Sensory discrimination and its relationship to the cerebral processing of infrequent stimuli

We recorded cerebral evoked potentials, back and forward averaged from the EMG onset of the responding muscle, in three reaction time tasks, each requiring an identical motor response to an identical stimulus but differing in the nature of the sensory discrimination required. Two types of stimuli were presented: a rare one to which the subject responded with finger-extension, and a frequent one to which no response was required. We found a close but variable relationship between the cerebral events associated with performance of a task and the timing of the motor response. As completion of the discrimination process was delayed relative to stimulus occurrence, EMG activity began later relative to the cerebral potentials. Moreover, we were able to record these cerebral events only from the response to the rare (unexpected) stimulus and not when subjects were required to respond to the frequent stimulus, suggesting that the sensory discrimination, in these experiments, is an event that occurred only in the processing of the unexpected stimulus.