In addition to adaptation and co-contraction, online feedback responses are essential mechanisms of motor control. Online responses occur at multiple time scales, ranging from short latency spinal responses (less than 50 ms in the upper extremity) to voluntary responses (more than 100 ms). In between these responses, the long-latency responses are interesting because they blend fast reaction times with flexibility of their output, but their neural correlates are not yet completely understood. We call this line of research StretchfMRI, because we aim to combine MRI-compatible robotics, fMRI, and EMG to study the neural correlates of long-latency responses. Utilizing surface EMG activity, we measure the LLR amplitude evoked from flexor and extensor muscles following the application of controlled angular displacements of the wrist using the MR-StretchWrist.
Our earliest work on this topic was on developing StretchfMRI, a measurement method that enables simultaneous measurement of fMRI and EMG signals during stretch-evoked muscle responses (see paper by Zonnino below). Our methods allowed us to establish for the first-time in human the organization of LLRs in the brainstem using fMRI.
In parallel to our methods development work, we have conducted a study to quantify how different behavioral factors including the neuromechanical state of the muscle prior to the application of a perturbation (i.e., muscle length and activation), the direction of perturbation (i.e., whether the perturbation stretches or shortens the muscle), the kinematic features of the applied perturbation (i.e., perturbation velocity, duration, amplitude, velocity profile), and the instructions provided to participants as to how to respond to the applied perturbations may affect the long-latency responses (LLR) amplitude evoked from the forearm muscles.
Our current work on this topic includes the development of a higher-power robot (the MR-StretchWrist v2.0) to study the responses under a “yield” and “resist” condition. Also, we are currently working on integrating TMS to our paradigm, which would allow to decouple the effects of corticospinal and reticulospinal circuits on the muscle and neural responses observable via StretchfMRI.