Effects of stimulus probability on encoding by mentally retarded and nonretarded persons.

A choice reaction-time experiment was performed in which probe stimulus probability and probe stimulus quality were orthogonally manipulated. Subjects were retarded and nonretarded junior high school students. The stimuli were random forms presented at four probability levels in order to assess the effect of stimulus probability on the encoding process. The results indicated that the retarded subjects appeared to be less sensitive than did the nonretarded subjects to stimulus-probability information, particularly under degraded stimulus conditions. The results were discussed in terms of the relative sensitivity of retarded and nonretarded persons to stimulus-probability information and the effect of such information on the encoding process.