CHAPTER 10- TEXTBOOK: Perceiving Depth & Size Depth
I. Cue approach to depth perception
Identifying information to the retinal image that is correlated with depth in the scene
II. Oculomotor: cues based on our ability to sense the position of our eyes and the tension in our eye muscles (incl. Convergence, accommodation)
- A. Convergence: inward movement of eyes when we look at nearby objects
- B. Accomodation: change in shape of lens when we focus on objects at various distances
III. Monocular: cues that work with one eye (incl. accommodation, pictorial, movement)
- A. Pictoral Cues: sources of depth information in a 2D picture.
- 1. Occlusion: when one object partially covers another object, the object that is covered must be farther.
- 2. Relative Height: objects with bases closer to the horizon are usually farther.
- 3. Relative Size: if 2 objects are of equal size, then the one that is farther will take up less of your field of view
- 4. Perspective convergence: things converge in the distance
- 5. Familiar Size: judge distance based on prior knowledge of the sizes of the objects.
- 6. Atmospheric Perspective: distant objects appear less sharp than nearer objects.
- 7. Texture Gradient: elements that are equally spaced in a scene appear to be more closely packed as distance increases
- 8. Shadows: indicate location of a 3D object.
- B. Movement Based Cues: sources of depth information created by movement.
- 1. Motion Parallax: when we move, objects nearby move QUICKLY past us, and further objects appear to move slower.
- 2. Deletion and Accretion: as observer moves sideways, some things become covered, and others become uncovered.
IV. Binocular Cues: cues that depend on both eyes
- A. Stereoscopic vision: two-eyed depth perception; has mechanisms that take into account differences in images formed on the left and right eyes
- B. Strabismus: misalignment of the eyes
- C. Binocular disparity: differences in the images on the left and right retinas
- 1. Stereopsis: the impression of depth that results from information provided by binocular disparity
- 2. Stereoscope: a device by Charles Wheatstone which produces a convincing illusion of depth by using two slightly different pictures
- 3. Passive method: two super-imposed polarized images viewed through polarizing glasses
- 4. Active method: alternates the left-eye and right-eye images on the screen 30 or more times a second
- 5. Lenticular projection: the screen is coated with a film that contains two sets of lenses that direct different images to the left and right eyes
- 6. Random-dot stereogram: two dot patterns presented to left eye and right eye; observers can perceive depth in displays that contain no depth information other than disparity
- D. Corresponding retinal points: points on the retina that overlap if the eyes are superimposed on each other
- E. Correspondence problem: How does the visual system match the parts of the image in the left and right eyes that correspond to one another?
- F. Binocular depth/disparity-selective cells: cell responds best when stimuli presented to the left and right eyes create a specific amount of absolute disparity
- G. Disparity tuning curve: disparity vs. firing rate; indicates the neural response that occurs when stimuli presented to the left and right eyes create different amounts of disparity
- H. Frontal eyes: have overlapping fields of view, can use disparity to perceive depth
- I. Lateral eyes: no overlapping visual fields, cannot use disparity to perceive depth
- J. Binocularly fixate: requirement for binocular disparity; two eyes are both looking directly at the object and two foveas are directed to the same place; infants cannot
V. Horopter: surface where objects fall on corresponding points
- A. Noncorresponding points: images of objects that are not on the horopter
- B. Absolute disparity: degree to which these objects deviate from falling on corresponding points
- 1. Angle of disparity: amount of absolute disparity
- 2. Relative disparity: the difference in absolute disparities of objects in a scene
- 3. Geometry: looking at where objects’ images fall on the retina
I. Holway & Boring Experiment:
- A. Observers sitting in intersection of two hallways
- B. Saw luminous test circle down right hallway; different sizes and distances
- C. Saw luminous comparison circle down left hallway; always 10 feet away
- D. Task: adjust size of comparison circle to match perception of test circle
- E. Depth cues present: observers view large distant circle and make comparison circle large; view small nearby circle and make comparison circle small; judgments match physical sizes
- F. Depth cues absent: judgments of test circles become less accurate; when all depth information is eliminated, perception of size is determined not by actual size but by relative size of the circle’s image on retina
II. Visual Angle
- angle of an object relative to the observer's eye
- extend lines from object to the lens of the observer's eye
- THE ANGLE IN BETWEEN THAT ^ IS THE VISUAL ANGLE
- depends on both size and distance
III. Size Constancy: perception of an object’s size is relatively constant even when viewed from different distances
- A. Size-distance scaling: maintain size constancy by taking an object's perceived distance into account. An object's perceived size, S, i determined by multiplying the size of the retinal image, R, by the object's perceived distance, D. S=K(R x D)
- As a person walks away from you, the size of the person's image on your retina gets smaller, but perception of the person's distance gets larger, so the two balance each other out and size stays the same.
- B. Emmert's Law: the farther away an afterimage appears, the larger it seems.
- Relationship between the apparent distance of afterimage and its perceived size.
CHAPTER 10- LECTURE: Depth Seeing a 3D World
I. Oculomotor Cues
- A. How does your visual system know the level of accommodation and convergence?
- Theory 1: proprioceptive feedback from muscles
- Theory 2: Efference copy (command signals)
- B. ARE ONLY USED FOR NEARBY OBJECTS!!!
II. Visual/Monocular Cues
- Pictorial: occlusion, relative height, relative size, aerial/atmospheric perspective, texture (density, size foreshortening), shadows
- Movement: motion parallax, deletion, and accretion
III. Visual/Binocular Cues
- Binocular disparity, crossed and uncrossed disparity, dependence on depth and distance, horopter
- Fusion (gives rise to stereo vision), suppression, diplopia, binocular rivalry
extra ocular muscle control, eyes move closer together when you bring your thumb closer to your eye
intra ocular eye, changes lenses to keep object in focus.
How does your visual system know the level of accommodation and convergence?
- Theory 1: Proprioceptive feedback from muscles
- touch sensitivity to the inside muscles
- if u close ur eyes, u can roughly feel where your fingers are
- monitoring the tension in different muscles
- strain gauge that tells you how much in extra ocular muscle
- measures how much strain in accommodating muscles
- could tell you what your eyes are doing therefore telling you how the object is
- Theory 2: Efference copy (command signals)
- Brain tells eye to converge, means what you are looking at is really close to u
- The motor centers in the brain sent one signal to move the eyes and eye muscles and copy of that signal to perceptual system, can use that to obtain depth
- OCULOMOTOR CUES ARE ONLY USEFUL FOR NEAR OBJECTS
- convergence changes as object gets closer or further
Depth Cues: Monocular depth cues
- Pictoral cues: could judge w one eye, a picture can tell you something is closer than another thing
- 1. Occlusion: bushes occlude mountains, mountains are further away, in movement; circles occluding each other look like they are moving closer or getting bigger
- 2. Relative size: something further away would be shorter. Closer object is bigger