Aging affects the direction selectivity of MT cells in rhesus monkeys.

The ability to accurately perceive the direction and speed of moving objects declines during normal aging. This is likely due to functional degradation of cortical neurons. Most neurons in the primate middle temporal area (MT) are direction-selective and their activity is closely linked to the perception of coherent motion. We investigated the mechanisms that underlie this age-related decline by comparing the proportions of direction-selective MT cells in old and young macaque monkeys, using in vivo single-cell recording techniques. Our results showed that the proportion of such cells was lower in old than in young monkeys. Moreover, one type of direction-sensitive cells, pattern cells, was especially sensitive to aging and was affected more severely than another class, component cells. We also found that direction selectivity was affected more severely in MT than in V1 of senescent monkeys. Thus, the functional degradation of MT and V1 cells may mediate perceptual decline in visual motion tasks in old primates.