Stimulus and response ERP analyses of a two-level reaction time task.

Higher cognitive processes include the ability to reliably transform sensory or mnemonic information. These processes either occur automatically or they are consciously controlled. To compare these two types of information processing, we developed a reaction time task that requires either a rule operation or else a direct sensory association. We were interested in evaluating the brain's electrical activity corresponding to both tasks, using event-related potentials (ERPs). In order to gain complete insight into the electrical activity of a stimulus-response segment, we analyzed the ERPs corresponding to the processing of the stimulus and the ERPs corresponding to the preparation of the response. To complete the analysis, we also evaluated the lateralized readiness potential (LRP) matched to the stimulus and to the response onset. Compared with the sensory association task, rule operation generated a higher negative potential field at frontocentral scalp areas in a latency range of 312-512 ms after the stimulus. In contrast, the LRP showed a negative component in the sensory association task which was absent during the rule operation; the latency of the difference was in the range 374-532 ms after the stimulus. The ERP component obtained by the response onset analysis was more negative in the rule condition up to a latency of -214 ms before the generation of the movement; the effect was localized at frontal and central scalp regions. We failed to find any significant difference in the LRP matched to the response onset. These results suggest that the brain computation of the rule operation takes place approximately in the middle of the stimulus-response time interval and that it is an additive process to the sensory association response.