Transcript lecture2
The nervous system, vision, and motor control Even the simplest action must involve linkage between memory, vision, eye movements, and body movements. from Land et al, 1999 Studying the integrated system Perception – Action Cycle recognize, locate cat cat image afferent signals compute movement neural activity 200 msec reach, pick up 500 msec efferent signals cat moves Levels of analysis Behavior: What are we trying to explain? Performance limits, loss of information Analytical vs. Ecological (Gibson) Neurophysiology & Anatomy: Structure and function of nervous system. What are the neural processes underlying behavior? Theory: Computer vision, Robotics, Artificial Intelligence. What are the principles governing brains/sensory systems? Need for all levels of analysis. What image information is needed for the things we do? How is vision used to acquire information from the world? QuickTime™ and a MPEG-4 Video decompressor are needed to see this picture. QuickTime™ and a MPEG-4 Video decompressor are needed to see this picture. Why do we move our eyes? - Image stabilization - Information acquisition The Eye and Retina Cone Photoreceptors are densely packed in the central fovea Visual Acuity matches photoreceptor density Scaled for equal visibility Why do we move our eyes? 1. To bring objects of interest onto high acuity region in fovea. Visual Projections Cortical Magnification Primary Visual Cortex: V1 The brain uses more physical space for signals from the fovea than periphery Why do we move our eyes? 1. To bring objects of interest onto high acuity region in fovea. 2. Cortical magnification suggests enhanced processing of image in the central visual field. Muscles that Move the Eye Why eye movements are hard to measure. A small eye rotation translates into a big change in visual angle Visual Angle x 18mm a d tan(a/2) = x/d a = 2 tan 1 x/d 1 diopter = 1/focal length in meters 0.3mm = 1 deg visual angle 55 diopters = 1/.018 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. Demonstration of “miniature” eye movements It is almost impossible to hold the eyes still. Stereotyped fixation patterns arise in different natural tasks Examples: Making tea or sandwiches Playing cricket, table tennis Driving Note: Real world activities almost always have some combination of saccades, smooth pursuit, VOR, & vergence Driving: fixate tangent point while driving around a curve Fixation density Eye movements in cricket bowler Bounce point batsman Land & MacLeod, 2000 Vision is Predictive Eye movements in cricket: Batsman anticipate bounce point saccade pursuit Better batsman arrive earlier bounce Land & MacLeod, 2000 Making tea: object oriented actions Land et al 1999 Specialized fixation patterns QuickTime™ and a MPEG-4 Video decompressor are needed to see this picture. QuickTime™ and a MPEG-4 Video decompressor are needed to see this picture. Main Insights from Natural Tasks Vision is active not passive. Specific information is usually acquired at the fixation point. Information is acquired “just-in-time”. TO DO: Read Land chapter on class website. Cortical specialization Imaging Jargon High Spatial Frequency/Resolution -Sharp/ In Focus- Low Spatial Frequency/Resolution -Blurry/ Out of Focus-