Transcript lecture10
Describe 2 functions of eye movements and give an example of each. Types of Eye Movement Information Gathering Voluntary (attention) Stabilizing Reflexive Saccades vestibular ocular reflex (vor) new location, high velocity, ballistic body movements Smooth pursuit optokinetic nystagmus (okn) object moves, velocity, slow whole field image motion Vergence change point of fixation in depth slow, disjunctive (eyes rotate in opposite directions) (all others are conjunctive) Fixation: period when eye is relatively stationary between saccades. Describe your results in the ball-catching lab. Describe the relation between the first and second labs. Too slow when an event is unexpected because of feedback delays. Experience allows prediction (feedforward response). Draw a sketch of the brain showing the structures involved in the generation of a saccadic eye movement. Specify the function of these structures (to the extent that this is possible) Function of Different Areas monitor/plan movements target selection saccade decision inhibits SC saccade command signals to muscles Sketch a muscle, including the muscle spindle and the sensory and motor nerve fibers. Secondary Encoding (Less Sensitive) MUSCLEFIBRE/SPINAL Spindles Golgi Dorsal Extrafusal Ventral Intrafusal How does the speed of a movement (e.g. reaching) affect its accuracy? Explain why. Role of Visual Feedback Question: why does error increase with speed? Note: 50 cm/sec = 5cm/100msec Draw a sketch of a feedback system and a feedforward system. Give an example of each (for visuo-motor control). 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 Describe the effect on reaching movements of large-fiber sensory neuropathy (degeneration of the afferent fibers from the muscles and skin) when visual feedback is eliminated? Sketch the movements to illustrate your answer. 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 Describe the problems that have to be solved in order to program a robot to flip an egg. Mention possible solutions. Flipping an Egg Autonomous control: robot is pre-programmed - no human input Problems to be solved: 1. Grasp spatula locate handle (vision) some mechanism to translate location into arm movement some mechanism for controlling fingers - “passive compliance” 2. Move to pan locate pan (vision) translate location into arm movement 3. Lower spatula to pan vision or proprioception: lower until force > 0 4. Flatten proprioception: rotate until forces on fingers are equal 5. Locate egg vision or proprioception: move forwards until horizontal force > 0 Flipping an Egg (ctd) Problems to be solved: 6. Lift need to keep spatula level: vision or proprioception (keep tension constant) 7. Flip need to learn how much to rotate hand: rotate until forces = 0 Limitations of autonomous control: inflexible - can’t adapt to changed circumstances requires high precision How can examples form robotics help us understand the human visuo-motor system? Give examples. Describe sequence of eye movements in an everyday task. Give reasons. 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. 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 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). 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.