Color Vision and Night Vision

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Chapter 10 Color Vision and Night Vision

Overview

Day vision and night vision are two separate modes of visual perception and the visual system shifts from one mode to the other based on ambient light levels. Each mode is primarily mediated by one of two photoreceptor classes in the retina, i.e., cones and rods. In day vision, visual perception is primarily cone-mediated and perceptions are chromatic. In other words, color vision is present in the light levels of daytime. In night vision, visual perception is rod-mediated and perceptions are principally achromatic. Under dim illuminations, there is no obvious color vision and visual perceptions are graded variations of light and dark. Historically, color vision has been studied as the salient feature of day vision and there has been emphasis on analysis of cone activities in color vision. Night vision has historically been studied in terms of rod activity and considerations of the shift from day vision to night vision.

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This chapter will review basic aspects of rods and cones and neural pathways that process rod and cone information. Measurement of sensitivity during dark adaptation is discussed as the established measure of the shift between day vision (cone vision) and night vision (rod vision). Clinical assessment of rod and cone sensitivities using dark adaptation function as a means of assessing retinal disease is also discussed. Color vision is discussed in terms of experimental paradigms and theoretical considerations and variations in human color vision are described. Evaluation of color vision can be helpful in understanding the underlying mechanisms of some retinal diseases and suggestions for clinical evaluation of color vision are offered. In the last section of this chapter, new developments in color vision research are discussed.

Rod and cone functions

Differences in the anatomy and physiology (see Chapters 4, Autofluorescence imaging, and 9, Diagnostic ophthalmic ultrasound) of the rod and cone systems underlie different visual functions and modes of visual perception. The rod photoreceptors are responsible for our exquisite sensitivity to light, operating over a 108 (100 millionfold) range of illumination from near total darkness to daylight. Cones operate over a 1011 range of illumination, from moonlit night light levels to light levels that are so high they bleach virtually all photopigments in the cones. Together the rods and cones function over a 1014 range of illumination. Depending on the relative activity of rods and cones, a light level can be characterized as photopic (cones alone mediate vision), mesopic (both rods and cones are active), or scotopic (rods alone mediate vision).1 In the literature, the terms photopic vision and scotopic vision are used to reflect cone and rod vision, respectively. Table 10.1 shows this overlapping range of activity.

Table 10.1 The dynamic range of the human visual system

image

Modified from Hood DC, Finkelstein MA. Sensitivity to light. In: Boff KR, Kaufman L, Thomas JP, editors. The handbook of perception and human performance, vol. 1. Sensory processes and perception. New York: John Wiley; 1986.

The distribution of rods and cones in the retina (see Chapter 4, Autofluorescence imaging) is also reflected in visual function. The greatest sensitivity to light occurs in the midperiphery of the visual field, which has a predominance of rods, while high-acuity and good color vision are mediated by the fovea, which has a predominance of cones. Nonetheless, the entire retina, with the exception of a very small area within the fovea, is capable of mediating night vision, and color vision is present throughout the visual field with daylight stimulation of the entire retina. The following will introduce rod and cone differences in light adaptation, spectral sensitivity, and spatial/temporal sensitivity.

Light adaptation

Photoreceptors, whether they are rods or cones, respond well to only a small range of variations in illumination within a steady adapting background.2 However, adaptation mechanisms adjust photoreceptor sensitivity so that this small range of responses is always centered near the current adaptation level, even though adaptation levels can vary over a wide range. This behavior forms the basis for the large operating range of the visual system.

It is possible to measure a threshold for the perception of an increment in light on a large, steady background field. As the background light level is increased, the increment threshold starts to increase. Rods and cones behave somewhat differently in this regard. For the rod system, as shown in Fig. 10.1A, the increment threshold increases steadily over almost a thousand-fold range. With further increases in background adaptation levels, an increment is not detected, no matter how much additional test light is presented as an increment, due to rod saturation. In comparison, the cone system, as shown in Fig. 10.1B, shows a continuous steady increase in the increment threshold with increases in background illumination, even at levels that bleach almost the entire amount of available photopigment. The portion of the curve that rises linearly with illumination levels is called the Weber region (Fig. 10.1). In the Weber region, an incremental light can be detected when it is a constant proportion (i.e., the Weber fraction) of the background light level. Different photoreceptor systems have a characteristic Weber fraction. Cones have lower Weber fractions than rods and the M and L cones have lower Weber fractions than S cones. Under optimal conditions, the cone system can detect a light level difference of 1%, while rods need a light change of 20%.

In addition to photoreceptor adaptational properties, other factors, including pupil size, the temporal and spatial summation characteristics, and photopigment depletion, can also contribute to extend the operating range of the visual system over a large luminance range. While some adaptation operates within the photoreceptors themselves, other properties of adaptation may reflect the effects of the complex neural circuitry of the retina.2

Spectral sensitivity

Day vision is primarily mediated by three types of cone photoreceptors with different but overlapping spectral sensitivities. Each is identified by the relative position of the peak in spectral sensitivity. The three cone types are called the long-, middle-, and short-wavelength-sensitive (L, M, and S) cones. When overall sensitivity to light is measured at the light-adapted fovea, a broad sensitivity spectrum peaking near 555 nm is found. This sensitivity spectrum represents the combined activity of the L and M cones and is called the V(λ) function. When sensitivity to light is measured in the dark-adapted peripheral retina, where rods dominate, a broad-sensitivity spectrum is found with a peak sensitivity at 507 nm. This rod spectral sensitivity function is called V’(λ) (Fig. 10.2A). Both V(λ) and V’(λ) functions have practical significance and have been accepted by the International Commission of Illumination as representative of human vision relative luminous efficiency at photopic and scotopic levels. They are also used to relate luminous (perceived energy of light) to radiant (emitted light) energy.

Estimates of the spectral sensitivities of the three cone types have been obtained from a variety of psychophysical procedures. One approach was to derive the spectral sensitivities of cones from analysis of color-matching data. Another approach used spectral bleaching lights to depress the responses of two cone types relative to the third so that measurements of the spectral sensitivity of the third cone type could be isolated. Figure 10.2B shows the relative spectral sensitivities of the three cone types. The S cones are most sensitive to light near 445 nm, with sensitivity declining rapidly at longer wavelengths. At 555 nm and longer wavelengths, the S cones are virtually unresponsive to light. The M and L cones have overlapping spectral sensitivities that span the entire visible spectrum. The M cones peak in sensitivity near 543 nm, while the L cones peak near 566 nm. The differential spectral sensitivity functions of the L, M, and S cones provide the foundation of early spectral processing.

Visual pathways for rod and cone functions

Retinal pathways

Rod signals are conveyed by two primary neural pathways that are dependent on the illumination level.5 One pathway is via ON rod bipolars, AII amacrine cells, and ON and OFF cone bipolars, which are all cells in the retina. This is a temporally sluggish pathway that mediates rod vision at low scotopic light levels. The second pathway transmits rod information via rod–cone gap junctions and ON and OFF cone bipolar cells in the retina. This is a fast pathway that mediates vision at higher scotopic and mesopic light levels. A third insensitive rod pathway between rods and OFF cone bipolars has been identified in rodents but, thus far, not in primates. The significant point here is that rods and cones share neural pathways and have joint inputs to retinal ganglion cells.

Retinogeniculate pathways

There are three major neural retinogeniculate pathways in primates that convey retinal information to the visual cortex.6,7 The pathways are named after the layers of the lateral geniculate nucleus (LGN) that receive input from distinct types of ganglion cells and project to different areas of the primary visual cortex. The MC layer of the LGN receives inputs from parasol ganglion cells. The MC pathway processes the summed output of the L and M cones to signal luminance information. The parvocellular (PC) layer of the LGN receives input from midget ganglion cells. The PC pathway mediates spectral opponency of L and M cones (discussed later) to signal chromatic information. The koniocellular (KC) layer of the LGN, which receives input from small bistratified as well as other ganglion cells, detects changes in S-cone signals compared to the sum of the L- and M-cone signals. These three pathways mediate different aspects of vision, with the MC pathway mainly carrying out luminance and motion processing, the PC pathway mainly processing red–green color, acuity, and shape information, and the KC pathway mainly handling blue–yellow color processing.

The sharing of neural pathways between rods and cones implies that rods should have input to the MC, PC, and KC pathways. Indeed, physiological studies have shown that there is strong rod input to the MC pathway, but weak input to the PC pathway.8 Demonstration of rod input to the KC pathways is less clear. An earlier study8 did not find rod input to the bistratified ganglion cells in the parafovea, while two recent studies demonstrated strong rod input in the peripheral retina.9,10 Figure 10.3 shows a schematic diagram of the visual pathways conveying rod and cone inputs to the MC, PC, and KC ganglion cells, which would then produce signals that are projected into the cortex to mediate different aspects of visual perception.

Dark adaptation functions: assessment of the shift from day vision to night vision

Measurements of sensitivity thresholds during adaptation to darkness have produced a characteristic biphasic function with an initial segment that is attributed to cone responses and a subsequent segment attributed to rod responses. Figure 10.4 shows a characteristic dark adaptation function measured in the peripheral retina. Thresholds decrease quickly initially and this rapid recovery is attributed to cones. Thresholds then reach a plateau in about 5 minutes and remain invariant for another 5 minutes (cone plateau, reflecting cone absolute thresholds). Then, there is a second rapid decrease in thresholds due to sensitivity recovery of the rods, referred to as the rod–cone break, to a new plateau that is reached in 40–50 minutes (rod plateau, reflecting rod absolute thresholds).

The shape of the dark adaptation function depends on testing parameters, including retinal location, wavelength, and temporal and spatial characteristics.1,11 The effects of these parameters on dark adaptation curves can be understood by the differences between rods and cones in terms of their distributions, spectral sensitivity, and spatial/temporal resolution characteristics. For instance, because there are only cones in the fovea, dark adaptation measured at the fovea using a small test light reveals a rapid monophasic branch attributable to the cones. On the other hand, because the rod and cone systems have similar absolute sensitivities at long wavelengths, dark adaptation measured with long-wavelength lights is monophasic, resembling the cone function. As the test wavelength is changed to shorter wavelengths, a biphasic curve emerges because the rods show greater absolute sensitivity than cones at shorter wavelengths.12

Clinical evaluation using dark adaptation functions

It is known that certain retinal disorders may selectively affect rods (e.g., retinitis pigmentosa) or cones (e.g., cone dystrophies). Clinically, rod and cone functions can be evaluated electrophysiologically by measuring rod and cone electroretinograms (ERG: see Chapter 7, Electrogenesis of the electroretinogram) or, psychophysically, by measuring dark adaptation functions. Dark adaptation functions quantify the ability of the rod and cone systems to recover sensitivity (i.e., regenerate photopigment) after exposure to light. The recovery is faster for cones, but the absolute level of sensitivity is greatest for rods. Variations in sensitivity and sensitivity recovery times can be used to characterize retinal disorders.

Clinical evaluation using dark adaptation functions involves a measure of the cone and rod absolute thresholds and the time of the rod–cone break. Specifically, rod absolute thresholds have been used as a psychophysical supplement to ERG measurement for night-blindness evaluation. The instrument for dark adaptation rod absolute threshold measurement is called a dark adaptometer and the most widely used is a Goldmann–Weekers dark adaptometer (Haag–Streit). This instrument is old, however, and finding replacement lamps is difficult. A new light-emitting diode-based dark adaptometer has recently become commercially available (LKC Technologies scotopic sensitivity tester-1: SST-1). The SST-1 dark adaptometer can determine a full dark adaptation curve as well as full-field scotopic sensitivities.

Color vision

Color vision refers to our ability to perceive colors based on spectral variations in light absorbed by the photoreceptors. Color vision includes both chromatic discrimination and color appearance appreciation. Color matching and color discrimination experimental tasks are two fundamental psychophysical procedures that have provided theoretical insights into the nature of color vision and have also been developed for clinical diagnosis of color vision. Color matching and color discrimination results, however, do not address questions of color appearance, for example, why an object appears red. Color appearance is far more complex because it depends on not only the chromatic properties of an object but also the spatial, temporal, and spectral characteristics of the neighboring objects.13 Neural processing beyond the retina is required for color appearance perceptions.

Color matching

Color-matching experimental techniques and data

Theoretically, a color match occurs when the photoreceptor quantal catch for the test stimulus and the quantal catch for the stimulus that is a combination of the three primary lights are the same. Color matches are unperturbed by changes in luminance levels, as long as no significant photopigment bleaching occurs, which means that the basic nature of trichromacy exists under wide variations in light levels. In a classical color-matching experiment, three spectral lights are chosen as the three primaries and the precise setting of the relative percentages of the three primaries to match a test light is the data for that match. When all three primaries are added together, they can be set to match a neutral white.

Procedurally, due to the mechanics of testing optical equipment, the spectral test color and one primary are made to appear in one field and the other two primaries appear in a second field with the two fields usually appearing as two halves of a circle. The observer’s task in a color-matching experiment is to make the two fields appear identical in color. Color-matching data are usually represented in one of two ways. In the first, the amounts of energy of each primary can be plotted as a function of the test wavelength. These are called color-matching or color mixture functions. In such plots, the primary that is added to the test color is given a negative value, and the other two primaries are given positive values. The second plotting method is to normalize the value of each matching primary relative to the sum of the three primaries, leading to the sum of the normalized primaries being equal to 1. Therefore, a plot of one primary value against a second primary value is sufficient to display all the information in the color-matching data. This kind of plot is called a chromaticity diagram, which shows the relative contributions of each spectral primary needed to match any spectral light.

The CIE colorimetric system

This spectral primary-based chromaticity diagram has proven to be very useful as a generalized color specification system. However, linear transformation is needed to compare data collected with different choices of primaries. In 1931, the Commission Internationale d’Eclairage (CIE) standardized the spectral-primary-based chromaticity system by adopting three imaginary primaries (X, Y, Z primaries) that are out of the spectral locus and therefore do not exist physically. The choices of the imaginary primaries were based on two important considerations: first, to ensure that the chromaticities of all physical lights have positive values and, second, to relate colorimetric functions to the previously adopted luminosity function, V(λ). In the system, the color-matching function for the Y primary is identical to V(λ) of the photometric system as designed.

Figure 10.5A shows the 1931 CIE chromaticity chart, in which the coordinates of the Y primary [y = Y/(X + Y + Z)] are plotted against those of the X primary [x = X/(X + Y + Z)]. The spectrum loci (their wavelengths are indicated on the graph) form the horseshoe-shaped curve. Equal-energy-spectrum (EES) light is plotted in the center, with the coordinates of x = 0.3333 and y = 0.3333. A straight line connects 400 nm to 700 nm for purples, which result from mixtures of short- and long-wavelength light. All possible lights occur within the boundaries of the spectrum locus and the purple line. Highly saturated colors occur near the locus and desaturated (pale) colors occur near the white point.

The 1931 CIE system was based on 2° field color-matching functions that were derived from color matches of many observers. The averaged color-matching function from these observers was treated as the standard. Therefore, an observer with the standard color-matching function is referred to as the standard observer. Since the color-matching data are affected by the size of the stimulus field presented to the observer, in 1964, the CIE also adopted a large-field XYZ system that was based on the 10° field standard observer color-matching functions. For large stimuli, such as those generated by Ganzfeld for ERG measurements, it is recommended to use the 1964 CIE chromaticities to reflect more accurately the large field color-matching functions.