Sensation and perception
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Sensation and Perception. AP Psychology. Sensation   is referred to as being bottom-up processing , detecting environmental stimuli from senses up to the brain. Sensing the World: Some Basic Principles.

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Sensation  is referred to as being bottom-up processing, detecting environmental stimuli from senses up to the brain.

Sensing the World: Some Basic Principles

  • An Absolute Threshold is the lowest amount of stimulus needed to notice it  50% of the time.

  • The Difference Threshold (just noticeable difference or jnd) is the lowest difference you can detect between 2 stimuli 50% of the time.

  • This is described by the Signal Detection Theory – predicting when we will notice a weak stimulus (signal).

Weber’s Law states that two stimuli must differ in percentages or ratios, not amount, for a person to detect it (jnd).

But your detection of a stimulus also depends on your state of arousal, expectations, experiences, and motivation.

A stimulus is Subliminal if it is below your absolute threshold, you detect it less than 50% of the time.  i.e., a microscopic cell is subliminal to you because you cannot see it with your naked eye.

Subliminal advertisements (Drink Coke, eat popcorn etc.), does have an affect on you but  do not persuade you.

Sensory Deprivation

  • (Blind Person Example)

    • Some aspects of perception might be innate

    • Others involve experiences

  • Critical period for development of sensation and perception is infancy

Sensory Adaptation –  lowered sensitivity due to constant exposure from a stimulus.

For example, 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;  it is irritating to be constantly reminded that your foot is in contact with the floor.

Your Senses

Transduction  refers to

Sensory energy

being converted (transformed) into

Neural energy/impulses.


Light is composed of electromagnetic waves with Wavelengths (distance from one peak to another peak on a wave) and Amplitudes (height of the wave)

WAVELENGTH  determines   HUE  (Color, i.e. Red, Blue, Green) and PITCH/FREQUENCY  in sound.

AMPLITUDE       determines   INTENSITY  (Brightness, i.e. Bright red, dark red) and  LOUDNESS in sound


~External Light entering the eye first travels through the Cornea (protective layer) 

~ Pupil  (an adjustable opening) control by Iris (muscle around the pupil)

~ Lens (an oval transparency) that changes shape to focus light by a process called Accommodation

~Light is then focused onto the back of the eye called Retina (multi-neuron surface).

The Retina has 2 types of receptor cells : 

Rods  (detect brightness of light, sensitive in dark),

Cones (detect color and detail, sensitive in daylight). 

Cells connecting these detectors form the Optic Nerve that sends the impulses to brain.



(Focal point)




Everyone has a Blind Spot, a small region in the visual field where nothing could be seen. 

This is because there are no receptor cells where the optic nerve leaves the eye in the retina. 

Normally, we don’t witness this effect because we have two eyes that compensate for each other’s blind spot, and the fact that our eyes are constantly moving.

Fovea is the region in the retina where light is centrally focused. 

The fovea has no rods, only cones.


There are 3 basic types of Acuity (how sharp/clear vision is) : normal,

nearsightedness (only see near things clearly), and farsightedness (only see far things clearly)

Nobel prize winners Hubel and Wiesel discovered Feature Detectors in the brain cortex that are sensitive to specific features in what we see (i.e. shape, color, depth, movement, form, and even postures, arm angle, gaze).

Parallel Processing -  Our brain Processes lots of information  simultaneously. 

For example, looking at an orange, the brain processes the orange color, the round shape, and the bumpy texture all at the same time.


Color processing is described in 2 stages :

1)  Young-Helmholtz trichromatic(three-color) theory – Light is detected by 3 types of cones each specifically sensitive to Red, Blue, or Green. Combinations of them produce intermediate colors (yellow, cyan, purple)

2)  Opponent-Process theory – Color is then processed by their opponent colors (red-green, blue-yellow, black-white). Some cells are excited by blue and inhibited by yellow, vice versa. Thus, you cannot see a bluish-yellow.

Color constancy  refers to the importance of surrounding background

effects on perceived color. 

Color constancy states that colors don’t look different even in different illumination (i.e. sunlight or dark room). Green leaves will still be green whether on a clear or cloudy day.


Sound is composed of electromagnetic waves with Wavelengths (distance from one peak to another peak on a wave) and Amplitudes (height of the wave)

WAVELENGTH  determines   HUE  (Color, i.e. Red, Blue, Green) and PITCH/FREQUENCY  in sound.

Frequency (Pitch) is the number of waves travelling through a point in one second, relates to how fast a wave travels.

AMPLITUDE  determines   INTENSITY  (Brightness, i.e. Bright red, dark red) andLOUDNESSin sound

Audition, or hearing, requires sounds waves converted into neural impulses, and this is done in the ear.


Sound travels through the 3 sections of the ear to the brain :

OUTER EAR :Auditory Canal

MIDDLE EAR: Ear drum (tight membrane) ~ Hammer, Anvil, Stirrup

(3 small bones connected to ear drum that vibrates when sound waves hit ear drum)

INNER EAR : Cochlea (coiled, fluid-filled tube) that contains the Basilar Membrane, which is lined with hair cells that vibrates to excite nerve fibers. 

The fibers form the Auditory Canal connecting to the brain.


Place theory

says that we hear different pitches because specific “places” in the cochlea are stimulated.

Frequency theory

says that we hear different pitches because the speed of neural impulses traveling to the brain matches the speed of the sound waves (“frequency”).


We can tell which direction a sound is coming from because if  it is closer to our right ear, the right ear will receive the sound slightly faster than left ear and the brain calculates this difference. 

Consequently, if the sound is directly behind or in front, where the distance between 2 ears is the same, then it is difficult to differentiate.

Conduction Deafness  – loss of hearing due to damage of eardrum, and/or

the tiny bones in middle ear. (Could be fixed by hearing aid)

Nerve Deafness – loss of hearing due to damage to cochlea, basilar membrane, and/or

hair cells in the inner ear. (Could be fixed by a bionic ear, implanting a cochlea)

Smell and Taste

Taste is a Chemical Sense composed of 4 basic senses : Sweet, Sour, Salty, and Bitter.

Taste receptors (taste buds) regenerate every 1 or 2 weeks, but age, smoking, and alcohol will lower taste bud number and sensitivity.

Smell or  Olfaction  is also a  Chemical Sense that  directly transmits information from nose to the temporal lobe.

**The only sense that doesn’t first relay impulses to the Thalamus! (smell)

Sensory Interaction is when one sense affects another sense, thus interacting. For example, tasting apples and potatoes seem the same if we cannot see it or smell it.

The OTHER Senses


is composed of 4 senses : Warmth, Pain, Cold, and Pressure 

Pressure is the only sense with identifiable receptors.

The other three don’t have specific receptors

Combinations of these create “feelings”:

Warmth and Cold = HOT

Pressure and Cold = WET

Pressure and Pain =  TICKLING ITCH

The OTHER Senses

Pain is merely a physical and psychological interpretation. 

Distraction methods, where attention is focused elsewhere, can ease the felt pain.

Phantom Limb Sensations  occur when pain is felt in a nonexistent limb.  Even though the leg is not present, the receptive neurons previously connected to them are still there and will fire, resulting in pain sensations.

The Gate-Control Theory states that the spinal cord has “gates” that open/close to transmit pain impulses. 

Small fibers open Gate = pain.  Large fibers close Gate = no pain.

The OTHER Senses

Kinesthesis(using sensors in muscles, tendons, and joints)

Vestibular sense (using fluids in semicircular canal, cochlea, and vestibular sacs in inner ear)

Both sense our position, movement, and balance.

Sidenote: I want these shoes SUPER bad 

Psychologists use REST (Restricted Environmental Stimulation Therapy), where you are put into a warm bath with eyes closed, or in a totally dark room, to lower stimulation and reduce stress, or unwanted behaviors.

Sensory Reduction


Much of our perception comes not just from world “out there”, but also from behind the eyes and between the ears 


is a top-down process: the brain interprets stimulation based on our own state of arousal, expectations, experiences, and motivation.

Experiences, assumptions, and expectations give us Perceptual set: mental set up to perceive one thing and not another


Selective attention:

focusing only on one thing at a time; focused awareness only on limited aspect of all that is capable of experiencing; you aren’t aware of nose in line of vision

Cocktail Party Effect: (example of selective attention) ability to focus only on one voice in a huge crowd

(Unnoticed stimuli has effect: women who had listened to tunes previously played to them while unnoticed preferred it later on)

Perceptual Illusions

Visual capture: phenomenon when a conflict occurs between vision and another sense, vision dominates; vision captures other senses (overrides)

In theaters, sound comes from behind (projector), yet perceive as from screen

Perceiving voice coming from ventriloquist’s dummy

Perceptual Organization

Humans organize clusters of sensation into gestalt: organized “whole”; human tendency to order pieces of info into a meaning picture

Gestalt is a psychology term which means "unified whole". It refers to theories of visual perception developed by German psychologists in the 1920s.

These theories attempt to describe how

people tend to

organize visual elements into groups or

unified wholes when certain

principles are applied.



First perceptual task: to perceive figure (object) as 

distinct from  ground (background)

Figure-ground: organization of visual field into the

figure(s) that stand out from the ground


Next, organize figure into meaningful form (color, movement, like-dark contrast)

To process forms, use grouping: rules mind follows to organize stimuli into logical groups

Grouped into  Proximity,  Similarity, Continuity, Closure, Connectedness

  • Similarity occurs when objects look similar to one another.

  • People often perceive them as a group or pattern.

The example above (containing 11 distinct objects) appears as a single unit because all of the shapes have similarity.

Unity occurs because the triangular shapes at the bottom of the eagle symbol look similar to the shapes that form the sunburst.

This displays uniformconnectednesssince all six buttons sharethe same enclosing shaded box. Additionally, within the box, this example displays uniform connectedness between each icon and the text with which it shares a button.

Continuityoccurs when the eye is compelled to move through one object and continue to another object.

Continuation occurs in the example (right) because the viewer's eye will naturally follow a line or curve. The smooth flowing crossbar of the "H" leads the eye directly to the maple leaf.

Proximityoccurs when elements are placed close together. They tend to be perceived as a group.

Closureoccurs when an object is incomplete or a space is not completely enclosed.

If enough of the shape is indicated, people perceive the whole by filling in

the missing information.

Perceptual Organization

Depth perception: ability to see objects in 3D even though image sensed by retina are 2D; allows distance judgment - partly innate (born with)

Gibson and Walker placed 6-14 months old infants on edge of a visual cliff (table half glass, half wood), making the

appearance of a drop-off.

Mother then tries to convince infant to crawl

pass the normal part of the table onto glass;

most refused, indicating innate

perception of depth

Visual cliff: laboratory device for testing

depth perception in infants/animals

Perceptual Organization

Binocular cues: depth cues that depend on both eyes

Eyes apart, slightly different images, brain sees difference

Retinal disparity: binocular cue in which the greater the difference between images, the closer the object

Convergence: binocular cue in which the more the eyes turns inward, the closer the object

Monocular cues: distance cue that are available to either eye; allow relative distance and depth to be judged

Examples: relative size, interposition,  relative clarity,  texture gradient, relative height, relative motion, linear perspective, relative brightness 

Retinal image size allow us to judge distance based on our past and present experience and familiarity with similar objects (same object but smaller = further away)

When objects of known distance subtend a smaller and smaller angle, it is interpreted as being further away. Parallel lines converge with increasing distance such as roads, railway lines, electric wires, etc

Monocular cues

linear perspective

relative size

relative height

Interposition cues occur when there is overlapping of objects. The overlapped object is considered further away.

Even at same size, past and present experience and familiarity with similar objects linear perspective causes one to see one object bigger

  • Texture:

  • coarse=close

  • fine=distant

  • Relative Clarity: past and present experience and familiarity with similar objects hazy object seen as more distant

Monocular cues

  • Relative Height: higher objects seen as more distant

  • Relative Motion: closer objects seem to move faster

  • Relative Brightness: closer objects appear brighter

  • Light and Shadow: nearby objects reflect more light to our eyes.

Muller lyer illusion
Muller- past and present experience and familiarity with similar objects Lyer Illusion

Perceptual past and present experience and familiarity with similar objects


Brain computes motion base partly on assumption that objects moving away is shrinking & vise versa

Brain reads rapid series of slightly different images as movement; phenomenon called stroboscopic movement

Another illusion of movement is phi phenomenon: perception of movement when lights blink one after the other; Police car lights

Perceptual past and present experience and familiarity with similar objects


Perceptual constancy:

perception that objects are not changing even under different lighting; allowing identification regardless of angle of view

[a door is a door even at 45 degree (shape constancy) angle or 20 feet away(size constancy)]

Perceptual Set past and present experience and familiarity with similar objects

Experiences, assumptions, and expectations give us

Perceptual set:

mental set up to perceive one thing and not another

Can’t resist finding a pattern on unpatterned stimuli

Impossible image
Impossible Image past and present experience and familiarity with similar objects

Impossible staircase
Impossible Staircase past and present experience and familiarity with similar objects

Parapsychology past and present experience and familiarity with similar objects

50% of Americans believe in extrasensory perception (ESP): claim perception occurring without sensory input

Parapsychology: study of paranormal phenomena (profession called Parapsychologists)

Is there extrasensory perception
Is there Extrasensory Perception? past and present experience and familiarity with similar objects

  • Extrasensory Perception: claim that perception can occur apart from sensory input.

    • Astrological predictions, psychic healing, communication with the dead

    • Telepathy- mind to mind

    • Clairvoyance- perceiving remote events

    • Precognition- perceiving future events

    • Psychokinesis- mind over matter

      • Example- light as a feather stiff as a board

Parapsychology past and present experience and familiarity with similar objects

Three varieties of ESP:

Telepathy (sending or reading thoughts),

Clairvoyance (perceiving an event unfolding),

Precognition (seeing future)

Vague predictions can later be interpreted to match events (Nostradamus claimed his prophecies could not be interpreted till after the event)

After many experiments, never had a reproducible ESP phenomenon or individual who can convincingly demonstrate psychic ability