Specialized nerve cells in zebrafish visual system enable conspecific recognition
Humans are notoriously social animals. But they are not the only ones who tend to team up with other individuals of the same species (conspecifics) to achieve their goals. In fact, flocks of mammals, flocks of birds or schools of fish are widely observed in nature. How does an animal’s brain recognize other animals of its species? Scientists at the Foundation’s Max Planck Institute for Biological Intelligence are studying this process in young zebrafish. They have now discovered a neural circuit that mediates social attraction. This specialized pathway, which runs from the retina deep into the brain, allows zebrafish to detect and approach nearby conspecifics.
Humans and many other animals live in society. At a fundamental level, social interactions require individuals to identify others as belonging to their own species. This usually happens within fractions of a second, often instinctively. Uncovering the neural circuits underlying this behavior is anything but trivial, however.
“There is an inherent challenge in studying social interactions: for us as observers, actions and reactions are intertwined, both in animal behavior and at the neural level,” says Johannes Larsch, head from project to Department of Herwig Baier. “It’s because the individuals who participate in these interactions influence each other. Both are both transmitters and receivers of social signals. It has been particularly difficult to study the role of the visual system and its associated brain areas.
Visual stimulus for shoaling behavior
Johannes Larsch’s team has nevertheless found a way to elucidate the importance of the visual system in social interactions. Scientists have developed an experiment virtual reality facility for zebrafish larvae which simulates congeners. All that’s needed is a projected dot on a screen, which – and importantly – moves across the screen with a jerky motion that’s stereotypical of swimming zebrafish. The animals cannot resist this signal: they follow it for hours, apparently confusing the mobile point with a true congener. The researchers had thus discovered a definite visual stimulus that triggers shoaling behavior.
The team could now study the neural processing of the stimulus. To do this, they extended their virtual reality setup allowing them to simultaneously measure activity in the brains of fish. The experiments revealed that a moving dot activates a specific set of neurons in a region of the brain known as the thalamus. The same area of the thalamus is activated when another zebrafish larva swims nearby.
“The thalamus is a sensory control center in the brain that integrates and relays sensory input,” explains Johannes Larsch. Sensory information is processed on its way to the thalamus, first in the retina and then in the tectum, a major visual center in the vertebrate brain. By the time the information arrives in the thalamus, it has already been filtered for social cues, such as the jerky movements of a potential conspecific.
Connection between the visual system and regions for social behavior
Nerve cells identified by the researchers in this region link the zebrafish visual system to other brain regions active during social behavior. “We already knew that these other regions of the brain play a role in controlling social behavior. However, the visual stimuli that activate them were unknown. Our work filled this knowledge gap and revealed the neural pathways that transmit the signals,” says Larsch.
The importance of the newly identified neurons was confirmed when the researchers specifically blocked the function of these cells. Zebrafish larvae lost interest in conspecifics as well as moving dots and hardly followed them anymore. “The neurons we discovered thus regulate social approach and affiliation in zebrafish,” says Johannes Kappel, graduate student and lead author of the study. “Humans also have a thalamus, and many neural processes have been conserved through evolution. We also have brain regions that are active when we perceive facial or bodily movements, but the importance of these regions for social behavior has not been explored.
The study by Kappel, Larsch, Baier and their collaborators shed light on a part of the brain whose activation provides the basic “glue” for bonding two zebrafish. Collectively, these small-scale interactions create schools of fish. Social behavior is driven by brain networks, which are themselves neural networks. Baier concludes: “Neurobiological discoveries, such as ours, can perhaps inspire and enrich the reflection on the self-organization of animal societies in general, which is currently the domain of other scientific disciplines.