Stimulation and control as components of stereotyped body rocking.

Severely and profoundly mentally retarded children who engaged in stereotyped body rocking were presented with all possible pairs of three conditions and made a choice as to which they preferred: a rocking chair that rocked freely under the child's control (active stimulation), a rocking chair that provided the same stimulation but was under the experimenter's control (passive stimulation), and a rocking chair that had been immobilized (stationary). Children significantly chose the active stimulation condition over the passive and the passive over the stationary; however, the degree of the effect was associated with developmental level. Results were interpreted as showing that self-stimulation involves at least two independent processes, control and stimulation.