AP Psych Chapter 03: Sensation and Perception


What is Sensation?

Transduction – converts physical energy into electrical signals that our brain processes.


Sensory Thresholds

Threshold is the point above which a stimulus is perceived and below which it is not perceived.  When we 1st become aware of a stimulus.

Absolute threshold is when the intensity of a stimulus is such that a person will have a 50% chance of detecting it.  Associate with Gustav Fechner.  The detection of cancer X-rays is an example of this.

Just Noticeable Difference (JND) is the smallest increase or decrease in intensity that a person is able to detect.  Example is lift weights and difference needed to feel a difference.

Weber’s law states the increase in intensity of a stimulus needed to produce a just noticeable difference grows in proportion to the intensity of the initial stimulus.  An example of this is the lowering of volume of a radio.  A few minutes after lowering it, it can sound just as loud as before.

Subliminal stimulus is the intensity that gives a person less than a 50% chance of detecting it.  0%-49%.


Habituation and Sensory Adaptation

Habituation lower centers of the brain filter sensory stimulation.  “Ignores” or prevents conscious attention to unchanging stimuli.

Sensory adaptation – decreasing response to a stimuli due to continuous contact.


The Sense of Sight

The Science of Seeing

Can see because of the visible spectrum – we see colors because light hits an object and bounces back with different wavelengths.

Can’t see gamma rays, X rays, or ultraviolet rays.

The psychological aspects to light: Brightness (amplitude of the wave), Color (the length of the wave), and Saturation (the purity of the color).

When we see images:

  • Image enters the eye – image is initially reversed.
  • Light waves are changed to broad beams.
  • Cornea – round, transparent part of the eye – focuses the image.
  • Pupil – round opening in the eye that allows light to enter the eye.
  • Iris – muscle around the pupil – controls the amount of light that enters the eye – in less light it relaxes letting light in – in bright cases it contracts preventing too much light from entering – gives your eye color.
  • Lens – bends and focus the light – focuses the image.
  • Retina – located in the back of the eye – contains the cells that are VERY sensitive to light called photoreceptors – transduction begins here.

Eyeball – Normal vision is 20/20 vision – Nearsighted results when the eye is too long – Objects that are near are easier to see than things far away – Farsighted means the eye is too short and objects that are far away are clearer.

Photoreceptors that respond to various light waves.  Located on the retina.

Rods – visual sensory receptors found at the back of the retina, responsible for non-color sensitivity to low levels of light.

Cones – visual sensory receptors found at the back of the retina, responsible for color vision and sharpness of vision.

Blind spot – area in the retina where the axons of the three layers of retinal cells exit the eye to form the optic nerve, insensitive to light.


How the Eye Works

Dark adaptation – the recovery of the eye’s sensitivity to visual stimuli in darkness after exposure to bright lights.  The longer the light was on, the harder it is to adjust.

Night blindness – some people lose dark adaptation as they get older.  It can result in them not being able to drive during the nighttime.

Light adaptation – the recovery of the eye’s sensitivity to visual stimuli in light after exposure to darkness.  Much faster than dark adaptation.


Perception of Color

Trichromatic theory – theory of color vision that proposes three types of cones: red, blue, and green.

Afterimages – images that occur when a visual sensation persists for a brief time even after the original stimulus is removed.

Opponent-process theory – theory of color vision that proposes four primary colors with cones arranged in pairs: red and green, blue and yellow.


Three Types of Color Blindness

Monochrome color blindness – either have no cones or cones not working.

Red-green color blindness – either red or the green cones are not working.

Sex-linked inheritance – recessive inheritance pattern.


The Sense of Hearing


Sound waves are vibrations of the molecules of air that surround us and Hertz (Hz) are the measurement of frequency.  Sound has the same properties as light waves: wavelength, amplitude, and purity. Sound waves are stimuli for hearing.

Amplitude is the height of the wave. It is associated with volume.

Wave length/Pitch is the subjective experience of a sound being too high or low (from a specific source).

Purity – timbre (richness in the tone of the sound).

Frequency is the speed of the waves in one second.

Measuring Sound

Decibel is a unit to measure loudness.  The human threshold goes from 0-140 dB.  Anything over 85-90 dB for a prolonged time can cause hearing damage.


Three Parts of the Ear

The Outer Ear

  • Pinna/external ear – picks up sound waves.
  • Auditory canal – funnels sound toward the eardrum.
  • Tympanic membrane – turns sound waves into vibrations (aka the eardrum).

The Middle Ear

  • Hammer, anvil, and stirrup – amplifies vibrations produced by the eardrum.

Inner Ear

  • Cochlea – receptors for hearing.  Transduction occurs here.
  • Auditory nerve – sends electrical signals to the brain for processing.


Theories of Pitch

Pitch is how high or low a sound is (subjective).  It corresponds to frequency of sound waves. Higher frequencies are perceived as higher pitches.

According to the place theory, pitch is determined by location of stimulation on the organ of corti. Hair cells near oval window are stimulated with high sound. Lower pitch located further out on organ.

According to the frequency theory, pitch is related to how fast basilar membrane vibrates.  Faster the vibration, the higher the pitch.

Accord in the volley principle, frequencies between 400Hz  and 4000Hz cause hair cells (auditory neurons) to fire in a volley pattern. Cells take turns in firing.


Hearing Impairment

Conduction hearing impairment occurs when vibrations cannot be passed from eardrum to cochlea.  It is caused by damaged eardrum or damage to bones of the middle ear.  Sound is very muffled depending on the degree of damage.

Nerve hearing impairment is an impairment with inner ear or auditory pathways and cortical areas of the brain. Normal aging or noise exposure can cause tinnitus which is a constant ringing in ears.


The Sense of Taste


Taste/gustation is a chemical sense on the surface of our tongues.  The receptors are also known as taste buds. This is where transduction happens.

Five types of taste are:

  • Sweet
  • Salty
  • Sour
  • Bitter
  • Umami – a pleasant savory taste.

Certain parts of taste overlap like sweet and salty.

A sweet tooth is a passed on.

Most people avoid sour tastes because many sour things are poisonous.

Stimuli (lemon) is chewed on and is broken down.  The molecules of the broken down food land in the salvia.

Taste buds are receptors for taste.  Chemicals in salvia are activated.  The brain changes it into a nerve impulse.  Taste buds are replaced about every 10 days.  At certain points in our lives we can have 10,000 to 500 taste buds.

25% of the population is super tasters.  They have 2-3 times as many taste buds as the normal person.  They can’t eat certain foods (spicy, chili, etc.).  The can taste the chemicals placed in foods.

Flavor is a combination of taste and smell.  A cold reduces flavor.


The Sense of Smell


The nose has 6 million receptor cells.  They are located in the upper part of the nose.  Cilia are the small hair projecting into nasal cavity and are the receptors for smell.  There are at least 10 million receptors in each cavity.

Olfaction is smell and the stimuli are carried by the air.

Transduction turns nerve impulses into smell.

Volatile substances are things that release molecules/smells into the air.  Examples are skunks, brownies, and fish.

Olfactory cells are covered in mucus.  As molecules absorb into the mucus the cells are stimulated.

People can lose the sense of smell by a virus or even a hit to the head.

Humans can identify up to 10,00 smells.  Smell also allows us to taste food.  Smells warns us of danger like if food went bad.  Smells is the strongest sense tied to memory.


The Sense of Touch

Somesthetic Senses

Somesthetic senses consists of three systems – skin senses, kinesthetic senses, and vestibular senses.

  • Skin senses includes touch, pressure, temperature, and pain.  Sensory receptors are located in the skin.  Pacinian corpuscle responds to deep pressure.  While free nerve endings respond to changes in temperature, pressure, pain.
  • Kinesthetic sense provides information about body’s movement and location in space.
  • Vestibular sense is a receptor structure in inner ear that provides a sense of balance.  It consists of 3 semicircular canals attached to the cochlea.  It is filled with fluid and moves in response to the head.  It senses the position of the head, keeps it upright, and maintains balance.

Motion sickness is when the vestibular system and the input made by the eyes don’t match up.  This causes people to get nausea and sick.

Vertigo is damage to the semicircular canals that cause dizziness and nausea.



Different types of pain:

  • Visceral pain is pain in organs.
  • Somatic pain is pain sensations in skin, muscles, tendons, and joints and is carried on large nerve fibers.

Congenital analgesia and congenital insensitivity to pain with anhidrosis (CIPA) is an inability to feel pain.

Gate control theory of pain says non-painful and painful nerve impulses compete to reach the brain.  As it goes to the brain, there is a bottleneck.  If you rub your arm after injury, it allows more non-painful receptors reach the brain.  If you play through the pain – the gate closes temporarily to pain.




Perception is when sensations experienced are interpreted and organized in meaningful fashion.


Perceptual Constancy

Perceptual constancy – tendency to perceive sizes, shapes, brightness, and colors as remaining the same even though their physical characteristics are constantly changing. Things morph and change yet we see them as a constant.  This is important because it takes ever-changing stimuli and transforms it into something comforting.

  • Size constancy – tendency to perceive objects as remaining the same size even when their image on the retina are continually growing or shrinking.  Learned from experience.  Car driving toward you isn’t increasing in size; it is simply getting closer to you.
  • Shape constancy – tendency to perceive an object as retaining its shape even though when you view it from different angle its shape is continually changing its image on the retina.  Looking at a book from different angles.
  • Brightness constancy – tendency to perceive brightness as remaining the same in changing illumination.
  • Color constancy – tendency to perceive color as remaining the stable despite differences in lighting.  Seeing a person’s yellow shirt in the day and at night.  You would still see it as yellow regardless.


Gestalt Principles

The whole is greater than the parts.  According to this principle, the brain follows rules on how individual elements for perception.  They say it is not the sum.  Rules for the brain seem more evident than the sum.

Rules of organization – how our brains combine and organize individual elements into meaningful perception.

  • Figure-ground – tendency to automatically distinguish between a figure and the ground.  The figure, with more detail, stands out against the background, which has less detail.  This is innate.  Reversible figures are visual illusions in which the figure and ground can be reversed.
  • Closure – tendency to fill in missing parts of a figure and see the figure as complete.  Example is a jigsaw puzzle.
  • Simplicity – stimuli that are organized in the simplest way possible.
  • Similarity – elements that appear similar are grouped together.  Example is seeing colors together.
  • Proximity is when objects are physically close to one another will be grouped together.
  • Continuity – tendency to favor smooth or continuous paths when interpreting a series of points and lines.
  • Contiguity – tendency to perceive events that happen close together in time as being related.


Depth Perception – Binocular

Depth perception is the ability of your eyes and brain to add a 3rd dimension, depth, to all visual perceptions, even though the images projected on the retina are in only two dimensions, height and width.

Binocular depth cues depend upon the movement of both eyes.

  • Convergence is a binocular depth cue for perception based on signals sent from the muscles that turn the eye.  To focus on near or approaching objects, these muscles turn the eyes inward, toward the nose.  The brain uses the signals sent by these muscles to determine the distance of an object.  Bring your finger closer to your nose.
  • Retinal/binocular disparity is a binocular depth cues that depends on the distance between eyes.  Because of their different position, each eye receives a slightly different image.  This difference is retinal disparity.  The brain interprets a large retinal disparity to mean a close object, and a small retinal disparity to mean a distinct object.


Depth Perception – Monocular

Monocular depth cues are visual signals produced from a single eye.  The way objects are arranged in the environment are the cues.

  • Linear perspective – cue that results as parallel lines converge in the distance.
  • Relative size – cue that results when we know two objects are the same size but are not.  Larger means it is closer and smaller means further away.
  • Interposition – cue that results when objects overlap.  In front means it is closer and behind means it is further away.
  • Light and shadow – brightly lit objects appear closer.  Objects in the shadows appear further away.
  • Texture gradient – the sharper, more detailed the object the closer it is.  Objects that are further away have less detail.
  • Atmospheric/Ariel perspective – cues provided by dust, smog, clouds, water vapors,  Hazier objects appear further away.
  • Motion parallax – objects appear to be moving faster the closer they are.  Objects appear to be moving slower, the further away they are.



Illusions are perceptual experiences in which we perceive an object as being strangely distorted that cannot and does not exist.  The brain can’t correctly interpret the space, size, and depth cues.

  • Hermann Grid – gray blobs or diamonds that fade away or disappear when focusing on grid
  • Muller-Lyer Illusion – line length is distorted by inward-turning or outward-turning corners on ends of lines. This causes lines of equal length to appear to be different.
  • Moon illusion is when the moon appears to be larger the closer it is to the horizon when in actuality it is the same size.
  • Stroboscopic motion – rapid series of still pictures will appear to be in motion.
  • Phi phenomenon – lights turned on in a sequence appear to move.


Influences on Perception

Cultural influence is pressure that encourages members of a society/ethnic group to conform to shared behaviors, values, and beliefs.  This can affect perception.  Example: A tribe that couldn’t distinguish between black and white photos and color photos of a dog and a cow.  The reason is because they never seen anything in black and white.  African pygmies that lived in dense jungle where removed.  When took to a field and saw buffalos miles away, the thought they were bugs.  They had no depth constancy.

Perceptual sets are learned expectations based on personal, social, or cultural experience.  It adds feelings to perception.