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