjects.
"The results are exciting not only because they show an odd case in which older people have better vision than younger people, but also because it may tell us something about how ageing affects the way signals are processed in the brain" says Patrick Bennett, the other senior author, also a Canada Research Chair at McMaster.
The results suggest that as we age, the ability of one brain cell to inhibit another is reduced. That sort of inhibition helps young people find an object hidden among clutter, but it can make it hard to tune into the clutter itself. When the young brain sees big, high-contrast bars, it effectively tunes out because there is no object hidden in the bars. But older brains do not inhibit information in the same way, so they do not tune out the bars, and they can actually perform the task better.
"As we get older, it becomes harder to concentrate on one thing and ignore everything else," says Bennett. "It takes more effort to tune out distractions. We've seen it in cognition and speech studies, and now we see it in vision. Although we don't know if those are all linked, we think the visual effect is due to changes in the effectiveness of inhibitory neurotransmitters in the brain." Neurotransmitters are chemical substances that can modify the way in which brain cells talk to one another. Some neurotransmitters enhance brain signals, and others inhibit them.
The study, conducted in collaboration with PhD students Lisa Betts and Christopher Taylor, suggests that one type of inhibitory neurotransmitter may not have as much effect in old brains as in young brains. However, the researchers caution that although such a change makes older people perform better on this task, the same change likely leads to increased difficulties in a much wider range of tasks.
"It's critical to understand how ageing affects vision and the brain. If we can characterize what is happening to our brains as we age, we'll be in a better position to help seniors see better for longer," says Sekuler.
The research was supported by funds from the Canadian Institutes of Health Research, the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation, the Ontario Innovation Trust, and the Canada Research Chairs program.
McMaster University, named Canada's Research University of the Year by Research InfoSource, has world-renowned faculty and state-of-the-art research facilities. McMaster's culture of innovation fosters a commitment to discovery and learning in teaching, research and scholarship. Based in Hamilton, the University has a student population of more than 20,000 and more than 112,000 alumni in 128 countries.
Journal
Neuron