Neuroscientists have taken a large step towards figuring out just what makes some memories vivid and others cloudy.

Researchers have now discovered that stimulating a brain area that releases a common neurotransmitter helps rats create more detailed memories. The finding will be published in the November issue of the journal Neurobiology of Learning and Memory.

“This is the first time that direct stimulation of a brain region has controlled the amount of detail in a memory,” Norman Weinberger, the lead author of the study and a neurobiologist at the University of California, Irvine, said in a press release.

The research links memory clarity to acetylcholine, a common neurotransmitter. Acetylcholine has been found to function in the acquisition of new memories by sending input to the cortex and hippocampus.
In Weinberger’s experiment, he and his team tested rats’ ability to remember specific frequencies of tones. While playing a tone, the researchers stimulated a rat’s nucleus basalis, a brain structure that releases acetylcholine. Some rats received more basalis stimulation than others, prompting them to release higher levels of the neurotransmitter. The following day, they determined how well the rats “remembered” the tones by measuring changes in the rodents’ respiration when the sounds were played.

The researchers discovered that when stimulation of the basalis cells had been weak, the rats were able to remember that they had heard a tone the day before but not the specific pitch.  When stimulation—and acetylcholine release—was higher during the learning task, the rats were able to remember which frequency they had heard. The results are the first to link acetylcholine levels to the specificity of memories, Weinberger said.

However, just how memories are acquired and stored is still a big question in cognitive neuroscience.

“No one knows where memories are stored in the short or the long term,” Weinberger said in a telephone interview. “But the evidence points to networks of neurons whose synaptic strengths have been altered and whose connections to other neurons form the totality of a memory.”

Weinberger’s findings may have important implications for memory disorders, particularly Alzheimer’s disease (AD).

“AD is associated with decreases in numbers of acetylcholine neurons in the nucleus basalis,” said Michael Hasselmo, a neuroscientist at Boston University who studies memory. “So this work is clearly relevant to possible changes in memory with AD.”

Determining exactly what role acetylcholine plays in learning and memory is crucial for treating Alzheimer’s and other memory-related diseases, Weinberger said.

Originally published October 30, 2006

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