Memories clutter brain in amnesia

In a paradoxical twist, people with amnesia can get bogged down by too many memories. Unwanted, irrelevant information crowds in and prevents amnesiac patients from recognizing objects, scientists report in the July 12 Neuron. The finding suggests that amnesia isn’t strictly a memory problem, and may even point out ways to help people with the disorder live more normally.

Most people consider amnesia  a breakdown of memory that leaves people unable to recall a conversation they had minutes earlier, says study coauthor Morgan Barense of the University of Toronto. While it’s true that people with amnesia have striking memory deficits, “the real picture is more complicated,” she says.

People with amnesia caused by damage to a brain region near the ears called the perirhinal cortex also have problems recognizing objects, Barense and colleagues found. In the study, two people with this form of amnesia assessed a series of pictures of two objects — squiggly blobs with distinctive patterns of lines. The objects, shown at different rotations, were either identical or slightly different.

At first, people with amnesia were just as good as people with functioning recall at deciding whether the two objects were the same. But as the experiment wore on, participants’ performance started to crash.

“They’re doing fine, they’re doing fine — and then all of a sudden, it was like a switch flipped,” says Barense.

After ruling out other possibilities, the researchers landed on what Barense calls a “wildly paradoxical conclusion” to explain the crash: too many memories. As the participants saw more and more objects, memories of irrelevant features of objects from previous pictures started to clog up the works, interfering with the present task.

These interfering memories weren’t for whole objects. Instead, small features common to many of the objects, such as the tilt of a line or the precise shading pattern, seemed to cause the problem.

When researchers removed some of these common features, participants’ performance shot right back up, suggesting that memories of the objects’ elements were indeed hindering performance.

For people with amnesia, these fine details may not coalesce in their minds as a coherent object, a deficit that could explain both the memory problems and the object perception problems. People who fail to form such a representation in their minds won’t be able to recognize that object or remember it later. And the perirhinal cortex, once thought to be used mainly for memory, seems to be a place where this coalescing happens, the researchers propose.

Reducing interfering clutter with measures like exchanging multiple complicated remote controls for one simple one might help some people with amnesia. “I find this so exciting because of the cognitive rehabilitation possibilities,” says neuroscientist Mark Baxter of Mount Sinai School of Medicine in New York City.