Transcript Session 10

Perception and VR
MONT 104S, Fall 2008
Lecture 10
Binocular Stereo
Visual Development
Thanks to website at University of Calgary:
http://www.psych.ucalgary.ca/PACE/VA-Lab/Marcela/Pages/index.html
1
Binocular Stereo
•The image in each of our two eyes is slightly different.
•Images in the plane of fixation fall on corresponding locations
on the retina.
•Images in front of the plane of fixation are shifted outward on
each retina. They have crossed disparity.
•Images behind the plane of fixation are shifted inward on the
retina. They have uncrossed disparity.
uncrossed (negative) disparity
plane of fixation
crossed (positive) disparity
2
Stereo processing
To determine depth from stereo disparity:
1) Extract the "features" from the left and right images
2) For each feature in the left image, find the corresponding
feature in the right image.
3) Measure the disparity between the two images of the
feature.
4) Use the disparity to compute the 3D location of the feature.
3
The Correspondence problem
•How do you determine which features from one image match
features in the other image? (This problem is known as the
correspondence problem).
•This could be accomplished if each image has well defined shapes
or colors that can be matched.
•Problem: Random dot stereograms.
Left Image
Right Image
Making a stereogram
4
Random Dot Stereogram
5
Problem with Random Dot
Stereograms
•In 1980's Bela Julesz developed the random dot stereogram.
•The stereogram consists of 2 fields of random dots, identical
except for a region in one of the images in which the dots are
shifted by a small amount.
•When one image is viewed by the left eye and the other by the
right eye, the shifted region is seen at a different depth.
•No cues such as color, shape, texture, shading, etc. to use for
matching.
•How does the brain know which dot from left image matches
which dot from the right image?
•Auto-stereograms (magic eye) are versions of random dot
stereograms.
6
Some neurons are sensitive to
disparity
Some neurons in V1 and MT are sensitive to the disparity of a
stimulus. In V1, most neurons prefer a disparity near zero.
Tuned Excitatory
Tuned Inhibitory
7
Near and Far cells
Some cells are broadly tuned for disparity, preferring either near
objects or far objects.
8
Testing for Disparity tuning
Movie: http://www.viperlib.org/
(General depth images, page 5)
Torsten Wiesel tests a cell for sensitivity to depth.
9
Columns in the cortex
The cortex is a folded sheet of cells, about 2 mm thick.
The cells form layers (6 layers in primary visual cortex).
If move perpendicular to the surface of the cortex, cells will
respond primarily to input from one eye (ocular
dominance).
The pattern of responses forms columns of ocular
dominance.
10
Ocular Dominance Columns
11
Orientation Columns
Preference for a given orientation also has a columnar structure:
12
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.
13
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.
14
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.
15
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.
16
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).
17
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.
18
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
19
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.
20
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.
21
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.
22