Analysis of the role of stimulus comparison in discrimination learning in Pigeons

In Experiment 1, pigeons were trained on a conditional discrimination in which presentations of a color and of a shape signaled that one response would be reinforced, and presentations of a different color and a different shape signaled that another response would be reinforced. For Group C (comparison), both colors were presented in some sessions and both shapes in others; for Group NC (no comparison), some sessions involved presentations of one color and one shape, other sessions of the other color and shape. The discrimination was acquired more readily by Group NC than by Group C and this difference between the groups was maintained in a further task (Experiment 2) involving a successive go/no-go discrimination in which pecking at one of the colors and one of the shapes was reinforced, response to the other color and shape being nonreinforced. Analysis of the details of the birds’ performance supported an explanation in terms of responses governed by the absolute properties of the stimuli. In contrast to what has been found for human subjects, there was no support for the notion that the opportunity to compare similar stimuli (available to Group C in Experiment 1) engages a perceptual learning process that enhances their discriminability.