Declining intelligence in old age linked to visual processing speed

August 7, 2014

The inspection time task. Participants focus on a cue, and are then shown one of the two possible stimuli, which is backward-masked after a brief exposure duration. The participant then indicates whether the longer line was on the right or left side of the stimulus (L/R; correct responses marked with an asterisk). Responses are not timed; only their correctness is measured. (Credit: Stuart J. Ritchie et al./Current Biology)

Age-related declines in intelligence are strongly related to declines on a very simple task of visual perception speed, researchers report in Cell Press journal Current Biology (open access) on August 4.

The evidence comes from experiments in which researchers 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 were tested 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,” says 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.”

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 explain. But before now, 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,” Ritchie says. “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.”

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 recognize as general intelligence. The results might also find practical use, given the simplicity of the inspection time measure, Ritchie says, since the test can be taken very simply on a computer and has been used with children, adults, and even patients with dementia or other medical disorders.

“Since the declines are so strongly related, it might be easier under some circumstances to use inspection time to chart a participant’s cognitive decline than it would be to sit them down and give them a full, complicated battery of IQ tests,” he says.

Abstract of Current Biology paper

Attempts to explain people’s differences in intelligence and cognitive ageing often hypothesize that they are founded substantially upon differences in speed of information processing. To date, there are no studies that fulfill the design criteria necessary to test this idea, namely: having a large sample size; being sufficiently longitudinal; and using measures of processing efficiency that have a tractable biological basis, are grounded in theory, and are not themselves complex or based on motor response speed. We measured visual ‘inspection time’, a psychophysical indicator of the efficiency of the early stages of perceptual processing, in a large (n = 628 with full data), narrow-age sample at mean ages 70, 73, and 76 years. We included concurrent tests of intelligence. A latent growth curve model assessed the extent to which inspection time change is coupled with change in intelligence. Results showed a moderate correlation (r = 0.460) between inspection time performance and intelligence, and a strong correlation between change in inspection time and change in intelligence from 70 to 76 (r = 0.779). These results support the processing speed theory of cognitive ageing. They go beyond cross-sectional correlation to show that cognitive change is accompanied by changes in basic visual information processing as we age.