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3Coding depth
4The physiological basis for stereopsis
• How can cortical neurons process disparity information?
•Barlow, Blakemore and Pettigrew discovered cortical neurons that could code for retinal disparity
•Their receptive fields were selective for the positions of the images on the two retinae
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•Individual neurons were “tuned” for different amounts and directions of disparity
6Hubel & Wiesel’s Depth-Sensitive Cortical Cells
7Clinical tests for stereopsis
•Can be tested by ophthalmologist/optometrist
•Polaroid or red/green filters
•May have poor stereopsis due to vision problems
–often due to binocular vision problems as a child
•Strabismus (eye-turn)
•Amblyopia (lazy eye)
•cataract
8Size Perception, Perceptual Constancies, and
Visual Illusions
9Size Constancy
•The perception of size and the perception of distance are closely related
•This relationship forms the basis for the phenomenon of size constancy. It allows for
the accurate perception of real size despite changes in the size of the retinal image
•In general, constancies are the visual system’s response to a continuously-changing
retinal image
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•Constancies represent a mechanism that takes into account the fact that an object
remains constant even though its image on the retina may change substantially
•A failure in the constancy mechanism may produce an illusion
•Several classes of constancy:
–Size
–Shape
–Lightness
