“We want to know how the brain cancels out irrelevant information from the senses, enabling us to focus on information that matters,” said Dr. Sawtell. “Electric fish turn out to be the perfect system for studying this.”

Dr. Sawtell has discovered a circuit of brain cells in the cerebellum that, like the noise-cancelling electronics in headphones, filter a fish’s senses. First the circuit makes a prediction about how the animal’s own electricity will be picked up by electrical sensors on its body. Then the circuit generates an opposite pattern in the brain that negates the would-be noise.

Understanding how the brain makes predictions to regulate our senses could have medical consequences. Consider tinnitus, an affliction that causes a continuous ringing in the ears.

“Tinnitus could be a problem caused by an internal prediction gone awry,” said Dr. Sawtell. Instead of filtering out unwanted noise, malfunctioning circuits in the brains of people with tinnitus may conjure up hallucinations of sound.

Decoding a simple fish circuit is only the first step in a much larger agenda. Scientists know that many animals, including humans, have similar circuits connected to hearing, vision and other senses. But researchers don’t know exactly how these groups of cells function.

“We want to make deep, satisfying links between the structures of brain circuits and their functions, starting with the cells that regulate our senses,” said Dr. Sawtell. “Unfortunately there are very few cases in which scientists can currently do this, even for the simplest known circuits.”

Dr. Sawtell is tackling a set of circuits that — despite having a seemingly straightforward structure and very few types of cells — have stubbornly resisted attempts to decode them. Scientists first mapped these groups of cells in the cerebellum in the 1960s. At the time, it was thought that the functions of these circuits would soon be known.

“People thought we were going to figure out the circuits of the cerebellum decades ago,” said Dr. Sawtell. “We didn’t, so people moved on.”

Dr. Sawtell, however, has committed himself to understanding this part of the brain. “If there is any major brain structure we should be able to decipher, it is the cerebellum,” he said.

The cerebellum, so large in the electric fish, has also been supersized in the human brain by evolution; it is deeply connected to the rest of the brain and could play a role in everything from skilled actions such as swinging a Ping Pong paddle to the regulation of thought and emotion. For Dr. Sawtell, the cerebellum is a beachhead for launching an all-out assault on decoding other parts of the brain.

“Though its circuitry is relatively simple and uniform, the cerebellum is connected to many different parts of the brain," he said, "Hence understanding how cerebellar circuitry works would provide major insights into a wide range of issues related to both normal brain function and neurological disorders."