Procedure for optogenetic stimulation of neurons that induce klinotaxis-like curving. a, Schematic diagram of klinotaxis. Worms gradually curve toward the side with higher (or lower) chemical concentrations. b, Schematic diagram of the photoactivation pattern. In the illustration, the animal is stimulated when it bends its neck to the left side. The left curve is ipsilateral to the stimuli. The right curve is contralateral to the stimuli. c, Quantification of head swing. Neck angle h is defined as the angle between two lines. One links the 5 and 20% points of the animal's midline and the other links the 20 and 35% points. The 0 and 100% points are set to the head and tail, respectively. The head-swing criterion is |h| > 10°. In the illustration, the animal is assumed to be bending its neck to the left side. d, Setup for automated tracking and stimulation. The control software decides whether to stimulate or not by the criteria (c) every 50 ms during an assay. e, Definition of the curving rate. See the Materials and Methods for a description.