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DEPTH

DEPTH . PERCEPTION. Depth perception. Translation of 2 dimensional (retinal) image into a 3 dimensional image Retina only codes in 2 dimensions But we live in 3-d world Most perception requires assessing depth Particularly hand eye coordination!. Two kinds of cues for depth.

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DEPTH

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  1. DEPTH PERCEPTION

  2. Depth perception • Translation of 2 dimensional (retinal) image into a 3 dimensional image • Retina only codes in 2 dimensions • But we live in 3-d world • Most perception requires assessing depth • Particularly hand eye coordination!

  3. Two kinds of cues for depth • Monocular cues • One eyed! • Several different categories • Binocular cues • Requires coordination of both eyes • Depends on retinal disparity: each eye sees a slightly different image • Use this feedback to determine depth

  4. Monocular cues • Three kinds • Accommodation • Kinetic or movement cues • Accommodation • Accommodation: • When you shift your focus from near to far or far to near, requires movement of eye muscles • Especially ciliary muscles • Feedback from these muscles provide a cue for depth

  5. Pictorial Cues • Picture cues • Available via stationary inspection (holding still!) • Often used by artists, architects, animation, etc. • Obey Gestalt Rules • Use 2-D image to imply a 3-D image

  6. Pictorial Cues • Size: • As objects move farther away the size of the retinal image gets smaller in direct proportion to the distance • This becomes a cue for depth • Note: assumes Object constancy • objects don’t change size • Compare ‘familiar’ size to retinal size

  7. Pictorial Cues • Interposition: • If 2 opaque objects are in same line of sight, the one in front will occlude the one behind • The front object blocks the back object • Interesting cultural differences: • Individuals with no formal education can see in real life but NOT see it in book or picture form • Why?

  8. Pictorial Cues • Light and Shadow • Objects that have depth cast shadow • Use shadow to judge depth and distance • Movement of shadow can even tell movement

  9. Pictorial Cues • Perspective • Lines or texture converge with distance • Linear perspective • Texture perspective • Also aerial perspective: higher objects appear fuzzy • Interesting applications: • Learning to fly and aerial perspective • Visual Cliff research • Animals who are precocial show fear from birth • Animals who are not precocial show fear once they begin moving on their own • Why?

  10. Kinetic or motion cues • Motion parallax • Assume observer is moving, not the world • Get this when sitting at stop light and other car moves slightly • Feel like are moving backward • Relative movement of objects differ with distance: • Close objects move fast • Far objects move slowly • Kinetic depth effect • Depends on moving objects • objects change shape with movement and distance

  11. Binocular Cues • Convergence • eyes move inward for near objects • Eyes move outward for far objects • This feedback does two things: • Changes degree of disparity • Movement of eye muscles is cue for depth

  12. Binocular Cues • Binocular disparity • Each eye sees slightly different visual field • The degree of overlap gives clue to distance • Little overlap: object is far away • Lots of overlap: object is near • Can actually calculate depth/distance this way: • triangulate the distance • This is how surveyors work • You have a dominant and non-dominant eye: • How to tell? • Hold figure about 12” from nose, line it up with distant vertical object • Close left eye, leaving right open; note how far finger “jumps” • Close right eye, leaving left open: note how far finger “jumps” • The eye for which the finger “jumps” the least is your dominant eye • Why? This is the eye the brain uses most, the other eye becomes the comparator • When there is too much disparity, the brain only attends to dominant eye • Ignores other eye • If this continues for too long- the pathway to the non-dominant eye dies

  13. Why is it important to “use” our senses? • Binocular disparity drives this home • The brain receives confusing signals, it will stop attending to the “bad” signal • If a sense is not used, the brain will begin to ignore it: • Brain tissue will die off or be used for other senses • Thus, it is not that people who are blind hear better, but their brain stops using optical information and only attends to hearing • Is really that they pay better attention to hearing • A variety of research has shown: Use it or lose it!

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