Vision is the ability to detect light patterns from the outside environment and interpret them into images. Humans are bombarded with sensory information, and the sheer volume of visual information can be problematic. Fortunately, our visual system is able to attend to the most-important stimuli. The importance of vision to humans is further substantiated by the fact that about one-third of the human cerebral cortex is dedicated to analyzing and perceiving visual information.
As with auditory stimuli, light travels in waves. The compression waves that compose sound must travel in a medium—a gas, a liquid, or a solid. In contrast, light is composed of electromagnetic waves and needs no medium; light can travel in a vacuum (Figure 12.4.1). The behavior of light can be discussed in terms of the behavior of waves and also in terms of the behavior of the fundamental unit of light—a packet of electromagnetic radiation called a photon. A glance at the electromagnetic spectrum shows that visible light for humans is just a small slice of the entire spectrum, which includes radiation that we cannot see as light because it is below the frequency of visible red light and above the frequency of visible violet light.
Certain variables are important when discussing perception of light. Wavelength (which varies inversely with frequency) manifests itself as hue. Light at the red end of the visible spectrum has longer wavelengths (and is lower frequency), while light at the violet end has shorter wavelengths (and is higher frequency). The wavelength of light is expressed in nanometers (nm); one nanometer is one billionth of a meter. Humans perceive light that ranges between approximately 380 nm and 740 nm. Some other animals, though, can detect wavelengths outside of the human range. For example, bees see near-ultraviolet light in order to locate nectar guides on flowers, and some non-avian reptiles sense infrared light (heat that prey gives off).
Wave amplitude is perceived as luminous intensity, or brightness. The standard unit of intensity of light is the candela, which is approximately the luminous intensity of a one common candle.
Light waves travel 299,792 km per second in a vacuum, (and somewhat slower in various media such as air and water), and those waves arrive at the eye as long (red), medium (green), and short (blue) waves. What is termed “white light” is light that is perceived as white by the human eye. This effect is produced by light that stimulates equally the color receptors in the human eye. The apparent color of an object is the color (or colors) that the object reflects. Thus a red object reflects the red wavelengths in mixed (white) light and absorbs all other wavelengths of light.
Anatomy of the Eye
The photoreceptive cells of the eye, where transduction of light to nervous impulses occurs, are located in the retina(shown in Figure 12.4.2a) on the inner surface of the back of the eye. But light does not impinge on the retina unaltered. It passes through other layers that process it so that it can be interpreted by the retina (Figure 12.4.2b). The cornea, the front transparent layer of the eye, and the crystalline lens, a transparent convex structure behind the cornea, both refract (bend) light to focus the image on the retina. The iris, which is conspicuous as the colored part of the eye, is a circular muscular ring lying between the lens and cornea that regulates the amount of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil at its center. In conditions of low light, the iris relaxes and the pupil enlarges.
The main function of the lens is to focus light on the retina and fovea centralis. The lens is dynamic, focusing and re-focusing light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through it to focus it sharply on the retina. With age comes the loss of the flexibility of the lens, and a form of farsightedness called presbyopia results. Presbyopia occurs because the image focuses behind the retina. Presbyopia is a deficit similar to a different type of farsightedness called hyperopia caused by an eyeball that is too short. For both defects, images in the distance are clear but images nearby are blurry. Myopia (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry but images nearby are clear.
There are two types of photoreceptors in the retina: rods and cones, named for their general appearance as illustrated in Figure 12.4.3. Rods are strongly photosensitive and are located in the outer edges of the retina. They detect dim light and are used primarily for peripheral and nighttime vision. Cones are weakly photosensitive and are located near the center of the retina. They respond to bright light, and their primary role is in daytime, color vision.
The fovea is the region in the center back of the eye that is responsible for acute vision. The fovea has a high density of just cones. When you bring your gaze to an object to examine it intently in bright light, the eyes orient so that the object’s image falls on the fovea. This is the area of the retina that gives us high clarity of vision. However, when looking at a star in the night sky or other object in dim light, the object can be better viewed by the peripheral vision because it is the rods in higher concentrations in the other regions of the retina, rather than the cones at the center, that operate better in low light. In low-light conditions, the rods allow us to see in shades of gray because cones require bright light to be stimulated and don't respond in low light conditions.
Review the anatomical structure of the eye, clicking on each part to practice identification.
Processing Visual Input
There are three types of cones (with different photopsins), and they differ in the wavelength to which they are most responsive, as shown in Figure 12.4.4. Some cones are maximally responsive to short light waves of 420 nm, so they are called S cones (“S” for “short”); others respond maximally to waves of 530 nm (M cones, for “medium”); a third group responds maximally to light of longer wavelengths, at 560 nm (L, or “long” cones). With only one type of cone, color vision would not be possible, and a two-cone (dichromatic) system has limitations. Primates use a three-cone (trichromatic) system, resulting in full color vision.
The color we perceive is a result of the ratio of activity of our three types of cones. The colors of the visual spectrum, running from long-wavelength light to short, are red (700 nm), orange (600 nm), yellow (565 nm), green (497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans have very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and 20 steps of saturation, or about 2 million distinct colors.
Visual signals leave the cones and rods, travel to the bipolar cells, and then to ganglion cells. A large degree of processing of visual information occurs in the retina itself, before visual information is sent to the brain.
The myelinated axons of ganglion cells make up the optic nerves. Within the nerves, different axons carry different qualities of the visual signal. Some axons constitute the magnocellular (big cell) pathway, which carries information about form, movement, depth, and differences in brightness. Other axons constitute the parvocellular (small cell) pathway, which carries information on color and fine detail. Some visual information projects directly back into the brain, while other information crosses to the opposite side of the brain. This crossing of optical pathways produces the distinctive optic chiasma (Greek, for “crossing”) found at the base of the brain and allows us to coordinate information from both eyes.
Once in the brain, visual information is processed in several places, and its routes reflect the complexity and importance of visual information to humans and other animals.
Vision is the only photo responsive sense. Visible light travels in waves and is a very small slice of the electromagnetic radiation spectrum. Light waves differ based on their frequency (wavelength = hue) and amplitude (intensity = brightness).
In the vertebrate retina, there are two types of light receptors (photoreceptors): cones and rods. Cones, which are the source of color vision, exist in three forms—L, M, and S—and they are differentially sensitive to different wavelengths. Cones are located in the retina, along with the dim-light, achromatic receptors (rods). Cones are found in the fovea, the central region of the retina, whereas rods are found everywhere else throughout the retina.
Visual signals travel from the eye over the axons of retinal ganglion cells, which make up the optic nerves. Ganglion cells come in several versions. Some ganglion cell axons carry information on form, movement, depth, and brightness, while other axons carry information on color and fine detail.
Figure Which of the following statements about the human eye is false?
- Rods detect color, while cones detect only shades of gray.
- When light enters the retina, it passes the ganglion cells and bipolar cells before reaching photoreceptors at the rear of the eye.
- The iris adjusts the amount of light coming into the eye.
- The cornea is a protective layer on the front of the eye.
Why do people over 55 often need reading glasses?
- Their cornea no longer focuses correctly.
- Their lens no longer focuses correctly.
- Their eyeball has elongated with age, causing images to focus in front of their retina.
- Their retina has thinned with age, making vision more difficult.
Why is it easier to see images at night using peripheral, rather than the central, vision?
- Cones are denser in the periphery of the retina.
- Bipolar cells are denser in the periphery of the retina.
- Rods are denser in the periphery of the retina.
- The optic nerve exits at the periphery of the retina.
- (cd) unit of measurement of luminous intensity (brightness)
- describes a time cycle about one day in length
- weakly photosensitive, chromatic, cone-shaped neuron in the fovea of the retina that detects bright light and is used in daytime color vision
- transparent layer over the front of the eye that helps focus light waves
- region in the center of the retina with a high density of photoreceptors and which is responsible for acute vision
- (also, farsightedness) visual defect in which the image focus falls behind the retina, thereby making images in the distance clear, but close-up images blurry
- pigmented, circular muscle at the front of the eye that regulates the amount of light entering the eye
- transparent, convex structure behind the cornea that helps focus light waves on the retina
- (also, nearsightedness) visual defect in which the image focus falls in front of the retina, thereby making images in the distance blurry, but close-up images clear
- visual defect in which the image focus falls behind the retina, thereby making images in the distance clear, but close-up images blurry; caused by age-based changes in the lens
- small opening though which light enters
- layer of photoreceptive and supporting cells on the inner surface of the back of the eye
- main photopigment in vertebrates
- strongly photosensitive, achromatic, cylindrical neuron in the outer edges of the retina that detects dim light and is used in peripheral and nighttime vision
- superior colliculus
- paired structure in the top of the midbrain, which manages eye movements and auditory integration
- suprachiasmatic nucleus
- cluster of cells in the hypothalamus that plays a role in the circadian cycle
- tonic activity
- in a neuron, slight continuous activity while at rest
- sense of sight
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