voluntary movement i. psychophysical principles & neural control of reaching and grasping claude...
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Voluntary Movement I.
Psychophysical principles& Neural control of
reaching and grasping
Claude Ghez, M.D.
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• Reflex and voluntary movements are sensorimotor transformations.– Feedforward vs. feedback control.
• Reflex control differs from voluntary control– Spatial organization of reflexes reflects hard-wired connections vs.
behavioral demands and context: decisions. Voluntary movements are organized to be appropriate to address behavioral goal: kinematic vs. dynamic transformations - internal models.
– 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.
– Neural organization of voluntary movements is highly dependent on learning and plasticity. Adaptability is critical over long term.
Introduction
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1. Voluntary movements require decisions and information processing: Latency and duration:
• Reaction time.• Parallel processing.• Speed-accuracy tradeoffs.
2. Sensorimotor transformations in reaching and grasping.
• Kinematics: visuomotor transformations. Movement vectors• Dynamics: internal models.• Role of vision and proprioception in feedback and feedforward
control
3. Organization of motor cortical areas for reaching and grasping
• Multiple motor areas• Somatotopic organization• Redundancy
Today
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Latency or “Reaction time” depends on decision
Reaction time paradigm• Warning -> cue= go signal•Simple RT: single or predictablecue Subject knows what response to make in advance•Choice: multiple unpredictable cues (e.g. colors, symbols, spatial locations) each requiring different responses.
Stage theory of reaction time
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Unpredictable stimuli
Predictable sequence
“Reaction time” depends on practice and learning
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Tones
Elbow forceMeasured byStrain gauge
>200
100-200
<100
Resting force
Target
Synch Cue
High
Middle
Low
High
Middle
Low
High
Middle
Low
High
Middle
Low
Right direction
Wrong direction
Response features can be processed in parallel
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Speed-Accuracy tradeoff (Fitts’ law)Error varies with speed
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Reaching reflects several sensorimotor transformationskinematic and dynamic planning
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Reaching: hand and joint kinematics are planned independently
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Extent and direction are planned in advance
Paths are straight Hand trajectories:Speed & acceleration Scale with distance
Reaching involves scaling a ‘trajectory primitive’ to target distance. Learned scaling factors& reference frame
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Mobility
Accuracy requires knowledge of mechanical propertiesof the limbs (“the plant”): role of proprioception
Normal control Patient without proprioception
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Patient MAControl MFG
Extent
Peak Acceleration
900
0Movement Direction
PK Acceleration
Movement time (ms)
0
300
Movement Direction
Peak Acceleration
Directional variations in inertial resistance arecorrected by compensatory variations in movement time
Proprioception is critical
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Proprioceptive information is used for feedforward control: Internal models
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Reaching reflects several sensorimotor transformationsKinematics and dynamics
Parallel planningof concurrent actions:hand preshaping
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Macaque Monkey Human
Prefrontal cortex
Premotor cortex
Supplementary motor area (SMA)
Central sulcusPosterior Parietal
CortexPremotor
cortex
Corpus Callosum
Supplementary motor area (SMA)
Primary motor cortex
Primary motor cortex
Primary sensory cortex
Motor areas of the brain
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Somatotopic organization revieled by electrical stimulation of the cortical surface
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Electrical stimulation is medically usefulEarly experiments
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Electrical stimulation is medically useful:More recent experiments
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Deltoid Ext. Carpi Rad.
Posterior
Anterior
Medial Lateral
Representation of individual muscles is patchy and distributed
Posterior
Anterior
Medial Lateral
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Cell activity associated with movement of individual digitsis broadly distributed: convergence of spinal projections