Transcript Session 9

Perception, Illusion and VR
HNRS 299, Spring 2008
Lecture 9
Visual Development
Thanks to website at University of Calgary:
http://www.psych.ucalgary.ca/PACE/VA-Lab/Marcela/Pages/index.html
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Experience is Necessary
Visual experience is necessary for normal visual development
in infants and young animals.
Cats:
•If kitten wears goggles with vertical stripes, the orientation
columns in the cortex don't form normally.
•Most neurons are tuned to vertical orientations.
•Very few, if any, are tuned to horizontal orientations.
•If one eye is covered in kittens, the ocular dominance columns
don't form normally.
•Most neurons only respond to input from the seeing eye.
•Very few respond to input from the covered eye.
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Critical Period
Visual experience must occur at the correct time during
infancy for normal visual development. The window of
time is known as the critical period.
•In the orientation experiments in cats, if the goggles
removed early enough, the orientation columns re-establish
themselves.
•If they are removed later, the abnormal columns never
recover.
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Human stereo development
•Some babies are born with "strabismus", a misalignment of the eyes.
•Vision in one eye becomes dominant, and vision in the other eye is
suppressed to prevent double vision.
•Stereo vision does not develop.
•Strabismus can be corrected by surgery.
•The surgery must be performed early (less than 2 years of age) for
stereo vision to develop.
•The critical period for stereo vision is about the first two years in
people.
4
Testing infant vision
There are several methods for testing infants for visual function.
Among them are:
Surprise: Infants can show surprise if something does not appear as they
expect.
Example: If a teddy bear moves behind and screen and a firetruck comes out
the other side, older babies (1 year or more) show surprise. (Younger babies
don't).
Preferential Looking: Babies will look at novel visual stimulus.
Example: If show gray on one side and grating on the other, the infant will
preferentially look at the grating, if he can see it.
This can be used to measure the contrast needed for babies to see the grating.
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Properties of Infant Vision
The retina is not completely developed at birth.
Cones are shorter and have less pigment than adult cones.
Cones are evenly distributed across retina.
As infant grows, the cones elongate and migrate toward the
fovea, where they become densely packed.
Color vision is not fully developed at birth.
At birth, infants have greatest sensitivity to yellows (medium
wavelengths).
The L and M cones develop first.
The S cones develop later (by about 2 months of age).
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Infant Acuity
Infants have much lower visual acuity (ability to see fine
features) than adults.
•Newborns have acuity of about 20/400 to 20/800 (Legally blind
for adults is 20/200).
•At 6 months of age, acuity is about 20/25
•Normal adult acuity of 20/20 reached at about age 7 years old.
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Sensitivity to Contrast
Infants need much higher contrast to see gratings at birth.
Infants can only see gratings with wide bars.
Vertical gratings
Thick
Thin
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The world is blurrier for infants
Because of their lower sensitivity to contrast and their inability to
see fine features (thin gratings), the world seen by infants is
blurrier.
9
Face Perception
Newborns prefer to look at their mother's face (as early as 2 days
old).
Preference for normal faces appears at 2 months of age.
10
The Visual Cliff
Infants who are at the
crawling stage, can
perceive a visual cliff.
They will not crawl over
the drop-off, even with
their mother encouraging
them.
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Exam Review Topics
There is a list of exam review topics at:
http://mathcs.holycross.edu/~croyden/hnrs299/notes/exam1_review.html
Expect short answer questions on any of the topics listed.
Examples:
1) Why is vision considered an "ill-posed" problem? List 2
assumptions made by the visual system to deal with this
problem.
2) What is the difference between rods and cones in terms of
their sensitivity to light?
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More Sample Questions
3. Describe the columnar structure of the cortex? Name two
properties that are processed by columns.
4. Describe the Craik-Obrien-Cornsweet illusion. What does
it tell us about how the brain processes changes in contrast?
5. Diagram the neural circuit for direction selectivity. Briefly
explain how it works.
6. Why is it that we can match any perceived color by a
combination of red, blue and green lights?
7. List 3 monocular cues to depth. Briefly explain each one.
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Problem 1.
Compute the 2D image positions for the following 3D points.
Assume the image plane is 2 cm behind the pinhole (center of
projection). All units are given in cm.
Point 1: (5, 10, 50)
Point 2: (16, 24, 8)
Point 3: (25, 45, 50)
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Problem 2.
-2
-2
-2
+6 +6 -2
-2
+6 +6 -2
-2
-2
-2
-2
-2
-2
The receptive field shown
is partially in light and
partially in dark. What is
the change in firing rate of
the neuron with this
receptive field for this
light stimulus? Assume
that the partially covered
squares are 1/2 covered.
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