Comprehensive Guide to Human Sensory Systems: Vision, Hearing, Taste, Smell, Touch, and Pain

Sensation

The sense organs' detection of external physical stimulus and the transmission of information about this stimulus to the brain.

Perception

The processing, organization, and interpretation of sensory information in the brain; these processes result in an internal neural representation of the physical stimulus.

Sensory receptors

Sensory organs that detect physical stimulation from the external world and change that stimulation into information that can be processed by the brain.

Transduction

A process by which sensory receptors change physical stimuli into signals that are eventually sent to the brain.

Absolute threshold

The smallest amount of physical stimulation required to detect a sensory input half of the time it is present.

Difference threshold

The minimum difference in physical stimulation required to detect a difference between sensory inputs.

Weber's law

A principle that states the difference threshold is a constant proportion of the original stimulus.

Just-noticeable difference

The smallest change in a stimulus that can be detected half the time.

Taste

1 teaspoon of sugar in 2 gallons of water.

Smell

1 drop of perfume diffused into the entire volume of six rooms.

Touch

A fly's wing falling on your cheek from a distance of 0.04 inch.

Hearing

The tick of a clock at 20 feet under quiet conditions.

Vision

A candle flame 30 miles away on a dark, clear night.

Sensory adaptation

A decrease in sensitivity to a constant level of stimulation.

Color perception

You perceive color based on physical aspects of light.

Visual organization

You perceive objects by organizing visual information.

Depth perception

When you perceive depth, you can locate objects in space.

Motion perception

Cues in your brain and in the world let you perceive motion.

Object constancy

You understand that objects remain constant even when cues change.

Light detection

Sensory receptors in your eyes detect light.

Iris

A circular muscle that gives eyes their color and controls the pupil's size to determine how much light enters the eye.

Lens

An adjustable, transparent structure behind the pupil that focuses light on the retina, resulting in a crisp visual image.

Retina

The thin inner surface of the back of the eyeball that contains the sensory receptors.

Rods

Sensory receptors in the retina that detect light waves and transduce them into signals processed in the brain as vision; they respond best to low levels of illumination and do not support color vision or detection of fine detail.

Cones

Sensory receptors in the retina that detect light waves and transduce them into signals processed in the brain as vision; they respond best to higher levels of illumination and are responsible for color and fine detail.

Fovea

A small region near the center of the retina where cones are densely packed.

Optic Nerve

A bundle of axons from ganglion cells that carries information from the eye to the brain.

Blind Spot

Areas in the left and right visual fields where the optic nerve exits the retina.

Thalamus

A brain structure through which information from the optic nerve passes before reaching the primary visual cortex.

Primary Visual Cortex

The area in the occipital lobes of the brain where visual information is processed.

Visible Light

Electromagnetic waves ranging in length from about 400 to 700 nanometers.

Amplitude

The height of the light wave from base to peak, experienced as brightness.

Wavelength

The distance from peak to peak of a light wave, determining the perception of hue.

Hue

The distinctive characteristics that place a particular color in the spectrum.

Trichromatic Theory

The theory that there are three types of cone receptor cells in the retina responsible for color perception, each responding optimally to different but overlapping ranges of wavelengths.

Opponent-Process Theory

The idea that ganglion cells in the retina receive excitatory input from one type of cone and inhibitory input from another type, creating the perception that some colors are opposites.

Gestalt Psychology

A psychological approach that postulates laws to explain how our brains group perceived features of a visual scene into organized wholes.

Figure and Ground

The distinction between an object (figure) and the background (ground) in a visual scene.

Grouping

The visual system's organization of features and regions to create the perception of a whole, unified object.

Bottom-Up Processing

The perception of objects due to analysis of environmental stimulus input by sensory receptors, influencing more complex, conceptual processing of that information in the brain.

Top-down processing

The perception of objects is due to the complex analysis of prior experiences and expectations within the brain; this analysis influences how sensory receptors process stimulus input from the environment.

Perceptual sets

Tendencies to perceive stimuli in specific ways that make sense given prior experiences and expectations.

Binocular depth cues

Cues of depth perception that arise because people have two eyes.

Monocular depth cues

Cues of depth perception that are available to each eye alone.

Binocular depth perception

We use both eyes to perceive depth through binocular disparity, where each retina has a slightly different view of the world.

Example of binocular depth perception

Glasses for watching 3D movies. Those glasses are polarized to allow different types of lights through, so each eye sees a slightly different version of the film.

Motion aftereffects

May occur when you gaze at a moving image for a long time and then look at a stationary scene.

Waterfall effect

A momentary impression that the new scene is moving in the opposite direction from the moving image.

Stroboscopic motion

Movies are made up of still images, each of which is slightly different from the one before it. When the series is presented fast enough, we perceive the illusion of motion pictures.

Color constancy

The perception of colors as constant even under varying lighting conditions.

Eardrum

A thin membrane that marks the beginning of the middle ear; sound waves cause the eardrum to vibrate.

Cochlea

A coiled, bony, fluid-filled tube in the inner ear that houses the sensory receptors.

Basilar membrane

Running through the center of the cochlea, it is a thin membrane.

Hair cells

Sensory receptors located in the cochlea that detect sound waves and transduce them into signals that ultimately are processed in the brain as sound.

Noise-induced hearing loss

Affects more than 5 million young people—nearly 13 percent of American children between the ages of 6 and 19.

60/60 rule

A guideline to protect hearing by listening to music at no more than 60% volume for no longer than 60 minutes.

Loudness in Decibels

A measurement of sound intensity, where different sounds have varying decibel levels.

Whisper

Safe for any length of time, approximately 30 decibels.

Normal conversation

Safe for any length of time, approximately 60 decibels.

Dishwasher

Safe for any length of time, approximately 70 decibels.

Vehicle traffic

85 decibels, safe for 8 hours.

Gas lawnmower

85 decibels, safe for 2 hours.

Earbuds or headphones

Listening to music on devices can reach 95-115 decibels, with damage occurring in 2 minutes.

Sirens

110-129 decibels, with damage occurring in less than 1 minute.

Jet engine

140 decibels, causing immediate damage without hearing protection.

Gunshot

165 decibels, causing immediate damage without hearing protection.

Rocket launch

180 decibels, causing immediate damage without hearing protection.

Frequency

The time between peaks in wavelength, measured in hertz, determining the pitch of sound.

Temporal coding

The perception of lower-pitched sounds based on the rate at which hair cells are stimulated by lower frequency sound waves.

Place coding

The perception of higher-pitched sounds based on the point on the basilar membrane where hair cells are stimulated.

Localization

The ear estimates the location of a sound based on when the sound arrives and the amplitude of the sound wave.

Vestibular sense

Allows us to maintain balance using information from receptors in the semicircular canals of the inner ear.

Taste (gustation)

The sense that detects flavor, processed through taste buds located in papillae on the tongue.

Taste buds

Structures located in papillae on the tongue that contain sensory receptors for taste.

Papillae

Structures on the tongue that contain groupings of taste buds.

Five main tastes

Sweet, sour, salty, bitter, and umami are the five basic taste qualities.

Supertasters

Individuals with nearly six times as many taste buds as normal tasters, highly aware of flavors and textures.

Taste preference

Influenced by the texture of food and cultural factors, beginning in the womb.

Olfaction

The sense of smell, which is the main way dogs perceive the world.

Olfactory receptors

Specialized receptors that detect odorants, with dogs having 40 times more than humans.

Olfactory epithelium

A thin layer of tissue, deep within the nasal cavity, containing the olfactory receptors; these sensory receptors produce information that is processed in the brain as smell.

Olfactory bulb

A brain structure above the olfactory epithelium in the nasal cavity; from this structure, the olfactory nerve carries information about smell to parts of the brain, including the primary olfactory cortex.

Odorants

Chemical molecules that are detected by olfactory receptors.

Human odor detection

According to a recent estimate, humans can distinguish more than a trillion odorants.

Prefrontal cortex

Information about whether a smell is pleasant or unpleasant is processed in the brain's prefrontal cortex.

Amygdala

The smell's intensity is processed in the amygdala, a brain area involved in emotion and memory.

Warm receptors

Sensory receptors in the skin that detect the temperature of stimuli and transduce it into information processed in the brain as warmth.

Cold receptors

Sensory receptors in the skin that detect the temperature of stimuli and transduce it into information processed in the brain as cold.

Pressure receptors

Sensory receptors in the skin that detect tactile stimulation and transduce it into information processed in the brain as different types of pressure on the skin.

Cranial nerves

The sensation of touch above the neck is sent to the brain directly via cranial nerves.

Spinal nerves

The sensation of touch below the neck is sent to the spinal cord, and then spinal nerves transmit the information to the primary somatosensory cortex in the brain.

Primary somatosensory cortex

The area of the brain that processes touch information.

Kinesthetic sense

Our kinesthetic sense tells us how our body and limbs are positioned in space.

Fast fibers

Sensory receptors in the skin, muscles, organs, and membranes around both bones and joints; these myelinated fibers quickly convey intense sensory input to the brain, where it is perceived as sharp, immediate pain.

Slow fibers

Sensory receptors in the skin, muscles, organs, and membranes around both bones and joints; these unmyelinated fibers slowly convey intense sensory input to the brain, where it is perceived as chronic, dull, steady pain.

Gate control theory

We experience pain when pain receptors are activated and a neural 'gate' in the spinal cord allows the signals through to the brain.

Distraction and pain perception

Distraction can reduce your perception of pain; listening to music is an extremely effective way to reduce postoperative pain, perhaps because it helps patients relax.