Subclasses of event-related potentials: response-locked and stimulus-locked components

The manner in which information processing proceeds and the relationship of the components of the cerebral "event-related" evoked potential (ERP) to the presumed stages of such processing are uncertain. We studied these relationships by recording long-latency evoked potentials from the scalp in conjunction with electromyographic (EMG) recording in experiments in which subjects were required to make a motor response to one of two auditory stimuli. The cerebral response was averaged from the tone onset (stimulus-synchronized average) and the EMG onset (response-synchronized average). We found that N2 was quite distinct from the other ERP components (P165 and P3) in its lack of a close relationship to the motor response required in a sensory discrimination task. This suggests that, contrary to general belief, the P165-N2-P3 components of the ERP are not generated in an orderly sequence, but rather are generated separately and are possibly related to different cognitive processes.