1. 1 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

2. 2 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.

3. 3 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.

4. 4 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.

5. 5 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.

6. 6 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).

7. 7 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.

8. 8 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

9. 9 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.

10. 10 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.

11. 11 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.

12. 12 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?

13. 13 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.

14. 14 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)

15. 15 Problem 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.