Flies walked on a flat, round platform with a diameter of 95 mm surrounded by a cylindrical arena (360° in azimuth) that was constructed from panels identical to those used for tethered walking. However, the LED light was not filtered, which resulted in a slight green shift of the stimulus light and many times greater stimulus intensity. The walking platform was actively maintained at the same temperature as was used for the tethered experiments (21 °C). The flies were enclosed by a heated metal ring with a height of 3.8 mm that supported a glass plate coated with Sigmacote (Sigma-Aldrich; arena design: T. Ofstad & M. Reiser, unpublished). We placed three 2–4-day old flies at a time on the platform, and presented stationary, clockwise and counterclockwise stimulation with the same pattern used in the tethered walking experiments. Each stimulus condition lasted 15 s and the stimulus sequence was repeated for 20 minutes. We tracked the positions of walking flies at 15 fps with a camera (Basler 602f) from above. We obtained walking trajectories for each fly using Ctrax software48 (link), and calculated the translational and rotational velocities based on the distance moved between subsequent frames (75 ms). For averaging, we excluded parts where the fly was stationary (< 1 mm/s translational velocity). Velocities of tethered flies were calculated as the change in position during 75 ms, and were smoothed using Savitzky-Golay filtering with a span of 150 ms.