Extinction-induced mirror responding as a baseline for studying drug effects on aggression.

Pigeons worked individually in a chamber containing a response key and a mirror. Responding on the key was controlled by a multiple schedule in which a brief period of continuous food reinforcement alternated with a 5 min period of extinction. Under baseline conditions, aggressive behavior (responding on the mirror) occurred at the onset of each extinction period. After 10 saline control sessions, 5 mg/kg of chlordiazepoxide was injected IM 30 min presession for 60 daily sessions. The drug initially produced a marked decrease in aggressive behavior but had little or no effect on key pecking. The aggressive behavior generally remained suppressed during the chronic drug regimen and returned to control levels when the drug was withdrawn. It was concluded that the technique of extinction-induced mirror responding in pigeons provides a stable, sensitive and recoverable baseline for objectively assessing selective drug effects on aggression.