By Watching Fish Tail Movement, Researchers Examine Memory

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Fish moving their tails like a metronome, rhythmically to the blinking of light-by studying this mechanism, UC Berkeley scientists are beginning to understand more about how the fish brains process short-term memory.

UC Berkeley researchers collaborated with scientists from UC San Francisco to look at the brain activity in zebrafish larvae.

When the scientists blinked light onto the fish at a steady frequency, they found that brain activity and tail movement corresponded to the rhythm of the light. After the scientists stopped blinking the light, the brain activity of the fish still exhibited this rhythmic frequency for about 20 more seconds.

Scientists said this indicates that the fish still remember the stimulus of the light, and compared the response of the fish to the human brain's reaction upon hearing a musical beat.

"If you do repetitive light stimulation, then the fish tends to remember that rhythm you apply," said Mu-Ming Poo, a professor of neurobiology at UC Berkeley who oversaw the study. "If you have the music stimulus, a beat, your brain will respond during the training to every stimulus you apply, like a wave of activity. After the training, the fish remembers."

This discovery can help explain how the fish are able to respond quickly to oncoming prey. When the fish detect changes like a blinking light, they can respond accordingly and escape in a matter of seconds.

By flashing the light, researchers were able to mobilize the fish and monitor their brain activity and tail behavior, Poo said.

Researchers used a strong microscope and high-resolution camera to see the amount of activity the fish brain exhibited. By injecting a molecule that emits fluorescent light when bound to calcium, researchers could then use the microscope to observe the signal that corresponded to the frequency of the blinking light.

After the blinking stopped, the scientists could see the same frequency of brain activity for about 20 more seconds, but at a lower amplitude.

The behavior presented in this study could help scientists begin to unravel the mysteries of short-term memory.

While long-term memory, which consists of the 24-hour circadian rhythm, is well-documented, scientists currently don't have the tools to accurately measure the changes the human brain undergoes within the narrow time frame of short-term memory.

The next step will be to learn more about how the human brain processes a smaller magnitude of time, in terms of seconds and minutes.

"We want to figure out how the neuro-circuit codes time, like, where is the clock-the clock that keeps the rhythm," Poo said. "That is totally mysterious. We do not even have a good hypothesis about how it works, how the circuit codes time and keeps track of the time interval in seconds."

Nicholas Spitzer, a professor of biology at UC San Diego, thinks the study might be an entryway to understanding more about how the brain codes time.

"One of the theories about how short-term memory works is that one has networks of neurons in the brain and electrical signals racing through the circuits," Spitzer said. "(There's) a lot of discussion about the brain having the reverberating circuit. This study is a wonderful sort of opening for research, to try to understand more of the details that underlies it. It could be an interesting opportunity of studying this process of short-term memory."


Contact Christine Chen at [email protected]

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