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Basic Principles of Sensation and Perception. Sensation vs. Perception. Basketball Study:

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sensation vs perception
Sensation vs. Perception
  • Basketball Study:
  • Inattentional Blindness = inability to see an object or a person in our midst. Intuitively, we think that as long as our eyes are open, we are seeing. Research beginning with Ulrich Neisser’s “basketball study,” has clearly indicated that visual perception is not like a videotape.
sensation vs perception1
Sensation vs. Perception

Sensation /Bottom-Up Processing

Perception/Top-Down Processing

The process by which the brain organizes and interprets the data received from the senses, enabling us to recognize meaningful objects and events.

Interpreting what we sense based on prior knowledge or context

  • The stimulation of sensory receptors (cilia in ears, rods/cones in vision, taste buds) by the properties of the stimulus (sound waves, light energy, chemicals) and the transmission of sensory information into the central nervous system

Psychophysics = study of the relationship between physical characteristics of stimuli and our psychological experience of them (light  brightness, sound  volume, pressure  weight, taste  sweetness)

sensation vs perception2
Sensation vs. Perception

Bottom-Up Processing

Analysis of the stimulus begins with the sense receptors and works up to the level of the brain and mind.

Picture is really black and white blotches broken down into features by the brain

Top-Down Processing

Information processing guided by higher-level mental processes as we construct perceptions, drawing on our experience and expectations.

Based on expectations, context or “love of dogs”, we make meaning out of the black and white blotches and perceive a dog.

What is this picture?

Bottom-Up Processing

Analysis of the stimulus begins with the sense receptors and works up to the level of the brain and mind.

Letter “A” is really a black blotch broken down into features by the brain that we perceive as an “A.”

Sensation vs. Perception

Top-Down Processing

Information processing guided by higher-level mental processes as we construct perceptions, drawing on our experience and expectations.


sensation perception
  • Transduction: sensory receptors (eyes, ears, nose, skin, tongue) convert the stimulus into neural impulses which are sent to the brain
  • EX: Receptor cells in the inner ear convert sound waves/vibrations into electrochemical signals. These signals are carried by neurons to the brain
process of sensation and perception
Process of Sensation and Perception
  • Any change in energy in environment creates stimuli (light waves, sound vibrations, pressure)…
  • which activates receptor cells of sense organs (eye, ear) to trigger electrical signals or impulses…
  • which are transformed by the brain into…
  • sensations or meaningless bits of sensory information…
  • to which experience automatically adds meanings, feelings, and memories…
  • which result in meaningful patterns or images known as perceptions.
sensation perception don t just happen
Sensation & Perception Don’t “Just Happen”


  • Light bounces off Lemon
  • Light forms image of Lemon on retina (upside down)
  • Image generates electrical signals in receptors
  • Signals travel along nerve fibers to the brain...


Signals are processed and you “perceive” Lemon










Observer’s Response


Tell when you (the observer) detect the light (50% of the time).

absolute thresholds


of correct












Intensity of stimulus

Absolute Thresholds
sensing the world basic principles
Sensing the World: Basic Principles
  • Absolute Threshold = the lowest amount of stimulus needed to notice it 50% of the time.
    • Measure absolute threshold by recording the stimulation needed for us to pinpoint its appearance 50% of the time
    • As stimulus intensity increases, subjects’ probability of responding to stimuli gradually increases
    • There is no single stimulus intensity at which the subject jumps from no detection to completely accurate detection  not really “absolute” or constant. Threshold varies within a person over time – due to changing psychological states, hormone levels, sensory adaptation, etc
    • EX: You turn down the radio to a point where you only hear the faint sound half the time. Then that loudness (decibel) is your absolute threshold for sound.
    • EX: The level of heat on a car heat warmer to feel it half the time.
    • EX: Lemon Lab – the number of lemons it takes to smell the scent of lemons in a room half the time
sensing the world basic principles1
Sensing the World: Basic Principles
  • Difference Threshold (just noticeable difference or jnd) = the lowest difference between two stimuli that person can detect 50% of the time.
  • EX: A musician must detect minute discrepancies in an instrument’s tuning
  • EX: A wine taster must detect the slight flavor difference between two vintage wines
  • EX: Parents must detect the sound of their own child’s voice amid other children’s voices
  • EX: Lemon Lab – students must detect the slight weight, firmness, size, etc between the lemons.
difference threshold
Difference Threshold

Difference Threshold: Minimum difference between two stimuli required for detection 50% of the time, also called just noticeable difference (JND).






Observer’s Response

Tell when you (observer) detect a difference in the light. (50% of the time)

Light intensity – the two light bulbs must differ by 8% (Weber’s Law)

sensing the world basic principles2
Sensing the World: Basic Principles
  • Weber’s Law = regardless of magnitude, two stimuli must differ by a constant proportion for the difference to be noticeable.
      • Light intensity – 8%
      • Tone frequency - .3%
      • Weight – 2%
    • EX: Lemon Lab – if you lemon weighs 6 oz then the next lemon will have to weigh .12 oz heavier or .12 oz lighter in order to detect the difference between lemon
    • JND varies according to the strength or intensity of the original stimulus. The greater the stimulus the greater the change necessary to produce JND
      • EX: If a farmer grows giant lemons, a greater difference threshold will be needed to determine a change from a 500 oz lemon, such as a change of 10 oz versus .12 oz with a 6 oz lemon.
sensing the world basic principles3
Sensing the World: Basic Principles
  • Fechner’s Law – larger and larger increases in stimulus intensity are required to produce perceptible increments in the magnitude of sensation. Constant increments in stimulus intensity produce smaller and smaller increases in perceived magnitude of sensation.
  • Scene #1: dark room – add one light bulb – difference in light is striking
  • Scene #2: same room – add a second light bulb – the amount of light is doubled but the room does not seem twice as bright
  • Scene #3: same room – add a third light bulb, it adds just as much light as the second, but you barely notice the difference
  • Three equal increases in stimulus intensity produces progressively smaller differences in the magnitude of sensation
sensing the world basic principles4
Sensing the World: Basic Principles
  • Sensory Adaptation = lowered sensitivity due to constant exposure from a stimulus. After constant exposure to a stimulus, our nerve celss fire less frequently
    • EX: when you go into someone’s house you notice an odor…but this only lasts for a little while because sensory adaptation allows you to focus your attention on changing environment
    • EX: forget your sunglasses are on the top of your head.
    • EX: Lemon Lab – students toward the end had a harder time detecting their lemon
sensing the world basic principles5
Sensing the World: Basic Principles
  • Signal Detection Theory – predicting when we will notice a weak stimulus (signal). Detecting a weak signal depends on:
    • Signal’s strength
    • Our internal psychological state (experience, motivation, and fatigue)

 Absolute threshold is not really “absolute”! Absolute Threshold varies depending on the level and nature of ongoing sensory stimulation; differs moment to moment and person to person

    • EX: exhausted parents of a newborn will notice the faintest whimper from the cradle, while failing to notice louder, unimportant sounds.
    • EX: On a dark night, on a lonely street, a twig snapping might trigger a stimulus that wouldn’t fire if it were light and busy.
    • EX: Lemon Lab – fatigue, embarrassment, motivation among students can influence the detection of the lemon
sensing the world basic principles6


of correct












Intensity of stimulus

Sensing the World: Basic Principles
  • A stimulus is Subliminal if it is below your absolute threshold, you detect it less than 50% of the time.

Not always about unconscious processing, just means below absolute threshold (consciously detect a weak stimulus some of the time)

Visual Processing: SENSATION->light waves cornea pupil (iris) lens retina (rods and cones – Begin Colortrichromatic theory bipolar ganglion – 1st stage of Coloropponent process) optic nerve (blind spot) thalamus occipital lobe (visual cortex – end of Coloropponent process ) feature detectors abstraction (cells in parietal and temporal lobe combine info from feature detectors) PERCEPTION
Acuity = sharpness of vision- Nearsightedness = nearby objects seen more clearly; lens focuses image of distant objects in front of retina- Farsightedness = faraway objects seen more clearly; lens focuses near objects behind retina
  • Farsighted Nearsighted Normal Vision Vision Vision
Nearly a million messages can be sent by the optic nerve at once, through nearly 1 million ganglion fibers.

One cone often synapses onto one bipolar and ganglion cell, while the axons of many rods have to share one bipolar and ganglion cell  allows cones to be more senstivie to detail.

Union of Opposites: Rods and cones are responsible for transduction- the transformation of stimulus energy (sights, sounds, smells) into neural impulses.
Blind Spot: Point where the optic nerve leaves the eye because there are no receptor cells located there.

Blind Spot Activity – Cover one eye and hold up a finger at arm’s length. Have them focus straight ahead and move the finger about two palm widths to the side until it disappears.

hermann grid1
Hermann Grid

4 bright patches in the inhibitory surround  inhibits cell, less neural activity so seems less bright

2 bright patches in the inhibitory surround  less inhibition so more neural activity

Look directly at intersection, the OFF and ON regions are so small that both fit within the width of a strip. Thus, all the cells around the region of fixation give the same response, whether in the intersection or not

motion aftereffects



Motion Aftereffects
  • Waterfall Illusion:
    • Fixing gaze  sensory adaptation (over-stimulate cells that detect outward movement)
    • Shift gaze  when the outward-movement detectors stop firing, there is a tendency for inward-movement detectors to start firing for a few seconds

*Motion Blindness:


Activated by outward-movement

Activated by inward-movement

Parallel Processing
    • simultaneous processing of several aspects of a problem simultaneously
    • the brain divides a visual scene into subdivisions such as color, depth, form, movement, etc.
  • Feature Detectors
    • nerve cells in the visual cortex respond to specific features
    • shape
    • angle
    • movement
visual information processing
Visual Information Processing

Retinal Processing

Rods & Cones èBipolar Cells èGanglion Cells

Feature Detection

Detector cells respond to elementary features


High-level cells respond to combined info

from feature-detector cells


Brain matches the constructed image with stored images

color vision
Is a lemon “yellow”?

Light has no color – brain constructs color from the variations in light waves reflected from objects


Is a chili pepper “red”?

Color Vision

Do objects possess color?

Two basic types of color mixing: Psychophysics!
    • subtractive color mixture - combining different color paints
  • Different pigments subtract different wavelengths: red subtracts all
  • but red, blue all but blue, green subtracts blue and red, etc…
    • 2. additive color mixture - combining different color lights.
    • By combining lights of different wavelengths we can create the perception of new colors. Examples: red + green = yellow; red + blue = purple; green + blue = cyan; red + blue + green = white
Hue (color) = dimension of color determined by the wavelength of light (the distance from the peak of one wave to the peak of the next wave). Visible light has wavelengths from about 400nm to 700nm
  • Intensity (brightness) = amount of energy in a wave determined by the amplitude.
  • Saturation = richness or purity of light determined by the smoothness or complexity of the waves

Most humans can distinguish 7 million different color shades

Wavelength (hue) - different wavelengths of light result

in different colors.








400 nm

700 nm

Short wavelengths

Long wavelengths

Intensity (brightness) - Blue color with varying levels of intensity. As intensity increases or decreases, blue color

looks more “washed out” or “darkened.”

color perception pg 418
Color Perception pg 418
  • Humans are able to discriminate 7 million different hues.
  • Colors convey important information:
    • Ripeness of food
    • Danger signals
  • Trichromatic theory (1st stage, occurring at the level of cones)
    • Eye contains 3 different color sensitive elements
      • Blue, green or red elements
      • Trichromatic theory accounts for color mixing of lights.
  • Opponent-Process theory (2nd stage, occurring further on in the visual system
    • Visual system is organized into red-green, blue-yellow and black-white units.
      • Theory can account for negative color afterimages.
trichromatic theory of color vision
Helmholtz 1852Trichromatic Theory of Color Vision

Human eye has 3 types of cone

receptors sensitive to different

wavelengths of light.




People see colors because the

eye does its own “color mixing”

by varying ratio of cone

neural activity

Theories of Color Vision: Trichromatic Theory

Wavelength Input


Signal to Brain




Equal Parts Red and Green =



Theories of Color Vision: Trichromatic Theory
  • Trichromatic Theory can explain some aspects of colorblindness:
    • most of us are trichromats
    • someone missing one of the three cone types is a dichromat
      • dichromats have only two primaries: any color they can see can be matched with differing proportions of the two wavelengths to which they are sensitive
      • most common is deuteranopia (~3% of men, <1% of women) - missing “green” cones
      • cannot see color difference between reds and greens - but they can see luminance difference
    • someone missing two is a monochromat
    • someone missing all cone types is called a rod monochromat (very poor vision!)

People who suffer red-green blindness have trouble perceiving the number within the design

Some Views With and Without Color Vision

Link Jay and Maureen Neitz Color Vision Page

Theories of Color Vision: Opponent-Process Theory











Double Opponent Cells in V1

opponent process afterimage effect
Opponent Process- Afterimage Effect

Gaze at the middle of the flag for about 30

Seconds. When it disappears, stare at the dot and report

whether or not you see Britain's flag.

explaining complementary afterimages
Explaining Complementary Afterimages
  • white normally stimulates the red and green cells equally
  • exposure to green fatigues the green cell while the red cell rests
  • exposure to white NOW causes red receptor to respond but green receptor is “tired”
  • we see red instead of white
Color Constancy
    • Perceiving familiar objects as having consistent color, even if changing illumination alters the wavelengths reflected by the object
    • Visual Pathway Review Activity
if a tree falls
If a tree falls in the forest and there is nobody

around to hear it…

Does it make a noise?

“If a tree falls…”

NO…Sound (like color) is all in your head!

pitch high or lowness of sound
Pitch – high or lowness of sound
  • The greater the number of cycles per second, the higher the pitch. Longer the wave = lower the pitch / Shorter the wave = higher the pitch
  • Frequency – number of cycles per second as expressed in the unit Hertz.
  • Hertz – A unit expressing the frequency of sound waves. One Hertz, or 1Hz, equals one cycle per second.
    • Human hearing  detect sounds ranging in frequency from 20Hz – 20,000Hz
pitch and age
Pitch and Age

Presbycusis: Older people tend to hear low frequencies well but suffer hearing loss for high frequencies

  • The higher the amplitude of a wave, the louder the sound.
  • Amplitude – strength or height of wave.
  • Decibel – A unit expressing the loudness of a sound, abbreviated dB.
    • Perceived loudness doubles about every 10 decibels. The absolute threshold for hearing is arbitrarily defined as 0 decibels.
loudness of sound
Loudness of Sound

Richard Kaylin/ Stone/ Getty Images



the ear
The Ear

Outer Ear - acts as a funnel to direct sound waves towards inner structures

Middle Ear - consists of three small bones (or ossicles) that amplify the sound

Inner Ear - contains the structures that actually transduce sound into neural response

outer ear
Outer Ear

Pinna - collect and direct “sound” into auditory canal

Auditory Canal (ear canal) - amplify and funnel “sound to tympanic membrane

Tympanic Membrane – collect “sound” and vibrate ossicles

middle ear
Middle Ear

Malleus (Hammer) - vibrate & move the Incus

Incus (Anvil) - vibrate & move the Stapes

Stapes (Stirrup) - vibrate against Oval Window of Cochlea

inner ear
Inner Ear

Cochlea - filled with fluid & contains receptors for hearing (Hair Cells)

Basilar Membrane – divides length of cochlea and holds the hair cells

*Semicircular Canals-

=Vestibular Senses (Balance and Equlibrium)

closer look at the cochlea
Closer Look at the Cochlea
  • The structures of the ear transform changes in air pressure (sound waves) into vibrations of the Basilar Membrane.
  • As the Basilar Membrane vibrates it causes the hairs in the Hair Cells to bend.
  • The bending of the hairs leads to a change in the electrical potential within the cell
perceiving pitch class demo
Perceiving Pitch (Class Demo)

Place Theory (Traveling Wave Theory)

= pitch determined by point of maximal vibration on basilar membrane.

Different pitches activate different places of the cochlea’s basilar membrane.

Only applicable to high pitched sounds – over 5000 Hz

(low pitched sounds do not localize as well)

Frequency Theory

= frequency of a tone (or pitch) matches the rate

at which the hair cells fire or the rate of nerve impulses

traveling up the auditory nerves (i.e., 1KHz tone cause

hair cells to fire 1k times/sec)

Only applicable to sounds under 1000 Hz

(individual neurons cannot fire faster than 1000 times/sec)

Therefore Volley Theory (1000-5000Hz)

= receptors in the ear fire in sequence. Several

neurons together, firing in sequence, can send a

more rapid series of impulses to the brain than one.

localization of sounds sound shadows class demos clack tube
Localization of SoundsSound Shadows (Class demos) Clack & Tube

We locate a sound by sensing differences in the speed and intensity with which it reaches our ears.

The head acts as a “shadow” or partial sound barrier. A sound that comes from directly ahead will be harder to locate than a sound that comes from off to one side.

Because we have two ears, sounds that reach one ear faster than the other ear cause us to localize the sound.

Time differences as small as 1/100,000 of a second can cause us to localize sound.

conduction deafness
Conduction Deafness
  • Caused by the failure of the three tiny bones inside the middle ear to pass along sound waves to the inner ear or the failure of the eardrum to vibrate in response to sound waves
  • Possible cause is a build-up of fluid
  • Hearing aids - amplifies sound (many people lose sensitivity to soft sounds but not loud sounds – unfortunately, some hearing aids amplify all sounds)
  • Normal hearing may return.
sensory neural deafness
Sensory-Neural Deafness
  • Damage to the inner ear. Most often caused by loss of hair cells that will not regenerate.
  • Damage to the auditory nerve.
  • Cochlear implants can help patients with this form of deafness.
stimulation deafness
Stimulation Deafness
  • Exposure to very loud sounds
  • Prolonged exposure to 85 dB can cause stimulation loss.
  • Tinnitus - ringing sound can mean hair cells have been damaged

Sensory receptors located around the roots of hair cells fire when surface of skin is touched (mechanical and thermal energy).

The sense of touch is a mix of four distinct skin senses—pressure, warmth, cold, and pain.

Only pressure has identifiable receptors. All other skin sensations are variations of pressure, warmth, cold and pain.

Two pathways: #1 – signals from thermal receptors + pain signals; #2 – signals from tactile stimulation (pressure)

Hot = warm ( firing) +cold ( firing)

Wet = pressure + cold

Tickling itch = pressure + pain

sensory homunculus
Sensory Homunculus

Homunculus - Latin for "little human“; any representation of a human being.

The Motor Cortex is the area at the rear of the frontal lobes that control voluntary movements. The Sensory Cortex (parietal cortex) receives information from skin surface and sense organs.

sensory homunculus1
Sensory Homunculus
  • Two-Point Threshold – to assess sensitivity to pressure - the least distance by which two rods touching the skin must be separated before the subject will report that there are two rods, not one, on 50% of occasions
  • Most sensitive – fingertips, lips, noses and cheeks
    • nerve endings are more densely packed in the fingertips and face than in other locations
    • a greater amount of sensory cortex is devoted to the perception of sensations in the fingertips and face
gate control theory
Spinal cord contains neurological “gates” that either block pain or allow it to be sensed.Gate-Control Theory
  • Small fibers (pain + temp) = open gate = pain. When tissue is injured, the small fibers activate and open the neural gate
    • Slow pathway – lags a second or two behind the fast system; longer lasting, aching pain
    • Fast pathway – registers pain and relays it to the cortex in a fraction of a second
  • Large fibers (tactile – pressure or vibration) = close gate = no pain.
    • Stimulate (massage, rub, acupuncture) gate closing activity to treat pain.
    • Also closed by signals from the brain – attention and expectations
  • Endorphins can also close gate
  • Brain (attention and expectations) also close gate
phantom limbs
Phantom Limbs
  • How do you Amputate a Phantom Limb?
  • Phantom Limbs = involves feeling pain in a limb after it has been amputated
  • The awareness we have our physical self is constructed by the brain. How our body feels depends on the maps of the body that are held within our brain and emerges as a conscious output. People with pathological pain have distorted maps of the body
body integrity identity disorder
Body Integrity Identity Disorder
  • BIID = relentless desire to amputate healthy limbs.
  • Lobby for surgery as a safe and legal option to remove body part
  • Medical Mysteries Clip
  • Determined to Amputate: One Man's Struggle With Body Integrity Identity Disorder
Traditionally, taste sensations consisted of sweet, salty, sour, and bitter tastes. Recently, receptors for a fifth taste have been discovered called “Umami”.Taste




Umami (Savory/Meaty)


Study: Tastes Form in Infancy


Like taste, smell is a chemical sense. Odorants enter the nasal cavity to stimulate 5 million receptors to sense smell. Unlike taste, there are many different forms of smell.

smell and memories
Smell and Memories

The brain region for smell (in red) is closely connected with the brain regions involved with memory (limbic system). That is why strong memories are made through the sense of smell.

sensory interaction
Sensory Interaction

When one sense affects another sense, sensory interaction takes place. So, the taste of strawberry interacts with its smell and its texture on the tongue to produce flavor.


  • Temperature
  • Odor
  • Texture
  • Taste

(I T.O.T.T. you about flavor)

KINESTHESIS – The sense that informs us about the positions and motion of parts of our bodies

VESTIBULAR SENSE – The sense of equilibrium that informs us about the positions of our bodies and our heads relative to gravity; sense of balance

Read “The Remarkable Case of Ian Waterman” on page 31

visual capture
Visual Capture
  • When vision competes with our other senses, vision usually wins – a phenomena called visual capture.