Changes in Summated Visual Potentials (Lambda Waves) during Mental Tasks Using the Stroop Test

In normal Ss, a parietal-occipital located response (lambda wave) is evoked by scanning a printed block and word material (colored, and black and white) of the Stroop test. When this response is summated with a computer triggered by the eye movement, a highly reliable triphasic wave appears which can be subjected to accurate latency and amplitude measurements. The shortest latency of this response was observed when blocks vs words were presented, but there were no differences in latency in spite of increasing difficulty of material. This indicates that the structure of the material rather than mental activity affected the latency. Likewise, no changes were noted with colored vs black material. There was, however, a small increase in amplitude of the lambda wave response in the most difficult (word interference) task—a result that could be attributed equally to increase of muscle potentials and to the effect of mental activity.