04a Sensation

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Sensation and PerceptionChapter 3

Part I

William G. Huitt

Last revised: May 2005


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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 thathas registered in the brain but has not beeninterpreted

Perception

The process by which sensory information isactively organized and interpreted by the brain


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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 insensation 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


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Ernst Weber

Observed that the JND for all the senses

depends on a proportion or percentage ofchange rather than a fixed amount ofchange

Observation known as Webers law


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Sensory receptors Specialized cells in the sense organs that detect

and respond to sensory stimulilight, sound,

odorsand transduce (convert) the stimuli intoneural impulses

Provide the essential link between the physicalsensory world and the brain

Transduction Process where the receptors change or convert thesensory stimulation into neural impulses


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Vision


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Vision


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Vision


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Vision

Rods

Allow humans to see in black, white, and shades ofgray 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 inadequate light, but that do not function in dim light

Mostly in the fovea

Adapt fully to darkness in 2 3 minutes


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Vision

Trichromatic theory

First proposed by Thomas Young in 1802 andmodified by Hermann von Helmholtz about 50 years

later

The theory of color vision suggesting that there arethree types of cones, which are maximally sensitiveto red, green, or blue, and that varying levels of

activity in these receptors can produce all of thecolors


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Vision

S-Cones

(Sensitive to blue)

M-Cones

(Sensitive to Green)

L-Cones

(Sensitive to Red)

Three Types of Cones


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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 colorare 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 andthat corresponds to the amplitude of the light wave


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Vision

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

firing rate to signal one color and decrease theirfiring 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 theopponent cell begins to fire, producing theafterimage


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Vision


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Hearing


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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 thepitch of the sound

The human ear can hear sound frequencies from lowbass tones of around 20 Hz to high-pitched sounds ofabout 20,000 Hz


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Hearing

Amplitude

Measured in decibels, the magnitude or intensity ofa sound wave, determining the loudness of the

sound; the amplitude of a light wave affects thebrightness of a visual stimulus

The measuring unit used, bel, is named afterAlexander Graham Bell

Decibel A unit of measurement of the intensity or loudness

of sound based on the amplitude of the sound wave


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Hearing


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Hearing

Timbre

The distinctive quality of a sound that distinguishesit from other sounds of the same pitch and loudness

Human voices vary in timbre, providing us with away of recognizing individuals when we cant see

their faces

Timbres also vary from one instrument to another


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Hearing

Inner ear

The innermost portion of the ear, containing thecochlea, the vestibular sacs, and the semicircular

canals Cochlea

The snail-shaped, fluid-filled chamber in the inner ear thatcontains the hair cells (the sound receptors)

Hair cells Sensory receptors for hearing, found in the cochlea


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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 topof each nasal cavity, which together contain about

10 million olfactory neurons, the receptors for smell Olfactory bulbs

Two matchstick-sized structures above the nasalcavities, where smell sensations first register in thebrain


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Smell and Taste


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Smell and Taste

Pheromones

Chemicals excreted by humans and other animalsthat 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 consciouslyaware of it, respond to pheromones when it comesto mating


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Smell and Taste

Gustation The sensation of taste

Five basic tastes Sweet

Sour

Salty

Bitter

Umami Triggered by the substance glutamate (monosodium

glutamateis commercial product)


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Smell and Taste


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Skin Senses

Skin The largest organ of your body

Performs many important biological functions while

also providing much of what is known as sensualpleasure

Tactile Pertaining to the sense of touch

Information that is conveyed to the brain when anobject touches and depresses the skin, stimulatingone or more of the several distinct types ofreceptors found in the nerve endings


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Skin Senses


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Skin Senses

Pain

Motivates us to tend to injuries, to restrict activity,and to seek medical help

Teaches us to avoid pain-producing circumstancesin the future

Chronic pain

Pain that persists for three months or more

Three common types

Low-back

Headache

Arthritis


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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 transmitthem to the brain

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

Contend that messages from the brain to the spinal cord caninhibit the transmission of pain messages at the spinal gateand thereby affect the perception of pain


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Skin Senses

Endorphins

Chemicals, produced naturally by the pituitarygland, that reduce pain and positively affect mood

Some people release endorphins even when theyonly think they are receiving pain medication but aregiven, instead, a placebo in the form of a sugar pillor an injection of saline solution


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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 eachother and the movement of the entire body and/orits parts

Vestibular sense

The sense that provides information about thebodys movement and orientation in space throughsensory receptors in the semicircular canals and thevestibular sacs, which detect changes in themovement and orientation of the head

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