How stimulus direction determines the trajectory of the Mauthner-initiated escape response in a teleost fish.

Fishes use the Mauthner-initiated C-start for short-latency evasion of predators. C-starts consist of a sudden turn (stage 1) and a rapid acceleration (stage 2). We analyzed high-speed ciné films of goldfish C-starts elicited by dropping a ball into the water. It was previously thought that stage 1 angle does not vary concomitantly with the angle of the threatening stimulus relative to the position of the fish. We found, however, a significant inverse relationship between the direction of the impact of the ball and the angle turned by the end of stage 1. When starting near a wall, or when its usual trajectory was blocked by a wall, the fish used an escape route that was not predictable from the stimulus angle. The fish did not appear to correct its trajectory if it began to turn towards the ball. This behavioral evidence supports the previous notion that the underlying neural command is ballistic and does not use sensory information from the stimulus once the movement begins. If this is so, the fish probably utilizes information on obstacle location in the interval leading up to the trigger stimulus.