Conditioned reinforcement in the goldfish

Goldfish were trained to press a lever on a 10:1 fixed-ratio schedule of reinforcement. They were extinguished under three conditions. Responding was followed by (i) solenoid noise and water delivery formerly associated with food reinforcement, (ii) solenoid noise only, or (iii) nothing. The number of extinction responses was largest in condition 1, less in condition 2, and smallest in condition 3, thus providing evidence for conditioned reinforcement in goldfish.