Neuron selectivity: down memory lane.

Neuron selectivity: Down memory lane

Researchers have recorded the activity of single neurons within the medial temporal lobe--a brain region important in memory formation -- and found the first evidence in humans that individual nerve cells apparently recognize specific words and are important in short-term memory.

Eric Halgren of the University of California at Los Angeles and his colleagues took advantage of electrodes temporarily implanted in the brains of 10 patients undergoing neurosurgery for epilepsy. They asked patients to memorize 20 abstract words shown on a video monitor, then gave them recognition tests and recorded their neuronal responses.

Most neurons tested, the team found, displayed a preference for specific words. For instance, one cell fired repeatedly at "luck," another at "woe."

"A striking feature of these data," notes Michael Rugg of the University of St. Andrews, Fife, Scotland, in a commentary accompanying the report in the June 23 NATURE, "is the frequency with which word-specific responses were observed." In 75 percent of neurons tested, the nerve cells preferred one or more of the words listed. This high rate with the small number of cells tested does not mean that each cell memorizes just one word. Instead, say the researchers, it means that a word evokes activity in many neurons, only a few of which were tested.

The group had set out to learn how damage to the medial temporal lobe causes a peculiar inability to recall words and faces seen very recently, while the ability to use language remains. The findings support the view that structures within the medial temporal lobe help a person remember the context in which a complex stimulus, such as a word or face, was encountered--but not those aspects of the stimulus that remain constant, such as meaning or pronunciation. The researchers say their data suggest the medial temporal lobe "contributes specific information rather than diffuse modulation to the encoding and subsequent recognition of a stimulus during recent memory." In other words, says team member Gary Heit, "it's like shining a flashlight on one picture rather than turning on all the lights in the room."

Other implications of the work are open to interpretation. Contrary to what might have been expected, the researchers found a neuron's response to a word unaffected by repetition: Whether the word "luck" was shown once or 10 times, neuronal firing remained the same. Rugg questions the authors' explanation that repetition does not affect the output of information from this area to other brain regions.