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Chapter 10

Sensation and Perception Psych 367 chapter 10 perceiving depth and size.docx

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Douglas Wylie

Chapter 10 perceiving depth and size - Cue approach to depth perception o Focuses on identifying information in the retinal image that is correlated with depth in the scene - Occlusion: is a signal or cue that one object is in from of another - Divide these cues into 3 major categories o Oculomotor: cue based on our ability to sense the position of our eyes and the tension in our eye muscles o Monocular: cue that works with one eye o Binocular: cue that depends on two eyes Oculomotor cues - Created by o Convergence, the inward movement of the eyes that occurs when we look at nearby objects o Accommodation, the change in the shape of the lens that occurs when we focus on objects at various distances - We can feel the inward movement of the eyes that occurs when the eyes converge to look at nearby objects, and we feel the tightening of eye muscles that change the shape of the lens to focus on nearby objects - Feelings you experience as you move your finger closer are caused by o The change in convergence angle as your eye muscles cause your eyes to look inward o The change in the shape of the lens as the eye accommodates to focus on near objects Monocular cues - Work with only one eye - They include accommodation and movement based cues Pictoral cues - Sources of depth information that can be depicted in a picture Occulusion - Occurs when one objects hides or partially hides another from view - Partially hidden object is seen as being father away - Occlusion does not provide information about an objects absolute distance only indicate relative distance Relative height - Objects that are below the horizon and have their bases higher in the field of view are seen as being more distant - When objects are above the horizon like clouds, being lower in the field for view indicates more distance Relative size - When two objects are of equal size, the one that is farther away will take up less of your field of view than the one that is closer - This cue depends to some extent on a person’s knowledge of physical sizes Perspective convergence - When parallel lines extend out from an observer, they are perceived as converging – becoming closer together as distance increases Familiar size - When we judge distance based on our prior knowledge of sizes of objects - Epstein: o Shows that under certain conditions, our knowledge of an objects size influences our perception of that objects distance o The stimuli in the experiment were equal sized photographs of a dime a quarter and half dollar which were positioned the same distance from the observer o Epstein created the illusion they were real coins o They estimated that the dime was closest o Observer judgements were influenced by their knowledge of the sizes of real dimes, quarters and half dollars o This result did not occur however, when the observer viewed the scene with both eyes because the use of both eyes provided information indicating the coins were at the same distance o Cue about familiar size is most effective when other information about depth is unknown Atmospheric perspective - Occurs when more distant objects appear less sharp and often have a slight blue tint - The farther away an object is the more air and particles we have to look through making objects that are farther away look less sharp Texture gradient - Elements that are equally spaced in a scene appear to be more closely packed as distance increases Shadows - Shadows that are associated with objects can provide information regarding the location of these objects - Shadows also enhance the three D of objects Motion produced cues Motion parallax - Occurs when as we move, nearby objects appear to glide rapidly past us but more distance objects move more slowly - Because the image of the near object travels a large distance across the retina, it appear to move rapidly a the observer moves - Motion parallax is one of the most important sources of depth for many animals Deletion and accretion - Covering up is deletion - Uncovering is accretion - Deletion and accretion are related to both motion parallax and overlap because they occur when overlapping surfaces appear to move relative to one another Depth information 0-2 meters 2-20 meters Above 30 meters Occlusion yes yes Yes Relative size Yes yes Yes Accommodation and convergence Yes Motion parallax yes Yes Relative height yes Yes Atmospheric perspective yes Binocular depth information Binocular disparity - Is the difference in the image in the left and right eye - When you switched from looking with your left to your right eye, you noticed that your finger appeared to move to the left relative to the far objects - This occurs because you were looking right at both objects so their images would fall on the foveas - Corresponding retinal points: the places on each retina that would overlap if one retina could be slid on top of the other - Horopter: which is an imaginary surface that passes through the point of fixation and indicates the location of objects that fall on corresponding points on the two retinas - The difference between where Caroles image falls on the right eye and the corresponding point of the left is called angle of disparity - Absolute disparity is important because it provides information about the distances of objects - The amount of absolute disparity indicates how far an object from the horopter - Greater disparity is associated with greater distance - Properties of absolute disparity is that it changes every time the observer changes where he or she is looking - Every new fixation establishes a new horopter, this mean that the absolute disparities for every object in a scene have to be constantly recalculated - Relative disparity: the difference between two objects absolute disparities - The difference between them remains the same - As long as the objects stay in the same position relative to an observer , the difference in the disparities remains the same no matter where the observer is looking Connecting disparity information and the perception of depth - Stereopsis: the impression of depth that results from information provided by binocular disparity - Stereoscope: a device introduced by the physicists Wheatstone, which produces a convincing illusion of depth by using two slightly different pictures o The left and right are presented simultaneous on the screen, slightly displaced from one another to create disparity o These images can be presented separately to the left and right eyes by colouring one red and other green and viewing the film through glasses o Another way is to separate the images to create left and right images from polarized light o One image is vertical the other is horizontal - In order to show that disparity alone can result in depth perception, Julesz created a stimulus called the random dot stereogram which contained no pictorial cues - These patterns were constructed by first generating two identical random dot patterns on a computer then shifting a square shaped section of the dots one or more units to the side - The effect of shifting one section of the pattern in this way is to create disparity - Disparity alone must be causing the perception of depth The correspondence problem - In order for the visual system to calculate this disparity, it must compare the images of the cactus on the left and right retina and the image of the window on the left and right retinas - This is the correspondence problem o How does the visual system match up the images in the two eyes o May match the images on the left and right retina on the basis of the specific feature of the object o No good answer yet Depth information across species - To make use of binocular disparity, an animal must have eyes that have overlapping visual fields - Frontal eyes, result in overlapping fields - Lateral eyes, such as rabbits, to not have overlapping views and therefore cannot use disparity to perceive depth - Psychophysical experiments have shown that the pigeon does have a small area of binocular depth right at its beak - Movement parallax is probably insects most important method of judging distance - The locust, uses a peering method: moving its body from side to side quickly to create movement of its head o Locust swayed more when object was farther away o May be judging distance by how much swaying needed to have the image move on the retina - Sonar works by sending out pulses of sound and determining distance by information in the eco - Echolocation used by bats - The timing of these echoes provides the information the bats need to locate objects Physiology of depth perception Neurons that respond to pictorial depth - Tsutsui: o Studied the physiology of neurons that respond to the depth indicated by tex
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