This is What a Fish Thought Looks Like

It’s an old cartoon trope that a light bulb will appear over a character’s head when they’re struck by a brilliant idea, but that kind of tangible indication of mental activity only exists in animated illustration—at least, it did until Japanese researchers Akira Muto, Koichi Kawakami, and Junichi Nakai pioneered new technology enabling them to observe neural activity occurring in the zebrafish brain in real time, as described in a recent article published in Current Biology.

In its embryonic and larval stages, the zebrafish's body remains transparent, making it an ideal candidate for the fluorescence imaging study undertaken by scientists at Japan’s National Institute for Genetics. That unique property allows researchers to observe the body's underlying structures directly, either with the naked eye or under magnification. By developing a chemical marker that can be inserted directly into the relevant neurons of interest and detected by a fluorescent probe, the scientists enabled a close study of the activity occurring within the zebrafish brain at the level of a single cell. They introduced a new version of GCaMP, a genetically encoded calcium indicator that glows green in the presence of calcium, signaling a quantifiable increase in brain activity. As areas of the fish’s brain lit up in response to a moving stimulus, the researchers were able to keep track of neural firing at any given moment, tracing the path of the fish’s thought as it occurred.

In order to make sure they would be able to monitor the correct parts of the working zebrafish brain, the scientists first identified the relevant neurons that became active in response to a moving object and created a model of how they anticipated the neurons would react to other patterns of movement. They then tempted their subject by releasing single-celled paramecia, a common zebrafish food source, into its environment. The expected neurons glowed in accordance with the researchers’ forecast, thereby validating their predictive model.

Observing the hunger responses of a 2-inch minnow is a far cry from unlocking the secrets of human cognition, but the developments with the zebrafish indicate potential for an expansion into research on other animals' neural patterns, including humans. Co-author Kawakami optimistically predicts that “in the future, we can interpret an animal’s behavior, including learning and memory, fear, joy, or anger, based on the activity of particular combinations of neurons.” Even if we never get there, maybe we’ll finally prove, on a neurochemical level, that goldfish shouldn’t get such a bad rap for their terrible memories.