Sensation and PerceptionChapter 3

Part I

William G. Huitt

Last revised: May 2005

Sensation and Perception

• Sensation– The process through which the senses pick up

visual, auditory, and other sensory stimuli and transmit them to the brain; sensory information that has registered in the brain but has not been interpreted

• Perception– The process by which sensory information is

actively organized and interpreted by the brain

Process of Sensation

• Absolute threshold– The minimum amount of sensory stimulation that

can be detected 50% of the time

• Difference threshold– The smallest increase or decrease in a physical

stimulus required to produce a difference in sensation that is noticeable 50% of the time

– Just noticeable difference (JND)– The smallest change in sensation that a person is

able to detect 50% of the time

Process of Sensation

• Ernst Weber– Observed that the JND for all the senses

depends on a proportion or percentage of change rather than a fixed amount of change

– Observation known as Weber’s law

Process of Sensation

• Sensory receptors– Specialized cells in the sense organs that detect

and respond to sensory stimuli—light, sound, odors—and transduce (convert) the stimuli into neural impulses

– Provide the essential link between the physical sensory world and the brain

• Transduction– Process where the receptors change or convert the

sensory stimulation into neural impulses

Process of Sensation

• Sensory adaptation– The process of becoming less sensitive to an

unchanging sensory stimulus over time– Allows you to shift your attention to what is most

important at any given moment

Vision

Vision

Vision

Vision

• Rods– Allow humans to see in black, white, and shades of

gray in dim light– Mostly in the periphery– Take 20 – 30 minutes to fully adapt to darkness

• Cones– Enable humans to see color and fine detail in

adequate light, but that do not function in dim light– Mostly in the fovea– Adapt fully to darkness in 2 – 3 minutes

Vision

• Trichromatic theory– First proposed by Thomas Young in 1802 and

modified by Hermann von Helmholtz about 50 years later

– The theory of color vision suggesting that there are three types of cones, which are maximally sensitive to red, green, or blue, and that varying levels of activity in these receptors can produce all of the colors

Vision

S -C o n es(S en sit iv e to b lue)

M -C o n es(S en sit ive to G ree n)

L -C on es(S en sit ive to R e d)

T h ree T yp es o f C on es

Vision

• Hue– The property of light commonly referred to as color,

determined primarily by the wavelength of light reflected from a surface

• Saturation– The degree to which light waves producing a color

are of the same wavelength; the purity of a color

• Brightness– The dimension of visual sensation that is dependent

on the intensity of light reflected from a surface and that corresponds to the amplitude of the light wave

Vision

• Opponent-process theory– The theory that three classes of cells increase their

firing rate to signal one color and decrease their firing rate to signal the opposing color (red/green, yellow/blue, white/black)

• Afterimage– After you have stared at one color in an opponent-

process pair (red/green, yellow/blue, black/white), the cell responding to that color tires and the opponent cell begins to fire, producing the afterimage

Vision

Hearing

Hearing

• Audition– The sensation of hearing; the process of hearing

• Robert Boyle– Demonstrated that sound requires a medium through

which to move, such as air, water, or a solid object

• Frequency– Measured in the unit called the hertz, the number of

sound waves or cycles per second, determining the pitch of the sound

– The human ear can hear sound frequencies from low bass tones of around 20 Hz to high-pitched sounds of about 20,000 Hz

Hearing

• Amplitude– Measured in decibels, the magnitude or intensity of

a sound wave, determining the loudness of the sound; the amplitude of a light wave affects the brightness of a visual stimulus

– The measuring unit used, bel, is named after Alexander Graham Bell

• Decibel– A unit of measurement of the intensity or loudness

of sound based on the amplitude of the sound wave

Hearing

Hearing

• Timbre– The distinctive quality of a sound that distinguishes

it from other sounds of the same pitch and loudness– Human voices vary in timbre, providing us with a

way of recognizing individuals when we can’t see their faces

– Timbres also vary from one instrument to another

Hearing

• Inner ear– The innermost portion of the ear, containing the

cochlea, the vestibular sacs, and the semicircular canals

– Cochlea• The snail-shaped, fluid-filled chamber in the inner ear that

contains the hair cells (the sound receptors)

– Hair cells• Sensory receptors for hearing, found in the cochlea

Smell and Taste

• Olfaction– The sensation of smell; the process of smelling– You cannot smell a substance unless some of its

molecules vaporize

• Olfactory epithelium– Two 1-square-inch patches of tissue, one at the top

of each nasal cavity, which together contain about 10 million olfactory neurons, the receptors for smell

• Olfactory bulbs– Two matchstick-sized structures above the nasal

cavities, where smell sensations first register in the brain

Smell and Taste

Smell and Taste

• Pheromones– Chemicals excreted by humans and other animals

that act as signals to, and elicit certain patterns of, behavior from members of the same species

– Used by animals to mark off territories and to signal sexual receptivity

• Karl Grammer– Suggested that humans, although not consciously

aware of it, respond to pheromones when it comes to mating

Smell and Taste

• Gustation– The sensation of taste

• Five basic tastes– Sweet– Sour– Salty– Bitter– Umami

• Triggered by the substance glutamate (monosodium glutamate is commercial product)

Smell and Taste

Skin Senses

• Skin– The largest organ of your body– Performs many important biological functions while

also providing much of what is known as sensual pleasure

• Tactile– Pertaining to the sense of touch– Information that is conveyed to the brain when an

object touches and depresses the skin, stimulating one or more of the several distinct types of receptors found in the nerve endings

Skin Senses

Skin Senses

• Pain– Motivates us to tend to injuries, to restrict activity,

and to seek medical help– Teaches us to avoid pain-producing circumstances

in the future

• Chronic pain– Pain that persists for three months or more– Three common types

• Low-back• Headache• Arthritis

Skin Senses

• Melzack and Wall– Gate-control theory

• Contend that there is an area in the spinal cord that can act like a “gate” and either inhibit pain messages or transmit them to the brain

• You feel pain when pain messages carried by the small, slow-conducting nerve fibers reach the gate and cause it to open

• Contend that messages from the brain to the spinal cord can inhibit the transmission of pain messages at the spinal gate and thereby affect the perception of pain

Skin Senses

• Endorphins– Chemicals, produced naturally by the pituitary

gland, that reduce pain and positively affect mood– Some people release endorphins even when they

only think they are receiving pain medication but are given, instead, a placebo in the form of a sugar pill or an injection of saline solution

Spatial Orientation Senses

• Kinesthetic sense– The sense providing information about relative

position and movement of body parts– Gives the position of body parts in relation to each

other and the movement of the entire body and/or its parts

• Vestibular sense– The sense that provides information about the

body’s movement and orientation in space through sensory receptors in the semicircular canals and the vestibular sacs, which detect changes in the movement and orientation of the head