Ch 6: The Visual System pt 3

Seeing ColorThe technical term for color

is hueThe color you perceive is

largely due to the wavelength of the light reflected from the object

Theories of Color VisionComponent theory (trichromatic theory)

3 different types of cones, each with a different spectral sensitivity

The color of an object is determined by the ratio of activity of those 3 types of receptors

Based on the observation that any color can be made by mixing together 3 different wavelengths of light in different amounts

Theories of Color VisionOpponent-process theory

There are 2 different classes of cells in the visual system for encoding color & another for encoding brightness

Each of these 3 encodes 2 complementary colors Complementary colors are pairs of colors that

produce white when combined in equal measure (and white represents presence of all colors in equal amounts)

Theories of Color VisionOpponent-process theory cont.

One class signals red by changing its activity in one direction & signals green (red’s complementary color) by changing its activity in the other direction

Another class signals blue & its complement yellow

3rd, brightness class, signals black & whiteBased on the observations that

complementary colors cannot exist together (ex: no such thing as bluish yellow) & that the afterimage of red is green and of yellow is blue

Afterimages

So which theory is right??There are 3 kinds of cones & each has

different photopigment & its own characteristic absorption rate (sensitive to different wavelength ranges)

At the level of the cones, coding color seems to operate on a purely component basis

At the subsequent levels of the visual processing system (retina-geniculate-striate system), cells respond according to the opponent-process

Animal visionMostly trichromats (3

photopigments)

Dichromats Difficulty seeing long

wavelengths (red end)

Some have a 4th photopigmentAllows them to detect UV light

Animal Vision

http://webecoist.momtastic.com/2009/01/14/animal-vision-color-detection-and-color-blindness/

Color ConstancyColor constancy: perceived color of an

objects is not purely a function of the wavelengths reflected by it

The wavelengths of ambient light can alter the way a color looks

Allows us to recognize and remember objects even when we see them at different times/locations

Normally goes undetected by usA color will always seem its same color as

long as the ambient light has some short, medium & long wavelengths; and that the object is viewed in a scene

Color Constancy

Retinex TheoryStates that the color of an object is determined by its

reflectance (the proportion of light of different wavelengths that a surface reflects)Visual system calculates reflectance & perceives color

by comparing the light reflected by adjacent surfaces in at least 3 different wavelength bands (short, med, long)

Suggests that there is one type of cortical neuron involved in color visionNeurons that should be responsive to color contrast

These type of neurons do existEx: dual-opponent color cells in the monkey cortex

turn on

Cortical Mechanisms of VisionThe primary visual cortex is not the only area

involved in human vision (retinal-geniculate-striate pathway)

All of the occipital cortex and large parts of the temporal & parietal lobes are involved

Secondary visual cortexReceives input from primary visual cortex

Visual association cortexGets input from secondary visual cortex &

other sensory systems

Location of Visual SystemPrimary

Posterior area of occipital lobeSecondary

Prestriate cortex Band of tissue that surrounds

primary visual cortexInferotemporal cortex

AssociationPosterior parietal cortex

*Flow of information goes along this direction*

Damage to the Visual SystemScotoma: an area of blindness caused by

damage to an area of the primary visual systemBlind area corresponds to area of the

contralateral visual field of both eyesCan be tested with a perimetry test

Used to map the visual field of each eyeMany people with scotomas don’t realize they

have deficient visionHemianopsic: A patient with a scotoma

covering half of the visual field

Damage to the Visual SystemJust because you can see something doesn’t

necessarily mean you are consciously aware of seeing it

Blindsight:Ability of patients with scotomas to respond to visual

stimuli in their scotoma even though they are not aware of the stimuli

When damage is done to the primary visual cortex, movement/motion is the characteristic of visual stimuli likely to still be perceivedEx: a patient may grab an object that moves through

his scotoma, despite reporting they saw nothing

2ndary & Association CortexDifferent areas of the visual cortex are

involved in different functions of visionEx: Some areas associated with visual

properties of color, movement, shape, etc.But keep in mind that brain areas and

functions are all very interconnected & can potentially “pick up the slack” for one area if it is damaged

Dorsal & Ventral Streams2 main pathways that conduct neural signals

about vision from the primary to secondary/association cortices

DorsalFrom the primary to dorsal prestriate cortex to

posterior parietal cortex [secondary to association]

VentralFrom primary to ventrial prestriate cortex to

inferotemporal cortex [secondary]

Dorsal & Ventral StreamsDorsal

Neurons here respond strongly to spatial stimuli, location of objects & direction of movement

VentralResponds to characteristics of objects (color, shape,

etc.)There are clusters of neurons that respond to specific

classes of objects; ex: faces, bodies, letters, animals, tools

“Where” vs “What” TheoryDorsal & ventral streams have different visual

functions

Dorsal & Ventral Streams

Dorsal & Ventral Streams“Where” vs “What” Theory

Dorsal & ventral streams have different visual functions

“Control of Behavior” vs “Conscious Perception” TheoryDifference between the streams is not the kind

of info they carry, but what they do with itSuggested that this theory encompasses the

above theory