Age-related declines in intelligence are strongly related to declines on a very simple task of visual perception speed, scientists say.
Researchers conducted experiments in which they showed 600 healthy older people very brief flashes of one of two shapes on a screen and measured the time it took each of them to reliably tell one from the other.
Participants repeated the test at ages 70, 73, and 76.
The longitudinal study is among the first to test the hypothesis that the changes they observed in the measure known as "inspection time" might be related to changes in intelligence in old age.
"The results suggest that the brain's ability to make correct decisions based on brief visual impressions limits the efficiency of more complex mental functions," said Stuart Ritchie of the University of Edinburgh.
"As this basic ability declines with age, so too does intelligence. The typical person who has better-preserved complex thinking skills in older age tends to be someone who can accumulate information quickly from a fleeting glance," Ritchie said.
Previous studies had shown that smarter people, as measured by standard IQ tests, tend to be better at discerning the difference between two briefly presented shapes, the researchers said.
However, no one had looked to see how those two measures might change over time as people grow older.
"What surprised us was the strength of the relation between the declines. Because inspection time and the intelligence tests are so very different from one another, we wouldn't have expected their declines to be so strongly connected," Ritchie said.
The results provide evidence that the slowing of simple, visual decision-making processes might be part of what underlies declines in the complex decision making that we recognise as general intelligence.
The study is published in the Cell Press journal Current Biology.
