Longitudinal tracking responses of the weakly electric fish, Sternopygus

Summary

The ability of Sternopygus, a South American electric fish, to track longitudinal motion of its surround was studied. Tracking responses were elicited by moving plexiglass enclosures which these fish readily occupy. The longitudinal position of these ‘shuttles’ was varied over time in a sinusoidal or linear fashion. The objectives were to quantify the metrics of longitudinal tracking responses and the roles of the various sensory modalities in this behavior. The following results were obtained:

• 1. In cases where the longitudinal position of the shuttle varied in a sinusoidal fashion over the time with a periodicity of approximately 0.25 Hz and with a peak to peak excursion of 6–8 cm, fish tracked with little or no phase lag and a gain of 0.8–0.9. For linear motion at 0.25 Hz, the gain of tracking responses was similar, however, a phase lag of approximately 7° was observed.

• 2. Restricting visual or electrosensory information markedly impaired tracking performance. Fish did not appear to rely on mechanosensory cues in tracking shuttle motion.

• 3. Jamming signals that were approximately half the amplitude of the fish's own discharges and 4, 8 or 16 Hz greater in frequency failed to impair longitudinal tracking performance.

• 4. For 6 cm peak to peak shuttle excursion, optimal tracking was observed at rates of motion of 0.1–0.2 Hz.

• 5. The electromotor component of longitudinal tracking responses was most clearly seen in fish that had impaired vision due to ‘frosted lenses’ over their eyes.