Effects of varying sample- and choice-stimulus disparity on symbolic matching-to-sample performance

Six pigeons were trained on a typical two-stimulus two-response symbolic matching-to-sample task involving the discrimination of sample and choice stimuli that were signaled by different probabilities that pixels in an area were lit on a computer screen located behind the response keys. The disparities of the sample and choice stimuli were systematically varied across five experimental parts by manipulating the probabilities of pixel illumination. Across conditions within parts, the ratio of reinforcers obtainable for matching responses was varied over five levels. A recent model of detection based on the discriminability between the stimulus-response relations and between response-reinforcer relations provided an effective description of the data. Consistent with this model, changes in the disparity of the sample stimuli led to decreases in stimulus-response discriminability and left response-reinforcer discriminability unchanged. Equally, changes in choice-stimulus disparity caused a decrease in estimates of response-reinforcer discriminability and not in stimulus-response discriminability. Parameter invariance was thus obtained, and the variables expected to affect these parameters (sample and choice stimulus disparity) were correctly identified. The reasons for the failure of two recently reported studies to support parameter invariance under this model are discussed.