Reaching and grasping Rosenbaum Ch 7 Factors in control of reach Learnt motor program for ballistic phase feedforward Feedback during reach from vision or proprioception Ballistic component might rely on a simple property of muscles that is muscles are like springs Planned component depends on current sensory evidence plus p experience Pre programmed component also depends of the goal of the movement Grasp height varies depending on height of platform where object is moved to Role of Visual Feedback Question why does error increase with speed Note 50 cm sec 5cm 100msec Mass Spring Model Muscles are like springs a spring has a resting length and stiffness length and stiffness of muscles can be neurally programmed Spring Equation Muscles can be modeled as springs F k x1 x0 Difference between current position and eq point k spring constant Evidence for Mass Spring Model Accurate reaching following deflection even when vision and proprioception were absent dorsal roots severed But is this consistent with the evidence from patients with large fibre sensory Neuropathy Velocity profiles for small and large movements note difference throughout movement Even at beginning Implications Fitt s Law Movement time increases with accuracy requirements Planning reaching movements Reaching movements are initially planned and represented in the brain in a simplified abstract form as vectors in which extent amplitude and direction is specified perhaps planned independently Locate target Locate hand Plan movement vector Planning reaching movements The variability in direction errors off axis are smaller than that of extent errors On axis error The patterns of errors reflect limitation of CNS provide clues to control strategy Mean endpoint Off axis error Are movements planned in joint space or hand space Straight hand paths People move their hand in a straight path even when they can t see their hand motion and so can only rely on proprioception Straight hand paths People move their arm so that the paths look straight even if it involves a curved path Grasping Reaching and grasping actions require close coordination but would seem to depend on different kinds of visual information Reaching egocentric where is the object relative to me Grasping object centred Reaching vs Grasping Peak deceleration time correlated with time to peak aperture Also when arm moves faster hand opens wider finger separation increases hand position hand velocity grip size aperture velocity Visuo Motor Relationships Plasticity and Development Problem of sensory motor coordination How do we relate the visual and motor worlds For reaching a visual signal about location must be transformed into a command to the arm and hand muscles This is not innate but must be learnt during development and maintained through adulthood Development of reaching Within first 2 weeks babies already directing arm towards objects Some crude control of reach direction Improves by the 5th month consistently touch targets Won t reach for targets beyond arm s length Catching and anticipating target motion at 6 months Distance accuracy develops more slowly improving by 7 months Development of reaching Within first 2 weeks already directing arm towards objects Improves by the 5th month consistently touch targets Catching and anticipating target motion at 6 months Distance accuracy develops more slowly improving by 7 months Visual information used early on to aid in sensory motor integration Increased use of visual feedback between 5 and 11 months Early reach movements Initially use the trunk shoulder proximal joints to reach for objects use elbow less frequently When babies do make large movements can t control intersegmental dynamics So hand oscillates Development of reaching Between 5 and 9 months see many changes to kinematics 1 Straightening of the hand path Development of reaching Between 5 and 9 months see many changes to kinematics 2 Reduced number of submovements 3 Reduced movement time Development of reaching Joint kinematics changes as well coordination among joints Development of grasping Newborns have grasp reflex clasp object brought against the palm disappears by 6 months Use palmar grasp until about 12 months then use fingers to grab Corresponds to rapid increase in the rate of myelination of corticospinal tracts at 12 months responsible for distal musculature Development of grasping At 5 months babies orient hand but only AFTER making contact with the object Predictive orienting starts at 9 months Development of grasping Tailoring of grasp to object size only after 9 months grip aperture wider for larger objects Still adjusting grip force by 7 8 years grip force larger for larger objects Reach and grasp development Integrate sensorymotor signals Pincer grasp Calibrating visual information to form grip Birth Increased myelination of corticospinal tracts months reach onset Direct hand to object fine tune reach Coordinated torque patterns joint patterns Continued refinement years More evidence that visuo motor coordination must be learnt during development Evidence Kittens given visual experience without opportunity for movement and motor experience without vision don t learn how to control their movements using vision Correlating the two is necessary Held Hein study Role of Experience in Development of Visuo motor coordination Held Hein 1 2 Both kittens get visual experience and motor experience K1 Visual experience correlated with motor commands proprioceptive feedback vision of limbs K2 Gets both but uncorrelated Kitten 2 abnormal visuo motor coordination Adaptation to different relation between vision and movement George Stratton Wore inverting lens for 8 days If he saw an object on the right he would reach with his right hand and discover he should have reached with his left He could not feed himself very well could not tie his shoelaces and found himself severely disoriented His image of his own body became severely distorted At times he felt his head had sunk down between his shoulders and when he moved his eyes and head the world slid dizzyingly around As time went by Stratton achieved more effective control of his body If he saw an object on the right he would reach with his left hand He could accomplish normal tasks like eating and dressing himself His body image became almost normal and when he moved his eyes and head the world did not move around so much He began to feel as though his left hand was on the right and his right hand
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