Arm movement maps evoked by cortical magnetic stimulation in a robotic environment

Abstract

Many neurological diseases result in a severe inability to reach for which there is no proven therapy. Promising new interventions to address reaching rehabilitation using robotic training devices are currently under investigation in clinical trials but the neural mechanisms that underlie these interventions are not understood. Transcranial magnetic stimulation (TMS) may be used to probe such mechanisms quickly and non-invasively, by mapping muscle and movement representations in the primary motor cortex (M1). Here we investigate movement maps in healthy young subjects at rest using TMS in the robotic environment, with the goal of determining the range of TMS accessible movements, as a starting point for the study of cortical plasticity in combination with robotic therapy. We systematically stimulated the left motor cortex of 14 normal volunteers while the right hand and forearm rested in the cradle of a two degree-of-freedom planar rehabilitation robot (IMT). Maps were created by applying 10 stimuli at each of 9 locations (3 × 3 cm grid) centered on the M1 movement hotspot for each subject, defined as the stimulation location that elicited robot cradle movements of the greatest distance. TMS-evoked movement kinematics were measured by the robotic encoders and ranged in magnitude from 0-3 cm. Movement maps varied by subject and by location within a subject. However, movements were very consistent within a single stimulation location for a given subject. Movement vectors remained relatively constant (limited to <90 degree section of the planar field) within some subjects across the entire map, while others covered a wider range of directions. This may be due to individual differences in cortical physiology or anatomy, resulting in a practical limit to the areas that are TMS-accessible. This study provides a baseline inventory of possible TMSevoked arm movements in the robotic reaching trainer, and thus may provide a real-time, noninvasive platform for neurophysiology based evaluation and therapy in motor rehabilitation settings.Corresponding Author Contact Details: George F. Wittenberg, MD, PhD, Baltimore VAMC, GRECC (BT/18/GR), 10 N Greene Street, Baltimore, MD 21201-1524, Phone: (410) Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. (Georgopoulos et al. 1992), but the study of multijoint movement end points has not, to our knowledge, been undertaken. Such simple end point measurements are currently used as markers for functional improvement by therapists (e.g. reaching towards a cup on a table top, collecting change from a counter top) and in upper extremity en...

Rapid plasticity of motor corticospinal system with robotic reach training

“…TMS evoked ballistic movements of arm, braked by the spring-field with passive return to home position, as previously reported (Jones-Lush, et al, 2010). An individual’s movements tended to cluster within a quadrant of the plane, but mean movement vector varied by individual subject.…”

“…The forearm was secured to the robot’s molded arm cradle with two straps (Jones-Lush et al, 2010) that maintained the participant’s elbow just below the horizontal plane as compared to the hand and shoulder (Figure 1A). Subjects were instructed to remain relaxed with their hand resting around the handle at the end of the cradle.…”

“…Arm movements were elicited by methodically stimulating the area over the dominant primary motor cortex (M1), guided by a 5 × 5 cm grid centered over the anatomical landmark of the hand knob and aligned with the anterior-posterior and medial-lateral axes of the head (Jones-Lush et al, 2010). Movement hotspots were located for each subject, defined as the location that, when stimulated, produced the largest movement recorded by the planar robot.…”

“…Previous research in our laboratory employed transcranial magnetic stimulation (TMS) applied over M1 to map TMS-evoked movement representation, using a robotic device to measure movement kinematics. These M1 movement maps varied between subjects and stimulus locations, but within a given location, the direction and extent of the evoked movements were remarkably consistent over time (Jones-Lush et al, 2010). The stability of TMS-evoked movements in the controlled environment of the rehabilitation robot therefore allows practice-related changes in motor representation to be characterized.…”

Timing of motor cortical stimulation during planar robotic training differentially impacts neuroplasticity in older adults

“…First, TMS-evoked movements in the upper extremity as an outcome measure for neuroplasticity has been established (Jones-Lush et al, 2010, Lewis et al, 2012). Intense robotic reaching training facilitated plasticity in TMS-evoked upper extremity movements when reaches were practiced in a direction opposite of the initial evoked movement (Kantak et al, 2013).…”