EFFECTS OF VARYING SAMPLE-STIMULUS AND CHOICE-STIMULUS DISPARITY ON SYMBOLIC MATCHING-TO-SAMPLE PERFORMANCE

Six pigeons were trained on a typical two-stimulus two-response symbol ic matching-to-sample task involving the discrimination of sample and choice stimuli that were signaled by different probabilities that pixe ls in an area were lit on a computer screen located behind the respons e keys. The disparities of the sample and choice stimuli were systemat ically varied across five experimental parts by manipulating the proba bilities of pixel illumination. Across conditions within parts, the ra tio of reinforcers obtainable for matching responses was varied over f ive levels. A recent model of detection based on the discriminability between the stimulus-response relations and between response-reinforce r 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-rei nforcer discriminability unchanged. Equally, changes in choice-stimulu s disparity caused a decrease in estimates of response-reinforcer disc riminability and not in stimulus-response discriminability. Parameter invariance was thus obtained, and the variables expected to affect the se parameters (sample and choice stimulus disparity) were correctly id entified. The reasons for the failure of two recently reported studies to support parameter invariance under this model are discussed.