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Chapter 12

Chapter 12 Depth and Stereopsis

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McGill University
PSYC 212
Evan Balaban

Chapter 12: Depth and Stereopsis Binocular vision - 2 eyes at 2 horizontal locations - produces cyclopean perception (unified view) A. Extra-Personal Space - object direction and distance 1. Spatial frames of reference - there is a set of axes to describe position and placement - allocentric frame = independent frame of reference to the viewer - egocentric = frame of reference dependent on viewer -> egocentric direction - reference point = visual egocentre “cyclopean eye” (midpoint between 2 eyes) 2. Head + Body Movements in Space Human Dynamics Proprioception - detection of muscle contraction and movement around a joint - efference copy: perceptual centers of brain receive a copy of motor commands Vestibular System - “balance organ” - detect head and body movement - in inner ear 1) semicircular canals (rotation) 2) otoliths (linear movement, for- ward, backward, vertical) - based on fluid movement which stimulates hair cells in those 2 regions (neu- rons) - brain knows we moved, not the world itself, so we see the world as stable 3. Absolute Depth Optical cue (retinal image size) - object distance changes retinal image size Accommodation - reflex that changes the optical power of our eyes - near object = increase in optical power to make a sharper image than far ob- jects - increases thickness of crystalline lens -> controlled by ciliary muscle - only useful for objects 3 m or closer Vergence (eye movements) - eyes rotate horizontally in sockets so the object falls on the fovea - convergence: both eyes roll inward (closer object) - divergence: both eyes roll outward (far object) Afference + Efference - both of the following are involved: - efference copies may be relayed to frontal cortex - afferent signal: signal sent from periphery (eye muscles) to brain B. Monocular Depth Perception & Cues - relative depths (where objects are located with respect to each other in distance along the 3rd dimension) 1. Pictoral Cues - stationary information in 2D scenes/pictures Occlusion - if an object covers another, the covered object is father away - not innate (learned at 5-7 months) Relative Size - the smaller an object appears, the farther away it is perceived to be Texture Gradient - foreground texture is coarse Linear Perspective - objects become smaller as they recede Aerial Perspective - hazy quality of distant environmental objects due to light scattering in atmosphere Shading and Shadows - light reflections of curved surfaces - “shape from shading” (gives shape cue) Image Blur - out of focus objects - as blur increases, relative depth in relation to focus object increases 2. Dynamic Cues Kinetic Depth - watching a moving object while you are stationary - closer objects appear to move faster Motion Parallax - moving viewer, stationary object - optic flow (relative movement of passing objects) - change in object’s direction of movement caused by self-motion is “motion parallax” - if observer is moving way and objects behind focus point seem to move magnitude of movement depends on how far the object is from the fixation point - Optic flow (James Gibson) - motion gradients caused by looming and expansion give depth information (also involved in balance and heading in movement) Accretion and Deletion - accretion - appearance of objects in front of an edge - deletion - disappearance of objects behind an edge - objects that disappear behind a curtain are farther 3. Size Perception and Constancy - how object distance affects size perception Size-Distance Relationship - eyeball image size depends on how far an object is - closer = larger image size Size Constancy - perceived size and object distance aren’t directly related - mental impression of size conforms to actual object size regardless of object distance Mechanisms: 1) We learn about true physical size through visual experience 2) Relative sizes to all other objects around Emmet’s Law - Afterimages (fleeting sight after prolonged staring) - perceived size of afterimage depends on distance @ which it was projected - a purely retinal phenomenon 4. Illusions of Size and Depths Absolute Depth Illusion - moon illusion -> moon largest @ horizon, smallest overhead (zenith moon) - due to apparent distance theory: humans believe the horizon is farther away than the sky - size-distance relationship: retinal images are smaller as the distance increases -> we see the moon to be larger on the horizon Relative D
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