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Lecture 6

Lecture 6.docx

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University of Toronto Mississauga
Stuart Kamenetsky

Lecture 6 Depth Perceptual Constancy - Shape - Color Visual system is keen component in maintaining the constancy of the perception Another property: constancy of size. It is an intrinsic property. We use information about the size of the retina image to get arepresentation of the actual size. Size constancy - Preserved size of an image The representation of 3-D space - Challenge: object is 3dimensional but we perceive it on 2 dimensional - The x-axis horitzontal location can be explicit representation on the retine o If the position of the object changes on the y axis, it can also be explicitly represented on the retina o Explicit representation rd - What about 3 dimension? o Z coordinate –ASSOCIATED WITH DEPTH  Cannot explicitly represent in the retina  The presence of the 3 dimension is represented only IMPLICITLY  It leaves cues  The tak of the brain is to use these cues in order to understand the 3dimensional property of the image o Challenge:  First image: both objects are identical  Second: • Two images will be identical in the retina • even though they are different • have to use all the cues available in the visual system in order to make the correct inference Occular motor cues - muscle related - related to activity of ocular motor Retinal cues - found oupon the retinal image - can be sub divided to monocular and binocular o monocular  only need one eye to extract o Binocular  Two eyes are looking at an image from two diff vantage points  So the right and left eye see things a little different  But visual system matches the small differences between the two views and from that – infer information about depth • Can be static or dynamic cues  Static • Can extract images even if they are static/frozen/unmoving o Position, size and lightning based cues  Cues that depend upon the relative location of  Size:  Lighting:  Dynamic • Occurs as a result of motion • Either environment relative to observer or vice versa or both • Motion in turn produces info about depth The visual system uses these cues, interprets them and constructs an assessment of the various cues. Occulomotor depth cues – Accomodation - Optical infinity: a few metres away and beyond - This is enough to project an image - Distant object to a closer object shifted gaze – the image of the object will be projected conceptually as a blurr - The focus of the spot of light will be beyond the retina conception - Increasing the curvature of the lengths helps - The image of the closer object is able to be projected on to the retina The curving of the lense is affected by the muscle that is controlled by the ligament - Contraction makes the lens change its shape - Potential cue to depth - If the muscles are to increase – acoomodation – the object is getting closer - The closer the object, greater muscual contraction needed – inverse relation between distance and contraction Convergence cue - If the object is closer and want to keep fixation – the closer the object the greater amount of convergence - Produced by contraction of the muscles - Another occulomotor cue - The shorter the distance, the greater the amount of contraction Occulomotoe cues - They are the only ones that provide information about absolute depth - Even though other cues can help, occulomotor are the only ones that provide info about absolute depth while the others are about relative depth - The operating range which they can provide information is limited: 2metres and few cm from nose As the object comes closer, accommodation has to increase and convergence has to increase – both lvels muscles have to increase Simple experiment Observer is presented with a lumionous disc – partion so each eye only sees one thing - Distance never changes - The physical size of the two discs never canges - If the distance never changes – accommodation cues are ruled out - The lens is flat - The only thing being changed is the distance of the two discs to the center partition - As the disc moves laterally, the eyes have to diverge and when they move closer, they have to converge - Estimate how you perceive the size - Is there any change in the perceived size - The phsycial size of the disc never changes - What is the only thing that changes? The discs move farth or close to the center What’s happening - The disc located in the farther location was perceived as significantly larger - Perceived size is larger BECAUSE: when the objects are located further – they objects have to diverge - The retina image knows there is no difference in size - Because the eyes diverge, you know its far - If you converge, you know its closer - The size of the retinal image doesn’t change but presumably one disc is father than the other because th eocculotor cues are telling you that Why should you perceive that one if physical larger than the other? - If you have an identical object and one if twice as farther away – what happens to the image? o As suggested by the convergence – the size when its farther is larger because it is its actual physical size o The brain produces this allusion because it is comparing retinal cues with o The representation is the outcome of a perceptual guess How could the cisual system gain information about these visual cues 1) If you bury receptors that control convergence of the eye 2) In order for the muscles to move the eye • Some order has to be in control • If you have a monitor that responds to the activity level – then this woukd another way the visual system keeps track 3) T There is evidence that both source smay be used in the visual system to keep track Monocular - Can be extracted from just looking through one eye Partial Occlusion - The assumption in the brain that most objects in the physical world are opaque - Very common - Very crude assessment of relative depth - Infants use this - They already respond to this cue Suitcase is in front of the person, umbrella in front of building, person in front of the car Red / Blue rectangles - Have to assume that both rectangles are the same size a) We will assume red is in front of it • Gesalt principles • Most objects tend to be symmetrical and simple in shape Relative Height - Another powerful cue - The line represents the horizon - Objects that are lower than the horizon tend to be perceived as closer - Beyond the horizon, obejcts that appear lower than the field - Cues vary depend on the image being below or above scene We use relative height info to make inferences about distance Familiar Size - We know what the true size of an object is - We know the range of average sizes when looked at closely - When we k
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