OPTO 5320 VISION SCIENCE I

Transcription

1 OPTO 5320 VISION SCIENCE I Monocular Sensory Processes of Vision: Color Vision Ronald S. Harwerth, OD, PhD Office: Room 2160 Office hours: By appointment Telephone: rharwerth@uh.edu

2 VISION SCIENCE I: Monocular Sensory Processes of Vision I. Color Vision A. Introduction - Normal color vision B. Colorimetry C. Retinal fundamentals D. Neural mechanisms E. Genetic color vision anomalies F. Clinical color vision tests G. Acquired color vision defects

3 I. Introduction A. What defines color vision? B. What defines normal trichromatic color vision? II. Scotopic vs. photopic vision III. Discriminations that depend on wavelength A. Color naming B. Wavelength discrimination C. Saturation discrimination D. Brightness discrimination 1. Heterochromatic photometry 2. Increment-threshold spectral sensitivity

4 What is normal color vision? Understanding normal color vision involves 1) physics of light 2) photobiology 3) neurobiology 4) perception The figure (Enigma) produces an illusion of motion when the center yellow spot is fixated. The illusion requires neural processing in extrastriate area V5, a color processing area.

5 How is chromatic vision different from achromatic vision?

6 Chromatic and achromatic information in normal vision Normal vision is a combination of chromatic and achromatic information. The Arnolfini Portrait painted by Jan van Eyck in 1434

7 Color Vision What defines color vision? Spectral response properties of vision. Discriminations based on wavelength.

8 Color Vision is a Property of the Cone Photoreceptors Cone phase - colored Human eye is duplex - rod and cone photoreceptors. Rod phase - colorless Time in the dark (min)

9 Color Vision is a Property of the Cone System Polyak s drawing of the primate retina to show the regional distribution of cones and rods, with an absence of rods in the central fovea. Osterberg s data on the densities of the different photoreceptor populations in the human retina show that the cone density is highest in the foveal pit and falls rapidly outside the fovea to a fairly even density into the peripheral retina.

10 Spectral response properties Photochromatic interval Photopic threshold Purkinje shift Scotopic vs. photopic vision Photopic thresholds are higher, especially in the shorter wavelengths - photochromatic (colorless) interval - mesopic vision. Lowest photopic thresholds are at longer wavelengths than scotopic - Purkinje shift. Scotopic threshold Wavelength (nm)

11 Spectral response properties The spectral response properties of individual cones are determined by the photopigments in the outer segments. Trichromatic vision requires cone photopigments with three different spectral responses. With three photopigments, any spectral stimulus can be matched by the appropriate intensities of three primaries, i.e., trichromacy. Subsequent neural processes form the perceptual spectral response properties.

12 Why Three Photopigments are Required for Trichromatic Color Vision. The absorption spectrum represents the probability of quantum capture as a function of wavelength. The quantum nature of light (E = h ) where: E = energy of a quantum h = Plank s constant = the frequency of light Quantum capture at two wavelengths can be equalized by appropriate intensities A photopigment only can signal intensity (the Principle of Univariance)

13 Matching Intensities of Two Wavelengths in a Retina with Two Photopigments Quantum capture at two wavelengths cannot be equalized by any combination of intensities. The intensities at two wavelengths may be adjusted so that the quanta caught are identical for either of the photopigments, but there is no intensity setting that equates the effects of both photopigments at the same time.

14 Color Matching of Wavelength in a Retina with Two Photopigments Requires Two Primaries. It is always possible to find intensities for two wavelengths so that the effects of their mixture will be identical to the effects of a third stimulus. The example illustrates the mixture of two wavelengths ( 2 and 3 ) to match a stimulus with a wavelength at 1.. Color matching for a retina with three photopigments requires three primaries.

15 Cone Photopigments for Color Vision Human color vision is based on three cone photopigments Short-wavelength sensitive (SW cones or blue cones) Middle-wavelength sensitive (MW cones or green cones) Long-wavelength sensitive (LW cones or red cones)

16 Photopigments and Color Vision Human color vision is based on the relative absorbance of three cone photopigments, but not all of color perception can be explained by the light absorbed by photopigments. A Land Mondrian is an example.

17 What Defines Normal Trichromatic Color Vision? 1. Three normal cone photopigments 2. Normal color matching functions 3. Normal color naming of spectral colors 4. Normal discrimination functions for the three perceptual attributes of the physical properties of color. Physical Wavelength Purity Intensity Perceptual Hue Saturation Brightness

18 Color Matching Functions A subject s discrimination of colors when they are added by physical superimposition or temporal mixture Any colored stimulus can be matched in its hue saturation and brightness by a combination of three properly chosen primaries. Color mixture by subtraction is a different process.

19 Discriminations that Vary with Wavelength

20 Color Naming Blue Green Yellow Red Across 100 languages, 11 basic color names have been identified. The English equivalents are white, black, grey, blue, green, yellow, red, purple, orange, brown, pink. Spectral colors can be named by combinations of four fundamentals Bluish Green?? Color score: Based on 3 points per sample, where: pure color = 3 dominant color = 2 modifier color = 1 Example for?? sample % red = 100 * (total red_score) / (trials *3)

21 Percent color score Color Naming Color naming requires 4 responses classes (blue, green, yellow, red) with modifiers e.g., greenish blue. Certain wavelengths elicit essentially pure color names. Some combinations do not occur as a single color, i.e., red / green and blue / yellow form opponent pairs

22 Wavelength Discrimination The smallest difference in wavelengths that is perceived as a difference in hue. C1 S C2 Wavelength discrimination for S = 520 nm = (C1 + C2) / 2 Wavelength (nm)

23 Wavelength Discrimination Normal color vision: Very small differences in wavelength (1 2 nm) can be discriminated on the basis of hue. Finest discrimination at 490 nm and 590 nm. Poorer discrimination in the middle of the visible spectrum (530 nm) and at the ends of spectrum Wavelength (nm)

24 Saturation Discrimination (colorimetric purity) Saturation discrimination is an ability to perceive differences between pure spectral color and colors with some white content. S C1 C2 Colorimetric purity (p ) for S = 520 nm p = L / (Lw + L ) where: L = luminance of S Lw = luminance of added white light

25 Saturation Discrimination (colorimetric purity) Wavelength (nm) Wavelengths in the region of nm (yellows) appear less saturated than those nearer the ends of the visible spectrum (blue or red).

26 Brightness Discrimination (luminous efficiency) C3 S C1 C2 Methods (step-by-step procedures): 1) heterochromatic brightness matching 2) flicker photometry 3) minimally distinct-border Relative luminous efficiency (V ) for C1 with respect to S = 520 nm V = (L(s) - L(c1)) / (L(s)) where: L(s) = luminance of S L(c1) = luminance C1 to match S

27 Brightness Discrimination (luminous efficiency) Ferry Porter law (CFF ~ log(i)). Critical flicker fusion is directly proportional to the logarithm of the light intensity Borders are least distinct when the boundary separates stimuli of equal luminance Maximum luminous efficiency for wavelengths near 555 nm. The V function is the basis of photometry. Flicker photometry or minimally distinct border methods produce smooth unimodal V functions, which represent the sum of the responses of the MW- and LW-cones.

28 Brightness Discrimination (increment-threshold spectral sensitivity) There are multiple detecting mechanisms in the retina, but the most sensitive mechanisms are used at any time. An adaptation field can be used to vary the sensitivity of the underlying mechanisms. Adaptation field Test field 1) White adaptation field (background) to obtain static photopic state. 2) Threshold - the smallest perceptible light increment of the monochromatic test field added to the adapting background. 3) Sensitivity = log (1 / threshold intensity).

29 Brightness Discrimination (increment-threshold spectral sensitivity) For normal color vision, increment-threshold spectral sensitivity functions have three peaks in the visible spectrum (440, 520, & 620 nm). The three peaks are produced by neural (subtractive) combinations of the responses of the three cone photopigments, but they are not the photopigment sensitivity functions.

30 Introduction - Normal color vision Recap Color properties of photopic vision Purkinje shift and photochromatic interval Discrimination functions for normal color vision Color matching Color naming Hue discrimination Saturation discrimination Brightness discrimination Luminous efficiency Spectral sensitivity