Fig. 1. Schematic diagram of the experimental arrangement (A) and examples of behavioral responses (B). (A) The experimental animal rested in a cage placed centrally, at half-water level, in a large tank. Two Ag/AgCl pellets, placed at the side of the fish at its trunk/tail region, could be connected, outside the tank, via a fast electronic switch. As a result of switch closure, a part of the electric organ discharge (EOD) current of the fish is redistributed to flow over the low-resistance path of the external circuit rather than through the water. This current could be monitored (cur) by inserting a current amplifier in the external circuit. As a reference of which electrocyte groups produced the current, head—tail EODs were recorded by two silver wires. To start an experiment, a rectangular pulse (en) simultaneously enabled a counter module (CT) and allowed EODs (sig) to be fed into a processor (PC). After 200 inter-EOD intervals were recorded, the counter module issued a reference pulse (ref), that signalled the onset of the 201st recorded EOD. By varying the delay of a command pulse (com) with respect to this reference pulse (DEL), the electronic switch could be closed during a selected phase of this particular EOD, or, as a control, in the silent time after it in which no EOD current is shunted. The processor continued to store 300 inter-EOD intervals that followed after switch closure. Prior to input to the counter module, the EODs were strongly amplified, filtered and converted to rectangular pulses (pulseformer; PF). The inset shows a head-to-tail EOD (sig; V₁-V₄ denote the major phases of the EOD) and a timing diagram of switch closure with the time course of the signals ref, com and cur. (B) Examples of four novelty responses to shunting of EOD current during particular phases of a single EOD. The switch was closed, for 100 µs, during EOD-phase V₄ (upper three traces) or during V₃ (bottom trace). The timing of switch closure is indicated by the arrow and the vertical grey line. Note the clear changes in interpulse interval. The abscissa (Time) shows the succession of the 200 pre- and 300 post-stimulus interpulse intervals.

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