PSYC 330: Perception

SEEING IN COLOR

PSYC 330: Perception

SeeinginColor

Color

Some more terminology…..scotopic – low light levels, appropriate for

rodsphotopic – higher light levels, appropriate

for cones

The Stimulus• Color dimensions– HUE wavelength

– BRIGHTNESS intensity/luminance– SATURATION mix of wavelengths for

intensity

Cones

• Why three kinds?– The problem of univariance

http://sites.sinauer.com/wolfe3e/chap5/univarianceF.htm

• 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

• The solution?– Trichromacy….. Three different cones for the detection of

color– Photopigments maximally responsive to given wavelengthhttp://sites.sinauer.com/wolfe3e/chap5/trichromF.htm

BUT we are usually seeing mixtures of light

Metamers

http://www.youtube.com/watch?v=w6ccBwnc5KU&feature=player_embedded

WTH????

• How the heck can trichromacy explain that?• Cone opponent cells– Retina

L-M and [L+M]-SM-L and [M+L]-SCenter/surround

– LGN

Opponent Process Theory

Color opponent cells and double opponents…. cortex

But that’s NOT all, folks!Culture effects – from the Dani to the Russians…. and beyond?

Color is a psychological experience!

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)

Color constancyhttp://sites.sinauer.com/wolfe3e/chap5/illusionsF.htm

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)

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….

Tracing the signal

Retina ganglion cells (M and P)M – large receptive field, fast responding,

sensitive at low illuminationP – small receptive field, slow responding,

require more illuminationLGN magno and parvocellular

Magno – peripheral vision, motion, broad form

Parvo – foveal vision, color, detail

LGN Striate cortex (PVC)

Columns

What can we actually “see” at this point?

• Topographical mapping?– LGN and PVC

• Simple, complex, hypercomplex (end-stopping)

• Separate channels for color, motion, location

Where?

What?

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)

A Quick Review

• Philosophy of perception– Sense data theories and naïve realism– Adverbial theories– Intentionalist theories (representation)– Alt theories

A Quick Review

• Physiology– Doctrine of Specific Nerve Energies– Sense receptors and transduction– Neuron (parts) – Resting and action potential– Synapse– Sensory cortex (lobes)

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

A Quick Review

• Brain or bust– Optic chiasm– LGN– PVC

• Visual fields and where the info goes• Magnocellular and parvocellular

• Visual stimuli (spatial frequency) more to come……