Chapter 3 Sensation and Perception
第三章感知觉
There are four modules in this chapter. Module 8 tells us the concept of sense; module 9 introduces vision of human and how do we see the world; module 10 tells us how can we hear and feel the world; at last, we can learn from module 11 that how do we construct our view of the world—perception.
MODULE 8 SENSING THE WORLD AROUND US
In all sensory processes, some form of energy stimulates a receptor cell in one of the sense organs. The receptor cell converts that energy into a neural signal, which is further coded as it travels along sensory nerves. By the time it reaches the brain, the message is quite precise.
Sensation is the experience of sensory stimulation. Perception is the process of creating meaningful patterns from raw sensory information. Stimuli are the energy that produces a response in a sense organ.
Sensory Thresholds
The energy reaching a receptor must be sufficiently intense to produce a noticeable effect. The least amount of energy needed to generate any sensation at all in a person 50 percent of the time is called the absolute threshold. The difference threshold or the just noticeable difference (jnd) is the smallest change in stimulation that is detectable 50 percent of the time. Generally speaking, the stronger the stimulation, the bigger the change must be to be sensed. According to Weber's law, the jnd for a given sense is a constant fraction of the original stimulus. In most cases, our senses adjust to the level of stimulation they are experiencing, a process known as adaptation.
MODULE 9 VISION
Unlike most animals, humans rely most heavily on their sense of vision to perceive the world.
The Visual System
In the process leading to vision, light enters the eye through the cornea, then passes through the pupil(in the center of the iris) and the lens, which focuses it onto the retina. The lens changes its shape to allow light to be focused sharply on the retina. Directly behind the lens and on the retina is a depressed spot called the fovea, which lies at the center of the visual field. The retina of each eye contains the two kinds of receptor cells responsible for vision: rods and cones. Rods, chiefly responsible for
night vision, respond to varying degrees of light and dark but not to color. Cones respond to light and dark as well as to color or different wavelengths of light, and operate mainly in daytime. Only cones are present in the fovea.
Rods and cones connect to nerve cells, called bipolar cells, leading to the brain. In the fovea, a single cone generally connects with one bipolar cell. Rods, on the other hand, share bipolar cells. The one-to-one connection between cones and bipolar cells in the fovea allows for maximum visual acuity, the ability to distinguish fine details. Vision is thus sharpest whenever the image of an object falls directly on the fovea; outside the fovea, acuity drops dramatically. The sensitivity of rods and cones changes according to the amount of available light. Light adaptation helps our eyes adjust to bright light; dark adaptation allows us to see, at least partially, in conditions of darkness. An afterimage can appear until the retina adapts after a visual stimulus has been removed.
Neural messages originating in the retina must eventually reach the brain for a visual sensation to occur. The bipolar cells connect to ganglion cells, whose axons converge to form the optic nerve that carries messages to the brain. The place on the retina where the axons of the ganglion cells join to leave the eye is the blind spot.
At the base of the brain is the optic chiasm, where some of the optic nerve fibers cross to the other side of the brain.
Color Vision
The human vision system allows us to see an extensive range of colors. Hue, saturation, and brightness are three separate aspects of our experience of color. Hue refers to colors (red, green, blue, etc.), saturation indicates the vividness or richness of the hues, and brightness signals the intensity of the hues. Humans can distinguish only about 150 hues but, through gradations of saturation and brightness, we can perceive about 300,000 colors. Theories of color vision attempt to explain how the cones, which number only about 150,000 in the fovea, are able to distinguish some 300,000 different colors. One clue lies in color mixing: Additive color mixing is the process of mixing only a few lights of different wavelengths to create many new colors; subtractive color mixing refers to mixing a few pigments to come up with a whole palette of new colors.
Based on the principles of additive color mixing, the trichromatic theory of color vision holds that the eye contains three kinds of color receptors that are most responsive to either red, green, or blue light. By combining signals from these three basic receptors, the brain can detect any color and even subtle differences among nearly identical colors. This theory accounts for some kinds of colorblindness. People referred to as dichromats have a deficiency in either red-green or blue-yellow vision; monochromats see no color at all. People with normal color vision are referred to as trichromats. By contrast, the opponent-process theory maintains that receptors are specialized to respond to either member of the three basic color pairs: red-green, yellow-blue, and black-white (dark and light).
