High doses of caffeine impair performance of a numerical version of the Stroop task in men.

The effects of caffeine ingestion on mid-morning cognitive performance were investigated in thirty-two male subjects. These were given drinks containing either no caffeine, 125 mg caffeine (mean dose: 1.38 mg/kg), or 250 mg caffeine (mean dose: 3.45 mg/kg) and were tested on three tasks: 1) free recall of supraspan word lists, 2) a response time (pointing) task and 3) a numerical Stroop task. There were no significant group differences on the recall task or in response times, but subjects having the higher caffeine dose were seriously impaired on the Stroop test, making particularly slow responses. Caffeine may have a deleterious effect on the rapid processing of ambiguous or confusing stimuli, and this may account for its clear effect on the Stroop test than on other cognitive tests used hitherto.