The effects of number of sample stimuli and number of choices in a detection task on measures of discriminability

Six pigeons were trained on a conditional discrimination task involving the discrimination of various intensities of yellow light. The research asked whether stimulus-response discriminability measures between any pair of stimuli would remain constant when a third or fourth sample and reinforced response were added. The numbers of different sample stimuli presented and different responses reinforced were two (Part 1), three (Parts 2 and 4), and four (Part 3). Across conditions within parts, the ratios of reinforcers obtainable for correct responses were varied over at least five levels. In Part 5, the numbers of sample stimuli and reinforced responses were varied among two, three, and four, and the reinforcer ratio between consecutive remaining samples was constant at 2:1. It was found that once a particular response had been reinforced, subjects continued to emit that response when the conditional stimulus for that response was no longer presented. Data analysis using a generalization-based detection model indicated that this model was able to describe the data effectively. Four findings were in accord with the theory. First, estimates of stimulus-response discriminability usually decreased as the arranged physical disparity between the sample stimuli decreased. Second, stimulus-response discriminability measures were independent of response-reinforcer discriminability measures, preserving parameter invariance between these measures. Third, stimulus-response discriminability measures for constant pairs of conditional stimuli did not change systematically as conditional stimulus-response alternatives were added. Fourth, log stimulus-response discriminability values between physically adjacent conditional stimuli summed to values that were not significantly different from estimates of the discriminability values for conditional stimuli that were spaced further apart.