Chapter 4Sensation and Reality

Psychophysics

• Study of relationship between physical stimuli and sensations they evoke in a human observer

• Absolute threshold: Minimum amount of physical energy necessary to produce a sensation

• Subliminal perception: Perception of a stimulus below the threshold for conscious recognition

• Difference threshold: A change in stimulus intensity that is detectable to an observer

General Properties of Sensory Systems

• Perceptual features: Basic stimulus patterns• Sensory coding: Converting important

features of the world into messages understood by the brain

Just Noticeable Difference (JND)• Any noticeable difference in a stimulus

Sensation and Perception

• Sensation: Information arriving from sense organs (eye, ear, etc.)

• Perception: Mental process of organizing sensations into meaningful patterns

• What can go wrong in these two processes?

p. 120

Absolute thresholds define the sensory worlds of humans and animals, sometimes with serious consequences. The endangered Florida manatee (“sea cow”) is a peaceful, plant-eating creature that can live for more than 60 years. For the last decade, the number of manatees killed by boats has climbed alarmingly. The problem? Manatees have poor sensitivity to the low-frequency sounds made by slow-moving boats. Current laws require boats to slow down in manatee habitats, which may actually increase the risk to these gentle giants (Gerstein, 2002).

Using Your Senses

In partners, I will give you pieces of paper with the names of different objects written on them. Do not to show the words to any other group. Have each group figure out how much sensory information there is in each of the written common objects. Use all 5 of your senses in their description.

Vision: The Key Sense

• Visible spectrum: Narrow spread of the electromagnetic spectrum to which the eyes respond

Parts of the Eye

• Lens: Structure in the eye that focuses light rays

• Photoreceptors: Light-sensitive cells in the eye

More Parts of the Eye

• Retina: Light-sensitive layer of cells in the back of the eye– Easily damaged from excessive exposure

to light (staring at an eclipse) • Cornea: Transparent membrane covering the

front of the eye; bends light rays inward

Accommodation

• Changes in the shape of the lens of the eye

Fig. 4-3, p. 123

Fig. 4-1, p. 122

Visual pop-out. Pop-out is so basic that babies as young as 3 months respond to it.

Vision Problems

• Hyperopia: Difficulty focusing nearby objects (farsightedness)

• Myopia: Difficulty focusing distant objects (nearsightedness)

• Astigmatism: Corneal, or lens defect that causes some areas of vision to be out of focus; relatively common

• Presbyopia: Farsightedness caused by aging

Light Vision

• Cones: Visual receptors for colors and bright light (daylight); 5 million in each eye

• Rods: Visual receptors for dim light; only produce black and white; about 120 million total

• Blind spot: Area of the retina lacking visual receptors

Fig. 4-7, p. 125

Anatomy of the retina. The retina lies behind the vitreous humor, which is the jelly-like substance that fills the eyeball. The rods and cones are much smaller than implied here. The smallest are 1 micron (one millionth of a meter) wide. The lower-left photograph shows rods and cones as seen through an electron microscope. In the photograph the cones are colored green and the rods blue.

Fig. 4-8, p. 126

Experiencing the blind spot. (a) With your right eye closed, stare at the upper-right cross. Hold the book about 1 foot from your eye and slowly move it back and forth. You should be able to locate a position that causes the black spot to disappear. When it does, it has fallen on the blind spot. With a little practice you can learn to make people or objects you dislike disappear too! (b) Repeat the procedure described, but stare at the lower cross. When the white space falls on the blind spot, the black lines will appear to be continuous. This may help you understand why you do not usually experience a blind spot in your visual field.

More on Light Control

• Visual acuity: Sharpness of visual perception• Fovea: Area at the center of the retina

containing only cones (50,000)• Peripheral vision: Vision at edges of visual

field; side vision – Many superstar athletes have excellent

peripheral vision• Tunnel vision: Loss of peripheral vision

Color Vision

Trichromatic Theory

• Color vision theory that states we have three cone types: red, green, blue– Other colors produced by a combination of

these

Opponent Process Theory

• Color vision theory based on three “systems”: red or green, blue or yellow, black or white– Exciting one color in a pair (red) blocks the

excitation in the other member of the pair (green)

– Afterimage: Visual sensation that remains after stimulus is removed (seeing flashbulb after the picture has been taken)

Example of Opponent ProcessTheory

• Negative afterimages. On the following slide, stare at the dot near the middle of the flag for at least 30 seconds. Then look immediately at a plain sheet of white paper or a white wall. You will see the American flag in its normal colors. Reduced sensitivity to yellow, green, and black in the visual system, caused by prolonged staring, results in the appearance of complementary colors. Project the afterimage of the flag on other colored surfaces to get additional effects.

Fig. 4-12, p. 128

Color Blindness

• Color blindness: Inability to perceive colors; lacks cones or has malfunctioning cones– Total color blindness is rare

• Color weakness: Inability to distinguish some colors– Red-green is most common; much more

common among men than women– Recessive, sex-linked trait on X

chromosome

Fig. 4-17, p. 130

Dark Adaptation

• Increased retinal sensitivity to light after entering the dark; similar to going from daylight into a dark movie theater

• Rhodopsin: Light-sensitive pigment in the rods; involved with night vision

• Night blindness: Blindness under low-light conditions; hazardous for driving at night

Light Adaptation

• In pairs, observe each other’s pupils. Flip a coin to select the “subject.” I will darken the room for a few minutes. Then turn on the lights and estimate how many seconds it takes your partner’s pupils to return to their original degree of constriction.

Hearing

• Sound waves: Rhythmic movement of air molecules

• Pitch: Higher or lower tone of a sound• Loudness: Sound intensity

Fig. 4-20, p. 133

Anatomy of the ear. The entire ear is a mechanism for changing waves of air pressure into nerve impulses. The inset in the foreground (Cochlea “Unrolled”) shows that as the stapes moves the oval window, the round window bulges outward, allowing waves to ripple through fluid in the cochlea. The waves move membranes near the hair cells, causing cilia or “bristles” on the tips of the cells to bend. The hair cells then generate nerve impulses carried to the brain.

Fig. 4-22, p. 134

This is how the cochlea looks “unrolled”. How does this diagram of sound waves explain why you can hear the bass from a big car stereo from far away?

How Do We Detect Higher and Lower Sounds?

• Frequency theory: As pitch rises, nerve impulses of a corresponding frequency are fed into the auditory nerve

• Place theory: Higher and lower tones excite specific areas of the cochlea

Hearing Loss• Noise Induced Hearing Loss

• Damage caused by exposing hair cells to excessively loud sounds

– Typical at rock concerts

– By age 65, more than 40% of hair cells are gone

• Sensorineural Hearing Loss• Caused by damage to hair cells or auditory nerve

– Hearing aids little or no help in these cases

– Cochlear implant: Electronic device that stimulates auditory nerves directly

• Conduction Hearing Loss:

• Poor transfer of sounds from tympanic membrane to inner ear

– Compensate with amplifier (hearing aid)

Fig. 4-25, p. 135

Smell

• Olfaction: Sense of smell• Dysosmia: Loss or impairment of sense of

smell• Lock-and-key theory: Odors are related to

shapes of chemicals and molecules• Pheromones: Airborne chemical signal

– Vomeronasal organ: Sense organ for pheromones

Smell Habituation

• Ammonia, rotten eggs, vinegar and other smelly materials can be brought to class. Students rate the smell at the start of class and every 10 minutes after, and should quickly experience olfactory adaptation (habituation).

• I didn’t want to stink up the classroom, but does anyone have any experience with this?

Gustation

• Sense of taste– Four taste sensations: sweet, salt, sour,

bitter – Most sensitive to bitter, less to sour, less to

salt, least sensitive to sweet– Umami: Possible fifth taste sensation;

brothy taste

How do these work together?

• As the text points out, “flavor,” as we experience it subjectively, is actually a combination of olfaction and gustation. This can be demonstrated relatively easily.

• I need a volunteer who can eat something.• Have a student volunteer taste bits of each while blindfolded.

Discriminating between different foods should be simple in this condition. Next, test the blindfolded volunteer while he or she pinches the nostrils closed. With more olfactory cues reduced, correctly identifying the food bits should be more difficult (although not impossible). Even if they correctly identify the foods in the second test, subjects will usually report greater difficulty. Typically, they must rely more on texture than on “taste” when olfactory cues are reduced

Somesthetic Senses

• Sensations produced by skin, muscles, joints, viscera, and organs of balance

• Skin senses (touch): Light touch, pressure, pain, cold, warmth

• Kinesthetic: Detect body position and movement• Vestibular: Balance, acceleration, and position in space• Sensory organs for touch, pressure, pain, cold, and warmth

Pain

• Visceral pain: Pain originating in internal organs

• Referred pain: Pain felt in one part of the body, but coming from another

• Somatic pain: Sharp, bright, fast; comes from skin, joints, muscles, tendons

Types of Pain

• Warning system: Pain carried by large nerve fibers; sharp, bright, fast pain that tells you body damage may be occurring (e.g., knife cut)

• Reminding system: Small nerve fibers; slower, nagging, aching, widespread; gets worse if stimulus is repeated; reminds brain that body has been injured

Vestibular System

• Otolith organs: Sensitive to movement, acceleration, and gravity

• Semicircular canals: Fluid-filled tubes in ears that are sensory organs for balance

• Crista: “Float” that detects movement in semicircular canals

• Ampulla: A wider part of the canal

Fig. 4-31, p. 141

Sensory Conflict Theory

• Motion sickness results from a mismatch between information from vision, vestibular system, and kinesthesis– After spinning and stopping, fluid in

semicircular canals is still spinning, but head is not

– Mismatch leads to sickness• Medications, relaxation, and lying down might

help

Gate Control Theory of Pain

• Gate control theory: Pain messages from different nerve fibers pass through the same “neural” gate in the spinal cord.– If gate is closed by one pain message,

other messages may not be able to pass through

Phantom Limb

• Missing limb feels like it is present, like always, before amputation or accident

Controlling Pain

• Fear, or high levels of anxiety, almost always increase pain

• If you can regulate a painful stimulus, you have control over it

• Distraction can also significantly reduce pain• The interpretation you give a stimulus also

affects pain

Coping With Pain

• Prepared childbirth training: Promotes birth with a minimal amount of drugs or painkillers

• Counterirritation: Using mild pain to block more intense or long-lasting pain

CUTANEOUS TWO-POINT THRESHOLD

• 1. Be sure the subject is securely blindfolded. • 2. Apply even, firm, but not excessive pressure with one point or

two, as directed. You should avoid causing pain to (or breaking the skin of) the subject.

• 3. Make exact measurements when setting the two points for each trial.

• 4. Do not repeat any trials.• 5. Ask the subject for a response after each application of pressure.• 6. Do not give hints or clues to help the subject. Encourage the

subject to make an immediate response.• For each response made by the subject, put a 1 or 2 in the appropriate

box.