Effects of stimulus contact on the feature-positive effect.

Fourteen pigeons were taught to discriminate displays differentiated by the presence or absence of a distinctive feature. Feature-positive (FP) birds were reinforced for responses to displays containing the feature, while feature-negative (FN) birds were reinforced for responses to displays containing only the elements common to both stimulus displays. Both control and experimental birds were required to respond to a spatially separate operant key, while experimental animals were also required to contact the individual elements of the display. A planned comparisons test indicated that FP performance was superior to FN performance only for the control animals. No difference was found between the performance of the experimental FP and FN birds. Consideration of the elements of the display toward which the FN birds orient was offered as an explanation of these results.