One of the brain's most important properties is its flexibility. Our cerebral circuitry changes constantly--every day, new links are made amongst the 86 billion individual neurons in our heads, and old connections are allowed to fall away. The result is a dizzyingly complicated network that is in a constant state of flux, rewiring itself on the fly in response to its environment and the life experience of its owner. The brain's ability to do this is called neuroplasticity, and it's what gives us the capacity to learn, grow, develop new skills and ideas, and adapt to the environment in which we live. We understand some aspects of neuroplasticity fairly well but others, including the reason that certain connections get made instead of others, remain deeply mysterious.

Imagine that while you are enjoying your morning bowl of Cheerios, a spider drops from the ceiling and plops into the milk. Years later, you still can't get near a bowl of cereal without feeling overcome with disgust. Researchers have now directly observed what happens inside a brain learning that kind of emotionally charged 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, imaging them under the microscope as they bloomed in beautiful fluorescent greens. From earlier work, they had expected the brain to encode the memory by slightly tweaking its neural architecture. Instead, the researchers were surprised to find a major overhaul in the connections.