Unit 2 Exam PSYC Sensation and Perception

Visible spectrum

Electromagnetic energy between 400 and 700 nm

Monochromatic light

Light containing only a single wavelength

Spectral colors

Colors associated with one monochromatic wavelength

Heterochromatic light

Light containing many different wavelengths

What does a prism do?

Disperses white light into its component colors

Spectral reflectance

The ratio of light reflected by an object at each wavelength (what we 'see')

Does the visual system respond to the total amount of light reflected or the percentage reflected?

The percentage reflected

Why do surfaces keep their color when everything gets dimmer?

The RATIOS of reflected light between surfaces stay constant (part of color constancy)

Fluorescent vs. incandescent light

Fluorescent peaks strongly in blue and green; incandescent rises gradually and peaks in long (red) wavelengths

Is color an inherent property of an object?

No; it's the brain's interpretation of the wavelengths reaching the retina

Hue

The color quality of light (red, green, blue): 'what kind?' A quality, not an amount

Saturation

The purity of light: 'how much hue?' An amount, not a quality; higher saturation = less white light mixed in

Brightness

The perceived amount or intensity of light present

Lightness

The psychological experience of light reflected by a surface

Achromatic surface

Reflects all wavelengths equally; no color (no dominant hue or saturation)

Reflectance of an achromatic surface seen as WHITE

90% or higher

Reflectance of an achromatic surface seen as GRAY

About 50%

Reflectance of an achromatic surface seen as BLACK

10% or lower

Color circle

Perceptual map: hue on the perimeter, saturation increases outward, neutral gray at the center

Order of colors from shortest to longest wavelength

Blue - Green - Red

Which wavelengths are most associated with 'red'?

Long wavelengths

Cone names used instead of blue, green, red

S (short), M (medium), L (long)

How is an object's color determined by cones?

The joint pattern of response across S, M, and L cones to the object's reflected wavelengths

Additive color mixing

Creating colors by ADDING wavelengths; R + G + B = white (e.g., laser light show)

Subtractive color mixing

Creating colors by REMOVING (absorbing) wavelengths; mixing paints, dyes, pigments; C + Y + M = near black

Mixing paints together is an example of…

Subtractive color mixing

Why can different combinations of wavelengths look like the same color?

Different ratios of wavelengths can produce colors that look the same, because color collapses to just 3 cone values

Principle of univariance

Many combinations of wavelength and intensity elicit the same response; one receptor can only signal the amount of light absorbed

Why does univariance require at least two cone types for color vision?

A single receptor loses wavelength information, so color needs a comparison between receptor types

What does univariance explain about scotopic (night) vision?

Why it lacks color: only one receptor type (rods) is active

Trichromatic theory (Young-Helmholtz)

Color is determined by the RATIO of activation of three cone systems

What does trichromatic theory explain?

Processing at the photoreceptor level and color blindness as missing cone types

Opponent-process theory (Hering)

Color arises from three opponent mechanisms: red-green, blue-yellow, black-white

What does opponent-process theory explain?

Why we don't see 'reddish greens' or 'bluish yellows,' why unique hues feel basic, and afterimages

Afterimages

Images seen after a visual stimulus is removed, appearing in the complementary color (opposite side of the color wheel)

How do trichromatic and opponent-process theories fit together?

Stage 1: cones are trichromatic. Stage 2: ganglion cells and LGN recombine signals into opponent channels

Red-green opponent channel: computation and pathway

L - M; parvocellular

Blue-yellow opponent channel: computation and pathway

S - (L + M); koniocellular

Light-dark opponent channel: computation and pathway

L + M; magnocellular

Cone-opponent cells (LGN)

Excited by input from one cone type in the center and inhibited by another cone type in the surround

Where are cone-opponent cells found?

LGN

Color-opponent cells (V1)

Excited by one color in the center and inhibited by its opponent color in the surround

Where are color-opponent cells found?

V1

Double-opponent cells

Cells that enhance color divisions at edges

Color vision in newborns

Limited chromatic vision that improves rapidly (nature + nurture)

Color vision and aging

Lens 'yellows,' reducing transmission of short wavelengths (less blue sensitivity); dark adaptation takes longer

Are most people with color deficiencies 'color-blind'?

No; a range of variation is common

Color deficiencies are often linked to…

The X chromosome (genetic)

Dichromacy

Two working cone systems; see color but can't make some discriminations trichromats can

Most common color deficiency

Red/green

Monochromacy

Two or all three cone types absent or nonfunctional; often with other problems like brightness sensitivity and reduced acuity

Color constancy

Inferring stable surface colors from a noisy, changing light signal (a red apple looks red in sunlight or a dim room)

How does the brain achieve color constancy?

Discounting the illuminant, ratios of reflected light, context and surrounds, top-down knowledge, chromatic adaptation in the retina

The depth problem

Everything along a cone of sight projects the same-size image on the flat retina, so the brain must infer size and distance

Cue approach

Depth is inferred by combining cues: aiding factors in a nonconscious, automatic decision

Three main categories of depth cues

Monocular, oculomotor, and binocular

Monocular cues

Depth cues needing only one eye; include pictorial and motion-based cues

Pictorial cues

Monocular depth cues from static (nonmoving) images

List the pictorial depth cues

Occlusion, relative height, relative size, familiar size, linear perspective, texture gradient, atmospheric perspective, shadowing

Occlusion (interposition)

A nearer object partially blocks a farther one

Linear perspective

Parallel lines (like railroad tracks) appear to converge toward the horizon

Motion-based monocular cues

Motion parallax, accretion, deletion, optic flow

Motion parallax

Objects at different depths move at different speeds relative to each other (e.g., looking out a side car window)

Accretion

Gradual REAPPEARANCE of a moving object after being blocked

Deletion

Gradual OCCLUSION of an object behind another as it (or the observer) moves

Optic flow

Pattern of image motion as we move forward or backward (e.g., through a car windshield)

Oculomotor cues

Depth cues from eye-muscle feedback to the brain as the eyes adjust near to far; contraction relates to distance; automatic

Accommodation

Oculomotor cue: muscles change the lens shape to focus; not precise (narrow pupil by day, poor acuity at night)

Vergence

Oculomotor cue: eyes rotate inward (converge) for near objects, outward (diverge) for far

How far is vergence a precise depth cue?

Up to about 2 meters; no vergence after 4 meters

Binocular disparity

Depth perception from the slightly different left- and right-eye views

Stereopsis

Sense of depth from the visual system comparing the two eyes' disparate images

Where is binocular vision possible?

Only where the visual fields of both eyes overlap

How do 3D movies with anaglyph glasses work?

Each lens filters out one of two overlapping images, so each eye gets a slightly different image (stereoscopic vision)

Corresponding points

A point on the left retina and a point on the right retina that match in location

Noncorresponding points

Points on the two retinae in slightly different locations

Diplopia

Double vision when the mismatch between the eyes' images is too large to fuse

Horopter

Curved region in space where objects fall on corresponding retinal points and appear single (fused)

Disparity on the horopter

Zero disparity

Crossed vs. uncrossed disparity

Crossed (+) = in front of the horopter; uncrossed (-) = behind it; both are noncorresponding points

A car image is 1 mm left of the fovea in the left eye and 2 mm left in the right eye. What kind of points?

Noncorresponding points

Correspondence problem (depth)

How the visual system knows which image in the left eye matches which in the right eye

Stereogram

One image to the left eye and one to the right, similar enough to combine into an illusion of depth

Random dot stereogram

Research tool with no recognizable object until the images are fused; shows depth processing precedes object recognition

Interocular velocity differences

An approaching object moves different amounts on each retina depending on its direction

Where are binocular disparity-sensitive cells found?

V1 (striate) and V4 (extrastriate)

Disparity-tuned neurons

Binocular cells with a receptive field for each eye that fire for noncorresponding points; tuned to crossed or uncrossed disparity

Size-distance invariance

The relationship between perceived size and perceived distance

Visual angle

The angle an object subtends at the eye; farther = smaller angle and retinal image, closer = larger

Size constancy

Perceiving an object as a fixed size despite changes in visual angle with distance

Muller-Lyer illusion

Lines look different lengths because angled fins signal an outside corner (closer) vs. inside corner (farther); misapplied size constancy

Ames room

Trapezoidal walls, floor, and ceiling make two people in opposite corners look drastically different in size

Motion

A relative change in position over time

Three dimensions of motion

Ahead, side, above/below

Minimum motion for detection (absolute threshold)

At least 1 minute of 1 degree across the retina

Are motion thresholds fixed?

No; they're context-dependent, influenced by brightness and size, and differ between periphery and fovea

Real motion

Continual change in an object's position relative to some frame of reference

Apparent motion

The appearance of motion with no corresponding real motion; a mismatch between senses and reality (illusion)

Beta motion

Sequential images perceived as realistic movement; basis of movies, flip books, scrolling LED displays

Phi motion

You perceive motion but can see the elements don't actually move; e.g., flashing perimeter lights