Fig. 4. Resetting the phase of swimming by stimulating sint1. The firing pattern in Lsint1 was recorded along with a record of behavior (upper trace in each panel) during a continuous episode of swimming. (A–C) Hyperpolarizing current pulses sufficient to prevent soma and axon spikes in Lsint1 were applied via the recording electrode. The timing and duration of pulses were varied (pulse timing indicated by solid lines below the voltage traces). In each experiment, the interval between the centers of Lsint1 bursts was measured during the 10 cycles preceding the onset of the stimulus (indicated by vertical strokes above the voltage recording). These measurements were used to calculate the means ± S.D. of the swim period, which were then used to predict the time of occurrence of the Lsint1 burst projected forward in time beyond the period of stimulation, based on the assumption that stimulation of Lint1 has no effect on pattern generation. The predicted times of the center of Lsint1 bursts are shown as vertical lines above the voltage recording, with the S.D. represented by horizontal tics through the lines. The results show that the assumption fails and that the phase of swimming behavior is reset by the stimulus. The difference between the predicted and actual time of occurrence of the burst provides a quantitative measure of phase resetting (see details in the text). (D) The experiment was repeated with an extra burst driven in Lsint1. The extra burst also reset the phase of swimming in a quantitative fashion (see text). The bridge circuit used to deliver stimulating currents was imperfectly balanced and, therefore, the absolute membrane voltage is not accurately represented during periods of stimulation. This experiment was conducted 28 times in four different whole animal preparations with consistent results.