The alpha block duration to photic stimulation was measured with an Offner Type T EEG from 11 normal and 11 retarded children matched on the CA. Three intensities (varied in three levels by delivering at 1.25-amp. current of 20, 60 or 120 volts to a 40-watt bulb) and three durations (.5, 3, and 15 sec.) were employed. Each S received all treatment combinations of stimulus intensity and duration.

As expected, higher intensity stimuli produced longer alpha blocks duration than lower intensity stimuli (p＜0.01). There was also a significant, duration effect (p＜0.01), i.e., longer stimuli resulted in longer block. The alpha blocks of retarded group were slightly longer than normals in higher and longer stimuli, but these results were reversed in lower and shorter stimulus conditions. The results of this experiment could not wholly support Ellis´ theory of stimulus trace deficit in mental retardates.