Evidence for response membership in stimulus classes by pigeons

Response membership in pigeons' stimulus-class formation was evaluated using associative symmetry and class expansion tests. In Experiment 1, pigeons learned hue-hue (AA) and form-form (BB) successive matching plus a modified hue-form (AB) task in which reinforcement was contingent upon a left versus right side-key response after the positive AB sequences. On subsequent BA (symmetry) probe trials, pigeons responded more often to the comparisons on the reverse of the positive than negative AB sequences and, more importantly, preferentially pecked the side key consistent with symmetry after the reversed positive sequences. In Experiment 2, the original three baseline tasks were supplemented by dot-white (CC) successive matching in which reinforcement was contingent upon a left versus right side-key response after the positive CC sequences. Class expansion was then tested by presenting nonreinforced CA and CB successive matching probes. Comparison response rates were mostly nondifferential on CA probes but were uniformly higher on CB probes that consisted of the C samples and B comparisons from the same, hypothesized class. Together, these results provide evidence that responses can become members of stimulus classes, as predicted by Urcuioli's (2008) theory of pigeons' stimulus-class formation and Sidman's (2000) theory of equivalence.