Figure 1.

Schematic representation of the shuttle box adapted to fish conditioning. A, Top view. B, Side view. The system for detecting the fish's passing across the barrier consisted of two red lights (24 V cc, 0.3 W) used as photoemitters, and a pair of photoresistors (3 V cc) used as photoreceptors, all on the barrier. The barrier was made of gray PVC. To deliver the electric shock (ES) as an unconditioned stimulus (0.39 V/cm, 50 Hz), two stainless steel bars in contact with four metal plates were used as electrodes. Two green lights [discriminative stimulus (DS)] were used as a conditioned stimulus (10 W, 220 V AC, 50 Hz), which was presented through the translucent end walls of the box.