Goldfish were placed in a shuttlebox. They had to respond if the lights colours at each end were the same by swimming to the other end. When they were wrong, they received an electric shock. Later, they had to respond if the colours of the lamps were different.
"Goldfish, trained in the avoidance shuttlebox with a variant of the linear discrimination procedure, learned to conditionally discriminate between color signals."
The goldfish were able to learn general rules to avoid
the pain of the electric shocks.
"Shock, in 200-msec pulses, was scheduled to occur at 12,14, and 15 sec after trial onset. If, however, the relationship between the color of the trial standard color and the end-lamp response-contingent color met the procedural criterion, shock was omitted. For the matching procedure, the criterion was a match (same color) of end and hurdle lamps. For the oddity procedure, the criterion was a difference (in color) between the end and hurdle lamps."
"These findings are consistent with the interpretation that goldfish learned the original conditional discrimination at a conceptual level, and learned about the general matching or oddity relationships between colors, rather than about a specific set of colors."