Sensation and Perception

THE BASICS

The Basics• Sensation the stimulation of sensory receptors

and the transmission of sensory information to the central nervous system– Sensory receptors are located in sensory organs such

as the eyes and ears– Stimulation of the senses is automatic

• Perception the psychological process through which we interpret sensory stimulation– Perception reflects learning, expectations, and

attitudes

Absolute Threshold

• Absolute Threshold the weakest amount of a stimulus that can be sensed

• The absolute thresholds for sight, hearing, smell, touch, and taste have been determined for humans

• However, each individual person may vary on their own absolute threshold– These differences stem from psychological and

biological differences

Absolute Thresholds for HumansSENSE STIMULUS RECEPTORS THRESHOLD

Vision Electromagnetic energy

Rods and cones in the retina

A candle fame viewed from a distance of about 30 miles on a dark night

Hearing Sound waves Hair cells of the inner ear

The ticking of a watch from about 20ft away in a quiet room

Smell Chemical substances in the air

Receptor cells in the nose

About one drop of perfume diffused through a small house

Taste Chemical substances in saliva

Taste buds on the tongue

About 1 teaspoon of sugar dissolved in 2 gallons of water

Touch Pressure on the skin Nerve endings in the skin

The wing of fly falling on a cheek from a distance of about .4in

Difference Threshold

• Difference threshold the minimum amount of difference that can be detected between two stimuli

Signal-Detection Theory• Signal-detection theory a method of

distinguishing sensory stimuli that takes into account not only their strengths but also such elements as the setting, your physical state, your mood, and your attitudes

• Also takes into account psychological factors such as motivations, expectations, and learning

Sensory Adaptation

• Sensory adaptation process by which we become more sensitive to weak stimuli and less sensitive to unchanging stimuli

• Examples: – Your eyes beginning to adjust to

the limited light in a movie theater– “Tuning out” background noises

such as traffic, waves at the beach, or “white noise”

VISION

Light• Light is electromagnetic energy, described in wavelengths• Not all light is visible to the human eye; in fact, the light that

humans can see only makes up a small portion of the electromagnetic spectrum

• Light in the visible spectrum can be broken down into colors

The Eye

How does the eye enable humans to see?

• Light enters the eye and then is projected onto a surface• The amount of light that enters the eye is determined by

the size of the pupil which adjusts automatically to the amount of light entering the eye

• Once light enters the eye, it encounters the lens which adjusts to the distance of objects by changing its thickness

• These changes project a clear image of the object onto the retina, which consists of neurons that are sensitive to the light called photoreceptors

• Once the light hits the photoreceptors, a nerve carries the visual input into the brain where the information is relayed to the visual area of the occipital lobe

The Blind Spot

• Located where the optic nerve leaves the eye• When light hits that point, the eye registers

nothing because that area lacks photoreceptors

• This blind spot is necessary for all humans to see; without it, no visual input would reach the brain through the optic nerve for interpretation

Rods and Cones

• Rods and cones are two forms of photoreceptors

• Rods are sensitive only to the brightness of light; they only allow us to see in black and white

• Cones allow humans to see in color

Visual Acuity• Visual acuity the sharpness of

vision• The Snellen Vision Chart

measures visual acuity• Ex: If you were standing 20ft

from the chart and could only read the T or Z, your visual would be 20/100

• This means that a person with “normal” vision could see what you see at 20ft away from, 100ft away

Color Vision

• People with normal vision can see any color in the spectrum of visible light

• Cones, the photoreceptors that allow us to see light, are sensitive to different colors

• Afterimage seen as the complement of the original color; occurs when you have viewed a color for a while and then the color is removed

Color Blindness

• People who do not have normal color vision are said to be color blind

• These people are partially or totally unable to distinguish color due to an absence of, or malfunction in, the cones

• People with total color blindness are sensitive only to light and dark and see the world as black-and-white– Very rare

• People with partial color blindness see some colors, but not all

HEARING

Pitch

• The pitch of a sound – how high or low the sound is – depends on its frequency (the number of cycles per second)

• The human ear can head sound waves that vary from 20 to 20,000 cycles per second

• Many animals, including dogs, can hear sounds well beyond 20,000 cycles per second

Loudness

• The loudness of a sound is determined by the height, or amplitude, of sound waves

• The higher the amplitude of the wave, the louder the sound

• The loudness of sound is measured in decibels (dB)

• Zero dB is the absolute threshold of sound

Decibel Ratings for Some Familiar Sounds

Sound Creator Pain Level

Jet Engine/Shotgun blast

Severe pain

Rock concert/Thunder

Pain Threshold

Subway Train/Average Car

Loud

Conversation at 3ft/Leaves rustling

Moderate

2 people whispering/Watch ticking

Hearing Threshold

-150-140-130-120-110-100-90-80-70-60-50-40-30-20-10-0

The Ear

How the ear hears sound

• Sound enters the outer ear and is funneled to the eardrum

• Inside the middle ear, the hammer, anvil, and stirrup vibrate, transmitting the sound to the inner ear

• Within the brain, auditory input is projected onto the hearing areas of the cerebral cortex

Deafness

• Conductive deafness occurs because of damage to the middle ear– Often helped by hearing aids

• Senorineural deafness usually caused by damage to the inner ear– Can also be caused by damage to the auditory

nerve through disease or prolonged exposure to very loud sounds

– Can be helped by a cochlear implant

OTHER SENSES

Smell• Smell is necessary for the

full use of the taste sense• Odors are detected by

receptors high in each nostril

• Receptors react with molecules of the substance in the form of a gas

• Olfactory nerve sends information from the nose to the brain

Taste

• Four basic taste qualities – sweet, sour, salty, and bitter

• Some scientists argue for a fifth taste quality called umami, a Japanese word meaning savory

• Taste is sensed through taste buds on the tongue

The Skin Senses• Sensory receptors are located around the roots

of hair cells and react to touch• Fingertips, lips, nose, and cheeks are more

sensitive to pressure• Sensation of temperature is relative; for example,

when you are feverish, you might perceive other peoples’ skin as cool

• Receptors for temperature are located just beneath the skin

The Skin Senses• Not all areas of the body are equally sensitive to pain; the

more receptors, the more sensitive that part is to pain• Pain originates from the point of contact, then a message is

sent from the point of contact through the spinal cord to the thalamus, next it is sent to the cerebral cortex where the person registers the location and severity of the pain.

• Gate Theory suggests that only a certain amount of information can be processed by the nervous system at a time

• Phantom Limb Syndrome people continue to feel pain and sensations of feeling in the place of an amputated limb

The Body Senses

• Vestibular sense tells you whether you are physically upright without your having to use your eyes– Sensory organs in your ears monitor your body’s

motion and position in relation to gravity• Kinesthesis the sense that informs people

about the position of motion of their bodies– Sensory information is fed to the brain from

sensory organs in the joints, tendons, and muscles

PERCEPTION

Rules of Perceptual Organization

• Closure – the tendency to perceive a complete or whole figure even when there are gaps in what your senses tell you

• Figure-ground perception – the perception of a figure against a background

• Proximity – the tendency to group together visual and auditory events that are near each other

Rules of Perceptual Organization

• Similarity – thinking of similar objects as belonging together

• Continuity – the tendency to group stimuli into continuous patterns

• Common fate – the tendency to perceive things as moving together as belonging together

Perception of Movement

• To be able to sense movement, humans need to se an object change its position relative to other objects

• Stroboscopic motion the illusion of movement is produced by showing the rapid progression of images or objects that are not moving at all– Movies are actually just 16-22 frames of images

per second

Depth Perception

• Depth distance away• Monocular cues need only one eye to be perceived– Create the illusion of three dimensions on a two-

dimensional, or flat, surface• Binocular cues need both eyes to be perceived• Retinal disparity a binocular cue for perceiving

depth based on the difference between the two images of an object that the retina receives as the object moves closer or farther away

Perceptual Constancies• Size constancy the tendency to perceive an object as

being one size no matter how far away the object is, even though the size of its image on the retina varies with its distance

• Color constancy tendency to perceive objects as keeping their color even though different light might change the appearance of their color

• Brightness constancy tendency to perceive an object as being equally bright even when the intensity of the light around it changes

• Shape constancy the knowledge that an item has only one shape no matter what angle you view it from