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

•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 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. 

Sensory Deprivation (Blind Person Example) 

Some aspects of perception might be innateOthers involve experiences 

Critical period for development of sensation and perception is infancy 

Your SensesTransduction  refers to 

Sensory energy being converted (transformed) into

Neural energy/impulses.

SIGHT 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

SIGHT~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.

#1

#4

#2

#3(Focal point)

SIGHTEveryone 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.

SIGHTThere 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).

SIGHTParallel 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.

HEARINGSound 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) and  LOUDNESS in sound

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

HEARINGSound 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”). 

HEARING

HEARINGWe 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 TasteTaste 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

Touch 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 SensesPain 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. 

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.

The OTHER Senses

Sidenote:  I want these shoes SUPER bad 

Sensory Reduction

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.

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

Perception 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

PERCEPTION

Lochness Monster or a Tree Trunk?

PerceptionSelective 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)

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 Illusions

Perceptual OrganizationHumans 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 (FIRST GESTALT PRINCIPLE…LETS MIND KNOW WHAT TO “GROUP”)

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

Perceptual Organization

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 uniform connectedness since all six buttons share the 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. 

Continuity occurs 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.

Closure occurs 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.

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

Perceptual OrganizationDepth 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

 

Cultural Influences on Depth Perception

•  

Perceptual OrganizationBinocular 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

relative size

relative height

linear perspective

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)

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

Even at same size, linear perspective causes one to see one object bigger 

Texture: coarse=closefine=distant

Relative Clarity: 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

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 Organization

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 Organization

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 

Perceptual Set

Impossible Image

Impossible Staircase

Parapsychology50% 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?

• 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 

ParapsychologyThree 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