CORTICO-MUSCLE ENCODING: EVOKING PRINCIPLES OF CORTICO-MUSCLE CONNECTIVITY WITH RL-ICMS

Abstract

The specific aims of this study focus on three interrelated issues utilizing long-duration intracortical microstimulation (RL-ICMS) to elicit the organization and function of the rhesus macaque primary motor cortex (M1). The first focus concerns the timing between cortical activity and corresponding muscle activity, of which previous studies have revealed a broad range depending upon the research methods. Our findings suggest that the latency of cortico-muscle transmission observed (11.5 ± 5.6 ms) approaches the stimulus-triggered average (StTA)-derived minimum conduction time (9.6 ± 2.1 ms). We conclude that, during active movement, the best estimate of the delay between cortical cell firing and muscle activity is the physical conduction time from cortex to muscle. Secondly we investigated the relationship between RL-ICMS parameters applied to M1 and resulting muscle activation and evoked movement. We applied stimulation as combinations of frequency, amplitude and duration while concurrently recording EMG activity of 24 forelimb muscles and stimulus-evoked kinematics using a Vicon motion capture system. Our results suggest optimal parameters for evoking forelimb movements and stable spatial end-points are 70 - 130 Hz, 70 - 130 µA and 750 - 1000 ms, with median successful parameters of 110 Hz and 110 µA and a mean spatial end-point stabilization time of 479.0 ± 178.9 ms. Establishing stimulation parameters that yield consistent stimulus-evoked end-points forms the basis for comprehensive mapping of movement space represented in M1. The third study objective was to acquire a comprehensive M1 representation map of RL-ICMS-evoked forelimb movement end-points. This study expands upon previous RL-ICMS studies by producing a systematic and comprehensive stimulus-evoked spatial end-point map of M1. The results reveal that stable end-points generated by stimulating adjacent M1 cortical sites tended to be spatially contiguous in the monkey's work space, yielding a cortical map of end-point positions in which the monkey's three-dimensional work space was represented on the two-dimensional M1 cortical surface. While there were distinct cortical topographical differences between monkeys, there were also clear similarities such as hand-to-mouth postures elicited from sites neighboring the M1 face representation. These results reveal an extensive movement reportoire that can be elicited by applying RL-ICMS to M1.