Sustained and transient contrast in human subjects

Twelve educable mentally retarded (EMR) adolescents and twelve 'normal' adolescents participated as subjects (Ss) in an experiment to determine the effect of mental age and sequence of stimulus presentation upon transient and sustained contrast effects. In Phase I of the experiment, all Ss pressed a button to obtain reinforcement on a mult (VI-25 sec., VI-25 sec.) schedule. Components 1 and 2 of the multiple schedule were signaled by a blue or yellow light respectively. Six EMR Ss (Group MR-1) and six normals (Group N-l) received an alternating sequence of the two components. Groups MR-2 and N-2 received a non-alternating sequence of the two components. Following demonstration of stable baseline performance in Phase I, all Ss were switched to a mult (VI-25, EXT) schedule in Phase II. Analysis of data from Phase II revealed a systematic increase of response rate to the constant component, between sessions, accompanied by a decrease in response rate to the EXT component between sessions. This sustained contrast effect was evident in each of the four groups. There was no evidence of transient contrast occurring within component presentations (cycles) in any of the groups. A transient sequential contrast effect was demonstrated between cycles within sessions for groups MR-2 and N-2. No significant effects as a function of mental age was found for either sequential or sustained contrast.