J Neurophysiol 92: 395–407, 2004.
Mapping From Motor Cortex to Biceps and Triceps Altered By Elbow Angle
Michael S. A. Graziano, Kaushal T. Patel, and Charlotte S. R. TaylorGraziano, Michael S. A., Kaushal T. Patel, and Charlotte S. R.Taylor.
Mapping from motor cortex to biceps and triceps altered byelbow angle. J Neurophysiol 92: 395–407, 2004. First publishedFebruary 25, 2004; 10.1152/jn.01241.2003. This experiment usedcortical microstimulation to probe the mapping from primary motorcortex to the biceps and triceps muscles of the arm in monkeys. Themapping appeared to change depending on the angle at which theelbow was fixed. For sites in the dorsal part of the arm and handrepresentation, the effects of stimulation were consistent with initiatinga movement of the elbow to an extended angle. Stimulationevoked more triceps activity than biceps activity, and this differencewas largest when the elbow was fixed in a flexed angle. For sites in theventral part of the arm and hand representation, stimulation had theopposite effect, consistent with initiating a movement of the elbow toa flexed angle. For these sites, stimulation evoked more biceps activitythan triceps activity, and the difference was largest when the elbowwas fixed in an extended angle. For sites located in intermediatepositions, stimulation evoked an intermediate effect consistent withinitiating a movement of the elbow to a middle, partially flexed angle.For these sites, when the elbow was fixed at a flexed angle, the evokedactivity was largest in the triceps, and when the elbow was fixed at anextended angle, the evoked activity was largest in the biceps. Theseeffects were obtained with 400-ms-long trains of biphasic pulsespresented at 200 Hz and 30 _A. They were also obtained by averagingthe effects of individual, 30-_A pulses presented at 15 Hz. How thisstimulation-evoked topography relates to the normal function of motorcortex is not yet clear.
One hypothesis is that these results reflecta cortical map of desired joint angle. In specific, we predicted that the patterns of muscle activityevoked by stimulation should change systematically along thecortical surface, reflecting the apparent map of arm posturesobtained in our previous study. Dorsal sites in the arm representationshould be associated with goal elbow angles that arefully or mostly extended. Therefore stimulation of these siteswhile the elbow is held stationary should evoke more tricepsthan biceps activity. Greater triceps activity should be evokedwhen the elbow is fixed in a flexed angle, far from the hypothesizedgoal, and less triceps activity should be evoked when theelbow is fixed in an extended angle, near the hypothesizedgoal. Ventral sites should be associated with goal angles thatare fully or mostly flexed, and therefore stimulation of thesesites should evoke mostly biceps activity. Greater biceps activityshould be evoked when the elbow is fixed in an extendedangle, and less biceps activity should be evoked when theelbow is fixed in a flexed angle. Sites at intermediate locationsshould be associated with intermediate goal angles. In thiscase, stimulation of the same site in cortex with the sameparameters should evoke greater activity in one or anothermuscle depending on arm position. Greater triceps activityshould be evoked when the elbow is fixed in a flexed angle, andgreater biceps activity should be evoked when the elbow isfixed in an extended angle. In effect, the mapping from thestudied site in cortex to the muscles should switch from theextensor to the flexor muscle depending on elbow angle.
FIG. 1. Brain (top) shows the locations of 6 example sites in primary motor cortex. The brain drawing is schematized; theenlarged drawing of the central sulcus is based on data from monkey 1. The site indicated by the circle and the arrow is illustratedin the histograms. The monkey was tranquilized with ketamine during testing. A: electromyographic (EMG) activity of the biceps(thick line in each histogram) and triceps (thin line in each histogram) evoked by 200-Hz stimulation at 30 _A when the elbowwas fixed at 4 different angles. Thick black line under each histogram indicates time of stimulation train. Each histogram is a meanof 15 trials. Evoked activity in both muscles was significantly affected by elbow angle (triceps, F _ 74.6, P _ 0.001; biceps, F _24.5, P _ 0.001). B: EMG activity evoked by 15-Hz stimulation. Vertical line on each histogram indicates time of biphasic pulsedelivered to brain. Time from 0.2 ms before to 1.5 ms after the pulse is removed from the EMG data to avoid electrical artifact.Each histogram is a mean of 2,000–4,500 pulses. Evoked activity in both muscles was significantly affected by elbow angle(triceps, F _ 101.1, P _ 0.001; biceps, F _ 125.3, P _ 0.001).
FIG. 3. EMG activity evoked by stimulation of another example site in primary motor cortex. Location of site shown in braindrawing (top). See legend to Fig. 1 for details. A: for 200-Hz stimulation, activity in both muscles was significantly affected by jointangle (triceps, F _ 2.96, P _ 0.039; biceps, F _ 27.71, P _ 0.001). B: for 15-Hz stimulation, activity in both muscles wassignificantly affected by joint angle (triceps, F _ 113.25, P _ 0.001; biceps, F _ 178.15, t _0.001).
FIG. 6. EMG activity evoked by stimulation of another example site in primary motor cortex. Location of site shown in braindrawing (top). See legend to Fig. 1 for details. A: for 200-Hz stimulation, activity in biceps but not in triceps was significantlyaffected by joint angle (triceps, F _ 1.03, P _ 0.388; biceps, F _ 4.91, P _ 0.005). B: for 15-Hz stimulation, activity in bothmuscles was significantly affected by joint angle (triceps, F _ 65.51, P _ 0.001; biceps, F _ 25.86, P _ 0.001).