Fig. 1. Measurement of the efficiency with which a given stimulus elicited a novelty response after a pause in the ongoing electric organ discharges (EODs). The stimulus was given twice (vertical arrows): (1) n EODs after the pause and (2) 10 000 EODs, or approximately 200 s, later when the fish had returned to steady firing. The response is a transient shortening in the interpulse interval between successive EODs (ordinate). In the schematic trace shown, a Gymnotus carapo fired with a stable interpulse interval of approximately 20 ms. The fish was then induced to pause its EODs for the period indicated in grey. For each of the two subsequently elicited responses, the maximum reduction T in interpulse interval from the pre-stimulus interval T was determined, and T was normalized to T. The efficiency of the stimulus when given n EODs after the pause, (n), was taken as the ratio of the two normalized interval modulations, as indicated.