Perception of electric properties of objects in electrolocating weakly electric fish: two-dimensional similarity scaling reveals a City-Block metric

During electrolocation weakly electric fish monitor their self-emitted electric signals in order to detect and evaluate nearby objects. Individuals of the mormyrid species Gnathonemus petersii were trained to discriminate between resistive and capacitive objects that differed only in their electric properties. Capacitive properties are found almost exclusively among living objects, and are thus of special importance to the fish. Resistive and capacitive properties of objects influence the amplitude and waveform of the perceived electrolocation signals in different ways. Resistive objects change only signal amplitude, whereas capacitive objects affect both amplitude and waveform. The electro-perceptual system of weakly electric fish was investigated by systematic variation of amplitude and waveform using objects of various electric properties as electrolocation targets. After training with a particular stimulus set, the fish reacted in a graded manner to differences in both signal parameters. The perception of each stimulus dimension was found to be independent of the other one. In a kind of ‘cross modality matching’ experiment, amplitude and waveform parameters were tested against one another. For each amplitude value there was a corresponding waveform value that was judged by the fish to be equally different from the training stimuli. Because of these results, a two-dimensional “perceptual space” is postulated with the two stimulus dimensions waveform and amplitude as its axes. A scaling procedure, using Minkowski metrics, was applied to determine the fish's “perceptual metric”. The City-Block metric, and not the Euclidean metric, provided the best description of the data. The two signal dimensions were found to be separable, i.e. to combine additively in complex stimuli. The results are discussed with regard to the discrimination of animate and inanimate objects in the natural environment of the fish.