Electroreception in catfish: Patterns from motion

An electrosensitive catfish, Ictalurus melas, was trained in a two-alternatives forced-choice conditioning paradigm to discriminate between the electric fields of two direct-current (dc) dipoles, spaced 12 cm apart, the dipole axes parallel to the swimming path of the subject. The dipole size could be varied between 1 and 10 cm. The dipole current was about 5 mu A. When two dipoles of different sizes were presented simultaneously, the subject's electrodiscrimination performance exceeded the 85\% correct choices level provided the dipole of 1 cm was tested against a dipole with a span of 1.5 cm or more. The average stimulus strength at 1 cm distance from the dipole axis ranged from 1 to 10 mV/cm. The swimming speed of the subject was 7 +/- 3 cm/s. The potential swing over the skin caused by the subject passing the dipole, matched the frequency band of the ampullary electroreceptor organs. Apparently motion of the fish with respect to a stationary direct-current stimulus source, or vice versa, generates st biologically adequate form of reafferent stimulation. Without relative motion an electrical de-source would remain unnoticed.