Retinal locus and the identification of tachistoscopically presented letters by retarded and nonretarded individuals

Two-letter stimulus displays, differing in the magnitude of the horizontal spatial separation between the letters, were presented tachistoscopically to retarded and nonretarded persons. Assuming that the spatial separations are linearly related to retinal location, I derived an estimate of the stimulus-recognition threshold for each group by employing the method of constant stimuli. The data indicated a similar stimulus-recognition threshold and a similar decrement in the accuracy of responding as the stimulus separations increased. These findings suggested that reported differences in iconic memory between retarded and nonretarded individuals are most likely related to a postretinal stage of cognitive processing. Studies of iconic memory must take into account not only the influence of retinal locus but also the finding that, as the task becomes more difficult, retarded individuals may be less willing to respond.