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Chapter 5-8 notes for Psych1000
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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
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