The "novelty response" in an electric fish: response properties and habituation

The electromotor behavior evoked by novel sensory stimuli in the electrogenic teleost fish Gnathonemus petersii was examined. Novelty responses (NRs) consisted of a transient accelerations of the rate of electric organ discharges following a change in sensory input. NRs were basically similar in nontreated and in immobilized (treated with curare) fish. NRs could be evoked reliably by brief novel stimuli of all four sensory modalities (acoustic, visual, electrical. electrolocation) used in this study. Stimuli of a duration longer than 5 s caused an on- and off-response. A sudden change in the quality of an ongoing sensory stimulus also evoked novelty responses. NR properties depended on the stimulus modality, stimulus intensity, stimulus duration, and on the prior stimulus history. Habituation of several response parameters of the NR (latency, duration, maximal amplitude, response probability) occurred within a series of repetitive stimuli of a given sensory modality. Each modality appeared to habituate separately. Rate of habituation depended on stimulus intensity and on interstimulus interval. A strong disruptive stimulus of another modality lead to dishabituation. The novelty response evoked by stimuli of low or medium intensities resembled an "orienting response" as described by Sokolov.