Color Vision PDF

INTRO TO COLOUR Colour vision: ability to see differences between lights of different wavelengths Objects don’t have colour but rather from the interaction between receptors in the eyes and the wavelengths of light reflected from surfaces of objects Light and Colour Electromagnetic spectrum...

INTRO TO COLOUR Colour vision: ability to see differences between lights of different wavelengths Objects don’t have colour but rather from the interaction between receptors in the eyes and the wavelengths of light reflected from surfaces of objects Light and Colour Electromagnetic spectrum: range of all types of electromagnetic radiation - the energy that travels and spreads out as it goes The only wavelengths that give rise to sensation and perception in humans are between 400 nm and 700 nm (visible light) Gamma waves: very high Radio waves: very low Light is an electromagnetic wave ◦ Light of different wavelengths is perceived as different colours ◦ Light is usually heterochromatic (many wavelengths) ◦ We perceive heterochromatic light to have the corresponding to its dominant wavelength Achromatic light: light with no dominant wavelength Perceived as colourless Additive colour Create the perception of different colours by mixing the primary additive colours Works best with pure monochromatic coloured light Used in digital displays (screens) Subtractive colour Relies on light being absorbed Colour is determined by what wavelength get absorbed Used by mixing paint or ink to make spectrum of possible colours Spectral power distribution: intensity (power) of a light at each wavelength in the visible spectrum Colour dimensions The hue = the colour most closely associated with the wavelength of light Saturation: purity/vividness of the hue Brightness: perceived intensity of the light Colour circle: 2D depiction in which hue varies around the circumference and saturation varies along any radius Colour solid: 3D depiction of colour circle + brightness varies vertically  CIE colour space: standardized colour space where any colour can be represented by a pair (x,y) of coordinates Colour & Wavelength Spectral re ectance Spectral reflectance: proportion of light that a surface reflects at each wavelength Perceived colour of things depends on how things reflect light ◦ Depends on molecular structure of the surface Determines reflectance SEEING IN COLOUR PRIMARY COLOURS Metamers: 2 stimuli that are physically different but perceived to be the same With 3 primary colours, we can match wavelengths but its not possible with only 2 CONES AND COLOUR Spectral sensitivity function: probability that a cone's photopigment will absorb a photon of light of any given wavelength and generate an action potential Or rate of an action potentials we expect from a given wavelength A single type of photoreceptor cant distinguish between different wavelengths of light ◦ The photoreceptor will generate the same neural signal fl  ◦ Less intense light at a wavelength that it is more sensitive to ◦ More intense light at a wavelength that it is less sensitive to Principle of univariance: principle that absorption of a photon of light results in the same response regardless of the wavelength of the light Also applies to rods: night vision is colourblind TRICHROMACY Most humans have 3 types of cones with different sensitivities Other mammals have 2 (dichromats) Some birds have 4 (tetrachromats) Mantis shrimp has 16 (hedecachromats) At least 2 cones with different sensitivities need to be stimulated to perceive colour ◦ To differentiate wavelength and intensity using relative sensitivities ◦ But some wavelengths can produce the same relative response Trade-off between seeing fine colour details and differentiating colours ◦ 2 cones with different sensitivities need to be illuminated to perceive the colour ◦ The more amount of cones illuminating = less spatial resolution Cone distribution in the retina: - Higher [] of L-cones - High [] of M-cones - Low [] of S-cones Color de ciencies Inherited de ciencies 1. Monochromacy (achromatopsia): total colour blindness ; Only having 1 type of cones, or only rods ◦ Very rare ◦ Everything in a shade of grey ◦ Rod monochromacy: rods only, no cones — poor spatial acuity, no colour vision ◦ Cone monochromacy: rods and 1 type of cone 2. Dichromacy: only has 2 cones, divided in 3 types depending on which cone is missing 1. Protanopia (X-linked): lacks L-cones 2. Deuteranopia (X-linked): lacks M-cones 3. Tritanopia: lacks S-cones 3. Anomalous trichromacy: if L and M cones are too close together ◦ Becomes harder to differentiate between their responses Cortical Achromatopsia Achromatopsia: loss of colour vision due to brain damage fi fi Colour Opponency No single cone can correctly signal for colour = comparison 3 mechanisms distinct from the 3 types of cones that results in opponency between ◦ Red/green ◦ Blue/yellow ◦ Black/white Hue cancellation Hue cancellation: experimental technique in which the person cancels out any perception of a particular colour in a test light by adding light of another colour 1. Start with yellowish green 2. Add blue light until the green appears to have no yellow 3. When green appears neither yellow nor blue, we are left with unique green Unique/Pure colour wavelengths Unique BLUE = 477 nm Unique GREEN = 510 nm Unique YELLOW = 580 nm Unique RED = not a point on the spectrum Opponent ganglion cells 3 types of cones (S, M, L) form excitatory or inhibitory connections to ganglion cells to form opponent channels Circuits act as the opponent mechanisms S= short wavelength M = medium L = Long Circuits +S-ML PATHWAY Fires above the baseline rate in response to short-w light (+S) and below its baseline in response to medium-w and long-w (-ML) Excited by blue Inhibited by green and red Spatially uniform receptive field +ML-S PATHWAY Fires above the baseline rate for medium-w to long-w (+ML) and below for short-w (-S) Excited by green and red Inhibited by blue +L-M PATHWAY Fires above the baseline for long-w (+L) and below medium-w (-M) Annular receptive fields similar to on/off center WITHOUT inhibition Excited by red Inhibited by green +M-L PATHWAY Fires above baseline for medium-w (+M) and below for long-w (-L) Excited by green Inhibited by red Colour afterimages and opponency Photopigment bleaching: photopigment molecule's loss of ability to absorb light for a period after undergoing photoisomerization Chromatic adaptation: kind of photopigment bleaching that results from exposure to relatively intense light consisting of a narrow range of wavelengths Colour contrast Colour contrast: perception of a surrounded colour as shifted toward the complement of the surrounding colour Provides further support for opponent mechanisms Colour assimilation: perception of a surrounded colour as shifted toward a noncomplementary surrounding colour (i.e. spreading effect) Constancy Colour constancy: tendency to see a surface as having the same colour under illumination by lights with different spectral power distributions Lightness: perceived reflectance of a surface Lightness constancy: tendency to see a surface as having the same lightness under illumination by very different amounts of light COLOUR IN LGN Parvocellular layers = process +L-M pathway Koniocellular layers = process+ S-ML pathway Magnocellular layers = do not process colour information, rather luminance COLOUR IN V1 Parvocellular projects to layer 4C of the striate cortex +L-M pathway Send axons to layer 3 within the blobs Koniocellular projects to layer 4 of the striate cortex directly +S-LM pathway Directly to within the blobs Double opponent cells in V1 are inhibited if the preferred center colour is presented in the surround Ideal to detect colour differences in environment, even with no difference in luminance 𝛽