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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.
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