| Abstract No.: | 311 |
| Country: | Canada |
| | |
| Title: | Old Ideas and New Ideas About the Role of Motor Cortex in the Control of Voluntary Movement |
| | |
| Authors/Affiliations: | John Kalaska
University of Montreal, Department of Physiology, QC, Canada |
| | |
| Content: | The primary motor cortex (M1) is arguably the first cerebral cortical area for which a distinct function was identified by experimental neurophysiological methods (e.g., surface electrical stimulation, surgical lesions) more than 100 years ago. Yet ever since that time, the role and the neural mechanisms by which M1 contributes to the control of voluntary movements has remained the subject of continuing lively debate.
It has long been known that M1 or capsular lesions lead to severe motor deficits, including paralysis, and that epileptic foci in M1 result in involuntary motor seizures. The earliest studies by Fritsch, Hitzig, Ferrier and Sherrington showed that electrical stimulation of M1 evokes muscle contractions. Subsequent studies showed that M1 contains a systematic, albeit complex, topographic map of descending corticospinal axons projecting into spinal interneuronal networks and directly onto spinal motoneuron pools. Single-neuron neurophysiological studies have repeatedly shown that the activity of many M1 neurons was correlated to movements of a specific part of the body, and in particular with the causal forces and single- or multi-muscle activity patterns that produce the motor output. These and other findings have all supported the widely held view that M1 generates a descending motor command that dictates to the spinal motor apparatus the complete pattern of muscle activity required to produce a voluntary movement.
Nevertheless, many other studies have reported findings that pointed to other roles and mechanisms. Single M1 neurons can show labile or context-dependent relationships to motor output, and even M1 neurons that synapse directly onto spinal motoneurons can show discharge patterns that do not bear any simple relationship to the contractile activity of their target muscles. Many studies have reported M1 activity that correlates with the spatial motions of the arm and hand rather to forces and muscle activity. Indeed, the recent advances in implantable brain-machine interfaces have exploited those correlations to extract control signals for neuroprosthetic robotic devices. Recent learning studies have shown that some, but not all, M1 neurons change their activity during the acquisition of a new skill. Furthermore, the response changes tend to lag the improvement in motor performance, and can remain after the subject returns to baseline conditions. Other learning studies suggest that M1 may be more important in the consolidation and recall of learned skills than in their original acquisition. None of these findings can be easily reconciled with a simple role for M1 as the feedforward (predictive) controller that sends a detailed motor command to the spinal motor apparatus that dictates exactly when and how hard to contract every muscle. The final muscle activity patterns are determined ultimately by the interactions between M1, the spinal cord and other motor structures.
This presentation will review some of these discrepant findings and their implications for the role of M1 in voluntary motor control, as well as some of the reasons why it has been so difficult to answer the seemingly simple question of what is the role of M1 during voluntary movement.
(Supported by the CIHR, FRSQ and NIH |
| | |
| Back |
|
