Download Link Between Size & Distance in Vision: Size Constancy & Depth Perception and more Study notes Brain and Cognitive Science in PDF only on Docsity! 1 1 2 3 Coding depth 4 The 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 5 • Individual neurons were “tuned” for different amounts and directions of disparity 6 Hubel & Wiesel’s Depth-Sensitive Cortical Cells 7 Clinical 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 8 Size Perception, Perceptual Constancies, and Visual Illusions 9 Size 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 10 • 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 2 – Colour 11 Size constancy Why does someone walking away not appear to shrink? 12 Size constancy and size perception • Refers to the tendency to see objects as their true size, despite changes in retinal image size • Requires that distance of targets be taken into account • Misplaced constancy can lead to visual illusions 13 Relationship between size perception and perceived distance: • Generate afterimage on retina • View afterimage against surfaces at different distances • Note changed size of afterimage 14 • This picture looks odd because the size and distance cues are in conflict 15 Size constancy: • Given the size of the image on the retina (visual angle) and its distance, it is possible to compute the physical size of an object • Size constancy is the mechanism that makes this computation • Holway & Boring demonstrated the crucial importance of depth perception in an experiment 16 The Holway-Boring experiment: • Observer views Test Disks located at different distances • Task is to adjust size of Comparison Disk to match physical size of Test Disk 17 • Test disks all set to subtend 1o of visual angle 18 Constancy scaling • Richard Gregory proposed that size judgements in general were controlled by a constancy scaling mechanism • In brief , this means that perceived size is a simple product of retinal image size and perceived distance S = k(R x D) where S = perceived size
