CHAPTER 5: SENSATION Sensing the World: Some Basic Principles To study sensation is to study an ageless question: How does the world out there get represented in here, inside our heads? Put another way, how are the external stimuli that strike our bodies transformed into messages that our brains comprehend? Thresholds Each species comes equipped with sensitivities that enable it to survive and thrive. We sense only a portion of the sea of energy that surrounds us, but to this portion we are exquisitely sensitive. Our absolute threshold for any stimulus is the minimum stimulation necessary for us to detect it 50 percent of the time. Signal detection researchers report that our individual absolute thresholds vary with our psychological state. Experiments reveal that we can process some information from stimuli too weak to recognize. But the restricted conditions under which this occurs would not enable unscrupulous opportunists to exploit us with subliminal messages. To survive and thrive, an organism must have difference thresholds low enough to detect minute changes in important stimuli. In humans, a difference threshold (also called a just noticeable difference, or jnd) increases in proportion to the size of the stimulus—a principle known as Weber’s law. Sensory Adaptation Sensory adaptation refers to our ability to adapt to unchanging stimuli. For example, when we smell an odor in a room we’ve just entered and remain in that room for a period of time, the odor will no longer be easily detected. The phenomenon of sensory adaptation focuses our attention on informative changes in stimulation by diminishing our sensitivity to constant or routine odors, sounds, and touches. Vision Each sense receives stimulation, transduces it into neural signals, and sends these neural messages to the brain. We have glimpsed how this happens with vision. The Stimulus Input: Light Energy The energies we experience as visible light are a thin slice from the broad spectrum of electromagnetic radiation. The hue and brightness we perceive in a light depend on the wavelength and intensity. The Eye After entering the eye and being focused by a camera-like lens, light waves strike the retina. The retina’s lightsensitive rods and color-sensitive cones convert the light energy into neural impulses, which are coded by the retina before traveling along the optic nerve to the brain. Visual Information Processing In the cortex, individual neurons called feature detectors, respond to specific features of a visual stimulus, and their information is pooled for interpretation by higher-level brain cells. Sub-dimensions of vision (color, movement, depth, and form) are processed separately and simultaneously, illustrating the brain’s capacity for parallel processing. The visual pathway faithfully represents retinal stimulation, but the brain’s representation incorporates our assumptions, interests, and expectations. Color Vision Research on how we see color supports two nineteenth-century theories. First, as the Young-Helmholtz trichromatic (three-color) theory suggests, the retina contains three types of cones. Each is most sensitive to the wavelengths of one of the three primary colors of light (red, green, or blue). Second, as opponent-process theory maintains, the nervous system codes the color-related information from the cones into pairs of opponent colors, as demonstrated by the phenomenon of afterimages and as confirmed by measuring opponent processes within visual neurons of the thalamus. The phenomenon of color constancy under varying illumination shows that our brains construct our experience of color. Hearing The Stimulus Input: Sound Waves The pressure waves we experience as sound vary in frequency and amplitude, and correspondingly in perceived pitch and loudness. The Ear Through a mechanical chain of events, sound waves traveling through the auditory canal cause minuscule vibrations in the eardrum. Transmitted via the bones of the middle ear to the fluid-filled cochlea, these vibrations create movement in tiny hair cells, triggering neural messages to the brain. Research on how we hear pitch supports both the place theory, which best explains the sensation of highpitched sounds, and frequency theory, which best explains the sensation of low-pitched sounds. We localize sound by detecting minute differences in the intensity and timing of the sounds received by each ear. Hearing Loss and Deaf Culture Hearing losses linked to conduction and nerve disorders can be caused by prolonged exposure to loud noise and by diseases and age-related disorders. Those who live with hearing loss face social challenges. Cochlear implants can enable some hearing for deaf children and most adults. But Deaf Culture advocates, noting that Sign is a complete language, question the enhancement. Additionally, deafness can lead to sensory compensation where other senses are enhanced. Advocates feel that this furthers their view that deafness is not a disability. Other Important Senses Touch Our sense of touch is actually four senses—pressure, warmth, cold, and pain—that combine to produce other sensations, such as "hot." One theory of pain is that a "gate" in the spinal cord either opens to permit pain signals traveling up small nerve fibers to reach the brain, or closes to prevent their passage. Because pain is both a physiological and a psychological phenomenon, it often can be controlled through a combination of physical and psychological treatments. Taste Taste, a chemical sense, is likewise a composite of five basic sensations—sweet, sour, salty, bitter, and umami—and of the aromas that interact with information from the taste buds. The influence of smell on our sense of taste is an example of sensory interaction. Smell Like taste, smell is a chemical sense, but there are no basic sensations for smell, as there are for touch and taste. Unlike the retina’s receptor cells that sense color by breaking it into component parts, the 5 million olfactory receptor cells with their 1000 different receptor proteins recognize individual odor molecules. Some odors trigger a combination of receptors. Like other stimuli, odors can spontaneously evoke memories and feelings. Body Position and Movement Finally, our effective functioning requires a kinesthetic sense, which notifies the brain of the position and movement of body parts, and a sense of equilibrium, which monitors the position and movement of the whole body.