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En route to thought: recognition and recall.

The monkeys roam around, indiscriminately picking up objects, edible and inedible, and putting them in their mouths. Again and again they perform this ritual with the same items. First considered a type of mania, this abnormal behavior occurs in monkeys sustaining damage to a brain structure called the amygdala. Scientists now propose a new explanation for the behavior, and for some other forms of brain damage, based on research aimed at understanding thought processes.

"For these monkeys, looking at the object gives no clue as to what the object feels like and picking it up gives no clue as to what it tastes like," says Mortimer Mishkin of the National Institute of Mental Health (NIMH) in Bethesda, Md. Experiments in his laboratory indicate that certain brain structures, including the amygdala, act as selector switches to allow the association of different types of stored information, Mishkin told journalists at a Cold Spring Harbor (N.Y.) Laboratory workshop.

Mishkin and his colleagues began their investigations of thought by studying how the brain processes sensory input. "The [brain] cortex is like a patchwork quilt," Mishkin says. Each patch performs a computation and sends the result on to the next station.

As Mishkin, Leslie Ungerleider, and other colleagues traced area-by-area the parts of the visual system that are used to identify objects, they were led from the back of the brain forward to the low, outside region called the temporal lobe. Other sensory systems have a similar organization. For the olfactory, gustatory, auditory and tactile systems, signals also travel station-to-station, eventually reaching areas in or adjacent to the temporal lobe.

The function of each area of the brain is determined by assessing monkey's performance on specially devised tasks, before and after the area is surgically removed. In a revealing test for "recognition memory," the animal is asked to distinguish a novel object from an object it has seen once before.

"Monkeys are incredibly good at this," Mishkin says. A normal monkey correctly selects the novel item more than 90 percent of the time.

The experiments revealed functional connections between the cortex, the outer and evolutionary newest portion of the brain, and two underlying structures, the hippocampus and the amygdala. These structures are part of the limbic system.

A monkey lacking both amygdala and hippocampus on both sides of the brain shows a severe deficiency in the recognition memory task. This finding was surprising because removal of either the amygdalas or the hippocampal structures had little effect.

The limbic system's role is not limited to visual-information recall, Mishkin and colleague Elisabeth Murray find. If the recognition memory task is performed in the dark, allowing the monkey to touch rather than see each object, the monkey selects accurately unless both amygdala and hippocampus have been removed.

In addition, the limbic system appears to be a site where information stored by different sensory systems can be combined. To test "cross-modal association," Mishkin and Murray presented monkeys with the series of objects in the dark, so the memories were established from tactile information. Then the monkeys made their choice in the light, without handling the objects.

While normal monkeys and monkeys lacking any hippocampus scored about 90 percent correct, monkeys lacking any amygdala were correct only 55 percent of the time, hardly better than chance. "In the absence of the amygdala, the brain can lay down both tactile and visual memories, but it can't compare them," Mishkin says.

The hippocampus serves as a different type of selector, Mishkin now proposes. It provides associations between memories of objects and of positions. In the visual system, for example, a series of cortex stations, distinct from those that run along the temporal lobe, process information about spatial relations.

Mishkin and John Parkinson of NIMH devised a task to test the combined use of spatial and object memory. In step 1, a monkey is shown an object in a given position on a tray. In step 2, the monkey is presented with two copies of that object and is rewarded for selecting the one ine the same position as the object in step 1. Normal monkeys, and monkeys without amygdalas, succeed about 80 percent of the time. But monkeys lacking any hippocampus choose correctly in only about 55 percent of their attempts.

The hippocampus and the amygdala share a common ability to store information but differ in which representations they can connect, Mishkin concludes.

What's beyond the limbic system? Connections run in both directions between the hippocampus and the amygdala and another group of deep-brain structures called the basal forebrain cholinergic system. The destruction of these structures in Alzheimer's disease (see story, this page) and other brain disorders may underlie the characteristic memory loss. The basal forebrain structures produce a neurotransmitter called acetylcholine. Mishkin and his colleagues find that a monkey's performance on the recognition memory task can be improved by a drug that increases the amount of acetylcholine available in the brain, and can be decreased by a drug that blocks the acetylcholine receptor.

In recent experiments Mishkin, Thomas Aigner of NIMH and Donald Price of Johns Hopkins University in Baltimore used a chemical to selectively destroy teh cells of monkeys' basal forebrains. This treatment impaired the monkey's ability to recognize whether an object is novel. Mishkin speculates that the basal forebrain plays a role in memory storage, perhaps relating it to emotional context. He says, "We're on the way to begin to think about higher level memory processes."
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Title Annotation:research on brain damage
Author:Miller, Julie Ann
Publication:Science News
Date:Dec 14, 1985
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