Positive effect of heterogeneity of difference on thesame-different disparity in letter matching

Subjects judged whether two adjacent letters were identical or different.Different pairs were similar, for example HM, DO, VY, or dissimilar, for example DY, HV,MO. According to the noisy-operator theory, increasing the heterogeneity of difference (external noise) by intermixing similar and dissimilardifferent pairs ought to produce faster but less accurate responses ondifferent trials. As predicted, the tendency to make more false-different responses (i.e., errors on same pairs) decreased when the similar and dissimilardifferent pairs were intermixed rather than presented in separate blocks. The fast-same effect did not change, however, seemingly due to criterion misadjustment. Proctor and Rao apparently found no effect of heterogeneity of difference on RT and errors because they used a less sensitive procedure (between-subjects design; different set of letters for more and less heterogeneous conditions). Consistent with the internal-noise principle, but not with the response-competition model of Eriksen, O’Hara, and Eriksen, the enhanced fast-same effect on blocks containing only similar (vs. dissimilar)different pairs was accompanied by an increased preponderance of false-different errors.