Older brains do not think as quickly as younger brains do. But does this cognitive impairment arise because processing speeds slacken or because the ability to block out irrelevant information falters? A recent study reconciles these two leading hypotheses: older brains have a harder time ignoring distractions in the initial stages of performing a task, which slows down processing.

Adam Gazzaley of the University of California, San Francisco, and his colleagues asked two groups—one made up of 19- to 33-year-olds and the other of 60- to 72-year-olds—to perform a memory task. The researchers used electroencephalography to record electrical signals from the participants’ brains in milliseconds during the task. In contrast to the younger adults, the older group could not suppress distracting stimuli during the first 200 milliseconds after exposure. “At later time points, the ability to ignore does show up,” Gazzaley says. “It’s not abolished, just delayed.” By then, however, the irrelevant information had interfered with the memory task, making the older group less accurate overall than the younger group.

On average, older adults display this cognitive deficit, but “not all older adults are impaired relative to younger adults,” Gazzaley notes. Dividing the older group in half according to high and low task performance indicated that only the low scorers had the problem. He adds that exploring the older adults’ differences may reveal ways to avoid or correct the deficit.

Note: This article was originally published with the title, "Slow to Ignore".