Graziano et al. (2002a) recently demonstrated that applying repetitive intracranial microstimulation (RL-ICMS) to cortical motor areas for long durations (500 ms), matching the duration of normal movements, produces natural appearing arm movements ending with the hand positioned in different parts of extrapersonal space, depending on the cortical subregion stimulated. Three subregions were identified as producing movements with different characteristics. RL-ICMS of primary motor cortex (M1) evoked movements ending with the hand positioned in central space immediately in front of the monkey's chest and the formation of various postures of the digits appropriate for object manipulation. It was also reported that the pattern of EMG activity from stimulation was arm posture dependent and could switch from excitation to inhibition depending on initial posture. These findings were viewed as consistent with the hand reaching the same final position independent of the initial starting position. More recently, Graziano et al. (2004), reported that stimulus evoked output effects were also highly joint angle dependent. These results are remarkable and suggest a novel view of frontal lobe motor function. RL-ICMS was viewed as engaging functional neural substrates for natural, purposeful movements. In this application we will use stimulus triggered averaging of EMG and focus on M1 cortex to investigate some of the findings from RL-ICMS and to test possible alternative explanations for the findings observed.
Four specific aims are proposed to rigorously test: 1) the extent to which EMG responses are limb posture and joint angle dependent, 2) the possibility that natural appearing responses observed with RL-ICMS can be explained by sustained, tonic coactivation of a particular set of muscles at each joint that simply achieve a final equilibrium position, and 3) the extent to which muscles activated with RL-ICMS can be explained based on detailed knowledge of M1 muscle maps and the pattern of spread of excitation through these muscle representations associated with ICMS. ? ?
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