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Processes in Biological Vision

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Processes in Biological Vision PROCESSES IN BIOLOGICAL VISION: including, ELECTROCHEMISTRY OF THE NEURON This material is excerpted from the full β-version of the text. The final printed version will be more concise due to further editing and economical constraints. A Table of Contents and an index are located at the end of this paper. James T. Fulton Vision Concepts [email protected]/vision April 30, 2017 Copyright 2000 James T. Fulton Environment & Coordinates 2- 1 2. Environment, Coordinate Reference System and First Order Operation 1 The beginning of wisdom is to call things by their right names. - Chinese Proverb The process of communication involves a mutual agreement on the meaning of words. - Charley Halsted, 1993 [xxx rationalize azure versus aqua before publishing ] The environment of the eye can be explored from several points of view; its radiation environment, its thermo- mechanical environment, its energy supply environment and its output signal environment. To discuss the operation of the eye in such environments, establishing certain coordinate systems, functional relationships and methods of notation is important. That is the purpose of this chapter. The reader is referred to the original source for more details on the environment and the coordinate systems. Extensive references are provided for this purpose. Details relative to the various functional operations in vision will be explored in the chapters to follow. Because of the unique integration of many processes by the photoreceptor cells of the retina, which are explored individually in different chapters, a brief section is included here to coordinate this range of material. 2.1 Physical Environment The study of the visual system requires exploring a great many parameters from a variety of perspectives. This situation involves employing and crossing many traditional lines of investigation. Terminology becomes difficult under these circumstances. For purposes of this work, the wide variety of disciplines associated with the study of animals will be divided into the major categories of morphology and physiology. Morphology will relate to the form and structure of the animal. Physiology will relate to the functions performed and processes used by the animal. Within these categories, the terms defined in Table 2.1 will be used. When investigating fields without a check mark, the designation of the nearest field should be used with a specific modifier if a more specific label is not available. TABLE 2.1 DISCIPLINES AND TERMINOLOGY USED IN THIS WORK Field Morphology Physiology Microscope used Shape Structure Function Process None light electron Anatomy * * * * histology * * * cytology * * * topography * * * * * topology * * * * * biochemistry * * 2.1.1 The Radiation Environment The terminology used to discuss the radiation environment applicable to vision is very awkward, especially with the introduction of photometry based on the presumed absorption spectrum of the human eye. The subject has been further complicated by the recognition that this absorption spectrum changes as the light level is reduced. Halstead 1Released: April 30, 2017 2 Processes in Biological Vision has recently provided a readable description of the units used to describe the radiation environment of vision, although he did not address the difference between scotopic and photopic units. He makes a clear distinction between a photometer designed to measure incident flux (an incident photometer reporting in lumens/m2) and a photometer designed to measure the emitted, or reflected, light (a luminance photometer reporting in cd/m2). The reader should note the critique appearing at the end of the paper. Ryer has provided a more extensive treatise on the subject than is readily available2. There is a problem with page 31 of Ryer that will be addressed in Section 17.1.5.3.1. 2.1.1.1 The Luminance Range In the most general sense, the radiation environment of the eye is limited by two external parameters; the spectral range of the electromagnetic spectrum which is transmitted by the atmosphere and the intensity range between that generated by the sun and the relative darkness of a cloudy winter night in the countryside. For aquatic animals, the spectral range is limited further by the spectral transmission of water. Cronly-Dillon & Gregory provide a figure adapted from Lythgoe that describes the marine transmission spectrum3. The detectable spectral range at the surface of the retina is then limited somewhat further by the transmissive elements of the eye itself. The detected spectral range is then determined by the product of the input radiance and the absorption spectrum of the individual photodetectors. Many animals utilize the 300-400 nm. portion of the ultraviolet range of the radiant spectra, although normal humans do not. Therefore, a limited sector of the radiance spectra has been defined as the luminance spectrum, extending from 400 nm. to 700 nm based on the effective absorption spectrum of the human species. The 400 nm. end-point is relatively abrupt since it is determined not only by the relevant chromophore but also by the transmissivity of the optical system of the eye. Aphakic human observers (those with the crystalline lens of their eye removed) report seeing wavelengths out in the 310-360 nm. range with a peak wavelength near 342 nm4. Such a peak is clearly not associated with the skirt of the S-channel chromophore. It appears that the aphakic human eye, and therefore the retina of all humans, contains UV-channel chromophores. The color reported for this spectral range by a very small group of humans is blue. The long wavelength limit of human vision is controlled primarily by the absorption coefficient of the L-channel chromophore; for sufficiently intense illumination, the human eye can detect light out to at least 900 nm. The illumination ranges in Figure 2.1.1-1 have been adopted primarily from Rubin & Walls5. The Rubin & Walls variant used only English units. Note that the table is based on the emittance of a surface, not the irradiance or illuminance of an aperture. It provides a calibration on the luminance values of various surfaces found in the real world. The extra annotation on the right is from a Norden-Ketay Corp. report from the 1950’s. Halsted republished their figure in 1993 with a few embellishments and using SI units6. The values in the table are only accurate to an order of magnitude (at best). The common definition of twilight, extending from the astronomical definition of sunset to the astronomical definition of twilight, is a wide imprecise range from 102 to 10-2 millilamberts. Land & Nilsson have published a variant of this table7, apparently accumulated from the same sources but also including the luminance level at the sea surface as seen from different depths in the clearest ocean water (attenuation coefficient, 0.032 /meter). A draft set of proposed Absolute Munsell Values has been added for purposes of discussion. The original Munsell scale was a relative logarithmic1.478 scale suited to the needs of artists of the 1900's. It only accommodated ten reflectance levels of pigment when illuminated such that the pigments were observed within the photopic region of human vision. Today, there is a need for an extended logarithmic scale that can accommodate the industrial and scientific communities (Sections 17.3.5.2 & 17.3.8). This extended scale makes it possible to interpret recent data acquired with robotic probes visiting other bodies in the solar system (Section 17.3.8.3). To keep the new scale compatible with both the conventional irradiance scales of industry and the astronomical community, a logarithmic 2Ryer, A. (1997) Light Measurement Handbook. International Light (Publisher) www.intl- light.com/handbook.html 3Cronly-Dillon, J. & Gregory, R. (1991) Evolution of the eye and visual system. Boca Raton FL: CRC Press pg. 407 4Wald, G. (1945) Human vision and the spectrum. Science. Vol. 101, No. 2635 pp. 653-658 5Rubin, M. & Walls, G. (1969) Fundamentals of visual science. Springfield, Il: Charles C. Thomas pg. 40 6Halstead, C. (1993) Op. Cit. 7Land, M. & Nilsson, D-E (2002) Animal Eyes. Oxford: Oxford University Press Page 20 Environment & Coordinates 2- 3 scale to the base 1.5849 has been introduced. While slightly different than that of the relative Munsell Value scale, it provides a greatly extended Absolute Munsell Value scale useful over the complete range of human vision. In the new Absolute Munsell Value range, the brightest highlight of the sample is taken initially as the Value on the proposed scale. If the brightest highlight of the sample is found to be equal to 10 on the Absolute Munsell Value scale, this value also corresponds to the relative Munsell Value of 10. As noted, when the Absolute Munsell Value range is reduced more than a factor of 10, the colors in the lower Value range (i.e., -15/ to -20/ and below) will exhibit color shifting due to the loss of “color constancy” within the human eye. Figure 2.1.1-1 Table of equivalent light levels drawn from the literature. An expanded scale of proposed Absolute Munsell Values have been added for discussion purposes. See text. The above luminance values can be compared with some common incidence values given by Ryer. Sunny day, 105 Lux; office lights, 100 Lux; full moon, 0.1 Lux; overcast night 10-4 Lux. 1 Lux equals 1 cd/m2. The region of optimum visual performance is difficult to define because of the many variables involved. Color constancy is particularly difficult to define because its evaluation is strongly dependent on the color temperature of the illumination source used. Individuals are not readily aware of the loss of performance in the purple region of the spectrum when using tungsten light sources with color temperatures below 3600 Kelvin. [xxx edit these values for consistency ] The theoretical range of the photopic regime presented in Chapter 11 of this work is in excellent agreement with this Table.
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