Sensation & Perception
Jan 13, 2016
Sensation &
Perception
Sensation & Perception
• Sensation– Recognition that something is there– Something triggers the a dendrite of the first
neuron
• Perception– Interpretation of a sensation– What is it that I am hearing, seeing, etc.
3 Key Principles
• There is no 1-1 correspondence between physical and psychological reality– Alternative perceptions of any given sensation– Yellow traffic light
• Sensation and perception are active processes– We focus on stimuli that are relevant to ourselves– Phenomenological world
• Joint product of external reality and one’s creative efforts to understand reality
• Sensation and perception are adaptive– We adapt to our environment
Sensing the Environment
• Sensory receptors– Specialized cells that transform energy (stimuli)
into neural impulses that can then be interpreted by the brain
• Transduction– Converting a sensation into an internal
electrical signal to convey to the CNS
Absolute Thresholds
• Absolute Threshold
– Amount of energy needed for someone to notice a stimulus
– Detect stimulus 50% of the time
– Varies from person to person
– Varies over one’s life span
Signal Detection Theory
• Sensitivity– How well one picks up a given stimulus
• Response Bias– One’s readiness to report detecting a stimulus
when uncertain• False alarm- Report a signal when not there
• Hit- Accurately reporting a signal
• Correct False- Accurately reporting no signal
Difference Thresholds
• Just noticeable difference (jnd) – The lowest level of stimulation needed for one
to detect a change in a stimulus’ signal or a difference between two signals
• The level of the original signal is key to detecting just noticeable differences
Weber’s Law
• The greater the original signal the greater the difference necessary to achieve a difference threshold
• A math equation used to calculate just noticeable differences
• For weight Weber’s fraction is 1/50– 50 pound bag= Add 1 pound to achieve jnd– 100 pound bag= Add 2 pounds to achieve jnd
Fechner’s Law
• Relationship between intensity one reports a sensation is related to the amount of the original signal
• Suggests that all just noticeable differences are created equal– Each additional jnd feels like one incremental unit in
intensity
• Logarithmic relations between subjective and objective stimulus
Steven’s Power Law
• As the perceived intensity of a stimulus grows arithmetically, the actual magnitude of the stimulus grows exponentially
Sensory Adaptation
• Our propensity to quickly adjust to a stimulus level– Walk into a dark theater – Adapt to a high sound level in a sports stadium
• Prevents sensory overload
Vision
• Light is detected by the retina– Cornea– Pupil– Lens
• Retina transduces light into electrical signal– Photoreceptors
• Rods= black and white• Cones= color
– Optic Nerve– Receptive fields
Vision
• Optic Nerve sends information on two pathways– Superior Colliculus in the midbrain– Lateral geniculate nucleus in the thalamus and then
onto the visual cortex
• Visual Cortex– Feature detectors
• Only discharge when a stimuli matches a given pattern
– What pathway• Jut what is an object
– Where pathway• Where is the object located in space
Color
• Trichromatic Theory– The eye has three type of receptors for
• red
• green
• blue
• Opponent-process theory– Three antagonistic color systems
• Blue-yellow
• Red-green
• Black-white
Audition
• Sound involves vibrations in the air from the source (sound waves)
• Sounds travel much slower than light– See lightning– Hear thunder seconds later
• Sound can travel through objects
Sound Waves
• Frequency (pitch)– Number of times particles oscillate per second
– Timber is the combination of oscillations for a given sound source
• Amplitude (loudness)– Height and depth of sound waves
The Ear
• Auditory Canal– Sound waves enter the ear through the auditory canal to
the eardrum
• Eardrum– The eardrum’s stirrup creates pressure in the fluid of
the cochlea
• Cochlea– Hair cells attached to the basilar membrane trandsduce
sound & fire sensory neurons in the auditory nerve
Two theories on sound
• Place theory– Different areas of the basilar membrane
respond to different frequencies
• Frequency theory– Rate of vibration of the basilar membrane
transforms frequency into pitch
Sound Localization
• Involves the identification of the location of a sound in space
• Binaural neurons that respond to relative differences in the loudness and timing of sensory signals transduced by the two ears
Olfaction (Smell)
• Enables us to detect both pleasant and potentially dangerous odors– Spoiled food
• Smell can take place through your nose and your mouth
• We smell gas molecules in the air
Olfaction
• Olfactory Nerve– Composed of axons from hundreds of different
olfactory receptors
• Olfactory Bulbs– Receive signals from the olfactory nerve and
transmits to the olfactory cortex in the brains’ frontal lobes
Taste
• Taste buds transduce chemical input from molecules and send them to the medulla and pons in the hindbrain and then on two pathways
• Primary Gustatory cortex– Identification of tastes
• Limbic system– Generates gut level reactions and learned responses to
tastes
Gustatory Systems Four Tastes
• Sweet
• Sour
• Salty
• Bitter
Touch
• We have approximately 18 square feet of skin
• We have approximately 5,000,000 touch receptors
• Touch involves three senses:
– Pressure
– Temperature
– Pain
Pressure & Temperature
• Pressure involves the displacement of skin– Varies widely across different parts of the body
– Very sensitive on fingertips• Pin prick hurts
– Little sensitivity on the buttocks• Large needle hurts less than pin prick on the finger
• Temperature– Difference between skin temperature and the
corresponding temperature on an object is the key
Pain
• Pain– Purpose of pain is to limit tissue damage– Varies by individual– No specific physical stimulus– Skin does not transduce pain waves– Free nerve endings transmit signals to the brain when a cell is damaged
• Experiencing Pain– Impacted by beliefs, expectations and emotional state– Anxiety can increase pain– Intense fear, stress, or concentrating on other things can inhibit pain
• Controlling pain– Organic source– Mental attitude
Proprioceptive Senses
• Regulate body position and movement
• Vestibular Sense– Provides information on the position of the
body by sensing gravity and movement
• Kinesthesia– Provides information about the movement and
position of the limbs and other body parts relative to one another
Perception
• Involves the organization and interpretation of sensations
• Organizes sensations into meaningful units
• Interpretation of the organized sensations– What does it mean
Perceptual Organization
• Integrate sensations into percepts– Meaningful perceptual units– Locates percepts in space– Preserves percepts meaning while analyzing
them
• Form Perception– Organizing sensations into meaningful shapes
and patterns
Phantom Limbs
• Damage to sensory systems leads to reorganization of those systems
• Phantom limbs involve people who have lost an arm or a leg sensing its presence
• Amputees experience phantom limbs for some time after surgery
Gestalt Principles of Perceptual Organization
• The German term Gestalt means “whole” or “form”
• Key is that the whole is greater than its parts
Figure- Ground
• Figure= Object they are viewing
• Ground= Background the object is embedded within
• Insert commercial example here
Similarity
• We tend to group similar elements together
Proximity
• We tend to group together objects that are close to one another
Good Continuation
• We organize stimuli into continuous lines or patters
Simplicity
• We tend to perceive the simplest pattern possible
Closure
• We tend to perceive incomplete figures as complete
Perceptual Illusions
• The brain’s efforts to organize sensations into coherent percepts fails
• Organization lead to perceptual misinterpretations
Depth Perception
• Binocular Cues– Visual input integrated from both eyes
• Monocular Cues– Visual input from one eye
Binocular Cues
• Retinal disparity– Results from our eyes being in slightly different locations on our
face
• Retinal disparity enables depth perception
• Binocular cells
– Located in the primary visual cortex, some of these cells respond more vigorously when the same input arrives from each eye
– Other binocular cells respond to the disparity between the eyes
Binocular Cues
• Convergence
– When you observe an object close to you your eyes come together (converge)
– When you observe more distant objects your eyes diverge
Monocular Cues
• Interposition– When one object obstructs
another the blocked object is perceived as more distant
• Elevation– Objects farther away are
higher on one’s plane of view and thus appear higher up toward the horizon
Monocular Cues
• Texture Gradient– Textured surfaces
appear coarser at close range and finer/more densely packed at greater distance
• Linear Perspective– Parallel lines appear to
converge in the distance
Monocular Cues
• Shading– The brain assumes light comes from above. Thus, we
interpret shading differently toward the top versus the bottom of an object
• Aerial Perspective– Distant objects appear fuzzier than closer ones
• Familiar Size– We assume an object is its usual size even though it
appears smaller as distance increases
Monocular Cues
• Relative Size– When we look at two objects known to be of
similar size, we perceive the smaller object as farther away
• Movement– When we move images of closer objects sweep
across our field of vision faster than objects more distant
Motion Perception
• Involves multiple sensory modes– Vision– Sound
• Two systems for processing movement– Motion of the object being observed
• Football wide receiver
– Motion of the person doing the observing• Quarterback
Perceptual Constancy
• Color constancy– We perceive objects (apple) as the same color
regardless on changes in illumination
• Shape Constancy– We perceive objects as the same shape regardless of
sensory input (position or angel)
• Size Constancy– Objects remain the same size regardless of distance
from the object such as a bridge on the highway
Perceptual Interpretation
• Involves generating meaning from sensory experience
• Influenced by experience
• Gibson’s Theory of Direct Perception– Meaning is immediate and obvious– Adaptive in nature
Perceptual Interpretation
• Bottom-up– Perception begins with multiple sensations and then
integrate the data in higher regions of the brain
– Inductive process
• Top-down– Perception begins at higher regions of the brain based
on past experience as soon as sensations begin coming in
– Deductive process
Perceptual Expectations
• Perceptual Set– We anticipate what we will perceive before any sensory data
received
• Context– We anticipate what we expect to perceive in a given situation
• People make passes in bars– Was a comment a pass or just being polite?
• Schemas– Schemas are patterns of thinking about something that enables us
to have a sense of predictability• Dogs bark, cats don’t bark
Motivated Perception
• We tend to perceive what we want to perceive