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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
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MPEG-4 Video decompressor
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QuickTime™ and a
MPEG-4 Video decompressor
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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-