Psy 102: Sensation

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Psy 102 Sensation Perception Lecture 10 Depth
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5. Perceiving the world through color alone
Division of labor video
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Adjust luminance of red and green until
matched Minimize the responses of the
non-opponent pathway Lose spatial acuity, motion
depth more than pattern
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2D shape is adequately encoded in color But
shadows have to be darker So 3D shape defined by
shadows not seen if R G
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Comments
Craik Cornsweet, why does it look different on
the page -- printer or our eyes. How is
brightness filled in? Color opponent combinations
at the retina Can someone lose cones through
environmental causes? If red-green (-R) then the
red in the picture disappears, (-G) green
disappears? Super Women People with cortical
color blindness see contrast?
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Depth
1. Accommodation 2. Convergence 3. Corresponding
points and disparity 4. Stereograms 5. Physiology
6. Autostereograms
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1. Accommodation
Not much use beyond 1m
Bring a target into focus Lens is stretched or
relaxed The visual system senses the degree of
strain on the lens Translates this into a
distance for the object
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2. Convergence
Not much use beyond 1m
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But we can see depth between two objects without
moving our eyes back and forth between the
m Hold out two fingers, notice depth between
them without going back and forth Now notice the
difference in the spacings between the two
fingers as seen in each eye This difference is
binocular disparity
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3. Geometry of binocular disparity
p
p
corresponding retinal points - same position on
each retina with respect to the point of fixation
(fovea)
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horopter - locus of all points in 3-D space that
fall on corresponding retinal points
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Binocular disparity
Hold up two fingers Fixate on the left one Close
one eye then the next Compare the spacing in the
two eye views
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Fixating here
perceived depth increases with increasing
disparity
fovea
fovea
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Stereo Methods
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Edgar BergenphotographsJayne Mansfieldin 3-D
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4. Stereopsis depth from binocular disparity
How does it work? Match fragments from each eye
and then measuring binocular disparity? Does
stereopsis comes after some kind of recognition
process?
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Random dot stereograms
Bela Julesz, 1956 emigré engineer from
Budapest First innovative use of a computer for
research in perception
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Bela JuleszInventor of the random-dot stereogram
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If fixating on background dots, they fall on
corresponding points in both eyes. But the dots
of displaced square do not. They have a
disparity in the two eye views therefore the
square is seen in depth
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How does stereopsis work ?
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5. Physiological mechanism of stereopsis
Recall some cells have identical RFs in each eye
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Binocular cells have receptive fields in each
eye These come with a range of different
separations between the receptive field centers
(relative to the fovea of each eye) These cells
are selective for disparity The separation
defines the preferred disparity for that cell
separation
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Binocular cells have receptive fields in each
eye These come with a range of different
separations between the receptive field centers
(relative to the fovea of each eye) These cells
are selective for disparity The separation
defines the preferred disparity for that cell

Left and right eyes aligned
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Summary

• Accommodation and convergence
• Corresponding points and binocular disparity
• Discover shape first then disparity
• No, disparity then shape random dot stereograms
• binocular cells with varying RF separations
  provide a neural account of stereopsis

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6. Magic Eye, autostereograms
With Magic Eye images, the two pictures are
superimposed, not adjacent as in wallpaper
illusion, period of repetition specifies
depth vary the period across the image to produce
changes in depth
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Left Eye
Right Eye
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Left Eye
Right Eye
Picture Plane
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Left Eye
Right Eye
Picture Plane
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Different spacing
Left Eye
Right Eye
Picture Plane
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Different spacing
Left Eye
Right Eye
Still fixated to align closer spacing, broader
spacing now has disparity, landing on
non-corresponding points so is seen at different
depth
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To view the 3D image you must converge your eyes
at a different plane of depth from the
picture This superimposes the picture on itself
with an offset now the disparity sensitive cells
can find matches in the nearby repeated patterns
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Uncrossed fusion
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Thank you.