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Visuo-Motor Relationships:Plasticity and Development Read: Rosenbaum Chapters 2, 6 + may.doc
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.
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 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 vision 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).
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.
Role of Experience in Development of Vision Molyneux’s Question: Can a person blind from birth, whose vision is restored, tell that a circle and a square are different shapes?
Mike May - world speed record for downhill skiing by a blind person. Lost vision at age 3 - scarred corneas. Optically 20/20 - functionally 20/500 (cf amblyopia) Answer to Molyneux’s question: Mike May couldn’t tell difference between sphere and cube. Improved, but does it logically rather than perceptually. (cf other cases) Color: an orange thing on a basket ball court must be a ball. Motion: can detect moving objects, distinguish different speeds. Note: fMRI shows no activity in Infero-temporal cortex (corresponding to pattern recognition) but there is activity in MT, MST (motion areas) and V4 (color). Other parts of brain take over when a cortical area is inactive. Cannot recognize faces. (eyes, movement of mouth distracting) Can’t perceive distance very well. Can’t recognize perspective. No size constancy or lightness constancy/ segmentation of scene into objects, shadows difficult. Vision most useful for catching balls (inconsistent with Held & Hein??) and finding things if he drops them.
MT/MST (motion) V4 (color) Infero-temporal cortex
Implications? Basic object perception (recognition and segmentation) requires experience. (Experience prior to 3 yrs not enough.) Geometric cues about scene structure (perspective, distance) also require experience. Color and motion more robust - either present at birth, or acquired before 3yrs, and preserved without continued experience.
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
Cortical cells are tuned to spatial frequency eg one cell might fire most to the left patch, another to the middle patch, and so on.