Ch 6: The Visual System pt 3

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 4thphotopigment • Allows them to detect UV light

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

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 ventrialprestriate cortex to inferotemporal cortex [secondary]

20. Dorsal & Ventral Streams • Dorsal • 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