CHAPTER 10

1. CHAPTER 10 Vision and visual perception Color Vision

2. Review: eye to Brain pathway • Rods or cones bipolar cells  ganglion cells • Ganglion cell  optic nerve; • Optic nerves  optic chiasm • Optic chiasm  optic tracts (L and R) • Optic tracts  lateral geniculate nuclei of thalamus • Optic radiations leave LGN  occipital cortex • Several areas of occipital cortex • Areas 17, 18 and 19

4. Color vision • Brain works as an on/off system • Some neurons are always on; turn off with stimulation • Others are always off; turn on with stimulation • This “flipping of switches” allows a pattern of signals to be created • But: need specialized receptors to detect particular kinds of stimuli • Two theories attempt to explain how we see color: • Trichromatic theory: based on three primary colors • Opponent Process theory: based on this on/off system • Which is correct?

5. Color Vision: Trichromatic theory • Young (1880s) and von Hemmholtz (1950s) • The trichromatic theory states that just three color processes account for all the colors we are able to distinguish: • Red • Green • Blue • considered the primary colors because observers cannot resolve these colors into separate components.

6. Color Vision: Opponent Process theory • Ewald Herring (about 1850) and Hurvich and Jameson (1957) • Opponent process theory attempts to explain color vision in terms of opposing neural processes. • The photopigment in the red/green receptor • broken down by red light • regenerates in the presence of green light. • The chemical in the yellow/bluereceptor • broken down in the presence of yellow light • regenerates in the presence of blue light. • This arrangement was proposed to explain the phenomenon of complementary colors, • colors that cancel each other out to produce a neutral gray or white.

8. Color Vision: Which Theory is correct? • After images: • Stare at a red stimulus for a minute, and you will begin to see a green edge around it • Stare at yellow; will see blue • black goes to white • And vice versa. • Afterimage illusion: • Look at white wall/sheet of paper after staring at image • you will see reverse version of the original object. • This experience is called a negative color aftereffect. • This is what one would expect if the wavelengths were affecting the same receptor in opposed directions.

11. Color Vision: both theories are correct • Young and von Hemmholtz: • Proposed three primary types of cones • Red, blue, green • Anatomical evidence supports this • Hurvich and Jameson (1957) proposed that there are three types of color receptors in brain • red-sensitive • green-sensitive • blue-sensitive • interconnected in an opponent-process fashion at the ganglion cells.

12. Color Vision • Long-wavelength light • excites “red” cones • and the red-green ganglion cell, • Yields sensation of red. • Medium-wavelength light • Excites “green” cones • inhibits the red-green cell, reducing its firing rate below its spontaneous level • Signals “green” to the brain. • Short-wavelength light • excites “blue” cones • inhibits the yellow-blue ganglion cell • Result: sensation of blue.

13. Color Vision: Explaining yellow • What about other colors? • Light midway between the sensitivities of the “red” and “green” cones would stimulate both cone types equally. • The firing rate in the red-green ganglion cell would not change, because equal stimulation and excitation from the two cones would cancel out. • Note: Cones’ connections to the yellow-blue ganglion cell are both excitatory, • combined excitation would produce a sensation of yellow • Explains how we see “yellow” if not a primary color!

15. Color Vision We see entire color range by combinations of firing rates in opponent system The system must compare activity in all three types of cones to determine which wavelengths of light you are seeing. The “comparison” is an automatic neural process; it does not occur at the level of awareness. We LEARN the names of colors; not the color itself

17. hereditary color deficiency • Red or green cone peak sensitivity is shifted OR Red or green cones absent. • Several different kinds: • Deuteranomaly: green shifted toward red; 5% of males • DeutanDichromat: no green cones; only red and blue; 1% of males • Protanomalous; red shifted toward green; 1% of males • ProtanDichromat: no red cones; only green and blue; 1% of males • Thanks to Jeff Rabin, OD, PhD, Chief, Visual Function Laboratory, Ophthalmology Branch USAF School of Aerospace Medicine

18. Why do colors that look different to us appear the same to color deficient individuals? The two spots appear different in color because differences in R-G wavelength stimulation is larger for or one, and small for the other. Bigger separation = more difference If missing a cone or unable to detect differences, makes smaller difference thresholds Thus, perceive red-green as similar

19. The two spots appear different in color because R-G is large for one, and small for the other. Small difference in stimulation Large difference in stimulation of green and red cones G Consider a green vs. yellow light… B R Color Normal Individual

20. Small difference in stimulation G Small difference in stimulation  Look the same! Each spot produces the same R-G stimulation and thus looks the same! B R Deuteranomaly (the green sensitivity curve is shifted toward the red)

21. Some Views With and Without Color Vision Link Jay and Maureen Neitz Color Vision Page

22. Drs. Jay and Maureen NeitzDepartment of Cell Biology, Neurobiology & AnatomyDepartment of OphthalmologyMedical College of Wisconsin

23. Color Labeling • Color deficients rely heavily on context and learning—apple is “red” because patient learns to call it red —same hue may appear gray when presented without other cues. • For wavelengths beyond 545, relative brightness, context, and learning play a significant role verbal label and response.

24. Similar process for line orientation • 8-10% of males and 1/200 females (0.5%) are born with red or green color deficiency. • Sex-linked recessive condition (X chromosome). • Protanomaly—red cone peak shifted toward green (1%) • ProtanDichromat—red cones absent (1%) • Deuteranomaly—green cone peak shifted toward red (5%) • DeutanDichromat—green cones absent (1%) • Hereditary tritan defects are rare (0.008%)

26. Similar process for line orientation • We see entire range of line orientations by combinations of firing rates in opponent system • Horizontal to vertical line • Everything in between • On the next slide: see how neurons respond to different orientations • vertical hatchmarks represent neural responses • yellow line underneath indicates when the stimulus occurred • Response greatest when the line closest to the cell’s “preferred” orientation (vertical) • Response least when orientation most discrepant (horizontal). • In the last example: response diminished because stimulus failed to cover all of the cell’s field (indicated by the stimulus being off-center of the crosshair).

28. Processes for many different kinds of stimuli Lines Figure/ground Color Faces Textures Why might a variety of innate “feature detectors” be critical? Why don’t we have many feature detectors for specific shapes/stimuli?