Transcript lecture13
Describe how reaching and grasping abilities develop in the first year of life. 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 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. Describe Stratton’s experiment with inverting lenses. What are the implications? Adaptation to different relation between vision and movement. George Stratton – Wore inverting lens for 8 days – Was able to adapt to the new visuo-motor mapping – Believed that Alternatively… Implications: Either we learn visual directions by associating visual experiences with other forms of sensory feedback (e.g. proprioceptive). or Adaptation results from learning correlation betweeen vision and actively generated motor commands (Held, 1965). Why might we remain adaptable to new visuo-motor relationships? What is the evidence for this adaptability? Where in the brain might the adaptation be occurring? 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. Ability to adapt to new relationships requires cerebellum Prism adaptation What does change blindness imply about vision? What are some examples? Change Blindness: insensitivity to changes in visual scenes made during an eye movement/transient occlusion. Change blindness challenges idea that perception delivers a comprehensive representation of world. What is represented? Attended objects/regions of central interest? Implications Task specificity of computations explains “Change Blindness.” (effectively “blind” to irrelevant information) What are the advantages and disadvantages of using a virtual reality environment to study the brain and behavior? Can do experiments in naturalistic environments. Control of stimulus. Important in complex environments. Can do impossible manipulations. Unrealistic in many ways: small field of view, latency of display following movement, stereo cues weak, other cues missing. Force feedback: no tactile stimulation Discomfort. Environment might change the behavior - eg heavy helmet suppresses head movements. What is the conclusion from Held & Hein’s experiment with kittens (putting one kitten in a carousel driven by the other kitten.)? How would you do this experiment today, using virtual reality technology? 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. What is meant by “top-down” and “bottom-up” processing? Give examples of both. Briefly summarize the driving experiment by Jovancevic, Hayhoe & Sullivan. What did they find? Probability of Fixation During Collision Period Pedestrians’ paths Colliding pedestrian path 1 No effect in Leader condition 0.8 Probability of fixation More fixations on colliders in normal walking. Normal Walking 0.6 No Leader Leader 0.4 Follow Leader 0.2 0 Controls Colliders Why are colliders fixated? Small increase in probability of fixating the collider. Failure of collider to attract attention with an added task (following) suggests that detections result from top-down monitoring. Detecting a Collider Changes Fixation Strategy 1 Timeoffixating Sum Pedestri annormal Fi xationspedestrians Foll owi ng a Detection of a of Colli detection a der collider Normal Walking 0.8 Fixation durations (s) following 0.6 No Leader Follow Leader Leader 0.4 0.2 0 “Miss” Not Fixated “Hit” Fixated Longer fixation on pedestrians following a detection of a collider Subjects rely on active search to detect potentially hazardous events like collisions, rather than reacting to bottom-up, looming signals. To make a top-down system work, Subjects need to learn statistics of environmental events and distribute gaze/attention based on these expectations.