1. Ch 6: The Visual System pt 3

3. Seeing Color The technical term for color is hue
The color you perceive is largely due to the wavelength of the light reflected from the object

4. Theories of Color Vision
Component 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

5. Theories of Color Vision
Opponent-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)

6. Theories of Color Vision
Opponent-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 & white
Based 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

7. Afterimages

8. 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

9. Animal vision Mostly trichromats (3 photopigments) Dichromats
Difficulty seeing long wavelengths (red end)
Some have a 4th photopigment
Allows them to detect UV light

10. Animal Vision

11. Color Constancy
Color 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 us
A 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

12. Color Constancy

13. Retinex Theory
States 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 vision
Neurons that should be responsive to color contrast
These type of neurons do exist
Ex: dual-opponent color cells in the monkey cortex turn on

14. Cortical Mechanisms of Vision
The 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 cortex
Receives input from primary visual cortex
Visual association cortex
Gets input from secondary visual cortex & other sensory systems

15. Location of Visual System
Primary
Posterior area of occipital lobe
Secondary
Prestriate cortex
Band of tissue that surrounds primary visual cortex
Inferotemporal cortex
Association
Posterior parietal cortex
*Flow of information goes along this direction*

16. Damage to the Visual System
Scotoma: an area of blindness caused by damage to an area of the primary visual system
Blind area corresponds to area of the contralateral visual field of both eyes
Can be tested with a perimetry test
Used to map the visual field of each eye
Many people with scotomas don’t realize they have deficient vision
Hemianopsic: A patient with a scotoma covering half of the visual field

17. Damage to the Visual System
Just 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 perceived
Ex: a patient may grab an object that moves through his scotoma, despite reporting they saw nothing

18. 2ndary & Association Cortex
Different areas of the visual cortex are involved in different functions of vision
Ex: 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

19. Dorsal & Ventral Streams
2 main pathways that conduct neural signals about vision from the primary to secondary/association cortices
Dorsal
From the primary to dorsal prestriate cortex to posterior parietal cortex [secondary to association]
Ventral
From primary to ventrial prestriate cortex to inferotemporal cortex [secondary]

20. Dorsal & Ventral Streams
Neurons here respond strongly to spatial stimuli, location of objects & direction of movement
Ventral
Responds 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” Theory
Dorsal & ventral streams have different visual functions

21. Dorsal & Ventral Streams

22. Dorsal & Ventral Streams
“Where” vs “What” Theory
Dorsal & ventral streams have different visual functions
“Control of Behavior” vs “Conscious Perception” Theory
Difference between the streams is not the kind of info they carry, but what they do with it
Suggested that this theory encompasses the above theory