EP components, visual processing stages, and the effect of a barbiturate.

In a 2 x 2 x 2 factorial experiment, ten subjects carried out a visual choice reaction task. In addition to RT measurement, evoked potentials (EPs) were recorded from the central (Cz) and occipital (Oz) derivations. Independent variables were drug treatment (barbiturate versus placebo), visual stimulus intensity and visual stimulus degradation. The reaction time data showed that visual intensity and degradation had additive effects, which indicates that these variables affect independent stages, i.e. stimulus preprocessing and stimulus encoding. The effect of barbiturate on RT was additive with intensity but appeared to interact with degradation. This suggests a selective effect of that drug on the encoding state. EP components also showed selective effects of intensity, degradation and drug treatment. It was suggested that these components may be related to stages in the reaction process.