Summary: This study tracks how and where zebrafish brains convert dynamic visual motion into a directional choice, revealing how sensory evidence is accumulated across the vertebrate brain to drive behavior.
Source: Max Planck Institute
Everyday choices—whether to turn left or right, to speed up or wait, to attend or ignore—depend on the brain’s ability to evaluate incoming sensory information and then initiate an appropriate action. Researchers at the Max Planck Institute of Neurobiology have, for the first time, followed such a decision-making process across nearly the entire brain of a vertebrate. Using transparent larval zebrafish and whole-brain imaging, the team mapped how sensory motion is integrated over time and transformed into a choice to swim in a specific direction.
Larval zebrafish are ideal for whole-brain studies because their brains are very small and nearly transparent. This physical transparency allows investigators to record activity across most brain regions simultaneously while the animal performs a natural behavior. Elena Dragomir, a lead researcher on the project, explains that the first challenge was designing a behavioral paradigm suitable for studying decision making in zebrafish. Inspired by experiments in other species that use moving dot displays, the team adapted a visual motion discrimination task for the fish. Instead of training fish to perform an abstract response, the researchers exploited a robust natural behavior—the optomotor response—as a direct readout of the animal’s choice.
The optomotor response is a reflexive tendency for fish to swim with perceived optic flow: when the visual scene moves past the eyes, fish will swim in that same direction to stabilize their position and avoid being carried by currents. In the lab, full-field moving dots can elicit this response, causing the fish to turn left or right depending on the direction of motion. The experimenters controlled stimulus difficulty by changing motion coherence: if a higher percentage of dots moves coherently in one direction, the sensory evidence is stronger and the fish turns faster and more reliably in that direction. This manipulation enabled the team to vary the amount of information available and observe how the brain accumulates evidence over time before committing to a turn.
Using high-speed whole-brain functional imaging, the researchers observed that neural activity first reflects the momentary motion signals and then integrates those signals over seconds until a threshold is reached and the turning response is initiated. This slow buildup of activity distinguishes the behavior from a simple reflex and instead aligns it with decision-making processes seen in other animals. Importantly, the zebrafish preparation allowed the team to localize the distinct neural components that contribute to different stages of the sensorimotor decision.
Where in the brain does motion information flow?
The study identified functional clusters distributed across several brain regions that play complementary roles. Neuronal populations in the pretectum and thalamic areas appear particularly engaged in processing the incoming visual motion. Downstream motor-related circuits in the hindbrain are positioned to convert accumulated sensory evidence into the motor commands that produce turning and swimming. Notably, activity in the caudal interpeduncular nucleus (IPN), a ventral midline structure, tracked the instantaneous left and right turning rates, suggesting a role in encoding or shaping the motor output linked to the decision.
By combining a well-controlled behavioral assay with whole-brain optical recording and computational modeling of evidence accumulation, the team established a comprehensive view of how sensory information is collected, maintained, and translated into action in a vertebrate brain. This approach allowed them to characterize neural dynamics across multiple time scales and to map where momentary evaluation, integration of evidence, and motor execution are represented.
Source:
Max Planck Institute
Media Contacts:
Stefanie Merker, Ph.D. – Max Planck Institute
Image Source:
The image is credited to MPIN / Julia Kuhl.
Original Research: Closed access
“Evidence accumulation during a sensorimotor decision task revealed by whole-brain imaging”. Elena I. Dragomir, Vilim Stih, Ruben Portugues.
Journal: Nature Neuroscience. DOI: 10.1038/s41593-019-0535-8.
Abstract (summary)
This study introduces a sensorimotor decision-making assay in larval zebrafish driven by whole-field visual motion. Fish responded by swimming toward the perceived motion. Both the latency to initiate swimming and the proportion of correct turns depended on motion strength. Whole-brain imaging revealed neural signatures corresponding to different stages of decision making, including momentary evaluation and temporal accumulation of sensory evidence. These signals appear across distributed functional clusters with diverse time constants. The caudal interpeduncular nucleus reliably encodes left and right turning rates, linking distributed sensory processing to motor output.