To avoid injury, most organisms react reflexively to the approach of incoming objects. For instance, humans will, without thinking, dodge an oncoming car or bat away a small rock to evade collision. While typically unconscious, these essential defensive maneuvers rely on surprisingly complex computations in the eyes and brain. The brain must not only decide what is dangerous but also when danger is likely to impact the body, and these decisions must be made rapidly. To inform these decisions, the brain must make reliable estimations of the size, speed, and trajectory of objects in the environment, but very little is known about how the vertebrate brain achieves these tasks.
To crack the neural basis of object avoidance, Dunn et al. capitalized on the larval zebrafish, a vertebrate model organism that provides powerful opportunities for relating neurons to behavior. The authors first establish that young zebrafish can be startled by the simulated approach of objects or predators; when a looming, i.e. expanding, spot is projected onto a screen, the illusion of impending collision is sufficient to evoke ballistic escape maneuvers directed away from the stimulus. Via careful dissection of the behavior, the authors determine features of the stimulus that the brain extracts to guide subsequent locomotion. Most critically, it is revealed that zebrafish wait until the object spans a fixed portion of visual space, independent of absolute size or velocity, before initiating an escape. This feature is then used as a handle to investigate the underlying neural implementation.
To probe the neural circuits responsible for calculating critical size, the authors used fluorescent indicators of neural activity to image neuronal responses to looming visual stimuli across large areas of the fish brain. These experiments identified a specialized visual processing region as an important center sensing object approach. This center, the optic tectum, is highly conserved in vertebrates, suggesting that an analogous region in mammals, the superior colliculus, is most likely involved in similar types of computations that guide avoidance behavior in humans. The authors go on to show that within the optic tectum, hundreds of neurons work together to assess critical size and relay the “go” signal to downstream motor circuits shown to dictate the behavior. Furthermore, the authors use new transgenic zebrafish to tease apart fundamental functional components in the retina and in optic tectum interneurons that may interact to shape the ethologically relevant neural activity.
This work charts the neural circuits underlying an ubiquitous innate behavior and establishes a strong framework for exploring the ways in which populations of neurons in the vertebrate brain extract meaningful features from the visual world.