Essentials of Veterinary Ophthalmology. Kirk N. Gelatt
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of Vision
Introduction
The goal of this section is to describe the physical, anatomical, and physiological aspects of the process of vision, and it has been divided into two parts. The first part is devoted to light and vision and the different refractive structures of the eye. It covers the physical changes that light undergoes during its passage from the cornea, through the various structures of the eye, until it reaches the retina. The second part is devoted to the visual processing and describes what happens once light reaches the retina. This part is dedicated to the neuronal processes of vision and describes the generation, processing, and propagation of the visual signal in the retina, the visual pathways, and the cortex. Both these parts lay the foundations for understanding how optical and neuronal processes enable the detection of movement, details, and color that create the rich experience of vision described in last section of this chapter.
Visual Optics
Physical Optics
Light
Light has alternately been described as a wave or as photon particles. However, these descriptions are not mutually exclusive. Both models also are applicable in the eye: the wave theory explains the physical changes light undergoes during its passage through the eye, and the particle theory explains the energy transformation that occurs when light strikes the outer segments of the photoreceptors. Hence, the first part of this chapter discusses light as a wave, while the second part discusses it as a particle.
Light behaves as a wave as it passes through transparent media such as air, vacuum, or the visual axis of the eye. Much like a wave of water, a wave of light has two principal characteristics (Figure 2.8). Its amplitude, A, is the maximum value of the field generated by the propagating wave; it determines the wave's intensity. The wavelength, λ , is the distance between adjacent wave crests; it determines the wave's location in the electromagnetic spectrum. Light, which is the visible portion of the electromagnetic spectrum, occupies a small fraction of that spectrum, which ranges from cosmic and gamma rays ( λ < 10−10 m) to radio transmission ( λ > 103 m). In humans, visible light normally ranges in wavelength from 380 nm (i.e., deep blue) to 780 nm (i.e., deep red). However, additional wavelengths, outside the 380–780 nm spectrum, can be seen by other species. Many nonmammalian species, and some mammals, possess ultraviolet (UV) vision that allows them to detect light with a wavelength shorter than 380 nm, enabling them to see hues that are not perceived by humans; this capability is used in both foraging and courting behavior. The cat retina has also been shown to respond to infrared (IR) light (826–875 nm), though the functional and behavioral implications of this capability are not clear. This is not to be confused with the IR “vision” of snakes, which relies on the heat detection properties of the pit organs.
Figure 2.8 Representation of light as a wave, which is characterized by two parameters. Its amplitude (A) is the maximum value the wave obtains as it propagates. Its wavelength ( λ ) is the distance between two consecutive peaks.
At the same time, light also possesses the properties of particles, termed photons, which represent quanta of energy that can be emitted (at the light source) or absorbed (e.g., by retinal photoreceptors). The amount of energy in a given photon is inversely proportional to its wavelength; therefore, blue light possesses more energy than red light, which has a longer wavelength. An example of the particle nature of light is seen in the use of cobalt blue light to highlight fluorescein staining of corneal ulcers. Fluorescein sodium molecules absorb photons of blue light and reemit photons with lower energy content, in the yellow‐green portion of the spectrum, in a process known as fluorescence.
As light strikes the photoreceptor outer segments, it is absorbed by a visual photopigment. The function of this two‐part molecule reflects the principles of quantum physics, as it utilizes both the wave properties and the particle properties of light. The first part of the molecule, the opsin, determines the wavelength of the light that the photopigment will absorb, thus determining color vision. The second part of the molecule, the visual chromophore or retinal, uses the energy of the photon to undergo isomerization (from 11‐cis‐retinal into all‐trans retinal in the case of rhodopsin), thereby initiating conversion of a light stimulus into an electric signal. This process, the phototransduction process, which is discussed in detail later in this chapter, is the first step in the propagation of a visual signal.
Photometry
Photometry is the quantitative measurement of visible light. Photometry measures a number of interrelated properties of light, using a basic unit called a candela. Two important characteristics of light are its luminous intensity, which describes the intensity of a light source (as measured in candela), and its luminance, which describes its brightness reflected from a surface (as measured in foot‐lamberts or cd/m2). These two properties are related, but they are not necessarily proportional. A handheld transilluminator is a bright source of light, but it possesses low intensity and therefore cannot be used to illuminate a football stadium. On the other hand, a streetlight provides high‐intensity light, which illuminates a large area, but it is not bright and does not provide enough illumination to conduct cataract surgery (Table 2.12).
Table 2.12 Luminances of natural and artificial light sources.a
| Source | Luminance (cd/m2) |
|---|---|
| Sun | 109 |
| Car light | 107 |
| Incandescent tungsten lamp | 106–107 |
| Fluorescent lamp | 104–105 |
| Clear sky at noon | 104 |
| Full moon | 103 |
| Street lamp | 0.1–1.0 |
| Moonless night sky | 10−3 to 10−6 |
a In general, only the photopic system is active at a luminance >3 cd/m2; at a luminance <0.03 cd/m2, the scotopic system functions alone. Both systems are active at intermediate luminance values, which are defined as mesopic vision.
Luminance is measured using photometers, which are divided into two major classes. Visual photometers provide a subjective reading, because the observer compares the illumination of the measured light with that of a standard light. Photoelectric photometers convert the measured light into an electric current, which is displayed by the instrument. Photometry measurements are extremely important in electroretinographic (ERG) recordings because they are used to describe such variables as threshold, ambient light, and stimulus parameters.
Transmission and Reflection
Human vision is limited to a wavelength range of 380–780 nm. This limitation is a result of two factors: the first is the absorption spectrum of the opsin component of the visual photopigment, and the second limiting