What physical changes occur in the brain when a memory is made? A team of researchers at the University of Southern California has answered this question by inducing a memory in a larval zebrafish and mapping changes in their transparent heads with brain cells lit up like Times Square on New Year’s Eve. After six years of research, they made the groundbreaking discovery that learning causes synapses, the connections between neurons, to proliferate in some areas and disappear in others rather than merely changing their strength, as commonly thought. These changes in synapses may help explain how memories are formed and why certain kinds of memories are stronger than others. The study was published in the Proceedings of the National Academy of Sciences and was led by the University of Southern California’s Don Arnold, Scott Fraser, and Carl Kesselman. The study was made possible thanks to a new type of cell labelling and a custom-made microscope invented at USC. The researchers developed a cutting-edge way to track and archive the data collected to make their findings as accessible and reproducible as possible. Prior to their work, it was not possible to determine the location of a synapse in a living brain without modifying its structure and function, making comparisons before and after memory formation unfeasible. Through a multidisciplinary collaboration between the USC Viterbi School of Engineering, and the USC Dornsife College of Letters, Arts and Sciences, the teams were able to determine for the first time the strength and location of synapses before and after learning in the brain of a living zebrafish, an animal commonly used to study brain function. Zebrafish are large enough to have brains that function like our own, but small and transparent enough to offer a window into the living brain. By keeping the intact fish alive, they were able to compare synapses in the same brain over time, a breakthrough in the neuroscience field. In order to create memories to measure, the research team had to come up with new methods to induce a larval zebrafish to learn. They did this by training the 12-day-old fish to associate a light turning on with being heated on the head with an infrared laser, an action they sought to avoid by attempting to swim away. Fish that learned to associate the light with the impending laser would flick their tails, indicating that they had learned. Five hours of training later, the team was able to observe and capture significant changes in these zebrafish brains.