Methylmercury-induced changes in operant discrimination by the pigeon.

Pigeons were trained on a fixed consecutive number schedule of reinforcement, pecking eight or nine times on one key (a run) before making the single response on a second key that was reinforced if the number requirement had been met. A run of fewer than eight or more than nine responses reset the response requirement. They then were given methylmercury chronically until behavioral signs of poisoning occurred. Where possible, recovery was followed. Percentage of reinforcers earned and rate at which the birds pecked both decreased whereas variability of run length increased after enough methylmercury had been given to produce blood mercury concentrations between 13 and 27 ppm. Some birds also showed consistent shortening of run length throughout the time of maximum poisoning. Because ataxia was a common accompaniment of the changes in operant behavior, other methods of producing ataxia (hobbling one foot or dosing with ethanol) were also studied in some birds. The pattern of changes induced with these methods did not match that seen after methylmercury.