AP Psychology 1.6: Sensation

Sensation is the process of detecting information from the world around you. Each sense has receptor cells tuned to one kind of energy, such as light, sound waves or chemicals. When a stimulus is strong enough, the receptors transduce it: they turn it into neurochemical messages that travel to the brain for processing.

How strong is strong enough? The absolute threshold is the faintest level of a stimulus a person can detect at least 50% of the time. A whisper just below it goes unheard more often than not.

Noticing a change works the same way. The just-noticeable difference is the smallest change in a stimulus a person can detect. Weber's law says that difference is a constant proportion of the starting stimulus, not a fixed amount. If you can just feel 1 extra gram added to a 50-gram letter, you would need about 2 extra grams to feel a change in a 100-gram one.

Senses also turn themselves down. In sensory adaptation, sensitivity to a stimulus that does not change fades: the watch on your wrist, the smell of your own house.

And the senses never work alone. Sensory interaction is the way they constantly work together, so what you see can change what you hear. Synesthesia is rarer: one sense is experienced through another, as when a person sees a color every time a certain note plays.

The lens focuses light onto the retina, the light-sensitive surface at the back of the eye. To focus near or far, the lens changes shape: accommodation. When that goes wrong, nearsightedness (far things blur) or farsightedness (near things blur) can result.

The retina has two kinds of receptor cells. Rods lie toward the edges. They detect shape and movement but not color, work mainly in dim light, and play a role in dark adaptation (slowly seeing better in a dark room) and light adaptation (adjusting back to bright light). Cones are packed into the fovea, the center of the retina, and handle color and fine detail.

Two theories explain color vision. In the trichromatic theory, cones come in three kinds, responding best to short (blue), medium (green) or long (red) wavelengths. In the opponent-process theory, ganglion cells in the retina work in pairs: red/green, blue/yellow, black/white. Stare at red and its side of the pair tires, so a white wall then looks green: an afterimage. Damaged cones or ganglion cells cause color vision deficiency, such as dichromatism (only two working cone types) or monochromatism (little or no color).

Ganglion cells send messages along the visual nerve to the thalamus, then mainly to the occipital lobes. Where that nerve leaves the eye there are no receptors, the blind spot, and the brain fills the gap. Damage to the occipital lobes can cause prosopagnosia (face blindness) or blindsight, responding to things a person does not consciously see.

Sound is air molecules moving in waves. A wave's wavelength, the distance from one peak to the next, sets its pitch: shorter waves arrive more often each second and sound higher. Its amplitude, the height of each wave, sets its loudness.

Receptor cells in the inner ear transduce the vibrations. Three theories explain how pitch is coded:

• Place theory: each pitch most strongly stirs receptors at its own place in the inner ear. It works best for high pitches.
• Frequency theory: the nerve to the brain fires at the same rate the sound vibrates. It works best for low pitches.
• Volley theory: no single neuron can fire as fast as a higher sound vibrates, so groups of neurons take turns, and their combined firing keeps pace.

Sound localization is working out where a sound comes from. A sound on your left reaches your left ear a split second sooner, and a little louder, than your right, and the brain uses that difference. A sound straight ahead or straight behind reaches both ears at once, so those two are easy to confuse.

Hearing can fade with age or after damage. Conduction deafness comes from damage to the parts that carry vibrations into the inner ear, such as the eardrum. Sensorineural deafness comes from damage to the receptor cells of the inner ear or to the nerve that carries their messages, which years of loud noise can cause.

Smell (olfaction) and taste (gustation) are the chemical senses. Receptors in the nose transduce airborne chemicals. Smell is the only sense not processed first in the thalamus. Pheromones are chemicals an animal releases that carry messages to another's olfactory system.

Receptors on the tongue and in the mouth detect six tastes: sweet, sour, salty, bitter, umami (savory, as in broth) and oleogustus (fat). The more taste receptors on a person's tongue, the more sensitive they are: supertasters have the most, then medium tasters, then nontasters. The chemical senses also interact. Without smell, taste is muted or lost.

Receptors in the skin transduce touch, and the brain processes it. The feeling of hot comes from warm and cold receptors firing at once.

Pain is processed in both the body and the brain. Gate control theory proposes a gate in the spinal cord that lets pain messages through or blocks them; other signals, such as rubbing a bumped knee, or messages from the brain can close it. Phantom limb sensation is feeling sensation or pain where a lost limb used to be.

The vestibular sense controls balance. It is detected mainly by the semicircular canals of the inner ear and by structures in the brain. Kinesthesis is the sense of your body's movement. It lets you move in coordinated ways without watching your arms and legs.