1. PSYC 330: Perception Seeing in Color PSYC 330: Perception

2. Color
Some more terminology….. scotopic – low light levels, appropriate for rods photopic – higher light levels, appropriate for cones

3. The Stimulus Color dimensions HUE wavelength
BRIGHTNESS intensity/luminance
SATURATION mix of wavelengths for intensity

4. Cones
Why three kinds?
The problem of univariance
A single light gives us two dimensions (wavelength and brightness)
A single photoreceptor can vary in only one way (firing rate)
SO…. Different combinations of wavelength and intensities can yield identical firing rates in an individual cell…. therefore we can’t properly interpret color

5. The solution?
Trichromacy….. Three different cones for the detection of color
Photopigments maximally responsive to given wavelength

6. BUT we are usually seeing mixtures of light Metamers

7.
WTH????

8. Opponent Process Theory
How the heck can trichromacy explain that?
Cone opponent cells
Retina
L-M and [L+M]-S M-L and [M+L]-S
Center/surround
LGN

9. Color opponent cells and double opponents…. cortex

10. But that’s NOT all, folks!
Culture effects – from the Dani to the Russians…. and beyond?
Color is a psychological experience!

11. What is the problem?
Contrast and assimilation effects
(effects of nearby colors)
Related and unrelated colors (e.g. gray and brown only experienced when compared to lighter regions)
Variations in the illuminant
Cone output NOT equal, yet we still see color appropriately (usually)

12. Color constancy

13. How do we do it? Look for and learn predictable features of the world
e.g. sharp divisions (edges) between colors are usually due to surfaces, not illuminants
e.g. shades of a given color, on the other hand, are an exception (shadows)

14. Bloj et al (1999) Took card folded in half (red and white halves)
When folded to inward (like a greeting card), red reflects onto white and makes it pink. But observer subtracts that out and sees it as white.
When folded outward (like a roof) pink half looks pink to observer
Shows role of INFERENCES
In other words, we’re not done with vision yet….

15. Tracing the signal Retina  ganglion cells (M and P)
M – large receptive field, fast responding, sensitive at low illumination P – small receptive field, slow responding, require more illumination
LGN  magno and parvocellular
Magno – peripheral vision, motion, broad form
Parvo – foveal vision, color, detail

17. LGN  Striate cortex (PVC)

18. Columns

19. What can we actually “see” at this point?
Topographical mapping?
LGN and PVC
Simple, complex, hypercomplex (end-stopping)
Separate channels for color, motion, location

20. Where?
What?

21. A Quick Review
Psychophysics - Methodology: what are the methods for collecting data?
Results: what are the findings?
Thresholds (definition, kinds)
Psychophysical laws (Weber, Fechner, Stevens)
What do these tell us?
Signal detection theory (d’, B, ROC curves)

22. A Quick Review Philosophy of perception
Sense data theories and naïve realism
Adverbial theories
Intentionalist theories (representation)
Alt theories

23. A Quick Review Physiology Doctrine of Specific Nerve Energies
Sense receptors and transduction
Neuron (parts)
Resting and action potential
Synapse
Sensory cortex (lobes)

24. A Quick Review Physiology of vision Properties of light
Structures in the eye
How the structures focus light
Rods and cones
How they transduce light
Distribution on the retina
Functions/specializations
Other layers of cells and their functions

25. A Quick Review Brain or bust Visual fields and where the info goes
Optic chiasm
LGN
PVC
Visual fields and where the info goes
Magnocellular and parvocellular
Visual stimuli (spatial frequency) more to come……