PSYC 330: Perception Seeing in Color PSYC 330: Perception

Color Some more terminology….. scotopic – low light levels, appropriate for rods photopic – 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 wavelength http://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????

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

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 constancy http://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 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

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 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……