Action profiles of smoking and caffeine: Stroop effect, EEG, and peripheral physiology.

Twenty female regular cigarette smokers and coffee drinkers performed a numerical Stroop task in a 2 x 2 (caffeine x smoking) prepost crossover design. In the easier of the two different versions, caffeine and smoking reduced the reaction times (RT's) when given alone, but there was no additive effect. The Stroop effect itself (difference between RT's to numbers and RT's to symbols) was reduced by the two treatments only in the more difficult version, but the combination did not differ from the placebo condition. The physiological reactions to both treatments were additive, although the two reaction profiles were different. Smoking increased heart rate, blood pressure, finger vasoconstriction, respiratory frequency, EEG dominant alpha-frequency, and beta power and reduced respiratory amplitude, EEG delta and theta power. Caffeine increased blood pressure, finger vasoconstriction, motor activity, frontal EMG, and EEG theta power and decreased heart rate and EEG beta power.