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Final Review Questions and Answers

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University of Guelph
PSYC 2390
Lana Trick

1. Depth Perception. a) Describe the stages involved in calculating depth from binocular information. (Hint: You can find this if you look in your lecture notes, and look at the outline for the lectures on binocular perception that is posted on Courselink.) When calculating depth from binocular information, the process involves solving the stereo correspondence problem (which is done by determining which part of the visual field on the left eye matches which part on the right eye) and then calculating the retinal disparity (which is finding out how far apart the objects position is on the left eye as compared to the right). Incorrectly solving the stereo-correspondence problem could lead to illusions like magic eye The 3 possible outcomes when calculating retinal disparity are: 1. Crossed disparities: The reference point is on the same position on the left side of the left eye and the right side of the right eye (infront of the horopter) 2. Uncrossed disparities: Reference point is on the same position on the left and right eye (behind the horopter) 3. Zero disparity: reference point is in the middle of the eye for both eyes (on the horopter) b). When Elaine’s friend heard that Elaine was going to patch her eye for a Halloween party, she was horrified. She told Elaine that Elaine would permanently damage her depth perception capabilities if she wore an eye patch. There is an element of truth to her friend’s concerns, although they would not really apply in Elaine’s case. What would have to be true for visual deprivation of the type that Elaine experiences at the party to be damaging? Why is that type of deprivation damaging? In order for Elaine to damage her depth perception capabilities she would have to wear an eye patch during her critical period (4 months-5 years of age in humans) because during this time, if a patch is put on one eye she would never experience information from both the left and right eye at the same time and therefore the binocular cells would not form in the striate cortex and she will have trouble perceiving depth because she would not be getting correct depth information and cues c). In class we talked about the “Magic Eye” pictures that you often see in art stores or in the newspaper. They are also called autostereograms. In these pictures there are a series of dots or repeated elements. When people stare at these images for long periods of time, many can see a three-dimensional image “rise up” out of the flat picture. (This image is completely different than the one in the flat picture.) Why does this occur? Autostereograms cause people to see 3D that are completely different from the flat image because it happens when the Stereo-Correspondence problem is solved wrong—the wrong part of the left image is matched with the wrong part of the right image and your eye decides something is the same thing when its really not. By putting your eyes out of focuss it can encourage a mismatch to occur and gives you a depth illusion—we see a 3D image floating above the picture d). For 5 minutes Maurice stares at a grating made up of left tilting lines using only his right eye. (His left eye is covered up). Then his right eye is covered, his left eye uncovered, and he looks at a grating made up of vertical lines with only his left eye. To him, these vertical lines appear to be tilting to the right. Maurice has normal vision. Answer the following questions about this situation.... i. Why does Maurice perceive lines tilting to the right in the last phase even though they are really vertical? (Hint: This phenomena occurs even if the stimuli are viewed with both eyes and is consistent with a general principle in the visual system.) Maurice appears the lines to be tilted to the right in the last phase due to the tilt- after effect. The eye receptors had adapted to the initial tilted lines and therefore tired out and to compensate for being tired out, perceive the vertical lines as being tilted in the opposite direction ii. Why does this occur even though looked at the original left tilting lines with his RIGHT EYE then perceives the after-effect when using his LEFT EYE? This occurs in the left eye even though it had been covered because the receptors from the right eye adapted to the tilted lines and were tired out, these receptors are in the striate cortex and respond to both eyes (they are binocular cells) therefore the other eye perceives the tilt after-effect as well iii. How is this finding related to depth perception? This finding is related to depth perception because it shows that binocular cells are found in the striate cortex. It shows how important it is that both eyes work together in perception, as also evidenced in the kitten experiment where an eye patch was kept on a cat durings its critical period and also in strabismus (imbalance in eye muscles) which causes different information in both eyes that the brain cannot coordinate—depth perception is not developed normally when both eyes don’t function together correctly e). Theresa was putting on make-up in front of a mirror. She was focused on looking at the wall mirror, about 1 meter away. She was just bringing her hand up to her face to put on eye makeup when she noticed that she had double vision of the hand that was approaching her face to administer the mascara. (Theresa has normal vision.) Why did this occur? She experienced double vision of the hand due to panums fusion area: if something is too close from the horopter or too much farther away you will get a double image. Since her horopter was the mirror, her hand was too close and outside Panums fusion region it appeared double 2 The perception of form a). A complex scene is projected onto your retina. In class we discussed a 3-stage model that described how object form was derived, so that an object could be recognized and identified. In particular, indicate what specific processes are carried out at each stage (using example diagrams may help), and how the processes are carried out in the image. Also describe the characteristics of processes carried on at each stage. Stage 1: low level vision, AKA pre-attentive analysis, involves being spatially parallel and looking for particular colours, shapes and orientations all over the picture (discontinuities, AKA changes) to determine colours, edges, contrasts in brightness, etc—signaling the edges of whole objects is important *the stabilized image technique emphasizes that if there is a contour on the retina it would always fall on the same part regardless of how you move your eyes, since there is no change it would eventually disappear—in order for the visual system to work we need contour and change *we categorize discontinuities through feature analysis (horizontal, diagonal, black, red) Stage 2: visual routines, AKA analyses requiring spatial attention, involves being spatially serial which is noticing finer details in an image (recognizing a face, etc); this involves building an object file that will later be stored to long term memory Stage 3: visual cognition, AKA object recognition, you now have your object file so space is no longer relevant, this is when you store it in your long term memory and see if you recognize the object/image b). What is the difference between spatially parallel and spatially serial analysis? Describe one test that allows you whether a particular discrimination between items and other items requires parallel or serial analysis. Spatially parallel processing (pre-attentive) involves processing the entire image at the same time and spatially serial processing (attentive) involves analyzing one part of the image effortfully with the rest of the image receiving little attention The visual search task would allow you to make a discrimination between items and requires parallel or serial analysis—identifying a horizontal line among distractors involves spatially parallel processing (low level vision) but identifying a vertical line among other horizontal and vertical lines takes longer because it involves visual routines (spatially serial analysis) c). What are illusory contours? Give an example, and explain why they occur. Illusory contours refer to the fact that the contours (edges, lines) aren’t actually present in the physical stimulus even though we perceive a stimulus to be there, for example we perceive a square even though it is not really there. This is because of the way we perceive the black shapes by assuming they are circles being covered by a white square d). What is a Ganzfeld? What does it show about the perception of form? the ganzfeld experiment is when you tape ping pong balls infront of someones eye so that that they cannot see any discontinuities in brightness/no contours—it tells us that we need contours to anchor our eyes (form landmarks) and for our visual system to function, we need change or adaptation occurs e). Describe the Gestalt grouping principles, giving examples (drawings) of how they would work. The gestalt grouping princples are part of low level vision and involve: grouping by proximity grouping by symmetry grouping by continuation grouping by closure < > < > < > < > < > f) What are illusory conjunctions? Give an example of a situation in which an illusory conjunction may occur and indicate why it occurs in terms of processes in the visual system. Illusory conjunctions are when under some conditions, features associated with one object b
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