Motor Cortical Correlates of Arm Resting in the Context of a Reaching Task and Implications for Prosthetic Control

Abstract

Prosthetic devices are being developed to restore movement for motor-impaired individuals. A robotic arm can be controlled based on models that relate motor-cortical ensemble activity to kinematic parameters. The models are typically built and validated on data from structured trial periods during which a subject actively performs specific movements, but real-world prosthetic devices will need to operate correctly during rest periods as well. To develop a model of motor cortical modulation during rest, we trained monkeys (Macaca mulatta) to perform a reaching task with their own arm while recording motor-cortical single-unit activity. When a monkey spontaneously put its arm down to rest between trials, our traditional movement decoder produced a nonzero velocity prediction, which would cause undesired motion when applied to a prosthetic arm. During these rest periods, a marked shift was found in individual units' tuning functions. The activity pattern of the whole population during rest (Idle state) was highly distinct from that during reaching movements (Active state), allowing us to predict arm resting from instantaneous firing rates with 98% accuracy using a simple classifier. By cascading this state classifier and the movement decoder, we were able to predict zero velocity correctly, which would avoid undesired motion in a prosthetic application. Interestingly, firing rates during hold periods followed the Active pattern even though hold kinematics were similar to those during rest with near-zero velocity. These findings expand our concept of motor-cortical function by showing that population activity reflects behavioral context in addition to the direct parameters of the movement itself.

Brain-state classification and a dual-state decoder dramatically improve the control of cursor movement through a brain-machine interface

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Modulation of Spinal Motor Output by Initial Arm Postures in Anesthetized Monkeys

“…Therefore, reliable and reproducible neural prostheses using ISMS may not be achieved without knowing the relationship between the excitability of neurons around the stimulating electrode and the limb posture when ISMS is applied. Setting an initial value corresponding to the resting posture is also a challenge in a cortically controlled brain-machine interface (Velliste et al, 2014). Determining the neural mechanism that induces posture dependency during ISMS, therefore, should also advance the clinical application of ISMS for recovery from motor impairments.…”

Movement: How the Brain Communicates with the World

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