How does the brain of an animal recognize other animals of its own kind? Researchers studying this process in juvenile zebrafish have discovered a neuronal circuit that mediates social attraction. Zebrafish can detect and approach nearby conspecifics thanks to this specialized pathway that runs from the retina deep into the brain.
Humans are well-known for being social animals. However, they are not alone in their proclivity to collaborate with other members of the same species (conspecifics) to achieve their objectives. In fact, herds of mammals, flocks of birds, and shoals of fish are common in nature. How does the brain of an animal recognize other animals of its own kind? This process is being studied in young zebrafish by scientists at the Max Planck Institute for Biological Intelligence. They now know that a neuronal circuit mediates social attraction. Zebrafish can detect and approach nearby conspecifics thanks to this specialized pathway that runs from the retina deep into the brain.
Many animals, including humans, live in societies. Fundamentally, social interactions necessitate individuals recognizing others as members of their own species. This usually occurs in fractions of a second and is often instinctive. Uncovering the neuronal circuits that underpin this behavior, on the other hand, is far from simple.
“There’s an inherent challenge in studying social interactions: actions and reactions are intermingled for us as observers, both in animal behavior and at the neuronal level,” says Johannes Larsch, project leader in Herwig Baier’s department. “This is due to the fact that individuals involved in these interactions influence one another. Both are simultaneously senders and receivers of social signals. It has been especially difficult to investigate the role of the visual system and the brain areas associated with it.”
There’s an inherent challenge in studying social interactions: actions and reactions are intermingled for us as observers, both in animal behavior and at the neuronal level. This is due to the fact that individuals involved in these interactions influence one another. Both are simultaneously senders and receivers of social signals.Johannes Larsch
Visual stimulus for shoaling behavior
Despite this, Johannes Larsch’s team was able to demonstrate the importance of the visual system in social interactions. The researchers created a virtual reality experiment for zebrafish larvae that simulates conspecifics. All that is required is a projected dot on a screen that, most importantly, moves across the screen in a jerky pattern resembling swimming zebrafish. The animals are unable to ignore this cue, following it around for hours, apparently mistaking the moving dot for a real conspecific. Thus, the researchers had discovered a specific visual stimulus that causes shoaling behavior.
The team could now investigate the neuronal processing of the stimulus. To do so, they extended their virtual-reality setup enabling them to simultaneously measure activity in the fish brain. The experiments revealed that a moving dot activates a specific set of neurons in a brain region known as the thalamus. The same area of the thalamus gets activated when another zebrafish larva swims nearby.
“The thalamus is a sensory control center of the brain that integrates and relays sensory inputs,” explains Johannes Larsch. Sensory information is processed on its route to the thalamus, first in the retina and then in the tectum, a major visual center of 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 visual system and regions for social behavior
The nerve cells discovered by the researchers in this region link the zebrafish’s visual system to other brain regions active during social behavior. “We already knew that these other brain regions are involved in social behavior regulation. The visual stimuli that activated them, however, were unknown. This knowledge gap has been filled by our research, which has revealed the neuronal pathways that transmit signals” Larsch explains.
The importance of the newly identified neurons was confirmed when the researchers specifically blocked the function of these cells. Zebrafish larvae lost their interest in conspecifics as well as moving dots and hardly followed them around 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 possess a thalamus, too, and many neuronal processes have been conserved during evolution. We also have brain regions that are active when we perceive facial movements or body motion, but the significance of these regions for social behavior has not been explored.”
Kappel, Larsch, Baier, and their colleagues’ research has shed light on a part of the brain whose activation provides the basic “glue” for the bonding of two zebrafish. Such small-scale interactions result in shoals of fish. Networks of brains, which are networks of neurons, drive social behavior. “Neurobiological findings, such as ours, may inspire and enrich thinking about the self-organization of animal societies in general, which is currently the domain of other scientific disciplines,” Baier concludes.