Locus stabilization with parallel rectilinear dot-progressions

Illusory displacement measures ("straight" or "crooked") were compared for single vs parallel rectilinear light progressions in 24 educable mental retardates and 24 normals of equal MA. For both groups, frequency of perceived "straight" responses was greater for the parallel- than for the single-line progressions. It was concluded that concurrent stimulation by two proximate visual stimuli rather than the presence of supplementary interstimulus mediatory referents is sufficient to facilitate veridical perception of successive light positions. Comparable performance by the normals and retardates substantiates previous findings.