Human postdiscrimination gradients: The effects of three-stimulus discrimination training

Young adult subjects were trained in a discrimination involving three pure tones varying in pitch, with two positive stimuli and a negative stimulus located midway between the two. Subjects in the control conditions were trained only with the two positive stimuli. Generalization gradients were bimodal in all cases, but for the control subjects, the modes were at the positive stimulus values, while postdiscrimination gradients were displaced away from the negative stimulus toward the extremes of the continuum. Since, with this procedure, the training and test adaptation levels were the same, the observed displacements are difficult to explain in terms of an adaptation-level account of peak shifts. The observed shifts are more consistent with the conditioning-extinction model which assumes interactions between inhibitory and excitatory gradients. However, overall response rates did not reflect the action of summation processes.