PSYC 2220 Lecture Notes - Lecture 1: Visual Cortex, Psychophysics, Absolute Threshold

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13 Aug 2016
Department
Course
Professor
SnP Chapter 1 the perceptual process
notes of the process
does not have to follow this
order. i.e. one tends to think
perception of something has to
occur before recognition of
something, but it can actually
occur at the same time or in
reverse.
there are double arrows
between perception,
recognition and action because
they do not have to occur in
order i.e. 5,6,7 and one can
affect the others i.e. action of
looking closer at a stick
changes its recognition from a
stick to a snake, thus changing
perception.
stimuli
1. environmental stimuli
2. light is reflected and
transformed
1.light is reflected from the tree
(object outside) and enters eye
the nature of this reflected light
depends on: the properties of
light energy hitting the tree
(from the midday sun or light
from overcast day?), properties
of the tree (texture, fraction of
light hitting it that it reflects),
properties of medium through
which light is transmitted (i.e. a
foggy atmosphere)
principles of perception that
occur
1.Principle of transformation:
the stimulus is not the same as
the perceived stimulus because
the stimulus underwent
transformation from stimulus
(object)—> electromagnetic
energy (light) —> electrical
signals (firing neurons)
2. principle of representation:
everything a person perceives is
based not on direct contact
with stimuli but on
representations of stimuli that
are formed on the receptors
and on the activity in a person’s
nervous system.
physiology
3. receptor processes
sensory receptors: cells
specialized to respond to
environmental energy
(electromagnetic energy, light
waves)
visual receptors (in retina)
when these receive the
reflected light from an object
they: (1) transform the
environmental energy into
electrical signals (2)the way they
respond to stimuli shapes
perception
How are visual receptors able
to perform transduction (the
transformation of one form of
energy to another)? In this case
it’s from light to electrical energy
they do this via the light-
sensitive chemical in them that
is called VISUAL PIGMENT,
which reacts to light
how do visual pigments shape
perception?
one is able to see dim light if
there is a high concentration of
visual pigment
there are different types of
visual pigments; some respond
better to the blue-green part of
the spectrum, others to the
yellow-red part.
photoreceptors, the receptors
for vision (at back of eye).
Change light into electrical
impulses
rods
cones
auditory receptors respond to pressure changes in
air
touch receptors respond to pressure transmitted
through the skin
smell and taste receptors respond to chemicals entering
the nose and mouth
transduction occurs
(environmental energy/
electromagnetic energy (light) to
nerve impulses/ electrical
signals)
4. neural processing: the
changes that occur as signals
are transmitted through neurons
the image of the object,
represented in the electrical
signals, enter a neural network
that is present in the retina, at
the back of the eye, and goes
up to the brain
what does this neural network
do?
1. transmits electrical signals
from the receptors, through the
retina, through and within the
brain.
2. processes the electrical
signals as they’re being
transmitted (neural processing)
why are the electrical signals
processed?
because there are multiple
paths from the receptors to the
brain i.e. some signals can be
reduced, ignored or amplified.
contrast the electrical signals in
the receptors to the ones in the
brain?
differences: the patterns in the
electrical signals in the brain is
different from the signals left in
receptors.
similarity: although signals have
changed, still represent the
object
the cerebral cortex contains
primary receiving areas, that
receive the electrical signals.
Each primary receiving area is
different, depending on the
sense.
the cerebral cortex is a 2mm
thick layers responsible for
perception and other functions
like language, memory and
thinking.
what is the primary receiving
areas for each sense? Note:
primary receiving areas receive
signals sent by photoreceptors
vision most of the occipital lobe
hearing part of the temporal lobe
skin (temperature, touch, pain) area in the parietal lobe
all the senses frontal lobe
coordinates information
received through two or more
senses
perception (experience and
action)
5. perception
occurs as conscious
experience (awareness of the
object)
perception can occur without
recognition; i.e. visual form
agnosia is an inability to
recognize objects due to a brain
tumour.
example: I see something
6. recognition
2. placing an object in a
category so as to give it
meaning
1.we don’t need to name it to
recognize it; i.e. can recognize
it’s just there.
example: it’s a tree
7. action
this is tied to survival, since the
goal of visual processing was to
help the animal survive by being
able to control navigation, catch
prey etc
happens when motor activity is
initiated by perceiver in
response to recognition
example: let’s take a closer look
at that tree
extra: knowledge
information that a perceiver
brings to the situation. Could
have been recently acquired or
acquired years ago.
bottom-up/ data-based
processing
processing based on incoming
stimuli
this process can occur before,
and trigger, the steps in
perception
i.e. ‘it’s a moth’
most likely to occur when
witnessing simple stimuli like
spot of light
top-down processing/
knowledge-based processing
processing based on previous
knowledge (cognitive factors)
from perceiver
when someone labels what
they are seeing (recognition)
they access past info on this
label.
often is, not always, involved in
perception
if it is then the very last step to
occur i.e. pharmacist can
access past learning of drug
names to understand the image
(doctor note)
as stimuli becomes more
complex, the role of top-down
processing increases
i.e. ‘that moth is from the
species, X’
i.e. rat or man? Depends on
prior experience, i.e. if you saw
man first you’ll see man in the
ambiguous shape again.
top-down influences
people will see more or less
what’s actually in the stimulus,
so we can’t always predict what
people will see based on the
stimulus alone
the stimulus can be misleading michael bach illusions
stepping feet illusion
the different-coloured squares
seem to step alternatively, when
in reality the movement is
simultaneous (both both at
same speed)
Hinton’s lilac chaser see a rotating green spot that
sin’t actually there
bragman’s letters (B)
can affect any of the other
steps
measuring perception
visual searchprocedure?
subject has to find one
particular stimulus among
many, as fast as possible i.e.
find face among a crowd
reaction time, the time
between showing a stimulus
and a subject’s response, is
necessary to find more on
mechanisms important to
perception
phenomenological method,
often used for people with brain
damage
procedure?
person has to tell what they are
perceiving or indicate when a
particular perception occurs
i.e. might be asked to name
colour of light, if something is
sweet or sour etc.
perceptions above thresholds
(what can be easily seen/heard
etc), magnitude estimations: not
an ‘above threshold’ measure
but a ‘suprathreshold’ one.
SS sternest developed the
technique magnitude estimation
to relate stimulus intensity and
perceived estimation
Q: is we double the intensity of
a tone is it twice as loud?
can be done for many sensory
dimensions
method: magnitude estimations
(the relationship between
stimulus intensity and
perception)
results?
what do these two responses
illustrate?
the function of each sense
have adapted to how we
survive in the environment
i.e. because electric shocks
have an exponent of more
than one, a response
expansion occurs that serves
to warn us of the threat
shocks expose. (thus why we
withdraw from even small
shocks)
i.e. because light has an
exponent less than one,
response compression occurs.
This allows for more light data
to be received without blinding
you.
response expansion can
occur: perceived magnitude
increased more than intensity.
response compression can
occur: perceived magnitude
increased less than the intensity
conclusion: doubling in
intensity does not equal to
doubling in perceived
magnitude; response
compressions or response
expansion can occur
procedure?
each intensity can be shown
several times. Then the average
rating is taken for each intensity
subject is shown stimuli (lights)
at different intensities and has
to assign a perceived
magnitude (a value for how
bright they are in proportion to
the original light) to each in
relation to the ‘standard’
what’s the difference between
intensity and perceived
magnitude?
perceived magnitude: the
brightness, or, the perceptual
measure of what a subject
experiences
intensity: a physical measure of
how much energy there is in a
light (i.e. measured in watts)
how is stimuli presented?
experimenter shows a
‘standard’ stimulus (light) to
subject and subject assigns a
brightness value or perceived
magnitude to it i.e. 10
thresholds
threshold measurement can be
influenced by how a person
chooses to respond
people can have different
response criterion.
Someone’s response criterion
can be either low or high; low =
more willing to respond i.e. a
person can be more willing to
state she sees the light even if
it’s a glimpse of it. A high
response criterion= not so
willing to respond, i.e. only says
they can see light i.e. they are
completely sure only.
implications?
only matters when comparing
only two people’s responses.
Solution to this is the signal
detection theory (Appendix)
if testing many people and
averaging, also not important
if interested in only how one
person responds to different
stimuli, it doesn’t matter since
comparison is only within one
person
difference threshold (Ernst
Weber) : the minimum
difference between two stimuli
that must exist in order for us to
tell the difference or the
minimum difference between
two stimuli that can just be
seen
measured by comparing two
stimuli—a reference and a
comparison
Difference threshold, or DL, or
‘just noticable’ differentiation
(JND)’ for the senses:
weight
the ratio of the standard DL to
the standard (original weight) is
constant. Ie if weight doubles,
DL doubles too.
thus a DL/standard ratio that is
constant for all senses can be
made (called Weber Fraction)
each sense has its own weber
fraction/ JNDs
taste (salty).08
light intensity.08
sound intensity.04
lifted weight.02
electric shock.01
read as: ‘the intensity of an
electric shock would have to
increase by 1% before any
dierence is felt'
weber’s law: the weber
fraction remains the same even
as the standard is changed
this law is true for most
senses, as long as the
intensity of the stimulus isn’t
too close to its absolute
threshold
2%
absolute threshold: the
minimum stimulus intensity that
can just be detected or smallest
amount of energy needed to
detect a stimulus (i.e. smallest
amount of light needed to
detect a spot of light)
methods for measuring
thresholds? Note on thresholds:
it’s not a fixed point, it can vary
from depending on trial or
session
Gustav Fechner proposed three
main methods called classic
psychophysical methods, to
allow quantitative evaluation of
sensory thresholds. methods
still used today
all three method can be
affected by an observer’s
criterion
method of constant stimuli
(preferred method)
cons
prior knowledge on observer’s
performance
more time
pros
less chance of bias:
experimenter has control and
stimuli is randomized (observer
can’t expect)
precise estimates of observer’s
threshold, most precise of the 3
methods
procedure?
4. find threshold, which is 50%
point
3. plot the psychometric
function: plot the proportion of
times that the observer saw the
stimulus (light intensity)
2. present each test value in
random order (20 times) i.e.
present 7 intensities, 20 times
each, in random order
1. select a range of test values
(i.e. seven to eleven intensities
of light) that contain a subject’s
threshold (you need an idea of
where threshold will be)
to figure out range of rest
values, experimenters run a
method of adjustment
experiment
how is stimuli presented?
experimenter shows 5 to 9
difference stimuli in random
order, all varying in intensity
how does he choose them?
intensities in between are
detected on some trials but not
on others
the lowest intensity is one that
is never detected, the highest is
one that’s always detected
method of adjustment
cons
somewhat more variable than
other methods (less precise)
open to bias (observer controls
stimulus intensity, knows what’s
being measured)
pros
easy for observer to do
fast
procedure?
the settings (thresholds from all
the trials) are averaged to give
an estimate of the threshold
many settings are made for
each stimulus condition (trials)
observer adjusts stimulus
intensity until they can barely
detect it
how is stimuli presented?
in either ascending or
descending order
method of limits
cons
more time consuming than
method of adjustment
there still biases of expectation
because observer knows the
pattern of ascending, to
descending, to ascending etc
pros
an improvement over method of
adjustment; less bias since
experimenter has more control
over stimulus
procedure?
starting point varies from trial to
trial (because you don’t want
observer to come to get used
to it and expect it, this creates
bias)
ascending trials alternate with
descending trials
observer watches passively,
experimenter controls
presentation
crossover values (between YES
and no) used to calculate the
threshold
how is stimuli presented?
in either ascending (stimuli is
increased over time) or
descending (decreasing) order
what they all have in common:
the idea that human perception
is variable so measurements at
one point in time can differ from
measurements at another point
in time.
How is this variability taken into
account in Fechner’s methods?
By making subjects make
multiple judgements
examples?
for seeing light, the threshold
would be the intensity of light
that can just be seen
for oblique effect, the abs.
threshold would be the
minimum width of a line that
can just be detected. Because
at some point, the lines will
become so thin that the grating
will look homogeneous and so
orientation cannot be seen
it was found that when the lines
are horizontal or vertical, the
threshold was lower (the
minimum width of the line that
could be detected was smaller)
why?
the development of the nervous
system is influenced by
environmental stimuli. That is,
there are more vertical and
horizontal lines than oblique
and slanted lines in the
environment
how to approach the study of
perception
Problem: how might perception
be affected by outside factors?
cognitive influences on
perception: knowledge,
memories and expectations
influence perception.
researchers study these by
measuring how they aect all
three of the relationships
Q: what two approaches are
taken?
physiological approachrelationship(s) measured?
the relationship between
PHYSIOLOGY and
PERCEPTION
experiment?
results: brain response was
larger (physiology) when
humans were detecting
horizontals or verticals rather
than slanted lines (perception)
technique: brain scanner used
to detect brain activity while
human subjects carried out task
of detecting lines of different
orientations
the relationship between
STIMULI and PHYSIOLOGY
experiment?
results: vertical or horizontal
lines (stimuli) caused larger
brain responses (physiology).
technique: optical brain
imaging, measures large area
over visual cortex
theme: measure the stimulus-
physiology relationship in the
oblique effect (David Coppola
1998) in ferrets
psychophysical approach /
psychophysics
relationship(s) measured?
the relationship between the
STIMULI and PERCEPTION
Q: experiment?
results: patients saw the vertical
and horizontal lines in better
detail than slanted/oblique lines
oblique effect: people have
better detail vision for verticals
or horizontals than slanted/
oblique lines
theme: how well subjects could
see the fine lines in certain
stimuli, which were presented in
different orientation
Q: goal?
to understand how the steps in
the perceptual process leads to
perception
Q: what three relationships are
measured to study the
perceptual processes ?
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