To make good decisions, both humans and animals need to collect evidence and integrate new information and knowledge. Lots of research in psychology and the cognitive sciences has been done on understanding human decision-making. However, scientists still don't fully understand how decisions are made on a neuronal level: How do neurons in our brains and nervous systems guide our behavior?
To find answers to those questions, a pair of scientists from Harvard University combined behavioral experiments with brain imaging using larval zebrafish as a model organism. Zebrafish larvae are small, transparent vertebrates with roughly 100,000 neurons. With current imaging techniques, the entire volume of the zebrafish larval brain can be imaged non-invasively at high resolution over many hours.
The researchers saw that when zebrafish larvae were exposed to whole-field visual drift, they swam in the direction of the motion to stabilize themselves with respect to the visual environment. However, larvae don’t swim continuously. They make discrete movements that can each be considered an individual decision, raising the hypothesis that larvae accumulate evidence of the direction of the drift during motionless periods.
A popular decision-making paradigm (originally applied to humans and primates) is the random-dot motion discrimination task, which tests a viewer's perception of motion and direction. Using an adapted version of this random-dot paradigm, the scientists found that zebrafish larvae indeed accumulate information about motion direction over time and robustly swim in the direction they perceive their surroundings to be moving.
Further, the scientists created a whole-brain functional map and identified three clusters of neurons that relate to the behavioral choices of the zebrafish during the random-dot task. They propose a biophysical neural network in which a cluster of neurons, that integrates evidence of perceived motion direction over time, competes with another neuron cluster that represents a decision threshold. These two neuronal clusters are in a push-pull dynamic until a certain threshold is reached to finally activate the third cluster of motor neurons, which initiates movement in perceived direction.
With these findings, the researchers have shown for the first time that larval zebrafish integrate sensory information over time and that larvae are a suitable model organism to study questions about perceptual decision-making.