Human Ss [subjects] in a visual reaction-time experiment responded to stimuli of systematically varied luminance, area, and duration. Reaction time, EEG alpha blocking latency, and alpha blocking duration were recorded and measured. The major findings were: (a) Over a range of luminances (0.65-10.0 mL.) and exposure durations (10-200 msec.) constant I X t products result constant blocking latencies; i. e., Bloch''s law. (b) Constant products of I X A (Ricco''s law) do not lead to constant blocking latencies beyond areas of 1[degree]. (c) Reaction time decreases with increased luminance or area under equal-energy conditions and is independent of duration over the range of t employed. (d) Blocking duration increases with stimulus duration but is unaffected by luminance. (e) Correlations between reaction time and properties of the alpha rhythm are determined, in large part, by stimulus variables.