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Fig. 5 Model circuit of rheotaxis (A) Two-dimensional scheme of the model circuit represented within a larval zebrafish. Rotational flow (counter-clockwise) is represented by green circular arrows. A gradient of flow velocity is represented as a rainbow bar assigning the highest water-velocity gradient in red, and the lowest in blue). Here, only the positively stimulated HCs and LANs are shown. (B) The model circuit with transmission of rostrocaudal flow (repreesented as blue arrows within) and causdostral flow (red arrows within). Inter-hemispheric activity differences upon rotational flow and cross-hemispheric inhibition break symmetry of the system and solve vectorial ambiguity. Acronyms are spelled out on the right. (C) Top view of larval zebrafish performing rheotaxis under laminar water flow. A gradient of flow velocity is represented as a rainbow bar. As the fish swims in bouts across the horizontal plane, it will experience a decrease (red-to-blue transition) or increase (blue-to-red transition) in the magnitude of the rotational flow, which is directly linked to the gradient of bulk water flow. If gradient increases or decreases, the neuronal activity in the TNs will change (intensity differences in lilac) regardless of vorticity handedness. For simplicity, and because the system is lateral symmetric, only the active cells are depicted. (D) If the direction of rotation of local flow reverses when animals cross the midline of the water column (or experiences reversal of rotation flow upon its own directional changes across the horizontal plane), the system would simply undergo a mirror symmetric reversal of information reaching the brain. See also Figure S5.