Notes

Chapter 5 – Sensation and Perception

 Sensation – the stimulus-detection process by which our sense organs respond to and translate
environmental stimuli into nerve impulses that are sent to the brain
 Perception – active process of organizing the stimulus input and giving it meaning

Sensory Processes
 Stimulus detection – absolute threshold designated as the lowest intensity at which a stimulus can
be detected 50% of the time
 Signal detection theory – concerned with the factors that influence sensory judgments
o Decision criterion – standard of how certain a person must be that a stimulus is present
before they will say they detect it
o Increased rewards for noticing stimuli often results in lower detection thresholds
o Increased danger/punishment for noticing stimuli often raises detection threshold
 Difference threshold – smallest difference between two stimuli that can be perceived 50% of the
time (just noticeable difference – jnd)
o Weber’s Law – to perceive a difference between two stimuli, one must differ by a
constant ratio
 Value for weights = 1/50, therefore if 50 lbs. is lifted, increased weight will only
be detected at 51 lbs.
 Smaller fraction = higher sensitivity
 Doesn’t apply to extremely high or low stimulation intensities
 Sensory adaptation – the diminishing sensitivity to an unchanging stimulus
o Perception of stimuli will decrease if constantly present

The Sensory Systems

Vision
 The Human Eye
o Light enters eye through cornea (transparent protective structure)
o Pupil – adjustable opening that dilates or constricts to control amount of light entering
o Iris – controls the pupil
o Lens – elastic structure that becomes thinner to focus on distant objects and thicker to
focus on nearby objects
 Image flipped and reversed onto retina
 Ability to see clearly depends on lens’ ability to focus image onto retina
 Myopia (nearsightedness) – lens focuses image in front of retina
 Hyperopia (farsightedness) – lens focuses image behind retina
o Retina – multi-layered tissue at rear of eyeball
 Photoreceptors: Rods and Cones
o Retina covered in light-sensitive receptor cells
o Rods – black and white receptors
 Function best in dim light
o Cones – color receptors
 Function best in bright light
o In humans, rods are everywhere except fovea (direct center of retina)
 Cones decrease in concentration distant from the fovea
o Rods and cones send message to brain via two additional layers of cells
 Bipolar cells have synaptic connections with rods and cones
 Bipolar cells synapse with ganglion cells, whose axons form into optic nerve
o Cones in the fovea each have private line to a single bipolar cell (unlike others, which
have many rods/cones for each bipolar cell)
 Visual acuity (ability to see fine detail) increases with image directly on fovea
o Blind spot exists at point where ganglion cells exit to form optic nerve
 Transduction - process where characteristics of a stimulus are converted into nerve impulses

, o Rods and cones accomplish transduction through photopigments
o Absorption of light be photopigments increases release of neurotransmitters
 Brightness Vision and Dark Adaptation
o Dark adaptation – the progressive improvement in brightness sensitivity that occurs over
time in low illumination
 Cones adapt completely in 10 minutes
 Rods continue adapting for 30 minutes, allowing extreme sensitivity to light
 Color vision
o Trichromatic theory – three types of color receptors in retina (blue, green, red)
 All colors produced by combination of wavelengths between these three colors
 Flaws in theory:
 Yellow produced by red and green, yet people with red-green color
blindness can see yellow
 Color afterimage (image in different color appears after stimulus shown
for a while then withdrawn)
o Opponent-process theory – three color receptors, each responding to two different
wavelengths (red-green, blue-yellow, black-white)
 Explains color afterimage issue
o Dual processes in color transduction
 Modern dual-process theory combines both theories to account for color
transduction process
 Cones contain one of three different photopigments that are sensitive to blue,
green, and red
 Different combinations of intensities will produce different colors
 Opponent processes occur, but not in cones
 Ganglion cells respond in opponent-process by altering firing rate
o Color-deficient vision
 Dichromat – color blind to only one system (red-green or yellow-blue)
 Monochromat – completely colorblind (only sees black-white)
 Analysis and Reconstruction of Visual Scenes
o Feature detectors
 Optic nerve sends nerve impulses to brain (thalamus, then primary visual cortex)
 Groups of neurons in the cortex are organized to receive and integrate sensory
nerve impulses from specific regions of retina
 Feature detector cells fire selectively to stimuli that have specific characteristics
 Certain cells fire when horizontal line present, others when other angles
present
 Parallel processing – different cells analyze stimuli and construct
unified image of its properties
o Visual association processes
 Information analyzed and reconstructed in primary visual cortex is routed to
other regions known as visual association cortex

Audition
 Frequency – number of sound waves or cycles per second (Hz = one cycle per second)
 Amplitude – vertical size of the sound waves (decibels – db)
 Transduction system of ear is made up of bones, membranes, and tubes
o Sound waves vibrate eardrum, which vibrates three bones (hammer, anvil, and stirrup)
 Amplify sound waves more than thirty times
o Cochlea – coiled, snail shaped tube that contain basilar membrane (sheet of tissue)
o Organ of Corti rests on the basilar membrane
 Has thousands of tiny hair cells that are actual sound receptors
 Sound waves cause waves in liquid, which bend hairs, causing release of
neurotransmitters

,  Sound localization – ability to notice location of sound due to timing difference in sound wave
reception in ear
 Two types of hearing loss:
o Conduction deafness – problems involving the mechanical system that transmits sound
waves to the cochlea
o Nerve deafness – caused by damaged receptors within the inner ear, or damage to the
auditory nerve
Taste and Smell: The Chemical Senses
 Taste (gustation) and smell (olfaction) are sensitive to chemical molecules rather than energy
 Taste buds – chemical receptors along edges and back surface of tongue
o Consists of bitter, sour, salty, and sweet receptors
 Receptors of smell are long cells that project through the lining of the upper part of the nasal
cavity and the mucous membrane
 Pheromes – chemical signals found in natural body scents
o Menstrual synchrony – tendency of women who live together to have similar menstrual
cycles

The Skin and Body Senses
 Receptors in skin and internal organs sense pressure, pain, warmth, and cold
o Mixtures form other sensations, such as itch
 Kinesthesis – provides us with feedback about our muscles’ and joints’ positions and movements
o Cooperates with the vestibular sense (sense of body orientation or equilibrium)

Perception: The Creation of Experience
 Two different types of processing functions:
o Bottom up processing – takes individual elements of the stimulus and combines into a
unified perception
o Top down processing – sensory information is interpreted in the light of existing
knowledge, concepts, ideas, and expectations
 Accounts for psychological influences on perception

Role of Attention in Perception
 Attention involves processes of (1) focusing on certain stimuli, and (2) filtering out other
incoming information
o Studied through technique called shadowing
 Participants hear two messages simultaneously through earphones, and must
repeat one of the messages word for word
 Most can complete this, but cannot repeat second message
 Attention strongly influenced by nature and personal factors
o Internal factors (motives and interests) influence which stimuli are noticed

Perceptions Have Organization and Structure
 Synthesia – stimuli in one sensory modality give rise to perceptions in other modalities
 Gestalt theorists believe strongly in top-down processing
o Wholes perceived are often more than the sum of their parts
o Figure ground relations – perceptual organization in which a focal stimulus is perceived
as a figure against a background of other stimuli
o Gestalt laws of perceptual organization – four ways in which people group and interpret
stimuli
 Similarity – when parts of a perception are perceived as similar, they will be
perceived as belonging together
 Proximity – elements that are near one another are likely to be perceived as part
of the same configuration
 Closure – people tend to close the open edges of a figure or fill in gaps of an
incomplete figure, so that their identification of the form is more complete