Textbook Notes (363,236)
Canada (158,278)
Psychology (2,948)
PSY100H1 (1,804)
Chapter 5

Chapter 5 - PSYB51

7 Pages
Unlock Document

University of Toronto St. George
Mathias Niemier

Chapter 5 - The Perception of Colour Basic Principles of Color Perception  Color isn’t physical properly – related to physical property  Most of light we see is reflected light  More light absorbed, darker surface will appear  Problem of Univariance o Lights of 450 and 625 nm each elicit same response from photoreceptor  No way of distinguishing between 2 lights o When seeing color, output of single photoreceptor is ambiguous o Problem of variance: fact that infinite set of different wavelength-intensity combinations can elicit exactly same response from single type of photoreceptor. One photoreceptor type can’t make color discriminations based on wavelength Trichromacy Rods & Cones  Human retina contains 2 photoreceptors – rods and cones o Rods sensitive to low (scotopic) light levels  Scotopic: light intensities that are bright enough to stimulate rod receptors but too dim to stimulate cone receptors  All rods contain same type of photopigment molecule – rhodopsin – same sensitivity to wavelength  As consequence, though possible to tell light from dark under scotopic conditions, problem of univariance makes it impossible to discriminate colors – color is psychophysical o Cone photoreceptors sensitive to higher, daylight levels  Phototopic: light intensities that are bright enough to stimulate cone receptors and bright enough to “saturate” rod receptors  Come in 3 varieties, each containing slightly different photopigment  3 cone types named for where peak of sensitivity lies on spectrum  S-cone: 440 nm. Cone preferentially sensitive to short wavelengths; blue cone  M-cone: 535 nm. Preferentially sensitive to middle wavelengths; green cone  L-cones: 565 nm. Preferentially sensitive to long wavelengths; red cone  With 3 cones, can tell difference between lights of different wavelengths  Specific light produces twice as much M response as S response and twice as much S response as L response  Trichromacy/trichromatic theory of color vision: theory that color of any light is defined in visual system by relationships of 3 numbers, outputs of 3 receptor types now known to be 3 cones. Young-Helmholtz theory Metamers  Nervous system knows only what cones tell it  If mixture of “red” plus “green” light produces same cone output as single wavelength of “yellow” light, mixture and single wavelength must look identical  Metamers: different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of physical differences  2 warnings o 1. Mixing wavelengths doesn’t change physical wavelengths. Color mixture is mental event, not change in physics of light o 2. For mixture of “red” light and “green” light to look perfectly yellow, would have to have just right red and right green. Other mixes look bit reddish or greenish Lights, Filters and Finger Paints  Additive color mixture: mixture of lights. If light A and light B are both reflected from surface to eye, in perception of color effects of those 2 lights add together.  Subtractive color mixture: mixture of pigments. If pigments A and B mix, some of light shining on surface will be subtracted by A and some by B. only remainder contributes to perception of color Three Numbers, Many Colors  Color vision is based on output of 3 types of cone photoreceptors.  Rods make small, important contribution, but only in dim light  Because we have 3 types of cone photoreceptors, light reaching any part of retina will be translated to 3 responses, one for each local population of cones  Rest of nervous system can’t glean anything more about physical wavelengths of light – if light rays reflecting off 2 surfaces produce same set of cone responses, 2 surfaces must and will appear to be exactly same color – metamers  Color space: 3 dimensional space, established because color perception is based on outputs of 3 cone types, that describes set of all colors  Useful terms for describing perceived attributes of color are hue, saturation and brightness o Hue: chromatic (colorful) aspect of color (red, blue, green, etc.) o Saturation: chromatic strength of huge. White has zero saturation, pink is more saturated and red is fully saturated o Brightness: distance from black (0 brightness) in color space History of Trichromatic Theory  Theory that color vision can be explained by responses of 3 mechanisms with different sensitivities to wave lengths of light is known as “trichromatic theory of color vision”  Julie Schnaph managed to record activity of single photoreceptors  Jeremy Nathans found genes that code for different photopigments  In 1666, Newton understood that “rays to speak properly aren’t coloured. In them is nothing else than a certain Power and Disposition to stir up a sensation of this or that color” – knew it was mental event  3-D nature of experience of color was worked out in 19 century by Thomas Young and Hermann von Helmholtz o In honor, trichromatic theory is often “Young-Helmholtz theory”  James Clerk Maxwell developed color matching technique that was central to Helmholtz’s work on topic  Central observation from experiments was that only 3 mixing lights are needed to match any reference light o 2 primaries aren’t enough and 4 are more than are needed Opponent Processes Repackaging the Info  Difference between L and M responses contains considerable info about color  Other info comes from differences between L and Mn responses, and S-cone responses  Could create (L-S) and (M-S) sigals, but because L and M are similar, single comparison between S and (L+M) can capture almost same info that would be found in (L-S) and (M-S) singals  Combining L and M signals is good measure of intensity of light (S cones make small contribution to perception of brightness)  Might be wise to convert 3 cone signals into 3 new signals o (L – M), ([L + M] – S) and (L + M) Opponent Cells in Lateral Geniculate Nucleus  Earliest work on combination of cone signals was done with fish  Russell De Valois and others had begun to show that these sorts of signals actually exist in lateral geniculate nucleus (LGN) of macaque monkeys  Lateral geniculate nucleus (LGN): structure in thalamus, part of midbrain, receives input from retinal ganglion cells and has input and output connections to visual cortex  Some of retinal and LGN ganglion cells are excited by L cone onset in center and inhibited by M cone onsets in surround  Color opponent cell: neuron whose output is based on difference between sets of cones  There are also (M – L), ([M + L] – S(, and (S - [M + L]) cells – just sorts of cells we want to have to support 3 process opponent color system Pyschophysical Roots of Opponent Color Theory  Opponent color theory: theory that perception of color is based on output of 3 mechanisms, each based on opponency between 2 colors – red-green, blue-yellow, and black-white  Ewald Hering described curious feature of color vision – some combinations of colors are illegal o Can have bluish green, reddish yellow (“orange”), or bluish red (“purple”), but reddish green and bluish yellow have never been seen, because they don’t exist  Helmholtz was describing trichromatic theory with 3 basic colors (red, green and blue); Hering’s theory had 4 basic colors in 2 opponent pairs – red vs green, blue vs yellow o Black vs white component formed 3 opponent pair (“grey”)  Leo Hurvich and Dorothea Jameson revived Hering’s ideas and developed way to demonstrate opponency o Hue cancellation – start off with yellowish green color and cancel out yellow by adding opponent color – blue. Measure amount of blue light needed to remove all traces of yellow o Earlier – red + green = yellow but now making grey. Light from red phosphor on monitor, when added to light from green phosphor, will look yellow, but that’s because “Red” phosphor is actually orangish red o Hue cancellation used for lights across spectrum  Unique blue: blue that has no red or green tint  Unique hues: any f 4 colors that can be described with only single color term: red, yellow, green, blue. Other colors (purple and orange) can be described as compounds Afterimages  Afterimages: a visual image seen after stimulus has been removed  Adapting stimulus: stimulus whose removal produces change in visual perception or sensitivity  Negative afterimage: afterimage whose polarity is opposite original stimulus. Light stimuli produce dark negative afterimages. Colors are complementary; ex. Red produces green; yellow produces blue  Neutral point: point at which opponent color mechanism is generating no signal, if red-green and blue-yellow mechanisms are at neutral points, stimulus will appear achromatic (black-white process has no neutral point) Color in Visual Cortex  Psychophysicists found 3 basic color processes o red-green, blue-yellow, and black-white opponency  Physiologists found 3 types of LGN cells using each of 3 different combinations of cone outputs o (L – M), ([L + M] – S) and (L + M) cells o Sometimes described as red-green, blue-yellow and black-white) o Problem – ([L + M] – S) cell should be yellow-blue cell, maximally excited by unique yellow and inhibited by unique blue; but detailed look reveals that adding and subtracting cone sensitivities produces cells that respond maximally along an axis extending from purplish hue to yellowish green hue  (L – M) cells aren’t in right place either o L-cone end of axis is near perceptual red, but M-cone end is bluish green o End points – cardinal directions in color space  Mismatch between color percepti
More Less

Related notes for PSY100H1

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.