Perceiving Color - Cognitive Psychology - Lecture Slides, Slides of Cognitive Psychology

Download Perceiving Color - Cognitive Psychology - Lecture Slides and more Slides Cognitive Psychology in PDF only on Docsity! Chapter 9: Perceiving Color Docsity.com 400 500 600 700 Wavelength (nm) B 9.4 The visual spectrum. Docsity.com Color Mixing • Additive color mixture: – Mixing lights of different wavelengths – All wavelengths are available for the observer to see – Superimposing blue and yellow lights leads to white • Subtractive color mixture: – Mixing paints with different pigments – Additional pigments reflect fewer wavelengths – Mixing blue and yellow leads to green Docsity.com Figure 9.7 Color mixing with paint. Mixing blue paint and yellow paint creates a paint that appears green. This is subtractive color mixture. Docsity.com Trichromatic Theory of Color Vision • Proposed by Young and Helmholtz (1800s) – Three different receptor mechanisms are responsible for color vision. • Behavioral evidence: – Color-matching experiments • Observers adjusted amounts of three wavelengths in a comparison field to match a test field of one wavelength. Docsity.com Physiological Evidence for the Trichromatic Theory • Researchers measured absorption spectra of visual pigments in receptors (1960s). – They found pigments that responded maximally to: • Short wavelengths (419nm) • Medium wavelengths (551nm) • Long wavelengths (558nm) • Later researchers found genetic differences for coding proteins for the three pigments (1980s). Docsity.com Figure 9.9 Absorption spectra of the three cone pigments. From Dartnall, H. J. A., Bowmaker, J. K. and Mollon, J. D. (1983). Human visual pigments: Microspectrophotometric results from the eyes of seven persons. Proceedings of the Royal Society of London B, 220, 115-130. Docsity.com Response of Cones and Color Perception • Color perception is based on the response of the three different types of cones. – Responses vary depending on the wavelengths available. – Combinations of the responses across all three cone types lead to perception of all colors. – Color matching experiments show that colors that are perceptually similar (metamers) can be caused by different physical wavelengths. Docsity.com Color Experience for Monochromats • Monochromats have: – A very rare hereditary condition – Only rods and no functioning cones – Ability to perceive only in white, gray, and black tones – True color-blindness – Poor visual acuity – Very sensitive eyes to bright light Docsity.com Opponent-Process Theory of Color Vision • Proposed by Hering (1800s) – Color vision is caused by opposing responses generated by blue and yellow, and by green and red. • Behavioral evidence: – Color afterimages and simultaneous color contrast show the opposing pairings – Types of color blindness are red/green and blue/yellow. Docsity.com Figure 9.18 Color matrix for afterimage and simultaneous contrast demonstrations. Docsity.com Trichromatic and Opponent-Process Theories Combined • Each theory describes physiological mechanisms in the visual system – Trichromatic theory explains the responses of the cones in the retina – Opponent-process theory explains neural response for cells connected to the cones further in the brain Docsity.com Perceiving Colors Under Changing Illumination • Color constancy - perception of colors as relatively constant in spite of changing light sources – Sunlight has approximately equal amounts of energy at all visible wavelengths – Tungsten lighting has more energy in the long-wavelengths – Objects reflect different wavelengths from these two sources Docsity.com Possible Causes of Color Constancy • Chromatic adaptation - prolonged exposure to chromatic color leads to receptors: – “Adapting” when the stimulus color selectively bleaches a specific cone pigment – Decreasing in sensitivity to the color • Adaptation occurs to light sources leading to color constancy Docsity.com Lightness Constancy • Achromatic colors are perceived as remaining relatively constant. – Perception of lightness: • Is not related to the amount of light reflected by an object • Is related to the percentage of light reflected by an object Docsity.com Figure 9.28 A black-and-white checkerboard illuminated by (a) tungsten light and (b) sunlight. Docsity.com Possible Causes of Lightness Constancy • The ratio principle - two areas that reflect different amounts of light look the same if the ratios of their intensities are the same • This works when objects are evenly illuminated. Docsity.com Sources of Information About Illumination • Orientation of surfaces provides information about illumination and reflectance edges. • Perceptual organization of objects in a display affects perception of lightness. Docsity.com Figure 9.33 (a) Four dark discs partially covered by a white mist; (b) four light discs partially covered by a dark mist. The discs are identical in (a) and (b). Anderson, B. L., & Winawer, J. (2005). Image segmentation and lightness perception. Nature, 434, 79-83. Docsity.com Perceptual Experiences are Creations of the Nervous System • Physical energy in the environment does not have perceptual qualities. – Light waves are not “colored.” • Different nervous systems experience different perceptions. • Honeybees perceive color at 350nm, which is outside human perception. – We cannot tell what color the bee actually “sees.” Docsity.com