Visual perception of social groups in zebrafish

Many animals live in groups and often make decisions based on the behaviour of others, integrating information coming from multiple social partners. The number of conspecifics performing a given action is often a key determinant of individual behaviour. Accordingly, the nervous system must extract perceptual features that scale quantitatively with groups, enabling animals to evaluate and compare them rapidly in dynamic environments. Despite the extensive work trying to characterize group behaviours, still little is known about the underlying perceptual and neural processes at the individual level that allow to integrate multiple social signals and assess their quantity. In this thesis, I address these questions in adult and juvenile zebrafish by combining behavioural assays with two-photon calcium imaging. In particular, I investigate how specific group visual features - such as number of elements, speed and overall amount of motion - might influence shoaling behaviour and be integrated within a dorsal thalamic circuit for the detection of social cues. I show that visual motion cues alone are sufficient to elicit attraction and a preference for larger groups, and that behavioural choices are jointly shaped by the number of moving elements and their speed in a weighted manner. At the neural level, I found that previously characterized dorsal thalamic neurons are organized into two distinct populations, responding to bout-like and continuous motion respectively. These populations are anatomically segregated along the anterior-posterior axis and exhibit predominantly contralateral visual tuning. In this circuit, neural activity scales with group size through both increased recruitment of responsive neurons and graded increases in response magnitude. At the single-neuron level, number of elements and their speed are integrated within the same neurons and jointly modulate their activity. Together, these findings show for the first time how visual cues from social groups are integrated in a neural circuit for shoaling. These provide a foundation for understanding how early visual thalamic representations may be transformed into sensorimotor signals that guide group behaviour.