Transcript lecture9
Visuo-Motor Relationships: Plasticity and Development Demonstration: plasticity in relationship between vision and movements Ability to adapt to new relationships requires cerebellum Why do we need to retain plasticity for new visuo-motor relationships? 1. Need to adjust to changes in body size during development. 2. Need to adjust to damage/aging. 3. Need to adjust to environmental changes eg ice, loads etc. 4. Need to learn arbitrary mappings for tool use etc. 5. Need to acquire new motor skills. 6. Visuo-motor coordination is a computationally difficult problem for the brain. Need flexibility to correct errors. 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 Paths are straight even when the arm is loaded. Even when the arm is perturbed by forces acting on it, people learn to make straight hand paths. Grasping Reaching and grasping actions require close coordination, but depend on different kinds of visual information... Reaching: egocentric (where is object relative to me) Transform visual information about object location into movement direction. Grasping: object-centred Transform visual information about object properties (shape and size) into commands for grasping movements. Development of reaching Within first 2 weeks, 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. Increased use of visual feedback between 5 and 11 months QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. 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 inter-segmental dynamics. So hand oscillates. Development of reaching Between 5 and 9 months see many changes to kinematics: 1) Straightening of the hand path 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. 5 months: hand does not orient to object until contact 9 months: hand orients prior to contact (note visual information about orientation is available at 2 months). Pre-shape for object size. Still adjusting grip force by 7-8 years (grip force larger for larger objects). Use palmar grasp until about 12 months – then use fingers to grasp. Corresponds to rapid increase in the rate of myelination of corticospinal tracts at 12 months – responsible for distal musculature. 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 Role of Experience in Development of Visuo-motor coordination Held & Hein 1 2 Both kittens get visual experience and motor experience 1. Visual experience correlated with motor commands/proprioceptive feedback/vision of limbs 2. Gets both, but uncorrelated. Kitten 2 -abnormal visuomotor 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 on the left. If this new location of his body was vivid, the world appeared right side up, but sometimes he felt his body was upside down in a visually right-side-up world. After removing the prisms, he initially made incorrect reaching movements. However, he soon regained normal control of his body. Adaptation to different relation between visual and movement. George Stratton – Wore inverting lens for 8 days – Believed that we learn visual directions by associating visual experiences with other forms of sensory feedback (e.g. proprioceptive). – Alternatively… Adaptation results from learning correlation betweeen vision and actively generated motor commands (Held, 1965). Role of Experience in Development of Vision Mike May, and Molyneux’s Question Neural control of Grasping Neural control of Grasping Both vPM and AIP neurons fire for specific hand actions/objects. For example, this neuron prefers a precision grip. Precision grip Power grip Neural control of Grasping vPM neurons fire for grasping movements made in the light and dark. In contrast, AIP neurons fire far less when moving in the dark, and in general, AIP has more visual neurons than vPM. Neural control of Grasping Neural control of Grasping Neurons in the vPM also fires when perceiving, as well, as producing grasping movements!