This chapter covers the basic processes of sensation and perception and is divided into sections on sensory processes, the sensory systems, perception, illusions, and perceptual development.

The scientific area that studies relations between the physical characteristics of stimuli and sensory capabilities is called psychophysics. Psychophysicists are interested in studying both the absolute limits of sensitivity and the sensitivity to distinguish between different stimuli. The lowest intensity at which a stimulus can be detected fifty percent of the time is called the absolute threshold of the stimulus. Signal detection theorists study the factors that influence such sensory judgments. There has been a lot of study of subliminal stimuli, stimuli so weak or brief that it cannot be perceived consciously, since the 1950s. Such studies have indicated that behavior cannot be controlled subliminally, but subliminal stimuli can affect attitudes, at least in the laboratory. The difference threshold (also known as the just noticeable difference, or jnd) is defined as the smallest difference between two stimuli that can be perceived fifty percent of the time. Weber's law states that the jnd is directly proportional to the magnitude of the stimulus with which the comparison is being made. For instance, the jnd for weight is 1/50, so if one object weighs 50g, then a second object would have to weight at least 51g for you to notice a difference in weight (or if one object weighs 100g, then a second object would have to weigh 102g for you to notice it). Sensory adaptation is the phenomenon of diminishing sensitivity to an unchanging stimulus.

Psychologists study a number of sensory systems, including vision, audition, gustation, olfaction, and the tactile senses. The eye consists of several important structures such as the lens and retina. Nearsightedness, or myopia, occurs when the lens focuses the visual image in front of the retina; farsightedness, or hyperopia, occurs when the image is focused behind the retina. Rods are black-and-white brightness receptors in the eye. Cones are color receptors. Bipolar cells have synaptic connections with rods and cones and also connect to ganglion cells, whose axons bundle to form the optic nerve. Transduction is the process by which the characteristics of a stimulus are converted into nerve impulses. People must adapt to both bright and dark conditions. The progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination (as when you step into a movie theatre) is called dark adaptation. There are several theories of color vision. The trichromatic theory developed by Young and Helmholtz suggests that there are three types of color receptors in the retina that are sensitive to blue, green, or red. The opponent-process theory suggests that each of the three different cone types responds to two different wavelengths: one to red or green, a second to blue or yellow, and a third to black or white. Dual-process theory combines both theories, as evidence has been found for both. About 7 percent of males and 1 percent of females have some form of color deficiency. Dichromats are color deficient in only one of the systems. A few people are monochromats, sensitive only to black and white. The optic nerve sends visual information to the thalamus, which in turn sends information to the primary visual cortex in the brain. Groups of neurons within the primary visual cortex called feature detectors are organized to receive and translate nerve impulses coming from the retina. Visual association cortex is where the final processes of constructing a visual representation occur.

The stimuli for hearing are sound waves, which have two characteristics: frequency (measured in hertz [Hz]) and amplitude (measured in decibels [db]). The transduction system for audition occurs in the inner ear. Vibrating activity of inner ear bones amplifies sound waves. When sound waves strike the eardrum, pressure created by the inner ear bones sets the fluid inside the cochlea into motion. The fluid waves that result vibrate the basilar membrane causing a bending of the hair cells in the organ of Corti. This bending triggers a release of neurotransmitters into the synapse between the hair cells and neurons of the auditory nerve, and nerve impulses are then sent to the brain. To use sound, we must code both pitch and loudness. Loudness is coded by a greater bend by the hair cells, resulting in the release of more neurotransmitters and a higher rate of firing in the auditory nerve. The frequency theory of pitch suggests that nerve impulses sent to the brain match the frequency of the sound wave. The place theory of pitch suggests that the specific point in the cochlea where the fluid wave peaks and most strongly bends the hair cells serves as a frequency coding cue. We can localize sounds because we have two ears, which give us binaural ability. Sounds arrive first and loudest at the ear closest to the sound, allowing us to figure out where it is coming from. More than 20 million people in the U.S. suffer from hearing loss. Conduction deafness occurs when there is a problem in the system that sends sound waves to the cochlea. Nerve deafness is caused by damage to the auditory nerve or to receptors in the inner ear.

Gustation, our sense of taste, depends on taste receptors concentrated on the tongue. Through neural activity we combine the four taste qualities (sweet, sour, salty, and bitter) in complex ways to create a distinctive "taste" for a given substance. Olfaction refers to our sense of smell. Humans have about 40 million olfactory receptors. Pheromones, chemical signals found in natural body scents, may affect human behavior. For instance, some studies show that women who live together or are close friends develop similar menstrual cycles, a phenomenon called menstrual synchrony.

The human tactile senses perceive at least four sensations: touch, pain, warmth, and cold. The sense of kinesthesis provides us with feedback about the positions of our muscles and joints, allowing us to coordinate body movements. Our vestibular sense is the sense of body orientation or equilibrium.

Perception is an active, creative process which can cause different people to experience exactly the same stimulus in very different ways. To create perceptions the brain uses both bottom-up and top-down processing. In bottom-up processing, a stimulus is broken down into its constituent parts and then combined and interpreted as a whole. In top-down processing, expectations and existing knowledge are used to interpret new information. Because there are so many stimuli impinging on our senses, we can only pay attention to a small fraction of them. Experiments with a technique called shadowing suggest that we are incapable of attending to more than one stimulus at a time, but we can shift attention rapidly from one stimulus to another. Attention is affected by both the nature of the stimulus and by personal factors. People are especially attentive to stimuli that might represent a threat to their well-being. People tend to organize the world to make it simpler to understand. Gestalt theorists suggested that people use top-down processing to organize their worlds. For instance, we tend to organize stimuli into both a foreground and background, a process called figure-ground relations. People group and interpret stimuli according to the four Gestalt laws of perceptual organization: similarity, proximity, closure, and continuity. Recognizing an image requires that we have a perceptual schema (a representation of the image in memory) to compare it with. We make interpretations of stimulus input and sensory information based on our knowledge and experience. For instance, you can recognize what you're sitting on right now as a chair or sofa based on your experience with such objects in the past. Perceptual sets are sets of expectations that affect our perceptions. Perceptual constancies allow us to recognize familiar stimuli under varying conditions, allowing us to enter into different environments and be able to function. Without perceptual constancies, we would have to relearn what stimuli are in each environment we enter.

We perceive depth through both monocular (one-eye) and binocular (two-eye) cues. For instance, light and shadow (a monocular cue) helps us to see "depth" in paintings. Each eye sees a slightly different image (binocular disparity), and the resulting disparity is analyzed by feature detectors in the brain, which allow us to see depth. The perception of movement requires the brain to perceive various movement cues. Illusions are incorrect perceptions that often result from the inaccurate perception of both monocular and binocular depth cues.

Finally, some kinds of perception depend on cultural learning or other environmental factors. Critical periods are intervals during which certain kinds of experiences must occur if perceptual abilities and the brain mechanisms that underlie them are to develop. For example, congenitally blind people whose vision is restored in adulthood are unable to learn certain visual tasks that people with normal sight often take for granted.