Stimulus control in a two-choice discrimination procedure.

The relation between performance during discriminative training and subsequently obtained measures of stimulus control was investigated. Pigeons served as experimental subjects. In the discriminative training phase, a single peck on the center key, transilluminated by a bright or dim white light, resulted in the onset of the side keys, one red and one green. If the center key was brightly lighted, a response on the red side key was correct. A response on the green side key was correct if the center key was dimly lighted. Correct responses were reinforced on independently arranged variable-interval schedules. Following discriminative training, tests of stimulus control were administered during which white light of 11 intensities was projected on the center key and responses on the red and green side keys recorded. The proportion of correct responses in the presence of a bright or dim center-key stimulus decreased with decreases in the frequency of reinforcement of correct red or correct green responses, respectively. The slopes of the stimulus control gradients were related to the extent of response bias during training. The greater the bias to respond on the green key, the flatter the gradient showing the proportion of green-key responses to each stimulus and the steeper the corresponding gradient of red-key responses.