Categorical Discrimination of Sequential Stimuli: All SΔ Are Not Created Equal

Pigeons were exposed to a novel variation of a stimulus discrimination task previously used with rats as a slot machine analog. We sought to replicate and extend characterizations of structural characteristics of responding, determine relations between those characteristics and identify predictors of individual differences in performance. Pecking during a “collect” phase produced food if and only if the keylight was red for the entire sample phase. In other trial types, the keylight was green, or started red and turned green after one or two pecks. The opportunity to respond was available for 5 s in all collect phases, permitting multiple responses per trial. Pigeons were less likely to peck in collect phases when sample stimuli were not all red, and when they did peck, it was at a lower rate. However, consistent with reactions to near wins in other slot machine tasks, there was more responding in collect phases that followed sample phases when the red keylight turned green after two pecks. Among initial response characteristics, response rate in trials that resembled near wins was the best predictor of terminal responding. Supporting their characterization as measures of stimulus control, collect proportions were negatively correlated with response times in the sample phase. Supporting their characterization as measures of conditioned reinforcing value, latency and response rate in collect phases were positively correlated, but neither was systematically correlated with proportion or sample phase response times. Ultimately, isolating measures of stimulus control and conditioned reinforcement may help determine the mechanisms responsible for near-win effects.