Stimulus bias in the absence of food reinforcement.

Pigeons sometimes peck a key at different rates in the presence of different stimuli, even when the same schedule of reinforcement is correlated with each of these stimuli. The possibility that the occurrence of such stimulus bias is dependent on adventitious effects of food reinforcement in the presence of the stimuli was evaluated by correlating extinction with two stimuli. Both pigeons showed stimulus bias, indicating that the occurrence of this phenomenon is not critically dependent upon any effects of scheduled food reinforcements.