If your grandparents take a long to recognise known faces from a fleeting glance, that may well signal their declining intelligence.
The slowing of simple, visual decision-making processes might be part of what underlies decline in the complex decision making that we recognise as general intelligence, says a study.
As the basic ability to make correct decisions based on brief visual impressions declines with age, so does intelligence, said Stuart Ritchie from University of Edinburgh in Britain.
"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 added.
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 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.
"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," Ritchie noted.
The study appeared in the journal Current Biology.