The alpha block latency to photic stimulation was measured with an OFFNER TYPE T EEG from the parietal and occipital lobe of the left hemisphere of each subject. Twenty four children, half of whom were mentally retarded, were selected for this study. No children with known EEG abnormalities, gross sensory or motor handicaps, or with unscorable alpha rhythms was included in the sample. Subjects were presented with a series of photic stimulus varied in three levels by delivering at 1.25-amp. current of 20, 60, or 120 volts to a 40-watt bolb. The order of stimuli was randomized within each block for every subject, and each S received ten blocks for a total of 30 stimuli. Stimuli were presented when alpha rhythms appeared to be present.

The results show that higher intensity stimuli resulted in shorter alpha block latencies than low ones (p＜.00l); normals had shorter alpha block latencies than retardated ones (p＜.05), the two median latencies being .209 and .260 seconds. There was not, however, a significant interact.ion between intelligence and intensity