Three weakly electric fish (Gnathonemus petersii) were force-choice trained in a two-alternative procedure to discriminate between objects differing in their electrical characteristics. The objects were carbon dipoles in plexiglass tubing (length 2.5 cm, diameter 0.6 cm). Their electrical characteristics could be changed by varying the impedance of an external circuit to which they were connected (Fig. 1). In one (the 'capacitance dipole') the resistance was very low ( < 3 fl) and the capacitance variable. In the other (the 'resistance dipole') the resistance was variable and the capacitance low (< 50 pV).
Capacitances from several hundred pF ('lower thresholds', Fig. 2) to several hundred nF ('upper thresholds', Fig. 3) could be discriminated from both insulators and good conductors. In all cases the rewardnegative stimulus was the capacitance dipole, which was avoided by all fish spontaneously. Thresholds were defined at 70% correct choices.
The fish were then tested for their ability to discriminate between one object with a given capacitance and another with resistances varying from 3 [2 to 200 kf~. The capacitance dipole continued to be the negative stimulus throughout. All 3 fish avoided it in at least 80% of the trials at each stimulus combination (Fig. 4). This result suggests that Gnathonemus perceives the capacitance and the resistance of objects differentially.
The effect of the dipole-objects as well as some natural objects on the local EOD was recorded differentially very close to the fish's skin (Fig. 5). The amplitude of the local EODs was affected by all types of objects as they approached the skin. However, the waveform was changed only by capacitance dipoles and some natural objects (Figs. 6 and7). It appears that the fish perceive not only intensity changes in the local EOD but waveform deformations as well and can thus distinguish objects of different complex impedances.