-
Perception –
- study of how we notice,
- recognize & add meaning to the world around us
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-
Environmental Stimuli
- All the things in the environment we can
- possibly perceive
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Attended Stimulus
- The object that
- becomes the center of our attention
-
Stimulus on
Receptor
- Look directly at the
- object of interest – image is formed on receptors of the retina –
- transformation of object into an image
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Transduction
- Transformation of one form of energy to
- another
- E.g., ATM – when you press the “withdrawal”
- button on the screen, pressure exerted by your finger is converted into
- electrical energy – sends signals to mechanisms that push your money out
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Neural Processing
- Light energy entering
- retina – sends signals to activate neurons, which activate other neurons –
- “message is passed on”
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Perception
- Electrical signals
- are transformed by the brain into an “experience” (or awareness) or the object
-
Recognition
- Our ability to place the object in a category
- & give it meaning, e.g., “table”
Problem – Agnosia – inability to recognize objects
-
Agnosia
- inability to recognize
- objects
-
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Knowledge
- Any information that
- the perceiver brings to the situation – e.g., I already know the difference
- between a table & a chair
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Bottom-up (data)
processing
- –Understand based on
- incoming data
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Top-down
(knowledge-based) processing
- –Understand based on
- prior knowledge
- –What you already know
- influences what you learn from the environment
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Measuring Perception 5 levels
- (1) Description
- (2) Recognition
- (3) Detection
- (4) Magnitude
- estimation
- (5) Search
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Measuring Perception
(1) Description
- –Ask the observer to
- describe what they see – “all the people sitting in the student section are
- wearing red”
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Measuring Perception
(2) Recognition
- –Ask the observer to
- place the stimulus in a particular category – “Number 22 is the other team’s
- quarterback”
-
Measuring Perception
(3) Detection
- Absolute threshold
- Difference
- threshold (DL)
-
Measuring Perception
(4) Magnitude
estimation
- –How large/strong/bright
- is a stimulus?
- –The nature of light - at higher
- intensities – doubling the intensity leads to only a small change in perceived
- brightness – response compression – e.g., bright
- flashlight
- –The nature of electricity – doubling the
- strength of a shock more than doubles the perceived sensation of being shocked
- – response expansion
- –The nature of line lengths – perceived length
- is proportionate to changes in actual length
–Steven’s Power law
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Measuring Perception
(5) Search
- –In previous cases,
- observer has time to make leisurely responses
–
- –What about situations
- when there is time pressure? – e.g., air traffic controller, luggage screener,
- quality control inspector
–
- –Measure of perception
- – reaction time
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Measuring Perception
(3) Detection
Absolute threshold
3 levels
- –(1) Method of Limits
- –(2) Method of Adjustment
- –(3) Method of Constant Stimuli
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Measuring Perception
(3) Detection Absolute threshold
3 levels
1. Method of Limits:
- Present stimuli is
- ascending or descending order & observer says at what point the light
- becomes barely visible
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Measuring Perception
(3) Detection Absolute threshold
3 levels
2. Method of Adjustment:
- Observer adjusts
- stimulus energy in a continuous manner until the stimulus is barely detectable
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Measuring Perception
(3) Detection Absolute threshold
3 levels
3. Method of Constant
Stimuli
- Experimenter presents
- 5-9 different stimuli in random order – observer says “yes” when they detect
- the stimulus
- Threshold – the
- stimulus is detected 50% of the time
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Measuring Perception
(3) Detection Absolute threshold
3 levels
which is the most accurate but slowest?
–Method of constant stimuli
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Measuring Perception
(3) Detection Absolute threshold
3 levels
which is the least accurate but fastest?
Method of Adjustment
-
Difference
threshold (DL)
–
- minimum
- detectable difference between two stimuli
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Measuring Perception
(4) Magnitude estimation
The nature of light:
- at
- higher intensities – doubling the intensity leads to only a small change in
- perceived brightness – response
- compression – e.g.,
- bright flashlight
-
Measuring Perception
(4) Magnitude estimation
The nature of electricity:
- doubling
- the strength of a shock more than doubles the perceived sensation of being
- shocked – response expansion
-
Measuring Perception
(4) Magnitude estimation
The nature of line lengths:
- perceived
- length is proportionate to changes in actual length
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- Research on how the
- brain connects to perception – cerebral cortex, 2mm-thick layer covering brain
- surface
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Neurons
- Carry electrical
- signals from brain to the rest of the body
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-
Neurons
Electrical signals
- transmitted
- through a solution rich in ions (molecules that carry electrical charge) that surround neurons
-
Neurons:
solution outside axon is rich in:
solution inside the axon is rich in:
- outside:+ve charged sodium ions;
- inside: –ve charged potassiumions
-
resting potential
- Negative
- charge inside the neuron is called
-
“action potential” or “nerve impulse”
- If
- neuron’s receptor is activated, +ve
- sodium ions rush into the neuron & -ve
- potassium ions rush out –
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Permeability
- property
- of cell membrane – ease with which molecules can pass through membrane
-
Refractory period
- limit
- to the number of nerve impulses that can be carried down a neuron – 500-800
- impulses per second
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Spontaneous activity
- firing in the absence of stimuli from
- environment
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Synapse
- small
- space between neurons – how do nerve impulses flow across this space?
-
Neuron
releases a chemical called
neurotransmitters
-
“neurotransmitters” – flow into the
synapse into small areas called
receptor sites
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Excitatory
transmitters
cause the inside of neuron to become more positive – depolarization
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Inhibitory
transmitters
- cause
- the inside of neuron to become more negative - hyperpolarization
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Electromagnetic
spectrum
- light
- rays (electromagnetic energy) of varying wavelengths
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Visible light
- electromagnetic
- spectrum that humans can perceive
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Photons
small packets of energy
-
Wavelength + photons
important properties for perception
-
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Cornea & lens
(light rays first pass);
pupil (dilates in darkness or constricts in brightness);
retina (final surface on which image is projected)
–
- lens “accommodates” or
- adjusts its shape to bring the image to focus on the retina
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Ciliary muscles
- responsible for
- accommodation – closer objects-rounder shape, farther objects-flatter shape;
- natural resting point of the lens “resting state”; send message to receptors in
- the brain
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myopia
(nearsightedness);
-
presbyopia
(farsightedness)
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Near point
- distance at which your lens can no longer
- adjust to bring an object into focus – near point increases as a person gets
- older (presbyopia)
-
-
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Rods (night vision) and cones (day/bright
vision) – receptor cells in the brain. Properties:
- 1) Location: middle of retina (fovea) – only cones;
- periphery – more rods & few cones; cones decrease rapidly moving farther
- from fovea
- 2) One
- area in the retina – no receptors – place where optic nerve leaves the eye – “blind spot”
- 3) “Blind spot” –
- off to the side of our visual field, where objects are not in sharp focus
- 4) Most
- of the time, we don’t notice the blind spot – the brain “fills in” the gap from experience
- 4) Color sensitivity: rods are color blind – color perception
- decline in dim light & at night
- 5) Adaptation: when
- stimulated by direct light rods lose their sensitivity & take a long
- time to regain – temporary blindness – e.g., entering a darkened movie theater.
- Cones
- become hypersensitive in low light – glare from lights at night.
- 6) Differential
- wavelength sensitivity: rods are insensitive to long
- (red) wavelengths – red objects look black at night, illuminating objects in
- red light will not affect vision at night
-
“blind spot”
- One area in the retina –
- no receptors – place where optic nerve leaves the eye
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Color sensitivity
- rods are color blind –
- color perception decline in dim light & at night
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Adaptation
- when stimulated by direct
- light rods lose
- their sensitivity & take a long time to regain – temporary blindness –
- e.g., entering a darkened movie theater. Cones become hypersensitive in low light – glare
- from lights at night.
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Differential wavelength sensitivity:
- rods are
- insensitive to long (red) wavelengths – red objects look black at night,
- illuminating objects in red light will not affect vision at night
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Macular degeneration
- older people, a part of
- the cone-rich fovea & area surrounding it is destroyed – creates a “blind
- spot” in central vision
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Retinitis pigmentosa
- genetic – peripheral
- receptors are destroyed
-
- 1. Macular degeneration
- 2. Retinitis pigmentosa
-
Visual pigment molecules
- large protein called opsin; small light-sensitive
- molecule called retinal
-
opsin
large protein called
-
retinal
- small light-sensitive
- molecule
-
Measuring cone adaptation
- look directly at the test
- light so the light falls on the cone-rich fovea
-
Measuring rod adaptation
- can only be measured in
- people with no cones – a genetic defect called rod monochromats
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Cones are more sensitive in the
beginning – after ______mins, cones finish adapting & sensitivity
levels off
3-5
-
Rods catch up after about ___ mins
& continue to adapt for the next ____mins
-
rod-cone break
- Place where rods begin to
- determine dark adaptation
-
pigment bleaching
- When visual pigments
- absorb light, retinal separates from opsin & retina becomes lighter in color
-
visual pigment regeneration
- occurs in the dark –
- determines speed at which sensitivity is adjusted
-
Cone pigment – ____ mins to
regenerate completely
6
-
Rod pigment – 30
mins to regenerate
- Rod pigment – 30
- mins to regenerate
-
Rods are more sensitive to
___ wavelengths – most sensitive to light of 500 nm
short
-
Cones are sensitive to ____
wavelengths – most sensitive to 560nm
longer
-
Purkinje shift
- more sensitive to blue-green (short wavelengths) & less sensitive to red
- (long wavelengths) – effective night lights are generally
- shades of blue/green – this shift is called
-
-
- Bright light (photopic)
- Dusk (mesopic)
- Night (scotopic)
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Convergence
- when many neurons send
- signals to one neuron in the retina
-
convergence is greater in rods
- since all signals converge
- into one, there are fewer neural response units – poorer visual acuity
-
When searching for
something (familiar face), the image needs to be focused
on the
fovea for us to recognize it
-
cones have
better
- visual acuity (ability to
- see detail)
-
Foveal cones are more
____ packed than peripheral rods
- Foveal cones are more
- densely packed than peripheral rods
-
Rods take over from
____ in the dark
- Rods take over from
- cones in the dark – which is why vision is poorer in the dark
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Lateral Inhibition
- Activity in one
- neuron decreases activity in another
- – e.g., illumination of neighboring receptors inhibits firing of one receptor
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(1)Hermann grid: seeing spots at intersections
-
- Mach Bands: seeing borders more
- sharply
-
“dark band”
- area appears darker than
- it is
-
light band
- area appears lighter than
- it is
-
- Simultaneous
- Contrast:
- perception of brightness of one area or color is affected by the presence of an
- adjacent or surrounding area
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Receptive field
- of
- a neuron in the visual system is the area on the retina that influences the
- firing rate of the neuron
-
- Area A – not part of the neuron’s receptive
- field, stimulating this area causes no change in neural activity
- Area B – excitatory
- area –
- stimulating causes increase in neuron’s firing rate
- Area C – inhibitory
- area –
- stimulating causes decrease in neuron’s firing rate
- Center-surround receptive field – center region
- responds one way, surrounded by a region that responds another way
Excitatory-center-inhibitory-surround receptive field
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Selective adaptation
- when
- we view a stimulus with a specific property, neurons tuned to that property
- fire
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Method: Selective Adaptation
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Contrast sensitivity
- how
- sensitive people are to the differences between light & dark bars
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Orientation
- angle
- relative to horizontal
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Contrast
- difference in
- intensity of light & dark bars
- –After being adapted to a certain level of intensity,
- contrast sensitivity to dark & light decreases
-
Adapting to wide-grating bar decreased responding of neurons that respond best to wide bars – right bars appear more ____
narrow
-
Adapting to
narrow-grating bars decreased responding of
neurons that respond best to narrow bars – right
bars appear more ___
wide
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Neural plasticity
- response properties
- of neurons can be shaped by a person’s perceptual experience
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Selective rearing
- if
- an animal is reared in an environment that contains only certain types of
- stimuli, neural plasticity will cause neurons to respond best to that type of
- stimulus
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Maps
- surface
- of retina is mapped onto the visual cortex – adjacent
- points on the cortex correspond to adjacent areas on the retina
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Columns
- neurons
- with similar properties are organized as columns in the visual cortex – how an object is represented or what it “looks like
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Streams
- sequence
- of interconnected structures that serve specific functions – what to do, how to do
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Modules
- certain
- areas that are specialized to serve specific functions – recognizing faces, objects
-
Each point on the
object is imaged onto points in the
____
retina
-
Each point on the
retina corresponds to a point on the
lateral geniculate nucleus (LGN)
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Correspondence
between points on the LGN & points on the retina
- retinotopic map on LGN & in visual
- cortex
-
cortical
magnification factor
- Small fovea
- presenting a large image on the cortex – cortical magnification factor
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Positron
emission tomography (PET)
- person
- is injected with a small dose of radioactive tracer that is not harmful –
- enters bloodstream, accompanied by changes in blood flow, provides a measure of
- blood flow to track which areas of brain are being activated
-
Subtraction
technique
- brain
- activity is first measured in a “control” state before stimulation (object) is
- presented – then measured again when the person manipulates the object
-
Functional
magnetic resonance imaging (FMRI)
- similar
- to PET, but takes advantage of ferrous (iron/hemoglobin) molecules in the blood
- by presenting a magnetic field – causes ferrous (hemoglobin) molecules to line
- up like tiny magnets
-
How is an object
represented in the cortex?
- –We know that – each
- point on the object forms a corresponding retinotopic image on the LGN & visual cortex
-
–What does the image
of the object “look” like?
- –The image is distorted
- compared
- to the actual object – magnification factor allots more space on the cortex
- specifically to parts of the image that fall on the fovea
-
-
–Large stimulus
– stretchesacross the retina – stimulates a number of different orientation columns
-
–Cortical
representation of the stimulus does not have to
resemble the stimulus
-
–The length of the
peppermint stick is represented in different cortical columns in the visual
cortex – each column
contains neurons that fire
-
Object
discrimination (“what is it?”)
- –
- e.g., monkey – pick the object that contains food (object discrimination
-
Landmark
discrimination (“where is it?”)
- –
- e.g., monkey – pick the object that is closest in distance to the cylinder –
- (landmark discrimination)
-
the “what” pathway or “ventral pathway”
Temporal lobe
-
the “where pathway” or “dorsal pathway”
Parietal lobe
-
disrupted
object discrimination task
-
disrupted landmark discrimination task
–Parietal lobe –
-
The ventral &
dorsal pathways are ____
Signals flow in __
directions
- The ventral &
- dorsal pathways are connected
- Signals flow in both
- directions
-
Argument
- dorsal
- &
- ventral streams should also be called “how (instead of “where”) & “what”
- pathways
-
- Rod-frame illusion –
- matching task vs. grasping task
-
module
- A
- structure that is specialized to process information about a particular type of
- stimulus
-
Structures located
along the dorsal & ventral pathways are specialized to process information
about specific
visual quantities
- modularity
-
“Face neurons
- monkey’s
- cortex – some neurons fired in response to pictures of faces & did not fire
- in response to non-face stimuli (landscape, food)
-
Damage to temporal lobe – affects ability
to
recognize faces – prosopagnosia (difficultyrecognizing familiar faces, though they can recognize them easily when theyhear them talk)
-
prosopagnosia
(difficulty recognizing familiar faces, though they can recognize them easily when they hear them talk)
-
fusiform face area (FFA)
- also responds to
- pictures in which the presence of a face is implied – e.g., body with a blurred
- image where the face
-
Parahippocampal
place area (PPA)
– activated by indoor & outdoor scenes
-
Extrastriate body area (EBA)
– activated by body parts but not faces
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