Psy 280: Perception
Prof. Anderson
Department of Psychology
Week 2
Part 1:
The process of perception
Martians on earth?
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2 front facing spherical sensors
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2 fleshy antennae on side of
head
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Light reactive chemical
Pressure sensitive hairs
Cutaneous membrane
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Little detectors of different
shapes
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Bilateral air holes
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Temperature, pain, caress
Chemical sampling of gases
Retractable probe
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Chemical sampling of solids,
liquids
Perception is easy
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Misleading
World is projected on retina —> “vision”
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Discovery of depth in art, but not perception of depth
Computer vision
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MIT student summer project
No privileged access to how
perception works
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Perception is not just about introspection
Can’t introspect about working of the brain
directly
Have access to the results of complex
processes
What’s perception for?
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Evolutionary adaptionist view
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Shaped by environment to optimize fitness
Animal perception
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Olfactory sensitivity in dogs
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Bat “vision”
Bird brains: Magnetic fields
What do they help us do?
 Perceptual toolbox
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Work with environment
What is perception for?
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“smell face”
What's the problem to be solved?
What sense would you choose to lose?
Sensation vs. Perception
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Sensation: workings of peripheral sensory
receptors
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Receptors are like little specialized “peripherals”
Retina: Does your retina see?
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Hardwired?
Perception: working of the brain
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What’s frog vision like?
The big CPU
Influence of knowledge, learning
Consciousness
Sensation & Perception
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It’s not a clear distinction
The process of perceiving
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The big question: Where/when does
“perception” start?
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Start with the stimulus?
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Not exactly
A tree falls in the forest …
Don’t see things behind
your head?
Must being looking at
stimulus
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“Attention”
Receptors
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Stimulus must fall on receptors
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More restrictive for vision than hearing (< 180°)
What would vision be like if 360°?
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E. Abott’s “flatlands”
Don’t “perceive” what’s on receptors
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E.g., inverted retinal image
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Poke yourself in the eye!
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Don’t see world upside down
What’s up is down
What’s right is left
No need to “flip” image
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Or twist optic nerve
The magic of sensory
transduction
 Transference of energy from one form
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to another
Physical/environmental energy to
biological energy
Photosynthesis: Chemical energy
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Sensory transduction: Electrical energy
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Plants, bacteria
Sunlight—> sugar—>ATP (fuel)
Retina: Light—>chemical—>electrical
Sensation: Process of transduction for
different sensory systems
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E.g., Touch: physical compression —>
electrical energy
Neural processing I
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Where does it start?
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Definition of periphery vs.
brain
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Receptors—>Brain/neurons
Brain is where signal
becomes electrical
Retinal photoreceptors are
not part of the brain
Neural processing II
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If you think the retina is complicated …
Hundreds of thousands
of receptors in retina
Billions of neurons
Complexity comes from
the connections
Each neurons has
~ 1000 connections
 Not random connections
 Controls flow of
information
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Neural processing III
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What's the processing part?
Transformations and products
Malt—>Beer
 light—>sight of beer
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Perception I
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Transformation of electrical information into
perceptual experience
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Perception is conscious sensory experience?
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Depends on critical brain stages after receptor
transduction
Depends who you ask: Different research traditions
Can you have perception without awareness?
Subception (1940-50s)
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processing of emotionally significant information
unconsciously
Perception II:
Bottom-up vs. Top-Down
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Prof Anderson says: “This is where the cool stuff happens”
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Can perception take place without knowledge of the world?
Bottom-up
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Top down
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Information coming from receptors on up
Brain is not “Tabula rasa”
What effect does knowledge play in guiding the perceptual process
Knowledge is “higher-order”, receptors “lower-order”
Computer vision
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Goal: perception based entirely on bottom up analysis
Knowledge dependence of
perceptual processing
Is knowledge needed to
derive cup shape?
 Or can object be “drawn”
in minds eye without
knowledge?
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E.g. Gestalt grouping
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Good continuation
Computers find task very
difficult
Perception is much more than
the stimulus
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It’s a constructive creative process
T E
PE
TE MY T PE
Recognition I
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Perception’s contact with meaning
Can we dissociate perception and
recognition?
Yes and no
 Brain damage: Visual agnosia
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Vision w/out knowledge
Can you perceive without
accessing knowledge?
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Modularity
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Certain bottom-up perceptual
processes are not influenced
by knowledge
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Cognitive impenetrability
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Visual illusions still occur
despite knowing they are
illusions
A mind is a terrible
thing to waste (and taste)
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Its almost impossible to not use your
knowledge
Can’t ask you to not recognize a word
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Just process color, orientation of lines
Knowledge irrevocably changes the
process of perception
T E
PE
TE MY T PE
Where do top down
influences on perception
start?
 Does knowledge (cognition), desire (emotion)
influence how your retina works?
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If you’re hungry does your retina start looking for
steak? If you mentally imagine a steak does it
appear on your retina?
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Unlikely. Rather, occurs at some later (higher)
stage in your brain
Recognition II
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Can we have recognition
without perception?
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Access knowledge without
perceptual awareness?
Yes
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e.g., Amygdala unconscious
recognition of aversive stimuli
(associated with shock)
Unconscious semantic priming
Part 2: From brain to
perception and back again
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Levels of analysis
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Study experience (psychology)
Study brain (neuroscience)
Study relation between mind and brain
Psychophysical analysis (Fechner, 1860s)
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Phenomenology (quality of experience, W. Wundt)
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Wundt focused on qualitative studies
Quantify relation between “objective” stimulus energy
(physics) and “subjective” experience (psycho)
Physiological analysis (nonhuman animals)
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How does electrical activity in brain relate to presented
stimuli
Difficult to assess experience
Human functional
neuroimaging
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Can assess the relation between stimulus,
neural response, and experience
conjointly
What's the relation between
mind and brain
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Brain ultimately determines perception
Can look to the brain for physiological
characteristics to tell us how perception works
Can also look to perception for how brain is
organized
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E.g., weird illusions
Silly human tricks:
Psychophysical methods I
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Relation between stimulus and resulting
perception
The Phenomenological method
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Typically illusions
Detection
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Its all about thresholds
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Arcane fact # 1
Visual, auditory, touch, etc
Types of methods
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Limits (up/down staircases)
Adjustment (volume knob)
Constant stimuli (find amount of energy where 50% correct
detection)
Silly human tricks:
Psychophysical methods II
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Difference thresholds (Weber)
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Smallest detectable difference between
two stimuli
The Weber fraction
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5/100=.05
Amount of weight increases but proportion
stays constant
= Difference/standard
This applies to most senses
Think of it like the discovery of the
gravitational constant but applied to
mental activity
10/200=.05
Signal Detection Theory
Problems with sensory
thresholds
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Classical psychophysical
methods
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“I hear/see it” 50% of the
time
Represents true sensory
threshold?
100
Percent “yes” response
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Liberal
Conserv
No stimulus no response
(wrong)
0 Low
Sensory system vs observer
Liberal vs conservative
criteria
High
Tone Intensity
Signal detection experiment
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Trials with and without a stimulus (e.g.,
Tone)
How often respond to stimulus? Hits (H)
 How often respond to no stimulus? False
alarms (FA)
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Vito is liberal
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Mario is conservative
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90% H, 40% FA, 90 - 40 = 50%
60% H, 10% FA, 60 - 10 = 50%
Equal discrimination
Receiver operating
characteritics (ROC)
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Plot H vs FA
Liberal
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Sensitivity increases
with deviation from
diagonal
Observers falling on
curve have equal
threshold/sensitivity
Cons
Beyond thresholds
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Magnitude estimation
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“I love you this much”
I love my mom 2.46 times
as much as chocolate
Give standard (say 10)
Judge other stimuli
relative to standard
Stevens’s power law
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Compression vs.
expansion
Power functions
Perceived
magnitude=constant x
stimulus intensityn
Part 3: Neurons and
perception
The Neuron Doctrine
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Brain
Unicellular (common cytoplasm)
vs. Multicellular
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Neuron as basis for mind
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Neurons are special
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in brain, not other organs
Have special qualities that allow
for perception
Your liver or kidneys do not have
the capacity for perception
 Aristotle: Heart was the origin of
mind and soul
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Neurons and perception
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Cutaneous receptor neuron
Collection of sensory axons:
Nerve bundle
Axons from the the periphery
(receptors) are called nerves
Dendrites
Basic properties of neurons
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What are the fundamental units
of the neural code?
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What’s the neural code?
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Action potentials (AP)
How do neurons represent
stimulus intensity?
Neurons “fire” more APs
Do not fire larger APs
Analog to digital
Excitation: Increases # of APs
Inhibition: Decreases # of APs
Localization of brain function
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Perception is not created by a single neuron
It’s a result of many neurons
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Antilocalizationists (Holists)
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One brain system
One faculty (sensation, memory, emotion)
Localizationists
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Multiple brain systems
Multiple faculties
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Sensation, memory, emotion separate
Phrenology
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Franz Gall (Early 1800s)
Like palm reading
~35 specific functions
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Muscle metaphor
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1 vs many faculties
Bump on skull
Right and wrong
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Localization of function
Localization of perception:
Law of specific nerve
energies
 Mueller (early 1800s)
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Specific perceptual quality (hearing, touch,
sight, taste, smell) depends on stimulation
of specific nerves (“nerve energies”)
Critically depends on where in brain
nerves connect to
Intracranial stimulation:
Perception
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Epilepsy patients
Can activate sensations of smell, sight, touch, hearing,
by stimulating the brain regions directly
Perception happens in brain, not receptors
exposed cortex of
epilepsy patient
grid work of electrodes
laid over the surface for
stimulation and
recording
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Specific perceptual qualities
reside in distinct brain
regions
The primary cortical sensory regions
Taste
End of Section