Ch 10: Perceiving Depth and Size

oculomotor cues

physical feedback

non-visual depth cue (a feeling you get in your eyes)

monocular cues

gathered from one eye (one image)

used to perceive depth from a “flat” image

makes vision system robust (not entirely impaired if one eye is closed, covered, damaged)

binocular cues

gathered from both eyes (two images)

gives more information about depth (different info from each eye)

monocular depth cues

occlusion

relative height

familiar and relative size

perspective convergence

atmospheric perspective

texture gradient

shadows

occlusion

the hidden or partially hidden object is seen as being farther away (doesn’t tell use how far away, just the order)

relative height

objects higher in the visual field as farther away

below the horizon (on the ground), objects higher in the field of vision are more distant; above the sky (in the horizon), objects lower in the visual field are more distant

familiar size

judgement of distance is based on prior knowledge of the sizes of objects

relative size

when two objects are known or assumed to be equal size, the one farther away will appear smaller

perspective convergence

parallel lines in the distance converge as the distance increases

atmospheric perspective

further away the object is, then more air/dust/particles to look through

less shaper

slight blue tint

texture gradient

increased density of elements as distance increases

binocular depth perception

two images, one for each eye (each eye gets a mostly overlapping but ever-so-slightly different view of what’s in front of them)

stereoscopic vision

brain uses the differences to figure out distance

binocular/retinal disparity

the visual system sees every object from two distinct vantage points—the distance between retinal images of objects

but if an object is closer to your face, then your brain can’t reconcile the two images (finger sausage trick)

depth perception illusion

optical illusions arise when normal perceptual processes incorrectly represent distance between viewer and stimuli

how do 3D movies work?

two movies shown at the same time though two projectors; 3D glasses separate the two overlapping images (each eye receives one of the images)

binocular neurons

neurons have been found in the V1 (as well as along the ventral and dorsal pathways) that respond best to binocular disparity

• cells respond best to a specific degree of disparity between the images on the right and left retinas

selective rearing

cats from birth to six months

vision alternated between eyes every other day

• the cats had few binocular neurons

• the cats were unable to use binocular disparity to perceive death

perceiving size

perceiving distance and perceiving size are interrelated

we can misperceive size when accurate depth information is not present (ex: helicopter pilot flying in whiteout conditions in Antarctica)

classic experiment by Holway & Boring (1941)

Holway & Boring (1941)

observer at the hallway intersection: a luminous test circle: right hallway, 10 to 120 feet away; a luminous comparison circle: left hallway, always 10 feet away

the observer had to adjust the diameter of the comparison circle to match the perception of the test circle

part 1 of the experiment provided observers with depth cues (good depth cues → judgments of size matched physical size); part 2 of the experiment removed depth cues

visual angle depends on…

size of stimulus

distance from observer

size constancy

our perception of an object’s size is relatively constant even when we view the object from different distances

• object far away = small retinal change

• object near = larger retinal image

relative size

we can use the size of familiar objects to judge the size of other objects

illusions of depth and size

muller-lyer illusion

ponzo illusion

the ames room illusion

the moon illusion

Muller-Lyer Illusion

conflicting cues theory—perception of the line depends on the actual length of the lines and the overall length of the figure

The Ponzo (Railroad Track) Illusion

top object perceived as longer/larger, but it’s really the same size

The Ames Room

people are equal size but appear very different; the room is constructed so that the shape looks like a normal room when viewed from a particular observation point and one corner is twice as far from the observer as the other corner

The Moon Illusion

the moons size and distance from earth is same for horizon and elevated moon