Scientists Watch a Memory Form in a Living Brain

Imagine if a spider dropped from the ceiling into your milk while you were enjoying your breakfast. You can't get near a bowl of cereals without feeling disgust.

Researchers have directly observed what happens inside a brain when it learns that kind of emotional response. In a new study published in January in the Proceedings of the National Academy of Sciences, a team at the University of Southern California was able to visualize memories forming in the brains of laboratory fish, as they bloomed in beautiful fluorescent greens. They had expected the brain to change its neural architecture in order to decode the memory. The researchers were surprised to find a major change in the connections.

The view that memory is a complex phenomenon is reinforced by what they saw. It suggests that the type of memory may be critical to how the brain chooses to remember, a conclusion that may hint at why some traumatic responses are hard to unlearn.

Scott Fraser, a quantitative biologist at USC, said that it may be the equivalent of a solid-state drive in the brain. While the brain records some types of memories in a volatile, easily erasable form, fear-ridden memories may be stored more robustly, which could help to explain why some people can recall a memory as if reliving it.

The top of the mammal's brain is covered by the cortex, while the base of the brain is called the hippocampus. The amygdala, the brain's fear regulation center, has been examined less often. Associative memories are an important class of emotionally charged memories that link disparate things. This type of memory is very common, but how it forms is not well understood because it occurs in an area of the brain that is hard to access.

Fraser and his colleagues were able to learn more about associative memory formation by using zebra fish. The pallium is where associative memories form in fish, but they don't have an amygdala. Fraser explained that the pallium is more accessible for study than a developing mammal's brain is.

The brain can form memories by changing its synapses, the tiny junctures where the neurons meet. Fraser said that most people believe that it does so by tweaking the strength of the connections or how strongly one neuron stimulates the next.

To make that process visible, Fraser and his team genetically engineered zebra fish to produce neurons with a fluorescent protein marker bound to their synapses. Don Arnold, a professor of biological sciences and biological engineering at USC, created a marker that fluoresced under a custom microscope. To eavesdrop on something as it takes place, but use as little light as possible to avoid scorching the creatures, was the challenge. The brighter the light, the stronger the connection.