Sensation and Perception

Chapter 3

Sensation and perception are not the same thing!

§    The human brain is predisposed to find recognizable patterns

§    Our sensory and perceptual processes work together to help us sort out what we are experiencing


§    Sensation

§   a process by which our sensory receptors and nervous system receive and represent physical stimulus energy

§    Perception

§   a process of organizing and interpreting sensory information, enabling us to recognize meaningful objects and events

§     Physical energy is transformed into neural impulses, sent to the brain, then interpreted


•      Sensory receptors - specialized neurons.

•      Sense organs:

•    eyes

•    ears

•    nose

•    skin

•    taste buds



§    Top-Down Processing

§   information processing guided by higher-level mental processes

§   when we construct perceptions by drawing on our experience and expectations

§   Often imposes a meaning that does not exist in the physical stimulus (not veridical)



§    Bottom-Up Processing

§   analysis that begins with the sense receptors and works up to the brain’s integration of sensory information

§   reflects physical stimuli veridically



Sensation - Thresholds


•     Absolute threshold - The minimum amount of energy our senses can consciously detect 50% of the time.

•    Vision – candle flame at 30 miles

•    Hearing – ticking watch at 20 feet

•    Taste – 1 tsp sugar in 1 gal water

•    Smell – 1 drop perfume through 3 rooms

•    Touch – bee wing falling on face at 1 cm



§    Difference Threshold

§   Also called “just noticeable difference” or JND

§   the smallest difference between two stimuli that is detectable 50 percent of the time

§   Smallest detectable difference between two stimuli


§    Weber’s Law

§   Governs the JND – stimuli must differ by a constant proportion in order to be detected

§   light intensity-  8%

§   weight-  2%

§   tone frequency-  0.3%


Subliminal Sensations

•      Energy that is below absolute threshold (below conscious awareness)

•      no awareness of the stimulus

•      No enduring effect or persuasion on behavior



Sensory adaptation -  tendency of sensory receptors to fatigue and stop responding to an unchanging stimulus.


•      Given adaptation, why don’t objects just disappear when we stare at them?

•       Visual saccades – tiny eye movements that change the visual scene just enough to prevent adaptation

•       Advantage of adaptation?

•     Allows us to focus on informative changes


•      Lesson? Our perceptions are not always veridical, but they are (often) useful.






§    Habituation - tendency of the brain to stop noticing constant, unchanging information.

§   Example – your clothing, fan noise, perfume!


§    Psychophysics

§   study of the relationship between physical characteristics of stimuli and our psychological experience of them

§   Light-  brightness

§   Sound-  volume

§   Pressure-  weight

§   Taste-  sweetness

§   X-rays? Radio waves? Infrared light?


The visible spectrum

•      We see very little!

•      Bees see UV light, but not red

•      Dogs do not see red, but they do see some colors


Psychophysical Properties

§     Intensity

§    amount of energy in a wave determined by its amplitude (height of the wave)

§   Brightness for light waves

§   Loudness for sound waves


The Psychology of Color Vision

•      The human eye can see 7,000,000 colors

•      appropriate use of color can maximize productivity, minimize visual fatigue, relax the whole body

•    Yellow is the most fatiguing color because bright colors reflect more light, resulting in excessive stimulation of the eyes

•    Muted colors, such as blues and browns, are more soothing to the eye because it has to do less work

Color Perception Simplified


•      Sunlight (visible light) shines on the apple.

•      The apple’s surface absorbs all light wavelengths except for those corresponding to red; those are reflected to the human eye.

•      The eye receives the reflected wavelengths and sends a “red” color message to the brain.


Anatomy of the Human Eye

•       Pupil – aperture that admits light

•       Iris - round muscle (colored part) surrounding the pupil

•      Conracts to change pupil size, letting in more or less light

•       Cornea

•      clear membrane that covers the surface of the eye

•      80% of eye’s focusing power

•       Lens – uses accommodation to supply the other 20%

•      Visual accommodation - the process by which the eye’s lens changes shape to help focus near or far objects on the retina


The Retina

•      Light passes through the pupil and is focused by the cornea and lens onto the retina at the back of the eye

•      The retina consists of three layers of cells

1. Photoreceptor layer

2. Bipolar cell layer

3. Ganglion cell layer


•       Retina

•      the light-sensitive surface at the back of the eye

•      photoreceptors transduce the light stimulus into neural impulses

•     visual sensory receptors on the back of the retina; they account for nearly 3/4 of all human sensory receptors

•     Rods – photoreceptors responsible for noncolor and sensitivity in dim light

•    Cones - photoreceptors responsible for color vision and acuity (sharpness)


•       Retina (cont.)

•      Contains two additional layers of neurons that refine the neural processing of visual information

•    Ganglion cells

•    Bipolar cells

•      Optic nerve Bundle of ganglion cell axons that carries neural impulses from the retina to the visual cortex in the brain



The “Blind Spot”

•      Spot on the retina where the axons of the retinal cells exit the eye to form the optic nerve

•      insensitive to light (no rods or cones)

•      a small object that falls on the blind spot is “invisible”

•      What you “see” in your blind spot is an image created by your own brain, not an accurate reflection of the environment.  Think about what that implies.


Functions of Color Vision

•      Perceptual organization – organizing the visual world into separate entities

•    Signaling functions (contrived, red for danger)

•    Object discrimination (foreground/background)

•    Foraging, species survival, natural selection


Object discrimination

•      Functional importance for foraging individuals

•      Evolutionary importance of color vision

•      Dr. Nordby (monochromat from birth)

•      Mr. I (monochromat in adulthood)

•      What might be the underlying causes for each?


Trichromatic Theory

•      Reasons that the pattern of activity in 3 different retinal color receptors (cones) yields normal color perception

red green blue



Opponent Process Theory

•      Proposes FOUR primary colors arranged in pairs:

•     Red-green, blue-yellow

•     When one is activated, the other is inhibited

•     Lateral geniculate nucleus (LGN) of thalamus


                                    “ON”                “OFF”

                         when red is on,         green is off

                                    green              red

                                    blue                 yellow

                                    yellow blue

                                    black               white

                                    white               black



Color Deficient Vision

•      Monochrome colorblindess - either have no cones, cones that are not working, or damage to color-processing brain areas.

•      Red-green or blue-yellow color deficiency - either the red or the green OR ELSE the blue cones are not working.  Why not yellow?

•      Sex-linked inheritance.





Visual Processing in the Cortex

§    Feature Detectors

§   nerve cells in the cortex that respond to specific features of what you see

§   shape

§   angle

§   Movement

§   (think of a clock face)


Color Constancy

•      What happens to “color” under different types of illumination? (bright light vs. shadow)

•     Receptor sensitivity to higher intensity wavelengths quickly fatigues

•     In sunlight, all wavelengths are approximately equal in intensity

•     Red wavelengths are more intense in light bulbs

•     Demonstrate by adapting one eye to lamplight

•      Effects of Background - color constancy works only when the entire visual scene is equally affected


§    Color Constancy

§   Perceiving familiar objects as having consistent color, even if changing illumination alters the wavelengths reflected by the object


§    Parallel Processing

§   simultaneous processing of several aspects of a stimulus simultaneously


The Binding Problem

•      Electrical representations (e.g., color, shape, motion) are physically separated in the cortex

•      Time-locked responses from different areas indicate “whole-ness” of objects

•      Association areas and mental codes!

•      Some support from experimental studies



Sound & Audition!


Psychological Properties of Sound

•      Wavelength – interpreted as frequency or pitch (high, medium, or low).

•      Amplitude – interpreted as volume (how soft or loud a sound is).

•      Purity (aka complexity) – interpreted as timbre (a richness in the tone of the sound).

•      hertz (Hz) - cycles or waves per second, a measurement of frequency.



Anatomy of the Ear

•      Pinna (outer ear)

•     Channel to tympanic membrane (eardrum)

•      Auditory canal - short tunnel that runs from the pinna to the eardrum (tympanic membrane).

•      Tympanic membrane - thin section of skin that tightly covers the opening into the middle ear

•     sound waves hit the eardrum and vibrate three tiny bones in the middle ear (ossicles)

•    Malleus (hammer)

•    Incus (anvil)

•    Stapes (stirrup)


§     Middle Ear

§    chamber between eardrum and cochlea containing three tiny bones (hammer, anvil, stirrup) that concentrate the vibrations of the eardrum onto the cochlea’s oval window


§     Inner Ear

§    innermost part of the ear, containing the cochlea, semicircular canals, and vestibular sacs

§    Cochlea - snail-shaped structure of the inner ear that is filled with fluid.


•       Basilar membrane – long, thin membrane in the cochlea that moves up and down in response to sound vibrations; resting place for the organ of Corti


•       Organ of Corti – rests in the basilar membrane; contains receptor (hair) cells for sense of hearing.


•      Auditory nerve - bundle of axons from the hair cells in the inner ear; receives neural message from the organ of Corti.


Types of Hearing Impairments

•      Conduction hearing impairment - can result from either:

•     damaged eardrum (prevents sound waves from being carried into the middle ear properly), or

•     damage to the bones of the middle ear (prevents sounds from being conducted from the eardrum to the cochlea).





•      Nerve hearing impairment – can result from either:

•     damage in the inner ear (hair cells), or

•     damage in the auditory pathways and/or cortical areas of the brain.

Surgery to Help Restore Hearing

•      Cochlear Implant - a microphone implanted behind the ear picks up sound from the  environment.

•     Speech processor selects and arranges the sound

•     Implant receives the sound and converts it into electrical impulses sent to the cochlea

•    electrical impulses are converted to neural impulses and sent to the brain, as usual.


Gustation (Taste)

•      Taste buds – taste receptor cells in mouth; responsible for sense of taste

•    Five Basic Tastes:
•    Bitter
•    Sour
•    Salty
•    Sweet
•    “Brothy” msg



Olfaction (Smell)

•      Olfactory bulbs - areas of the brain just above the sinus cavity and just below the frontal lobes that receive information from the olfactory receptors

•      At least 1,000 olfactory receptors.

•      individual receptors are equally sensitive

•     only takes 1 odor molecule to cause transduction in an olfactory receptor.

•      humans can differentiate around 10,000 odors


Somesthetic Senses

•      Somesthetic senses - the body senses consisting of:

•     skin senses

•     kinesthetic sense

•     vestibular senses

1. Skin senses - the sensations of touch, pressure, temperature, and pain.

•     Sensory receptors in the skin


Pain Receptors

•      Receptors for pain (nociceptors)

•    Free nerve endings, networks within the skin that respond to intense pressure, heat, acids, and capsaicin (the active ingredient in hot peppers)

•    Pain serves a functional role for survival

•    Persons lacking pain receptors are at great risk

•    Pain is a motivational force that can activate behavior

•    Pain involves an emotional component that can increase or decrease pain perception


Gate-control Theory

•      Addresses both sensory & cognitive factors

•      theory that the spinal cord contains a neurological “gate” that blocks pain signals or allows them to continue to the brain

§   “gate” opens by activity in small nerve fibers

§   “gate” closes by activity in large fibers

•    Large fibers are activated by rubbing AND by pleasant thoughts, SO…mom was right!


Somesthetic Senses

2. Kinesthetic sense - sense of the location of individual body parts in relation to the ground and each other.

•     Proprioceptive receptors (proprioceptors)

3. Vestibular senses - whole body sensations of motion, balance, position arising from fluid motion in middle ear

sensory conflict theory - says motion sickness happens when information from the eyes conflicts with vestibular information, resulting in dizziness, nausea, etc.


Perceptual Organization

•      Top- down process (individualized by experience)

•      how your brain makes sense of the world, which includes “hiding” from you certain changes in stimuli so that the stimuli continue to appear constant to you


•      Size constancy - the tendency to interpret a familiar object as always being the same actual size, regardless of its distance from the eyes (retinal size)


Shape Constancy

•      the tendency to interpret the shape of a familiar object as being constant, even when its actual shape changes on the retina.


Brightness constancy

•      the tendency to perceive the apparent brightness of an object as the same even when the light conditions change



Gestalt Principles

•      Figure–ground - the tendency to perceive objects (figures) as standing out from their surroundings (ground). Reversible figures - visual illusions in which the figure and ground can be reversed.




Gestalt Grouping Principles

§    proximity-group nearby figures together

§    similarity-group figures that are similar

§    continuity-perceive patterns as continuous

§    connectedness-spots, lines, and areas are seen as a single, connected unit

§   closure-fills in the gaps; the tendency to complete figures that are incomplete.

§   Contiguity - the tendency to perceive two things that happen close together in time as being related (e.g., thunder and lightning).


Perception of Depth

•      Images on the retina are 2-D

•      How do we perceive 3-D (depth)?


•      CUE approach – we learn the connection between cues and depth through experience, yielding 3-D perceptions



Monocular Cues

•           Monocular cues (pictorial depth cues) – cues for perceiving depth based on one eye only.


Accommodation - as a monocular clue, the brain’s use of information about the changing thickness of the lens of the eye in response to looking at objects that are close or far away.


Linear perspective – the tendency for parallel lines to appear to converge as distance increases. (Items closer to convergence are perceived as being farther away.)


Relative size - when objects a person expects to be of a certain size appear to be smaller, they are interpreted to be farther away.


Interposition (overlap) - assumption that an object appearing to block part of another object is in front of the second object and closer to the viewer.


Relative clarity - the haziness that surrounds objects that are farther away causes the distance to be perceived as greater.


Texture Gradient - tendency for textured surfaces to appear to become finer (smoother) as distance from the viewer increases.


Motion parallax  - perception of objects in motion where close objects appear to move faster than far objects.

Binocular Cues

•      Binocular cues - cues for perceiving depth based on both eyes.

1.      retinal disparity

§   images from the two eyes differ

§   closer the object, the larger the disparity

2. convergence

§   neuromuscular cue

§   two eyes move inward (converge) more as objects get nearer


Perceptual Illusions

•      Mόller-Lyer illusion - illusion of line length where inward-turning or outward-turning corners on the ends of the lines cause two equal lines to appear to be different in length.


Moon Illusion

•      the horizon moon  appears larger than the zenith moon, but visual angles on the retina are identical!

•    Apparent distance hypothesis


Ponzo Illusion



Perceptual Illusions

•      Illusions of Motion:

•    autokinetic effect - a small, stationary light in a darkened room will appear to move or drift because there are no surrounding cues to indicate that the light is not moving.

•    stroboscopic motion - seen in motion pictures, in which a rapid series of still pictures will appear to be in motion.

•    phi phenomenon – lights turned on in a sequence appear to move.


Factors that Influence Perception

•      Top-down processing - the use of preexisting knowledge to organize individual features into a unified whole.

•     Perceptual set (perceptual expectancy) - the tendency to perceive things a certain way because previous experiences or expectations influence those perceptions.


Perceptual Set- tendency to perceive things in a particular way because of previous experience