Fig. 2. Smoothed startle trajectories displayed by juvenile roach in response to the initial half-cycle of an acceleration of about 6.7 Hz, at a stimulus level approximately 15 dB above the response threshold. Movements of the fish where measured in the horizontal plane from video frames recorded by a camera looking down on the fish through the transparent roof of the test chamber. The trajectories show movements of the head of the responding fish during a 160 ms period, i.e. from the video frame before stimulus onset (0,0) and through the subsequent four frames. (A) Trials in push mode with the initial acceleration to the left resulted in 36 startle responses from fish in the trailing (right) half of the chamber, which experienced compression, and no responses from fish exposed to rarefaction in the leading (left) half. (B) Tests in pull mode with the initial acceleration to the right resulted in 35 startle responses from fish in the trailing (left) half of the chamber and 3 startle responses (not illustrated) in the leading (right) half of the chamber. Startle responses in both stimulus situations were on average in the same direction as the initial acceleration.