Perception Chapter 7: Color Vision Color Vision: The reason why humans perceive different colors in the environment is because of the manner in which the visual system processes various wavelength energy present in the environment. It is likely that this capacity evolved in order to allow important discriminations to be made, such as a ripe apple from a rotten one, or a poisonous plant from an eatable one, sexually receptive female from non-receptive, etc. The term "color" is actually an imprecise term. Color is comprised of three distinct components: Components of color Hue: the perceptual pigmentation difference experience with varying wavelengths, this is what we are actually referring to when we use terms such as "green" or "blue." Brightness: refers to the intensity of the hue, varies in relation to the amplitude of the light. Saturation: refers to the degree of white light mixed in with the dominant wavelength. Perceptually experienced as the purity of the color. Composite vs. Pure light It was Sir Isaac Newton who made the first detailed observations of the composite nature of sunlight. By using a prism, Newton determined that sunlight was actually a mixture of many different wavelengths, each perceptually experienced as a different hue, when isolated. Pure light: perceptually this is light which can be experienced as only one hue. It cannot be broken down into any further component hues. Theoretically, this is light of a single wavelength, which is very difficult to achieve. Distinction is important as many wavelengths are experienced as the same hue. Composite light: light comprised of many different wavelengths, which if isolated would be perceptually experienced as different hues Spectral vs. Non spectral colors Originally Newton only included spectral colors (Roy G Biv); by combining short and long ends of spectrum he created non-spectral colors (purple, magenta, pink). Also, original circle had seven discrete colors and equal spacing. Later revisions changed this. Complementary colors across from each other. Newton’s color circle Hues blend one into another Highest saturation at rim, decreases as you move inward. Can be used to make color perception predictions (top) Brightness and saturation 1) Perceptual brightness and saturation: the photopic absorption curve shows the cones are maximally sensitive to wavelengths around 550 (greenish-yellowish color). It so happens that these hues also appear the brightest and the least saturated (i.e. least amount of added white light need to de-saturate). Even when their physical properties (intensity level, purity) are the same as other hues. This phenomenon reflects not only the differentially sensitivity of the cones, but also the functioning of cells in the LGN as we shall see later. Additive/Subtractive color mixing Color contrast & constancy Top: in constancy the visual system takes overall illumination into account. Constancy tends to occur when illumination changes in the character of its distribution, but still maintains a wide-band of wavelengths. Constancy tends to fail if illuminating light is restricted to a narrow band of wavelengths -hence, the strange clothing colors seen at amusement park rides when under blue light Bottom: in contrast, relative reflection of background affects perception. Reflects opponent nature of color processing. Most likely reason for this effect is the presence of double-opponent process cells in Primary Visual cortex which respond to the presence of a certain color in center, and to the presence of the complementary color in surround. Physiology of Color Vision: Retinal level: Trichromacy – why three cone types? If only one -- no differentiation between wavelengths at all, only able to distinguish intensity differences = monochromat. Why is this so? if one cone type with one absorption curve, then if Wavelength A is absorbed 25%, and B is 50%, and C is 50%. No way to distinguish if C is same as B or if it is A only at twice the intensity. Physiology of Color Vision: • What if two cone systems? Called dichromatic. More differentiation between wavelengths possible as each wavelengths is determined by the pattern of responding across two systems. However, a number of confusions are still possible as one wavelength, which has hi absorption in A system and low in B, could be mimicked by two wavelengths presented simultaneously, one of which has mid A and no B, the other of which has mid A and low B? • For example: • Wavelength 1 = 80% of A; 20% of B; confused with • Wavelength 2 = 40% of A; 0% of B + • Wavelength 3 = 40% of A; 20% of B Physiology of Color Vision: Trichromacy: seems to be natures balance between enough cone systems to provide adequate discriminative abilities (adequate for survival) and the anatomical constraints of retina. Note that some confusions still can occur, such as failure of color constancy, but most are only present in highly contrived environments. 3 cone systems: Studies using microspectrophotometry have demonstrated the maximal absorption curves for each of the three cone systems. Short system: max abs. at around 440 nm. Middle system: max abs. around 530 Long system: max abs around 570 Hence, most wavelengths will be characterized by a unique pattern of responding across the three cone systems Photopic absorption curves and 3 cone systems Apparent contradiction? remember that the peak of the photopic sensitivity curve was around 550. Yet the average of the sensitivities of the three cone systems is not going to be anything close to 550 (440+530+570/3=513). So how can this be? Two reasons: 1) Frequency distribution of cone systems: S cones are the rarest of all the cone systems. Only about 1 million of the 8 mil cones are of the S type. About 4.6 mil are of the L type, and 2.3 mil are of the M type. 2) Geographic distribution of cone systems: S cones are not found within the fovea of the retina, only M's and L's are found here. S cones are highly concentrated in a ring just outside of the fovea. This is why a blue dot, centered only on the fovea will actually take on a dark gray appearance. So -- the photopic sens. curve reflects measurements taken from the fovea, where S cones are not contributing to the over-all sensitivity, and where about twice as many L as M cones are present. Thus sensitivity is shifted to longer end of spectrum. Evolution of color vision Human eye has four photopigments (chemicals in photoreceptors that respond to light) Rhodopsin (in rods) 3 different cone pigments (S, L, M) Sometime prior to 500mybp: rhodopsin original cone pigment (peak sens. 510570nm) Sometime after 500mybp rhodopsin short pigment Around 50mybp original cone pig L and M pigments In primates, L,M split appears to be associated with the need to distinguish ripe fruit from leaves and un-ripe fruit Color blindness and deficiencies Color blindness is linked to X chromosome, so more common in males then females From Retina to LGN and PVC • • While retina tends to operate in trichromatic fashion, LGN (and PVC) tend to operate in more opponent processes fashion. Opponent process theory: idea first articulated by Ewald Hering in the 1800's, who, based on evidence from afterimages and color contrast effects, suggested that the visual system treats red-green, and blue-yellow as antagonistic pairs. Later research by DeValois and others have confirmed this notion by showing that cells in the LGN and PVC tend to respond vigorously to the presence of one color, and are inhibited by the presence of another complementary color. Note: achromatic channel sums responses of L and M cones.