1.6 Sensation

Transduction is converting one form of energy, like light or sound waves, into neural impulses. The absolute threshold is the weakest stimulus you can detect at least 50% of the time. The just-noticeable difference (JND), or difference threshold, is the smallest change you can notice 50% of the time.

Weber's law says the JND is a constant proportion of the original stimulus, not a fixed amount. If you can just notice adding 2 grams to 100 grams, you'll need about 20 grams to notice a change to 1,000 grams. Bigger starting stimuli need bigger changes.

Sensory adaptation is your reduced sensitivity to a constant, unchanging stimulus. You stop feeling your socks or smelling your own house because your receptors fire less. Senses also work together (sensory interaction): food tastes bland with a stuffy nose because smell is a big part of flavor. In synesthesia, stimulating one sense produces an experience in another, like seeing colors when hearing music.

Light passes through the cornea and pupil, and the lens focuses it onto the retina, the light-sensitive layer at the back of the eye. The lens changes shape to focus on near or far objects, a process called accommodation. If the eye focuses the image in front of the retina, distant objects look blurry (nearsightedness). If it focuses behind the retina, nearby objects look blurry (farsightedness).

The retina has two kinds of receptor cells. Rods sit mostly in the periphery, work in dim light, detect shapes and movement, and don't see color; they're key to adapting to darkness. Cones cluster in the fovea, the center of the retina, and handle color and fine detail in good light. Signals pass to ganglion cells, whose axons form the optic nerve. Where the optic nerve leaves the eye there are no receptors, creating a blind spot, which your brain fills in so you don't notice it.

Color vision needs two theories. Trichromatic theory says you have three types of cones, sensitive to short (blue), medium (green) and long (red) wavelengths, and their combined activity produces every color. Opponent-process theory says cells beyond the cones, starting with ganglion cells, work in opposing pairs: red versus green, blue versus yellow and black versus white. It explains afterimages: stare at a green shape, then look at a white wall, and you'll see a red one. Color vision deficiency comes from missing or irregular cones or ganglion cells. People with dichromatism have two working cone types (red-green deficiency is most common), and people with monochromatism see no color at all.

Damage to visual areas of the brain, mainly in the occipital lobes, can cause prosopagnosia, an inability to recognize faces, or blindsight, where a person reports seeing nothing in part of their visual field but can still point to objects there more accurately than chance.

Sound is vibrating air. The frequency of the waves determines pitch, and their amplitude (height) determines loudness. Sound travels through the eardrum and three tiny bones in the middle ear to the fluid-filled cochlea, where hair cells transduce vibrations into neural signals sent along the auditory nerve.

Place theory says different pitches vibrate different places along the cochlea's membrane; it best explains high pitches. Frequency theory says the auditory nerve fires at the same rate as the sound wave; it explains low pitches. Since a single neuron can fire only about 1,000 times per second, volley theory adds that groups of neurons take turns firing to signal higher frequencies.

You locate sounds because a sound reaches your nearer ear slightly sooner and louder. Conduction deafness comes from damage to the eardrum or middle-ear bones, which conduct sound. Sensorineural deafness comes from damage to the hair cells or auditory nerve, often from loud noise or aging.

Smell receptors in the nose send signals to the brain's olfactory areas. Smell is the only sense that doesn't route first through the thalamus, and its close links to emotion and memory areas help explain why a scent can trigger a vivid memory. Pheromones are chemical signals between members of the same species.

Taste (gustation) includes sweet, sour, salty, bitter, umami (savory) and oleogustus (the taste of fat). Taste receptors in taste buds on the tongue and mouth transduce chemicals. People with many taste receptors are supertasters who find bitter foods intense; medium tasters and nontasters have fewer. Without smell, taste is muted or lost.

Skin receptors detect pressure, warmth, cold and pain. 'Hot' isn't its own receptor: the burning-hot feeling comes from warm and cold receptors firing at the same time.

Pain is processed in both the body and the brain. Gate control theory proposes that the spinal cord has a neural 'gate' that can block or let through pain signals; rubbing a bumped elbow or being distracted can close it. Phantom limb sensation, when people feel sensation or pain in a limb they no longer have, shows that pain is created in the brain.

Your vestibular sense, detected mainly by the fluid-filled semicircular canals in the inner ear, keeps your balance. Kinesthesis is your sense of your body parts' position and movement, from receptors in muscles and joints, which lets you touch your nose with your eyes closed.