Sensation is the detection and direct experience of physical energy as a result of environmental or internal events. Perception is the process by which sensory impulses are organized and interpreted. Sensation begins with the sense receptors, which convert the energy of a stimulus into electrical impulses that travel along nerves to the brain. Separate sensations can be accounted for by anatomical codes (as set forth by the doctrine of specific nerve energies) and functional codes in the nervous system. Sensory crossover from one modality to another can sometimes occur, and in synesthesia, sensation in one modality consistently evokes a sensation in another, but these experiences are rare.

Psychological scientists specializing in psychophysics have studied sensory sensitivity by measuring absolute and difference thresholds. Signal-detection theory, however, holds that responses in a detection task depend on both a sensory process and a decision process and will vary with the person's motivation, alertness, and expectations.

Our senses are designed to respond to change and contrast in the environment. When stimulation is unchanging, sensory adaptation occurs. Too little stimulation can cause sensory deprivation.

Selective attention prevents us from being overwhelmed by the countless stimuli impinging on our senses by allowing us to focus on what is important, but it also deprives us of sensory information we may need, as in inattentional blindness.

The stimulus for vision is light, which is a form of electromagnetic radiation. Vision is affected by the wavelength, intensity, and complexity of light, which produce the psychological dimensions of visual experience—hue, brightness, and saturation.

The visual receptors, rods and cones, are located in the retina of the eye. They send signals (via other cells) to the ganglion cells and ultimately to the optic nerve, which carries visual information to the brain. Rods are responsible for vision in dim light; cones are responsible for color vision.

Specific aspects of the visual world, such as lines at various orientations, are detected by feature-detector cells in the visual areas of the brain. Some of these cells respond maximally to complex patterns, and three separate groups of cells in the brain help us identify faces, places, and bodies.

The trichromatic and opponent-process theories of color vision apply to different stages of visual processing. In the first stage, three types of cones in the retina respond selectively to different wavelengths of light. In the second, opponent-process cells in the retina and the thalamus respond in opposite fashion to short and long wavelengths of light.

Perception involves the active construction of a model of the world from moment to moment. The Gestalt principles (e.g., figure and ground, proximity, closure, similarity, and continuity) describe visual strategies used by the brain to perceive forms. We localize objects in visual space by using both binocular and monocular cues to depth. Perceptual constancies allow us to perceive objects as stable despite changes in the sensory patterns they produce. Perceptual illusions occur when sensory cues are misleading or when we misinterpret cues.

Hearing (audition) is affected by the intensity, frequency, and complexity of pressure waves in the air or other transmitting substance, corresponding to the experience of loudness, pitch, and timbre of the sound.

The receptors for hearing are hair cells (topped by cilia) embedded in the basilar membrane, located in the organ of Corti in the interior of the cochlea. These receptors pass signals along to the auditory nerve. The sounds we hear are determined by patterns of hair-cell movement, which produce different neural codes.

Gestalt principles (such as proximity, figure/ground, continuity, or similarity) help us to make sense of our auditory world. When we localize sounds, we use as cues subtle differences in how pressure waves reach each of our ears. A few blind people are able to use echolocation to navigate.

Taste (gustation) is a chemical sense. Elevations on the tongue, called papillae, contain many taste buds, which in turn contain the taste receptors. The basic tastes include salty, sour, bitter, and sweet. Umami, associated with the taste of protein, has also been proposed as a basic taste, but this is debatable. In most protein-rich foods, umami is not detectable; also, responses to umami vary among individuals, and the main role of umami appears to be in the gut, after protein is eaten and digested.

Smell (olfaction) is also a chemical sense. No basic odors have been identified, and up to a thousand different receptor types exist. But researchers have discovered that distinct odors activate unique combinations of receptor types, and they have identified some of those combinations.

The skin senses include touch (pressure), warmth, cold, pain, and variations such as itch and tickle. Receptors for some types of itching and a possible receptor for cold have been discovered.

Pain has proven to be physiologically complicated, involving the release of several different chemicals and changes in both neurons and glial cells. According to the gate-control theory, the experience of pain depends on whether neural impulses get past a “gate” in the spinal cord and reach the brain; in addition, a matrix of neurons in the brain can generate pain even in the absence of signals from sensory neurons. A leading theory of phantom pain holds that it occurs when the brain rewires itself after amputation of a limb or removal of a body organ.

Kinesthesis tells us where our body parts are located and equilibrium tells us the orientation of the body as a whole. Together, these two senses provide us with a feeling of physical embodiment.

Many fundamental perceptual skills are inborn or acquired shortly after birth. However, without certain experiences during critical periods early in life, cells in the nervous system deteriorate, change, or fail to form appropriate neural pathways, and perception is impaired.

Psychological influences on perception include needs, beliefs, emotions, and expectations (which produce perceptual sets).

In the laboratory, simple visual subliminal messages can influence certain behaviors and judgments, depending on a person's motivational state (e.g., thirst). However, in everyday life complex behaviors cannot be altered by the many kinds of “subliminal-perception” recordings on the market.