Reacting while moving: influence of right limb movement on left limb reaction.

An experiment was designed to determine whether the activation of a muscle group (flexors or extensors) used to produce an ongoing movement of one limb influenced the reaction time and associated initiation of elbow flexion or extension movements of the contralateral limb. Right-handed participants in the bimanual groups were asked to produce a pattern of flexion/extension movements defined by a sine wave (period = 2 s, amplitude = 16°) with the right limb. While performing the right limb movement, participants were instructed that they were to react as quickly as possible by making a flexion or extension movement with their left limb when the cursor they were using to track the sine wave changed color. Participants in the unimanual groups performed the left limb reaction time task but were not asked to make right limb movements. The reaction time stimulus occurred once in each trial and was presented at one of six locations on one of the six cycles comprising the sinusoidal waveform. Participants performed 7 blocks of 6 test trials. Reaction time was calculated as the time interval between the color change of the cursor and the initiation of the response with the left limb. Movement time was calculated as the interval of time between the initiation of the response and the left limb cursor crossing the upper or lower boundary line. Mean reaction of the left limb was significantly influenced by the concurrent type of movement (flexion/extension) of the right limb. Reaction times were shorter on trials in which both limbs were initiating movement with homologous muscles as compared to trials in which the limbs were initiating movement with non-homologous muscles. No differences were detected when the stimuli were presented during the ballistic phase of the right limb movement, and no differences at any position were detected for the unimanual groups. This result is consistent with the notion that neural crosstalk can influence the time required to react to a stimulus but this influence occurs when contralateral muscles are activated.