Effects of caffeine on functional asymmetry in a Posner letter-recognition task

A Posner task was used to investigate whether caffeine, in common with other drugs, has an asymmetric effect on cerebral functioning. Subjects consumed decaffeinated coffee either with or without added caffeine at 2 mg/kg body weight. They were then required to identify letter-pairs as the same or different. Same was defined as two identical letters irrespective of case (AA, Aa); different was defined as two different letters irrespective of case (AB, Ab). Main effects of stimulus type were found for both accuracy and speed of response. In the noncaffeine condition pattern-matching was faster by the right hemisphere and phonologic matching was faster by the left hemisphere. These results replicate much previous work, but under caffeine, a previously unreported reversal in the balance of hemispheric processing efficiency was found. An explanation is offered in terms of the disruption of the normal, optimum, rate of cerebral processing for each hemisphere.