Perceiving Visual Space & Visual Illusions

Cue Approach: focuses on identifying info in the retinal image that is correlated with depth in the enviornment
Ecological Approach: focuses not on retinal image but on what info exists directly in the environment
(associated with 'direct perception')

OCULOMOTOR CUES - sensing the position and muscle tension of our eyes
Convergence: eyes move inward to focus near objects
Accomodation: change in shape of the lens to focus near objects
These cues operate at distances less than 5-10 feet

PICTORIAL CUES (MONOCULAR DEPTH CUES)
1) Occlusion: one object occludes another
2) Relative Height: objects that are higher in field of view appear to be farther away
3) Relative Size: larger objects appear closer
4) Familiar Size: knowledge of object's normal size influences our perception of its depth
5) Atmospheric Perspective: distant objects appear less sharp, hazy
6) Linear Perspective: lines that are parallel in a scene converge as they get farther away
7) Texture gradient: elements that are equally spaced in a scene appear to be packed more closely as the distance increases

MOVEMENT-PRODUCED CUES
Motion Parallax: the difference in speed of movement between near and far objects; near objects go rapidly past, distant objects more slowly
e.g. looking out the window of a moving train

Deletion and Accretion: if two surfaces are at diff depths, any movement not perpendicular to the surfaces causes them to move relative to one another

BINOCULAR DEPTH CUES (Disparity and Stereopsis)
Stereopsis: impression of depth that results from two different images on the retina
Corresponding retinal points: places on the retina that connect to the same places in visual cortex
Recall that input from both eyes reaches V1, brain makes "comparison"
Horopter: imaginary circle that passes through the point of fixation
Noncorresponding points:
- object is either nearer than the horopter; that is Crossed Disparity / (image moves out to the side of the retina)
OR
- farther than the horopter; that is Uncrossed Disparity / (image moves inward on retina)
this provides disparity info for brain

<<< FIGURE: The HOROPTER >>>

Does disparity account for ALL perception of depth?

Random dot stereograms (Julesz 1971) - generate two identical displays of random dot patterns, then shift a subset of them on one of them
Proved disparity is enough for depth perception
Autostereograms do the same thing but use one image

PHYSIOLOGY of BINOCULAR VISION
Barlow, Blakemore, & Pettigrew (1967) found Binocular Depth Cells (AKA disparity detectors): cells in visual ctx that respond best to points with a specific angle of disparity on the retina
- These cells require coordinated visual input from both eyes during the sensitive period for the particular organism
Blake & Hirsch (1975) - raised cats to 6mos with only monocular vision (alternating right and left every other day) - reduces or eliminates binocular neurons in cortex
Behavioral testing - cats couldn't use binocular disparity to see depth
Good evidence that these neurons are responsible for depth perception
LeVay & Voigt (1988) - found V1 and V2 neurons responding to zero disparity, and crossed and uncrossed disparity
Others have found them in parietal ('Where') and temporal ('What') areas
Stimulus deprivation amblyopia: poor vision unrelated to physical eye structure, caused by closure of eye during sensitive period for human binocular development (1-2 yrs old)
- Tilt aftereffect transfer indicates functioning binocular cells, which respond to stimulation from both eyes; adapting pattern fatigues neurons, so they respond less to the test pattern

SIZE, ILLUSIONS, and ECOLOGICAL ASPECTS of PERCEPTION
Visual Angle - e.g. sun and moon have same visual angel; sun is much larger, but is farther away
Law of Size Constancy: we correctly perceive an object's physical size regardless of its distance or its image size on the retina
Size-Distance Scaling: links depth with size information; info from retina regarding an object's visual angle & distance of object are combined;
S = K (R * D)
Subject's perceived Size = Konstant (Retinal image size * Perceived Distance)
Emmert's Law: the farther away an afterimage appears, the larger it will seem
- retinal size is identical, perceived size is not

Size Illusions
Ames room: causes you to think you are seeing two people at same distance, which makes one with smaller visual angle appear to be shorter
If D is same for both Judy and Tom, but R is smaller for Judy, her S is smaller
Moon illusion
2 theories
1) apparent-distance theory: since the horizon moon and elevated moon have the same visual angle, the farther-appearing horizon moon (because it is amongst depth cues like trees) appears larger
2) angular size-contrast theory: horizon moon appears larger when surrounded by objects with small visual angles
(e.g., distant buildings trees)
Muller-Lyer and Ponzo illusions

Light and Color Constancy
Lightness constancy
ratio principle: 2 areas that reflect diff amounts of light will look the same if the ratios of their surround intensities are kept contstant
Color constancy
chromatic adaptation: prolonged exposure to one w can decrease sensitivity to that w
Zeki (1983, 1984) - green patch (med w ) from Mondrian display put into RF of an G+R- striate neuron and a V4 neuron
if illuminated w/ white light = both neurons fired
if " " long w light = G+R- was inhibited
V4 still fired
V4 could be a site for color constancy

Ecological approach of Gibson (1950-79) - WWII pilots and landing planes
Invarient Information: remains constant even when observer changes position or moves thru enviornment
Texture Gradient: equally spaced elements appear to be packed more densely as their distance increases (mentioned above)
Flow Patterns: the motion of objects in the enviornment as we move thru it
Horizon Ratio Principle: amount of an object that extends above the horizon divided by the amount it extends below it
If two objects in contact with the ground are the same size, their horizon ratios will be the same