Initiation of Bilateral Responses in a Kinesthetic Reaction Time Task

Two experiments are presented in which different mechanisms controlling each limb in a bilateral arm response to a unilateral kinesthetic stimulus are postulated to occur. In Experiment 1, the limb serving as the stimulus limb was described as operating with relative invariance by using a tightly coupled input-output reflexive pathway, whereas the nonstimulus limb appeared to be controlled by higher-order processing thought to be more susceptible to influences such as hemispheric specialization and stimulus expectancy. The differential control model was further tested in Experiment 2 by retaining the intrahemispheric pathway of the unilateral kinesthetic stimulus but experimentally uncoupling the reflex mechanism from the stimulus side. Analyses of bilateral EMG premotor latencies under these conditions revealed that each response side can be controlled at separate levels —i.e., by a reflexive type mechanism or by higher-order processing when one of the response limbs is also the stimulus limb. When the stimulus is on the same side but not generated by either responding limb, both sides reflect behavior that is best described by an information processing type of voluntary control.