Fig. 4. The dramatic post-pause reduction in the size of electrically elicited novelty responses is not an aftereffect of a pause-eliciting electrical `shock'. Post-pause efficiency, (100), was determined 100 electric organ discharges (EODs) after a pause. The stimulus used was an electrical pulse (one cycle of a 1 kHz sinewave, peak-to-peak field strength 235 mV cm<sup>-1</sup>). Pausing was elicited either by electrical shocks (`Electrical'; fish gc1, dark grey columns, 18 pauses; fish gc2, light grey columns, 10 pauses; fish gc3, black columns, 15 pauses) or, after appropriate training of the fish, by gently tapping on the wall of the tank (`Mechanical'; 12, 10 and 15 pauses, respectively). Values are means + S.E.M.