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Perception
Chapter 1
Why study perception
-philosophy, computer science, medicine
-hearing problems, speech recognition, robots that can see
The Perceptual Process
-perception is the end result of complex behind the scenes processes
Perceptual process: is a sequence of processes that work together to determine our
experience of and reaction to stimuli in the environment.
7 Perception
8 Recognition
9 Action
Experience
and Action
6 Processing
5 Transmission
6 Transduction
Electricity
Stimulus
1 Environmental
Stimulus
2 Attended
Stimulus
3 Stimulus on
the receptors
Stimulus
-exists both out there, in the environment and within the persons body
Environmental Stimuli and Attended Stimuli
Environmental: is all of the things in our environment that we can potentially perceive
Ex. Trees, path,
Attended stimulus: focusing on the stimuli and making it the center of attention.
-the attended stimulus changes from moment to moment
Ex. Moth
Stimulus on the Receptors
-the stimulus activates a receptor
Ex. Image of the moth is formed in the retina
-this image is a representation of the moth
Electricity
-everything we perceive is based on electrical signals in our nervous system
Transduction
-is the transformation of one form of energy into another form
Ex. Pattern of light in the retina is transformed into electrical signals
Transmission
-electrical signals active other neurons and eventually are transmitted to the brain
-if the signals don’t reach the brain there’s no perception
Processing
-As the signals are being transmitted they undergo neural processing interactions btw
neurons
*in the brain the original electrical representation of the stimulus that is created by the
receptors is transformed by processing into a new representation, and it is usually very
different from the original signal.
Experience and Action
-we are aware of this stage
Perception
-is conscious sensory experience
*Two additional steps: recognition and action are important outcomes of the process
Recognition
-is our ability to place an object in a category
Ex visual form agnosia- an inability to recognize objects
-this proves that perception and recognition are different processes
-Dr.P perceived parts of objects but couldn’t identify them because of a tumor
Action
-includes motor activities such as moving the head or eyes and locomotion through the
environment
- Milner & Goodale propose that the goal of visual processing was for survival
*overall perceptional process dynamic and continually changing
Knowledge
-is any information that the perceiver brings to a situation
- can affect numerous steps in the perceptual process
-Rat-man demonstration shows how recently acquired knowledge influences perception
Bottom-up processing: based on incoming data (Data-based processing)
*bottom-up is essential because the perceptional process begins with stimulation of
receptors
Top-down processing: based on knowledge (knowledge-based)
*thus bottom-up and top-down processing often work together to create perception
*as stimuli get more complex the more top-down processing is involved
Study of Perception
Psychophysical approach to perception: Gustav Fechner coined the term
psychophysicsuse of quantitative methods to measure relationships between stimuli
and perception.
Definition: Any measurements of the relationship between stimuli and perception
Physiologal approach to perception: measuring the relationship between stimuli and
physiological processes
. (A) Stimulusperception:
Two colored patches are
judged to be different.
(B) Stimulus-physiology: A
colored light generates a
Neural
response in the cat’s cortex.
(C) Physiology-perception:
Brain activity is monitored as
a person indicates
what he is seeing.
As we study perception we are also concerned with how knowledge, memories, and
expectations influence perception
Cognitive influences on perception: these factors are the starting point for top-down
processing
Measuring Perception
1. Describing
-indication characteristics of a stimulus
Phenomenological method: describing what he/she perceives is the first step of studying
perception
2. Recognition
-placing a stimulus in a specific category
Method: stimulus is presented and the observer indicates what is
Detection (3)
-becoming aware of a barely detectable aspect of a stimulus
Classic psychophysical method
I. Limits
II. Adjustment
III. Constant
Absolute threshold: the smallest amount of stimulus energy necessary to detect a
stimulus.
Method:
Limitsexperimenter presents stimuli in ascending or descending order
Find the Cross over point between yes and no
Adjustmentobserver or experimenter adjust the stimulus intensity continuously until
the observer can just barely detect the stimulus
Constant stimuliexperimenter presents five to nine stimuli with different intensities in
random order
-threshold is defined as the intensity that results in detection of 50% of the trials
*constant stimuli the most accurate b/c many observations and stimuli in random order,
minimizes observer judgment and expectation
*method of adjustment fastest
Difference Threshold: the smallest difference between two stimuli that a person can
detect
Method: same methods except participants are asked whether they detect a difference btw
two stimuli
-the difference threshold is the difference btw the standard weights and comparison
weights that the observer said “different”
*the magnitude of the stimulus increases so does the size of the DL
DL/S=K Weber’s Law
S is the value of the standard stimulus
K is Weber’s fraction
Magnitude Estimation (4)
-measuring above-threshold perceptions
Method: Stevens
-experimenter first presents a standard stimulus to the observer and assigns it a value (10)
he/she presents lights of different intensities and the observer is asked to assign a number
that is proportional to the brightness of a standard stimulus
- can do without standard
*doubling intensity does not necessarily double brightness
Response compression: as intensity increased, the magnitude increases but not as rapidly
as intensity. Ex. Brightness
Response expansion: as intensity is increased, perceptual magnitude increases more than
intensity. Ex. Electric shock
-the intensity of a stimulus and our perception of its magnitude follow the same general
equation for each sensepower functions
P=KSn Steven’s power law
P is perceived magnitude
S is stimulus intensity
-taking the log of the power functions results in straight lines
-the slopes of the straight lines indicate n
-response compression slope of less than 1
-response expansion slope of more than 1
Search
-looking for a specific stimulus ex. Visual search
-measuring reaction time important for mechanism of perception
Other methods
-same or different stimuli
*different people may have a different response criterion
-doesn’t matter if it’s the same person also not important if we are testing many people
and averaging their response
(Appendix) Signal detection theory: only one stimulus intensity is presented and
sometimes no stimulus is presented.
 When the tone is presented: saying yeshit
Saying nomiss
 When no tone is presented: saying yesfalse alarm
Saying nocorrect rejection
Chapter 2
Aristotle- heart was the seat of the mind and the soul
Galen- health, thoughts and emotions determined by spirits flowing from the ventricles
Descartes-flowing spirits, specified the pineal gland as the seat of the soul
Thomas Willis- brain responsible for mental function, different functions in different
regions, brain disorders due to chemistry
Structure of the nervous system
1. Reticular Theorylarge network of fused nerve cells
2. Neuron theorydistinct elements or cells, the discovery of staining proved this
theory
Golgi developed staining by immersing a thin slice of brain tissue in a solution of silver
nitrate created individual cells that were stained
Mueller: doctrine of specific nerve energies which stated that our perception depend on
nerve energies reaching the brain and that the specific quality we experience depends on
which nerves are stimulated
*nerves from each sense reach different areas of the brain
Edgar Adrian: recorded electrical signals from a single neuron
Basic Structure of the Brain
-cerebral cortex: covers the surface of the brain and contains machinery for perceptions,
language, memory, and thinking
Modular organization- specific functions are served by specific areas of the cortex
Ex. primary receiving areas: the first areas in the cerebral cortex to receive the signals
initiated by a sense receptor
Visionoccipital lobe
Hearingtemporal lobe
Skin senses (touch, temp, pain) parietal lobe
All sense, perceptions that involve coordinationfrontal lobe
Structure of neurons
Cell body: mechanisms for survival
Dendrite: receive signals from other neurons
Axon or nerve fiber: filled with fluid that conducts electrical signals
*variation in the structure
Receptors: type of neuron specialized to respond to environmental stimuli
a.) Light (vision)
b.) pressure changes in air (hearing)
c.) pressure changes in skin (touch)
d.) chemicals in the air (smell)
e.) chemicals in liquid form (taste)
*all respond to the environment and generates an electrical signal
Recording Electrical Signals
Nerve: consists of the axons of many neurons
Method: microelectrodes (small shafts of glass/metal) measure the difference in
charge between two electrodes
recording electrode: where the electrical signal will occur
Reference electrode: not affected by the electrical signal
-the differences btw electrodes is recorded on an oscilloscope
Resting potential: -70mV
Action potential: signal 40mV lasts 1ms
Chemical Basis
Action potential: sodium rushes in the neuron and potassium rushes out
-the flow of ions in and out of the neuron is due to permeability
Selective permeability: occurs when a membrane is highly permeable to one specific type
of molecule but not to others
-stimulation of the receptors triggers a process that makes the membrane selectively
permeable to sodium, so sodium flows in
-when the action potential reaches +40mV the membrane becomes selectively permeable
to potassium so potassium flows out
*sodium-potassium pump is necessary to maintain this gradient
Action Potentials
Properties:
1.) Propagated response: once the response is triggered it travels all the way down the
axon without decreasing in size
*important property of neurons because it enables them to transmit signals over long
distances
2.)Action potential remains the same size no matter what the intensity of the stimulus is
*increasing intensity changes the rate of firing (not the size of the action potentials)
Limit to increasing late of firing is due to the refractory period (1ms)
Refractory period- the interval between the time one nerve impulse occurs and the next
one can be generated in the axon.
Upper limit of rate firing= 500-800 impulses per second
3.) Spontaneous activity: neurons fire without stimuli and this establishes a baseline
level of firing for the neuron
*stimulation can be an increase or decrease from the baseline level of firing
These changes in nerve firing can provide information about the intensity of a stimulus
but it needs to be transmitted to other neurons, and the brain for it to be meaningful.
Synapse
Synapse: space between neurons
-action potentials triggers the release of neurotransmitters from the synaptic vesicles
-neurotransmitter flow from one neuron to another neuron’s receptor sites
Lock and Key principle: receptor is only sensitive to certain neurotransmitters therefore
the neurotransmitter must match in shape with the receptor as to trigger an action
potential.
Excitatory transmitterscause the inside of the neuron to become more positive
(depolarization (process of a neuron becoming positive) is the excitatory response)
*need enough excitatory neurotransmitters for action potential
Inhibitory transmitterscause the inside of the neuron to become more negative
(hyperpolarization (process of a neuron becoming positive) is the inhibitory response)
*The response of a neuron is determined by the interplay of excitation and inhibition
*inhibition is essential for the processing step
Neural Processing
Neural circuits: are groups of interconnected neurons
Circuit with no convergence: the response of one neuron is not affected by surrounding
neurons
Convergence: the synapsing of more than one neuron onto a single neuron
Circuit with convergence: The size of one neuron’s response increases when the stimulus
size (the number of receptors) is increased
Circuit with convergence and inhibition: the neuron responds best to a stimulus of a
specific size
Convergence and inhibition evidence has been obtained by measuring the property of
neurons called the neuron’s receptive field.
Receptive Fields
Receptive field: is the area on the receptors that influences the firing rate of the neuron
Method:
-Measure a receptive field of a neuron by stimulating a cat’s retina with light and
recording from a nerve fiver
-cat is stationary and is looking at a screen
-flash a small spot of light
Excitatory area: areas where there’s an increase in neurons’ firing rate
Inhibitory area: areas where there’s a decrease in neurons firing
Since a receptive field is any area where stimulation influences firing both an
excitatory and inhibitory area can make up a receptive field.
Center-surround receptive fields: a center region that responds one way, and a surround
region that responds the opposite way
Ex. Excitatory-center-inhibitory-surround receptive field
Ex. Inhibitory-center-excitatory-surround receptive field
Since the center and surround respond to in opposite ways it causes an effect called
center-surround antagonism
(Neuron is a type of cell)
Figure 3.21 Response of an
excitatory-center-inhibitory-surround
receptive field as stimulus size is
increased.
Color indicates the area stimulated
with light. The response to the
stimulus is indicated below each
receptive
Field. The largest response occurs
when the entire excitatory area is
illuminated, as in (b). Increasing
stimulus size
further causes a decrease in firing
due to center-surround antagonism.
Center-surround receptive fields also occur in the skin
Sensory Code
-receptive fields enable us to specify a neuron’s response
For the best response from a receptive field you need both the optimal location and the
size or shape of the stimulus.
How does the firing of neurons represent various characteristics: specificity coding?
Specificity Coding
Specificity Coding: is the representation of particular objects in the environment by the
firing of neurons that tuned to respond specifically to that object.
-idea that a single neurons response to a specific responseKonorski and Lettvin
Grandmother cell: according to Lettvin is a neuron that responds only to a specific
stimulus
Evidence:
-little evidence
-Quigora recorded from neurons that respond to very specific stimuli in patients with
epilepsy
-found that some neurons responded to a number of different views of just one person or
building
-neurons responded to different views of the stimulus, different modes of depiction, and
even words signifying the stimulus
-neurons respond to concepts
-these neurons were in the hippocampus and the MTL (medial temporal lobe) areas
associated with the storage of memories
-However they are not grandmother cells because:
 if it was only one neuron responding to a stimulus it would extremely difficult to
find
 neurons probably respond to different stimuli probably 50-150 different
individuals or objects
Distributed coding: the idea that a particular object is present not by the firing of a
single neuron but by the firing of a group of neurons
Distributed Coding
-is the representation of a particular object by the pattern of firing of groups of neurons
Advantage: it is doesn’t require a specialized neuron for every object
Sparse Coding
-Is the idea that a particular object is represented by the firing of a relatively small
number of neurons
-code for representing objects, tones, odours involve sparse coding
Mind-Body Problem
How do physical processes become transformed into the richness of perceptual
experience?
Neural correlate of consciousness (NCC): determining connections between stimuli and
the firing of neurons, where consciousness can be roughly defined as our experiences
Easy problem of consciousness: finding the NCC
Hard problem of consciousness: How do physiological response become transformed
into experience? I.e. how do sodium and potassium ions flowing across a membrane
become the perception of a persons face?
 most researchers focus on NCC