Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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