Visual Perception Week 5 Notes
Visual Perception Week 5 Notes PSYC 3124
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This 10 page Class Notes was uploaded by Freddi Marsillo on Thursday February 11, 2016. The Class Notes belongs to PSYC 3124 at George Washington University taught by Dr. John Philbeck in Spring 2016. Since its upload, it has received 48 views. For similar materials see Visual Perception in Psychlogy at George Washington University.
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Date Created: 02/11/16
Visual Perception Week 5 Notes 09/02/2016 16.13.00 Depth and Size Perceptions change based on state – for example, a hill might appear to be steeper if you are fatigued Equivalent Configurations There’s an infinite variety of things in thr world that can give rise to the pattern of light that’s on the back of our eye (an infinite variety of things can create the same or similar images to our eyes) – how is it that we’re able to narrow it down? How does our brain figure it ouDistance and depth information are extremely important The number of equivalent configurations is infinite if you can’t perceive distances The above image shows two different things in the real world creating the same pattern of light on the back of the eye Once you can see the depth information, you can narrow down the amount of equivalent configurations and allows you to differentiate between images Egocentric distance (or absolute distance) – distance between yourself and some object Exocentric distance (or relative distance) – the distance between two things out there in the world, not including yourself There is a distinction between distance and depth – they are related, but not exactly the same What we can do to gauge distance – Information about the above list: Absolute Distance Monocular cues – cues that can only come about if you have two eyes Accommodation o The process by which the eye changes optical power to maintain a clear image or focus on an object as its distance varies Absolute motion parallax (parallax = seeing something from two different locations) o Motion parallax comes about when your head or the object is moving from one location to another, i.e. from side to side Familiar size o Knowing the typical size of the object can help you gauge how far away it is Angular elevation o This has to do with how far down in your visual field you have to look to see an object – how high up in the visual field does the object appear? If the object is very low in your visual field, it must be close, and if it is very high up in your visual field, it must be farther away Binocular cues – cues you have if you cover up one eye Convergence o There is a muscular part and a visual part to the cue o Visual part of cue: parallax – you have two eyes at different locations o Muscular part of cue: you have to literally use your eye muscles and move your eyes to see object – a separate signal for your brain to gauge the distance. This is the signal your brain uses to decide how much you need to turn your eyes in o Convergence is not as good as angular elevation, but it’s better than the others Relative Distance Monocular cues Relative motion parallax o Also called “optic flow” – depth perception cue in which objects that are closer appear to move faster than objects that are further away o Focus of Expansion (FOE) = a point in the optic flow from which all visual motion seems to emanate and which lies in the direction of forward motion; it is the single point on the projected image where the object appears to be coming from Relative angular extent “linear perspective” texture gradient o Linear perspective: the appearance of lines tending to converge – the relative size, shape, and position of objects are determined by visible or imagined lines converging at a point on the horizon o Texture gradient: the distortion in size which closer objects have compared to objects farther away; groups of objects appear denser as they move farther away Angular elevation o Objects appearing closer to horizon tends to seem farther away, and objects farther away from the horizon tends to seem closer to you Aerial perspective o Useful for great distances – Has to do with the amount of water vapor in the air – things that are very far away will tend to seem more blue in color, and will tend to seem lower in contrast o This is because when something is farther away, it has to pass through more of the water vapor (almost like looking through an ocean) o For example, on a very humid day, objects will tend to seem farther away Interposition Lighting and shading Binocular cues Disparity o Our eyes get slightly different views of the world; our two eyes are separated just enough to get a slightly different perspective of the world o We have two eyes, but somehow our brain merges these two images into a single viewpoint that has depth to it o Disparity = the physical cue and quantity that can be measured o Stereopsis = Binocular disparity is the physical cue, and stereopsis is the perception of depth based on that cue o If you keep increasing disparity, you’ll still experience depth but you’ll start to see double images o For example, hold out two fingers, one on each hand. Now close one eye, and then the other. One eye sees a bigger distance between your two fingers than the other eye; this difference in distance is disparity Lighting and shading can also provide cues for distance perception Gives us a sense of 3D – Parts of the figure that are light at the top seem to be popping out of the screen, and parts that are darker at the top seem to be going into the screen Interposition – something appearing to be in front of another; e.g. the clouds are between us and the moon; clouds are in front of the moon and are therefore closer to us than the moon is Corresponding Points Representation of retinal surface from behind the eyes shows relative image locations of four peripheral objects Image location can be specified by distance (d) from fovea and whether they are situated in nasal (n) or temporal (t) retina Four pairs of images are formed on corresponding retinal points; distance from fovea is identical in the two eyes (d=d) If you’re getting the same image of an object in your two eyes, that is, if the image is formed on corresponding points between the two eyes, then the disparity is 0 Binocular Depth Perception Stereoscopic cues and binocular disparity: Horopters: o Vieth-Muller circle = a theoretical circle in space on which all objects produce optical images at analogous retinal points in the two eyes o Horopter = the set of environmental points that produce an image at analogous retinal sites for a given fixated object contains all those points in space whose images fall on corresponding points of the retinas of the two eyes o Horopter is a general term; Vieth-Muller circle is a type of horopter o This image is a top-down view of a pair of eyes fixated on object “F” o All objects situated on the Vieth-Muller circle project to corresponding points in the two retinas o Objects located behind the horopter create binocular images on non-corresponding retinal points Random-Dot Stereograms A stereo pair of images of random dots which when viewed with the eyes focused on a point in front of or behind the images produces a sensation of depth, with objects appearing to be in front of or behind the display level Correspondence problem = the problem of ascertaining which parts of one image correspond to which parts of another image Relationship between size, distance, and visual angle: if you change one of these variables, another one must also change Change: SIZE Hold constant: DISTANCE Visual angle must change Change: DISTANCE Hold constant: SIZE Visual angle must change Change: VISUAL ANGLE Hold constant: DISTANCE Size must change Emmert’s Law Relationship – you have a constant angle and you change the perceived distance of the object, so the perceived size also appears to change When the moon is on the horizon, it looks a lot bigger than it does when it’s up in the sky. It’s an illusion, because retinal image size is actually the same, regardless of where the moon is positioned Explanation: when the moon is on the horizon you actually perceive it to be farther away than it is when it’s directly overheard – consistent with the idea that we have more physical cues when we’re looking at the horizon – lots of distance cues near the horizon moon, so we perceive it to be farther away 09/02/2016 16.13.00 09/02/2016 16.13.00
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