Stimulus control in a discrimination based on a distinctive feature

Six pigeons were instrumentally trained to discriminate between two displays that differed only by the presence of a distinctive feature on the positive or food-correlated display. In accordance with previous studies, subjects learned the discrimination and, in the presence of the positive display, directed most of their responses to the distinctive feature, although responses to the common feature were also reinforced. Subsequent generalization tests revealed that on the positive display, both common and distinctive features produced decremental gradients, contradicting Farthing's (1971) statement that the common feature acquires a control function opposite that of the distinctive feature. Procedural differences probably caused the discrepancy in results; within a display, Farthing presented common and distinctive features successively; the present study used simultaneous presentations of common and distinctive stimuli.