Perception of Depth

Cues to depth: 1. Oculomotor 2. Monocular 3. Binocular

a) Convergence b) Divergence The Retinal Image is 2-D, but what information is available to construct the 3-D environment? Oculomotor Cues

c) Accommodation The shape of the lens changes as a function of the distance of an object

Monocular (pictorial) Cues a) Occlusion (overlapping object is closer) b) Size in the field of view (bigger is closer) c) height in the field of view (higher base is farther away) d) familiarity (sizes that we “know”)

Monocular (pictorial) Cues a) Occlusion (overlapping object is closer) b) Size in the field of view (bigger is closer) c) height in the field of view (higher base is farther away)

Pictorial depth cues: occlusion, “relative” size & height, shadows

Which coin is closer? Which one is looks bigger?

Which coin is closer? Effect of “Familiar Size”

Pictorial Cues Linear Perspective “Perceptual convergence”

Linear perspective: lines perceived as parallel that “travel” toward a “vanishing point”

Pictorial Cues texture gradient Relates to “ground” All other cues relate (size, height, linearity, occlusion, shadows, familiarity)

Relative height: objects higher (& smaller) in the picture are perceived as farther away

If the object is smaller and lower in the frame, the object is perceived as near, but relatively smaller

Pictorial Cues atmospheric perspective (aerial perspective) The higher & hazier contours are “hazier” because they are farther away (more air, moisture, dust to look through)

Higher & hazier Lower & sharper Pictorial cues: atmospheric cues

Implied lighting & shadows: shading Pictorial Cues

Motion Cues: reconfiguration of the visual field b) Deletion (covered)/Accretion

Motion Cues: reconfiguration of the visual field b) Deletion/Accretion (uncovered)

Motion Cues: Motion Parallax

Motion Cuesa) Motion parallax - When an observer moves, closer object moves a greater number of degrees of visual angle on retina than further objects - Subjective impression: objects nearer the observer move faster than more distant objects One Eye: Position 1 Position 2 T A H A T B H B

Monocular (pictorial) Depth Cues Occlusion Size Height Familiarity Linear Perspective Texture Gradient Atmospheric (aerial) Perspective Shading: Lighting & Shadows Movement Cues: (i) Deletion/Accretion; (ii) Motion Parallax

Test the size of your monocular and binocular visual fields Close one eye at a time Move your thumb across the visual field of each eye individually About how wide is your monocular field? About how wide is your binocular field? How different is the perspective of any one point in your binocular field when you switch from one eye to the other?

Binocular Disparity for Stereopsis

Depth cues with Bi (two) nocular (eyes) disparity Binocular disparity: difference (“disparity”) between the two points of view of the left and right eyes (retina) Stereopsis: Experience of depth from the joining together of left/right views at the level of the higher brain areas (LGN & cortex) Corresponding retinal points: specific location on each retina that communicates up to the identical receptive area of the cortex

Binocular Disparity for Stereopsis: The Horoptor and corresponding retinal points fixate blue object: image falls on corresponding points (identical locations) on both left and right retinas horopter – imaginary “circle” that passes through the point of fixation all images that are on the horopter fall on the exact same corresponding points on both retinas

Horopter

Images on the horopter share a corresponding retinal point Images not on the horopter are on different points on the retina

Binocular Disparity for Stereopsis angle of disparity images of objects not on the horopter fall on non-corresponding points on the retina Fovea

Binocular Disparity for Stereopsis the further away from the horopter, the greater the angle of disparity

Binocular Disparity for Stereopsis things in front of the horopter are in crossed disparity (you’d have to “cross” or converge your eyes to see it clearly) things behind the horopter are in uncrossed disparity (you’d have to “uncross” or diverge your eyes to see it clearly)

Disparity selective cell Binocular Depth Cells in Visual Cortex (striate cortex, V1) are disparity specific Also called “Disparity Detectors”

Perceiving relative size

Perceiving Size “Whiteout” is one of the most treacherous weather conditions possible for flying. Frank pilots his helicopter across the Antarctic wastes, blinding light, reflected down from thick cloud cover above, and up from the pure white blanket of snow below, making it difficult to find the horizon, or to know “up” from “down.” He thinks he can make out a vehicle on the snow below and he drops a smoke grenade to check his altitude. To his horror, the grenade falls only three feet before hitting the ground. Realizing that what he thought was a truck was actually a discarded box, Frank pulls back on the controls and soars up… drenched in sweat, he realizes how close he just came to a whiteout fatality…

Holway and Boring (1941): relation between size & distance (depth cues)? 1 deg comparison circle test circles Task for subjects: match the size of the “comparison” light exactly to the same size (diameter) of the test light. Condition 1: do matching with lots of depth cues Condition 2: do matching with fewer depth cues Note: the test stimulus light is always the same retinal size (1 degree) Where the subject was standing

Holway and Boring (1941) 1 deg comparison circle test circles With depth cues: Subjects accurately matched the physical size of the “comparison” and “test” stimulus lights Made match regardless of how far away (and despite the fact that retinal images were the same sizes) Without depth cues: Subjects routinely got the literal size of the stimulus wrong Matched the visual angle so that “comparison” and “test” lights were always the same size on the retina All “test” stimuli were believed to be the same size regardless of how close they were (all at 1-degree angle), so they looked the same Where the subject was standing

Conclusions from Holway & Boring (1941): Perceived Depth and Size of objects MUST Be codependent A A - We need depth information to accurately make judgments about size - I n the absence of depth information we determine the size of objects by the size of the image that they cast on our retina

Emmert's Law: size-distance scaling equation - Our perception of size equals the size of the Retinal image times the perceived distance away SpSp (R(RD p = ) X S = perceived size R = size on the retina D = perceived distance

Experiencing Emmert's Law:

Experiencing Emmert's Law first hand: - the farther away an afterimage appears, the larger we perceive its size If we look for the afterimage against a far wall the image looks much larger If we look for the afterimage on a piece of paper right in front of us, it looks smaller

Size of the afterimage, determined by how far away we look (or we think we look) to see the image Look at a piece of paper on your desk Look against a near wall Look against a far wall Emmert's Law: perceived size equals retinal size times perceived distance Bleached out cones in fovea (after-image)

Emmert's Law: size-distance scaling equation - Our perception of size equals the size of the Retinal image times the perceived distance away SpSp (R(RD p = ) X S = perceived size R = size on the retina D = perceived distance

Size Constancy: We perceive most object's physical size accurately regardless of it's distance from us A <A We can do this because of depth information - size-distance scaling mechanism - AND… FAMILIARITY

Most common index of size? Familiarity

Perception of Size (back to depth) Cues to depth: 1. Oculomotor 2. Monocular 3. Binocular

The Ponzo Illusion

Which cylinder in the “hallway” is perceived as larger?

Optical Illusions of Distance: Ponzo Illusion If object is perceived as further away, but it has same retinal size, it must be larger

Perceptual Illusions Which monster appears larger? The Ponzo Illusion

Perceptual Illusions The Ponzo Illusion

Ames Room: “throwing off” monocular depth cues

Ames Room The Ames room is designed so that the monocular depth cues give the illusion that the two people are equally far away

All roads lead back to Emmert's Law: size-distance scaling equation - Our perception of size equals the size of the Retinal image times the perceived distance away SpSp (R(RD p = ) X S = perceived size R = size on the retina D = perceived distance

The Ames Room Our perception of size equals the size of the Retinal image times the perceived distance away

Ames Room: “throwing off” monocular depth cues

Moon Illusion

Apparent-Distance Theory: Horizon v. Sky and the “flattened heavens” theory (Kaufman & Rock, 1962)

All roads lead back to Emmert's Law: size-distance scaling equation - Our perception of size equals the size of the Retinal image times the perceived distance away SpSp (R(RD p = ) X S = perceived size R = size on the retina D = perceived distance