POSITIVE SHIFTS: THE EFFECT OF TEST RANGE UPON REPORTED DIFFERENCES IN HUMAN AND INFRAHUMAN PERFORMANCES ON DISCRIMINATION TASKS

The relationship between shift magnitude and S+ S- separation on the stimulus continuum was assessed in experiments where post-discrimination gradients were obtained from women who had learned hue discriminations. The extent of test range was manipulated because it was a major methodological difference between animal literature which has reported an inverse relationship between shift magnitude and S+ S- separation and human literature which has reported a direct relationship. Such reports have led to speculation that human and infrahuman learning are based upon different foundations.;A broad range similar to those found in animal studies was used in Experiment 2. Under such testing conditions, the inverse relationship obtained in the animal investigations was observed with humans.;Experiment 3 demonstrated that the range used in the replication set up differential tests for the S- groups that experienced S+ S- differences of 10 and 20 nm, resulting in outcomes which could not be reliably interpreted. Experiment 4 examined the range used in the replication as it affected the S- group that experienced the 60 nm difference in training stimuli. Inclusion of the S- value into that range set did not result in a shift. When a range was constructed which included the S- value as well as values occurring between S+ and S-, that asymmetrical range led to displacements of response strength towards the center of an area of optimal spectral sensitivity.;It was concluded that discrimination training leads to the same type of shift activity in humans as in infrahumans when the methodologies under which each are collected do not differ. It was proposed that a similar, if not the same, type of process governs the performance of each.;By use of the narrow range used in a previous human study, Experiment 1 replicated the direct relationship when S+ S- differences were 10 and 20 nm. When the separation was 60 nm and S- was not included in the range, a shift did not occur.