Introduction Voluntary Movement I. Reflex and voluntary movements - PDF document

Introduction Voluntary Movement I. • Reflex and voluntary movements are sensorimotor transformations. – Feedforward vs. feedback control. Psychophysical principles • Reflex control differs from voluntary control – Spatial organization of reflexes reflects hard-wired connections vs. behavioral demands and context: decisions. Voluntary movements & Neural control of reaching are organized to be appropriate to address behavioral goal: kinematic vs. dynamic transformations - internal models. and grasping – Reflex latency & duration reflect mainly fixed neuromuscular constraints: conduction, transmission, contraction. These, also influence timing of voluntary movement, but information processing and accuracy constraints are the critical reaction time and movement duration. Claude Ghez, M.D. – Neural organization of voluntary movements is highly dependent on learning and plasticity. Adaptability is critical over long term. Latency or “Reaction time” depends on decision Today Reaction time paradigm • Warning -> cue= go signal 1. Voluntary movements require decisions and •Simple RT: single or predictable information processing: Latency and duration: cue Subject knows what • Reaction time. response to make in advance •Choice: multiple unpredictable • Parallel processing. cues (e.g. colors, symbols, spatial • Speed-accuracy tradeoffs. locations) each requiring different 2. Sensorimotor transformations in reaching and responses. grasping. • Kinematics: visuomotor transformations. Movement vectors • Dynamics: internal models. • Role of vision and proprioception in feedback and feedforward control Stage theory of reaction time 3. Organization of motor cortical areas for reaching and grasping • Multiple motor areas • Somatotopic organization • Redundancy Response features can be processed in parallel “Reaction time” depends on practice and learning Synch Cue Unpredictable stimuli Tones High Elbow force Target Middle Measured by Strain gauge Low Low Predictable sequence Middle High >20 100- <100 0 200 High Right direction Middle Low Resting force Low Middle Wrong direction High 1

Reaching reflects several sensorimotor transformations Speed-Accuracy tradeoff (Fitts’ law) kinematic and dynamic planning Error varies with speed Reaching: Extent and direction are planned in advance hand and joint kinematics are planned independently Paths are straight Hand trajectories: Speed & acceleration Scale with distance Reaching involves scaling a ‘trajectory primitive’ to target distance. Learned scaling factors & reference frame Accuracy requires knowledge of mechanical properties Directional variations in inertial resistance are of the limbs (“the plant”): role of proprioception corrected by compensatory variations in movement time Proprioception is critical Control MFG Patient MA Normal control Patient without proprioception Extent PK Acceleration 300 900 Mobility Movement time (ms) Peak Peak Acceleration Acceleration 0 0 Movement Direction Movement Direction 2

Proprioceptive information is used for feedforward Reaching reflects several sensorimotor transformations control: Internal models Kinematics and dynamics Parallel planning of concurrent actions: hand preshaping Motor areas of the brain Somatotopic organization revieled by electrical stimulation of the cortical surface Macaque Monkey Human Supplementary Primary motor Primary motor motor area (SMA) Supplementary Primary sensory cortex cortex motor area (SMA) cortex Central sulcus Corpus Callosum Premotor Posterior Parietal cortex Cortex Premotor cortex Prefrontal cortex Electrical stimulation is medically useful Early experiments 3

Electrical stimulation is medically useful: More recent experiments Representation of individual muscles is patchy and distributed Deltoid Ext. Carpi Rad. Posterior Posterior Anterior Anterior Medial Lateral Medial Lateral Cell activity associated with movement of individual digits is broadly distributed: convergence of spinal projections 4