Download 2320Lecture14

Careers for Psychology and
Neuroscience Majors
• Oct. 19th
5-7pm in SU 300 Ballroom B
WebCT and Your Exam
• Your grade as it currently appears on
WebCT might be off by up to 2 points.
• WebCT programmers are working to fix the
bug.
• Don’t panic.
“Top-Down” and “Bottom-Up”
Processes in Perception
A “Bottom-Up” System
Light & Dark
Line
Orientation
Surfaces and
Background
Identity/Meaning
Why should this model be called into question?
“Top-Down” and “Bottom-Up”
Processes in Perception
A Top-Down System
“Top-Down” or
“Look-up” System
-- Sensory System -Light & Dark
Line
Orientation
Surfaces and
Background
COMPARE
Identity/Meaning
Possible
Interpretations
Shape Identification Requires
Interaction between Top-Down and
Bottom-Up Processes
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Single Target - Identify shape
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Single Target - Identify shape
Single Shape/Brief Dots
• Shape is identified by top-down system
before information in the bottom-up system
gets changed
Single Shape/Brief Dots
• Shape is identified by top-down system
before information in the bottom-up system
gets changed
• Let’s push the system: overload top-down
system AND change the signal in the
bottom-up system
Single Shape/Brief Dots
• Shape is identified by top-down system
before information in the bottom-up system
gets changed
• Let’s push the system: overload top-down
system AND change the signal in the
bottom-up system
• What would you predict of a strictly bottomup system?
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Many targets - Identify shape inside dots
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Many targets - Identify shape inside dots
Object Substitution
• Strictly bottom-up system should still
identify the shape! but top-down model
accounts for the phenomenon:
• Bottom-up info gets changed before topdown process completes
• all that’s left in the bottom-up signal is the
four dots
More Depth Cues
•
•
•
•
Pictorial Depth Cues
Physiological Depth Cues
Motion Parallax
Stereoscopic Depth Cues
Physiological Depth Cues
–
Two Physiological Depth Cues
1. accommodation
2. convergence
Physiological Depth Cues
–
Accommodation
Physiological Depth Cues
–
Accommodation
– relaxed lens = far away
– accommodating lens = near
–
What must the visual system be able
to compute unconsciously?
Physiological Depth Cues
–
Convergence
Physiological Depth Cues
–
Convergence
– small angle of convergence = far away
– large angle of convergence = near
–
–
What two sensory systems is the brain
integrating?
What happens to images closer or
farther away from fixation point?
Physiological Depth Cues
–
Convergence and accommodation are
reflexively linked
Under what circumstances might this be a problem?
Motion Depth Cues
–
Motion
1. Parallax
Motion Depth Cues
–
Parallax
Motion Depth Cues
–
Parallax
– points at different locations in the visual
field move at different speeds depending
on their distance from fixation
Motion Depth Cues
–
Parallax
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Seeing
Seeing in
in
Stereo
Stereo
Seeing in Stereo
It’s
It’svery
veryhard
hardtotoread
readwords
wordsififthere
there
are
aremultiple
multipleimages
imageson
onyour
yourretina
retina
Seeing in Stereo
It’s
It’svery
veryhard
hardtotoread
readwords
wordsififthere
there
are
aremultiple
multipleimages
imageson
onyour
yourretina
retina
But how many images are there on your
retinae?
Binocular Disparity
• Your eyes have a different image on each
retina
– hold pen at arms length and fixate the spot
– how many pens do you see?
– which pen matches which eye?
Binocular Disparity
• Your eyes have a different image on each
retina
– now fixate the pen
– how many spots do you see?
– which spot matches which eye?
Binocular Disparity
• Binocular disparity is the difference
between the two images
Binocular Disparity
• Binocular disparity is the difference
between the two images
• Disparity depends on where the object is
relative to the fixation point:
– objects closer than fixation project images
that “cross”
– objects farther than fixation project images
that do not “cross”
Binocular Disparity
• Corresponding retinal points
Binocular Disparity
• Corresponding retinal points
Binocular Disparity
• Corresponding retinal points
Binocular Disparity
• Corresponding retinal points
Binocular Disparity
• Points in space that have corresponding
retinal points define a plane called the
horopter
The Horopter
Binocular Disparity
• Points not on the horopter will be
disparate on the retina (they project
images onto non-corresponding points)
Binocular Disparity
• Points not on the horopter will be
disparate on the retina (they project
images onto non-corresponding points)
• The nature of the disparity depends on
where they are relative to the horopter
Binocular Disparity
• points nearer than
horopter have crossed
disparity
• points farther than
horopter have
uncrossed disparity
Binocular Disparity
• Why don’t we see double vision?
Binocular Disparity
• Why don’t we see double vision?
• Images with a small enough disparity are
fused into a single image
Binocular Disparity
• Why don’t we see double vision?
• Images with a small enough disparity are
fused into a single image
• The region of space that contains images
with close enough disparity to be fused is
called Panum’s Area
Binocular Disparity
• Panum’s Area
extends just in front
of and just behind the
horopter
Stereopsis
• Our brains interpret crossed and
uncrossed disparity as depth
• That process is called stereoscopic depth
perception or simply stereopsis
Stereopsis
• Stereopsis requires that the brain can
encode the two retinal images
independently
Stereopsis
• Primary visual cortex (V1) has bands of
neurons that keep input from the two eyes
seperate
Stereopsis
• If the brain only
gets normal
signals from one
eye early in life,
that eye’s neurons
crowd out the
other eye’s
neurons
Amblyopia
• Amblyopia is a visual deficit in which
one eye has poor vision because the brain
never developed the ability to use signals
from that retina
Amblyopia
• Amblyopia is a visual deficit in which
one eye has poor vision because the brain
never developed the ability to use signals
from that retina
• Usually caused by
– strabismus - when eyes don’t lock onto the
same point
– anisometropia - when one eye has very bad
optics and the other is normal
Next Time:
• More about stereo vision
• MagicEye stereograms
• Pinker chapter