The relationship between the evoked potential and brain events in sensory discrimination and motor response

The relationship between the various components of the cerebral evoked potential, sensory discrimination and the initiation of motor responses is controversial. In order to study this relationship, we recorded long-latency evoked potentials from the scalp together with the EMG in a reaction time experiment where the subject was required to make a motor response to one of two auditory stimuli. The peak latencies of event-related components were all found to correlate with EMG response times. Similar results have been reported by others and have led to suggestions that one or another of these components reflects electrical activity in neural structures that are responsible for sensory discrimination. In our experiments, however, the EMG response generally preceded all these components. This observation, whether it relates to the simplicity of our task, the use of EMG to measure reaction time, or to some combination of these factors, implies these these event-related potentials cannot reflect cortical events involved in either sensory discrimination or the initiation of a motor response. Presumably these components must reflect subsequent neural events in the processing of infrequent target tones.