Fig. 1. Experimental design and automated tracking of the walked path. (A) Schematic drawing of the experimental set-up (not drawn to scale). A stick insect was tethered above a sphere on which it stood or walked. Stationary walking caused rotation of the sphere. Two optic sensors (Sx and Sy) detected all three degrees of freedom of rotation of the sphere, each of which could be attributed to one of three components of walking direction: turning, i.e. yaw rotation around the vertical axis (R), forward translation (Tx) and sideward translation (Ty). Arrows indicate the sign of the measurements taken. A large-field visual motion stimulus (M) elicited an optomotor turning response, causing the animal to walk on a curved path. The stimulus pattern was a vertical grating. (B) Each trial lasted 22.5 s and was divided into three periods of equal duration. A pre-stimulus period without stimulus motion was followed by two periods with constant stimulus motion. Diagrams show representative measurements of R, Tx and Ty in response to a step of stimulus velocity. Movement components were used to reconstruct the walked path (C), visualising the overall walking behaviour (same trial as in B). Each symbol indicates the location and orientation of the body axis. Filled circles label the head. Symbols are plotted every 200 ms. Vertical bar indicates stimulus onset. Grid width, 10 cm.