Speed-controlled visual stimuli modulate fish collective dynamics.

Fish movement is modulated by visual stimulation and associated parameters, such as size, speed, and orientation, yet the regulatory mechanisms remain unclear. Here, we experimentally investigate how systematically controlled stimulus speed influences the temporal and spatial collective dynamics of Hemigrammus bleheri. Using a customisable top-down projection setup, we displayed clockwise-rotating black-dot patterns at three distinct speeds to simulate complex environmental motion cues. We quantified individual temporal responsiveness through angular movements and evaluated group-level spatial organisation. Fish angular movements positively tracked the rotation direction of the visual patterns. Lower stimulus speeds elicited predominantly exploratory behaviour, while higher speeds induced a transition to strong motion-tracking behaviour, aligning fish orientation with the rotating patterns. Increasing stimulus speed also corresponded with greater nearest-neighbour distance, indicating reduced spatial cohesion. Notably, we observed transient schooling episodes not aligned with the rotational stimulus, reflecting periods in which internal social interactions outweighed stimulus-driven coordination. Using a newly defined free-schooling index (FSI), we show that such internally driven episodes were frequent at lower stimulus speeds but markedly reduced at higher speeds. Overall, our findings demonstrate that speed-controlled visual stimuli modulate both individual movement and group dynamics, offering new insights into how visual cues shape fish collective behaviour.