Making a lasting memory with half a brain
Memories can last a lifetime. In the November 2000 of Nature Neuroscience, Vol. 3, No. 11, pp. 1134-1142), Yasushi Miyashita and colleagues at the University of Tokyo describe what may be one of the first steps in making memories stick around. Whereas research on the cellular basis of memory often involves lower organisms and relatively simple tasks, the present study is noteworthy because the authors studied monkeys engaged in visual memory tasks akin to those that humans perform regularly.
Using so-called 'split-brain' monkeys, the authors measured whether a specific molecule (brain-derived neurotrophic factor, or BDNF, which is implicated in plastic changes in neural circuits) was selectively upregulated when a monkey learned to form associations between pairs of visual stimuli. Split-brain monkeys have had the connections between their two cerebral hemispheres surgically cut (as is sometimes done to treat human epilepsy); in such animals, the left side of the brain, which is concerned with visual objects in the right visual field, can no longer communicate with the right side of the brain, and vice versa. The authors were thus able to engage one half of the brain in the visual memory task, while the other half was engaged in a task that used comparable stimuli but which did not require memory formation. This meant that each monkey served as its own control, eliminating potentially confounding variables such as differences in genetics, attention, motivational states and levels of visual activity.
About halfway through training on the memory task (monkeys normally take about two weeks to become completely proficient at it), BDNF levels were measured in various parts of their brains. BDNF, but not other control molecules, was selectively upregulated in an area of cortex (inferior temporal) that is known to be involved in long-term memory of objects, in both monkeys and humans. BDNF was not upregulated in earlier stages of the visual pathway, which are involved in processing the stimuli but probably not in remembering them. Other studies have implicated BDNF in changes in neuronal morphology and physiology that are thought to accompany memory formation, and so the authors propose that memory tasks selectively induce upregulation of BDNF, which in turn could lead to the neuronal circuit reorganizations that underlie long-term visual memory.
Dr. Yasushi Miyashita
The University of Tokyo School of Medicine
Department of Physiology
tel: +81 3 5841 3457
fax: +81 3 5841 3325
(C) Nature Neuroscience press release.
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