The effect of warning and prior instruction on short-latency cerebral potentials produced by muscle afferents in man.

This study examined the effect of a warning and of prior instruction on the early components of the cerebral potential produced by stimulation of the left posterior tibial nerve at the ankle. Early components of this potential are dominated by the activity in muscle afferents from the small muscles of the foot. In 50% of presentations, the stimulus to the posterior tibial nerve was preceded by an auditory cue. In some sequences subjects were required to move toes on the left foot immediately the stimulus was detected. Although subjects responded more rapidly to shocks which were preceded by a warning there was no statistically significant effect of the warning on the amplitude or latency of the early components of the muscle afferent cerebral potential. Prior instruction to respond to the stimulus also failed to change the cerebral potentials. This study suggests that the ability to respond more rapidly to "warned" stimuli is due to events "upstream" of the sensorimotor cortex rather than to enhancement of the volley arriving at the cortex.