Tendon reflex asymmetry by voluntary mental effort in healthy subjects

The effect of voluntary mental influences on the tendon reflexes was examined in healthy subjects. The patellar reflexes were evoked by a method comparable with the clinical examination, and the reflexes were recorded by surface electrodes. Eighteen subjects were instructed to increase and then decrease the right patellar reflex by mental effort, without contracting any muscles. Most subjects increased the reflex in both conditions. Subsequently, ten subjects were instructed to increase the right patellar reflex and decrease the left by mental effort. Measurement showed reflex asymmetry in seven subjects consistent with the instruction. The experiment was repeated in another 20 subjects with symmetric reflexes at rest. Ten of these subjects were, after random assignment, instructed to increase either the right or the left knee jerk. All subjects (and an additional 15 asymmetric control subjects) were examined by a neurologist without knowledge of the instruction. Three of the ten instructed subjects were correctly judged to be asymmetric, but the agreement between instruction and neurologic judgment was not statistically significant. Mentally induced reflex asymmetry is possible, and may be clinically relevant in some cases.