Problems in motor control Rosenbaum Ch 2 Principals of motor organization Squire et al Ch 28 Kandel et al Ch 33 The degrees of freedom dof problem multiple ways to perform an action The body has a large number of dof or ways to move Arm has 7 dof 3 for shoulder 2 each for elbow and wrist Point in space is specified by 6 dof However humans typically don t take advantage of all dof eg move to touch the nose is typically straight ish and stereotyped The degrees of freedom problem some solutions Not a problem for the body because actions specified by goal state rather than the trajectory so having multiple ways of performing an action is an advantage Greater degrees of freedom allows flexibility Synergies a computational solution Examples L and R arm linkage head and hand linkage shoulder and wrist linkage Linkage reduces dof 4 dof 3 dof Eye head synergy eye head head eye Eye hand synergy hand The degrees of freedom problem Mechanical constraints http www andrew cmu edu user shc17 Robot Robot walkingVideos htm http userweb cs utexas edu jcooper walking Other optimization constraints Minimize energy optimize posture at end of movement minimum jerk derivative of acceleration Grasp may optimize position at end of movement or use spring like properties of muscles to assist in completion of the action Sequencing and Timing Speech errors spoonerisms this lecture is dearly none show that word segments are programmed partly in parallel Co articulation speech tulip typing finger movements initiated prior to completion of others fingers etc Coordination of eye head hand body Evidence suggests that motor system has knowledge of how long a movement will take The Perceptuo Motor Integration Problem How are actions guided by sensory information Pre programmed ballistic phase eg reach in the dark to a remembered target Slower feedback phase guidance at the end of the reach as the hand nears a visible target Schematic Representation of Feedback and Feed forward Systems Eg pursuit reaching grasping Eye velocity image velocity sensory Motor command retinal velocity delay Load fatigue current position Eg saccade throwing wind ballistic Learnt motor command guided Consequences of loss of feedback on reaching Large fibre sensory neuropathy leads to loss of proprioceptive feedback from muscles Errors in direction distance Normal proprioception only No vision or proprioception Vision compensates for lack of proprioception The Learning Problem Calibration what is the necessary command to move the eye to a certain location Need to experience the sensory visual and proprioceptive consequences of one s own movements in order to calibrate Even eye movements are sensitive to the need to maintain calibration Sensory feedback is needed to maintain calibration in addition to assisting in completion of the action as in reaching What will the visual image be as a consequence of that eye movement Developmental learning walking talking Ballistic component of an action must be learnt over trials repetitions Amount of practice Specificity of practice Shooting a basketball from the free throw line versus other distances Neural Plasticity If finger is amputated neighboring cortical regions take over that part of the projection from the fingers Merznick Motor rehab after stroke implies significant potential for cortical re organization Note phantom limbs pain Somatotopic organization of primary motor cortex Principles of organization of motor system Central Pattern Generations neural networks for control of rhythmic movements Central Pattern Generators Pathways for signals to muscles Hierarchical Organization of the motor system Planning sequences Somatosensory ctx Supplementary motor ctx Selection of trajectory Activity prior to movement Target selection Posterior parietal ctx Pre motor cortex M1 Primary motor ctx eyes Muscle commands Cortico spinal tract Initiation of movement Efferent copy Visual consequences of movement Smoothness timing Learning new skills Monitor feedback 80 msec Spinal feedback 30 50 msec Proprioceptive signals muscles joints Motor Programs Motor Equivalence evidence for motor programs Right hand Wrist immobilized Left hand Teeth Toe Movements are programmed with respect to the trajectory in space not the joint angles Kinematic data for hand paths c d and e shown in part B All paths are roughly straight and all hand speed profiles have the same shape and scale in proportion to the distance covered In contrast the profiles for the elbow and shoulder angles for the three hand paths differ The straight hand paths and common profiles for speed suggest that planning is done with reference to the hand because these parameters can be linearly scaled Planning with reference to joints would require computing nonlinear combinations of joint angles The average acceleration and velocity profiles are scaled linearly as a function of the extent of movement to the target The single peaks indicate that the extent of movement is specified prior to actual movement as a scaled impulse of force accelerating the limb Variation in reaction time with age and number of alternatives Variation in reaction time with number of alternatives and reduction with practice Supplementary Motor Area
View Full Document
Unlocking...