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Introduction
Visual PerceptionAditi Majumder, UCI
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Perception is taken for granted!
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Perception is very complex
PerceiveLocateIdentify/Recognize
Different objectsTheir relationship with each other
Qualitative and Quantitative
Act based on these information
It is a miracle that we do not cause accidents/mistakes every now and then
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Perceptual Process
Environmental Stimulus
Attended Stimulus
Stimulus on Receptors
Transduction
Processing
Knowledge Perception
Recognition
Action
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Perceptual Process
Environmental Stimulus
Attended Stimulus
Stimulus on Receptors
Transduction
Processing
Knowledge Perception
Recognition
Action
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Environmental Stimulus
Anything in our environment that we can perceiveCan be anything we can sense
See, hear, touch, smell
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Perceptual Process
Environmental Stimulus
Attended Stimulus
Stimulus on Receptors
Transduction
Processing
Knowledge Perception
Recognition
Action
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Attended Stimulus
A part of the environmental stimulusFocuses attention on this stimulus
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Perceptual Process
Environmental Stimulus
Attended Stimulus
Stimulus on Receptors
Transduction
Processing
Knowledge Perception
Recognition
Action
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Stimulus on Receptors
The attended stimuli excites the receptorsFor example
Visual stimulus forms a image on the retinaSound changes pressure to affect the ear drum
NoteWe do not `perceive’ the image on the retinaIt is just one of the initial steps of the process
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Perceptual Process
Environmental Stimulus
Attended Stimulus
Stimulus on Receptors
Transduction
Processing
Knowledge Perception
Recognition
Action
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Transduction
Transformation of one form of energy to otherEnvironmental energy transformed to electrical energy The image on the retina generates electrical signals in the tens and thousands receptors of the eye
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Perceptual Process
Environmental Stimulus
Attended Stimulus
Stimulus on Receptors
Transduction
Processing
Knowledge Perception
Recognition
Action
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Neural Processing
Neurons are elements of nervous systemInterconnected togetherProcessing of the electrical energy by the neurons while they travel through them
This changes the electrical energy in various ways
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Perceptual Process
Environmental Stimulus
Attended Stimulus
Stimulus on Receptors
Transduction
Processing
Knowledge Perception
Recognition
Action
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Perception
Conscious sensory experienceElectric energy transforms in brain to some experienceIs this the end of perception?
Recognition and action are important outcomes of the perceptual process
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Perceptual Process
Environmental Stimulus
Attended Stimulus
Stimulus on Receptors
Transduction
Processing
Knowledge Perception
Recognition
Action
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Recognition
Identifying the experience as somethingKnownSimilar to some experience before
Recognition and Perception are two separate processThe Man who Mistook his Wife for a Hat
By Oliver Sacks
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Visual form of agnosia
Dr. P found he cannot recognize students but can tell their namefrom their voicesStarted conversation with parking meters or expected furniture to talk to himWas he blind?
Eye examination showed no problemInability to recognize objects due to a tumor in brainWhen show a gloves, he told
a continuous surface, unfolded by itself, with five outpouchingsMaybe a container or a purse to keep coins to different sizes
He can perceive an object, identify parts of it, but cannot assemble the parts perceptually to recognize it
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Perceptual Process
Environmental Stimulus
Attended Stimulus
Stimulus on Receptors
Transduction
Processing
Knowledge Perception
Recognition
Action
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Action
Goal of perception is to create actionEvolutionary reason for development of perception
Motor activitiesResponse to perception and recognitionLeads to
New attended stimulusWhole cycle repeats
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Perceptual Process
Environmental Stimulus
Attended Stimulus
Stimulus on Receptors
Transduction
Processing
Knowledge Perception
Recognition
Action
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Knowledge
AffectsProcessing, Perception, Recognition
Identification from memoryOld knowledgeRecent knowledge
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Cognitive Influences on Perception
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Cognitive Influences on Perception
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Visual Perception
Vision plays preeminent role for humansEvolutionary history
Vision was developed to aid survival and successful reproduction
Used to get nourishing food, safe shelter and strong matesAlso avoid dangers and predators
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Why Vision is so Important?
Vision is preeminentOnly sense that provides accurate spatial information from a distance
Smell and hearing provides information from a distance but not accurateTouch and taste provide very accurate information but only on contact
Vision helps us most to keep away from dangerVision is a veridical perception
What you see is what you get
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Pathological Cases
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Studying Perceptual Process
Levels of Analysis (At different scales)Bottom Up
Starting from the stimulus towards perception
Top DownStarting from knowledge
We will study both and cross refer
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Studying Perceptual Process
PsychophysicalRelationship between stimulus and perceptionWhat?
PhysiologicalRelationship between the processes within a person and perceptionHow?
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Psychophysical Approach
DescriptionRecognitionDetectionMagnitude Search
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Psychophysical Approach
DescriptionDescribing the experiencePhenomenological Method
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Recognition
A stimulus is presented, the subject has to recognize it
Widely used by physicians and doctorsUsually qualitative
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Detection
Absolute thresholdSmallest amount of stimulus required to generate detectable perception
Difference thresholdSmallest difference between two stimulus that a person can detect
Sensitivity = 1/thresholdClassical Psychophysical Methods
Elements of Psychophysics by Fechner
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Absolute threshold
Measured byMethod of limitsMethod of adjustmentMethod of constant stimuli
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Absolute threshold
Measured byMethod of limits
Presents stimuli in ascending or descending order
Method of adjustmentMethod of constant stimuli
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Method of Limits
Results of using method of limits
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Absolute threshold
Measured byMethod of limits
Presents stimuli in ascending or descending order
Method of adjustmentSubject controls the strength of stimulus
Method of constant stimuli
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Absolute threshold
Measured byMethod of limits
Presents stimuli in ascending or descending order
Method of adjustmentSubject controls the strength of stimulus
Method of constant stimuli Presents stimuli of random strength in random order
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Method of Constant Stimuli
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Measuring Absolute Threshold
Method of limits and adjustmentErrors due to anticipationFast
Methods of constant stimuliMost reliableSlow
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Difference Threshold
‘Standard weight’ compared with `comparison weight’The `comparison weight’ is increased from ‘standard weight’ unless the change is detectedThe detectable change is a function of the `standard weight’
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Weber Law
Difference Threshold
Standard Weight K
Weber fraction
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Magnitude
Threshold measures detectable changesHow to measure what happens once we are past the threshold?Magnitude Estimation
Present subjects with different strength of stimuli at randomAsk them to number the strength of perceptionPlot these to estimate the magnitude
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Response
Response Expansion
Response Compression
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Steven’s Power Law
P = KS n
P = PerceptionS = Stimulus
Strength
n>1.0 (Expansion)
n<1.0 (Compression)
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Steven’s Power Law
P = KS n
P = PerceptionS = Stimulus
Strength
n>1.0 (Expansion)
n<1.0 (Compression)
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Adapted to Functions of Organisms
Bright sunlight would not appear very bright since n=0.6If n>1.0
Brightness of outdoor scenes would not allow us to see anything insideMay even impair our ability to see
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Adapted to Functions of Organisms
For electric shock, small changes cause large perception of painActs as a warning mechanismSo that we can react before much damage is done
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Searching
Can be easy if objects stand outDifficult if they mingle well with their surrounding
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Physiological Approach
Sensors have receptors that transduceenvironmental energy to electrical eneryNerves carry these signals to the brainProcessing in brain leads to the experience of perception
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Intrigued people for a long time ….
In mid 19th century by Johannes MuellerShowed that electrical energy flows down the nerveCan measure electrical signals in brain to assure brain activityNature of perception depended on which nerves were stimulatedQuality of perception depended on how these nerves were stimulated
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Neurons
Neuron: Basic element of the nervous system Three parts of a neuron
Cell Body: Has the elements to keep it aliveDendrites: Branches out from cell body to receive signals from other neuronsAxon: Tube filled with fluid that conducts the electrical signals
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Nerve
The nerve is formed by a bunch of axons
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Perception
Recording of electrical signal in receptorsTransmission of signals to the brain
Some low level processing occurs at this timeProcessing in brain
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Receptor Neuron
This is a special type of a neuron present in sensorsHave a receptor (transducer) in place of a cell body
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Recording the Electrical Signal
At the receptorElectricity is not carried in the nerves as electricity is carried to our homeNeurons are immersed in solutions rich in ions (molecules carrying electrical charge)The solution outside the axon is rich is Na+
The solution inside the axon is rich is K+
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Resting Potential
At normal state, the difference of charge is -70 mV
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Action Potential (Nerve Impulse)
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Back to Resting Potential
Sodium and potassium ion concentration goes back to normal by a process called sodium-potassium pump
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How is it caused?
Change in the permeability of the axon membraneBefore stimulus, the permeability to sodium and potassium is lowThe receptor when excited triggers a process which increases this permeabilityEverything happens in 1msPropagated response: This action potential travels through the axon
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Strength of Signal
Action potential magnitude is always sameHow is strength of stimulus denoted?
Rate of firing
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Rate of Firing
After every firing there is a refractory period of 1msThis limits the maximum firing rate to a maximum of 800 pulses per secondIn the normal state, there is some firing called spontaneous activity
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Transmission of Electrical Signals
From one neuron to anotherEnd of axon of one neuron to dendrite of another
No physical contact between neuronsSynapse: Small space between neurons
Discovered by Spanish anatomist Santiago Ramon CajalWon Noble Prize for this in 1906
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Synapse
Electrical signal triggers a chemical process in the synapse that is instrumental in transmitting the signalsPresynaptic Neuron: From whom the signal is transmittedPostsynaptic Neuron: The one who is receiving the signal
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Synapse
Axons of neurons have synaptic vesicles that can release neurotransmitters
Neurotransmitters can be of different shapesDendrites of neurons have receptor sites
Receptor sites can be of different shapes too
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Synapse
When electrical energy reaches the end of presynaptic neuron
Releases the neurotransmittersIf the shape matches the receptor site in postsynaptic neuron
Generates electric energy againTransmission is controlled by the type of neurotransmitters generated
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Synapse
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Synapse
Electrical signals transmitted may not be identical to the presynaptic neuronCan be excitation or inhibitionLow level processing while transmission
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Brain
Cerebral Cortex2mm thick layer that covers the surface of the brain Used for perception, language, memory and thinking
Modular organization
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Brain
Primary Receiving AreasFirst areas in Cerebral Cortex to receive the signalsOccipetal lobe: For eyeTemporal lobe: For earParietal lobe:For skin
Advanced processingInvolves other areas of the brain too
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Studying Brain Activity
NeuropsychologyStudying the effects of brain damage on behavior
Microelectrodes on cats and monkeysIntrusive
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Studying Brain Activity
Now can study on humans by less intrusive disk electrodes
Can study evoked potentialsRecord activity of thousands of neurons
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Neuroimaging
Using positron emission tomography (PET)Person is injected with low activity radioactive tracer (not harmful)This tracer indicates volume of blood flowBasic concept:
Activity in brain is accompanies with changes in the volume of blood flowMonitoring blood flow
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Subtraction technique
Find state before stimulusFind state after stimulusDifference gives what was cause by stimulus
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Functional Magnetic Resonance Imaging (fMRI)
Hemoglobin has ironIf presented with magnetic field, they line up like tiny magnets to indicate volume of blood flowEspecially, with activity hemoglobin loses some of it oxygen making it more magneticPopularly called Brain Scan
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THE END