Stimuli from the outside world is detected by sensory receptors
found in out our sensory organs. Sensation is the stimulation of
our sensory organs and the transmission of information to the
central nervous system.
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Stimulation of the sense organs is automatic Multiple sense
organs are being stimulated at any time We only focus on select
stimuli at any one time- this is known as selective attention
Selective Attention Experiment
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While the sense organs gather the information (sensation),
perception is the name given to the process during which the brain
organizes and interprets the information from the sense organs.
Perception givens meaning to the sensations.
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Sensation is the same for everyone (except the visually
impaired). Perception is not the same for everyone. List the
following: 1. Your favourite colour (sight) 2. Your favourite
vegetable (taste) 3. Your favourite muscial instrument (sound) 4.
Your favourite perfume/cologne (smell) 5. The temperature at which
you like to drink water (temperature)
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Absolute threshold- the smallest or weakest level of a stimulus
required for detection Differential threshold- the smallest or
weakest change in a stimulus that is required for the sensory
system to detect a change Note: The differential threshold is also
known as the just noticeable difference (JND)
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While there are 5 sensory systems we will focus only on the
visual perceptual system. The visual sense organ (the eye) collects
information in the form of electromagnetic radiation- light!
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The entire range of electromagnetic energy is known as the
electromagnetic spectrum. The human eye however can only detect a
small portion of this known as the visible spectrum.
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Electromagnetic energy travels in waves and therefore has
different wavelengths, frequencies and amplitudes. We see different
wavelengths/frequencies as different colours. We see different
amplitudes as different intensities of light.
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Ciliary muscles change the shape of the lens Distant objects =
flat and elongated lens Near objects = round bulging lens Remember
the lens aims to focus the image onto the retina.
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The lens also refracts (bends) light. This allows the image to
converge (come together) on the retina. The light bends (refracts)
as it passes through the bent surface of the lens. The light
closest to the ends of the lens are bent the most while the light
traveling through the middle is bent the least. The result is an
upside-down image.
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The receptor cells of the eye are found in the retina. They are
known collectively as photoreceptors. Photo = light There are two
types of photoreceptors: rods and cones. The area of the retina
where the optic nerve connects to the retina is known as the blind
spot because it is absent of any rods or cones.
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More sensitive to light therefore assist vision at night
Present in large numbers on outer regions are retina. Therefore
they play a large role in peripheral vision. Perceive shades of
grey Approx 120 million in number
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Sensitive to colour therefore involved in colour vision Most
effective in bright light Concentrated in the centre region of the
retina Responsible for visual acuity (high def) The fovea is a spot
in the retina that contains only cones. It is on this spot that an
image is focused to enable the best visual acuity (best detail).
Approx 8 million in number
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Rods Night and low light vision Shades of grey Found on outside
edge of retina Play a large role in peripheral vision More numerous
overall Cones Work best in bright light Colour vision Found in
centre of retina Fovea used for high acuity; contains cones
only
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The visual perceptual process is a six stage process that
describes how we see and interpret visual stimuli. While there are
six stages, it is a continuous process
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Visual perceptual principles guide the way in which visual
information is organized, grouped together and given meaning- the
selection, organisation and interpretation phase of the visual
perceptual process. They are like a set of instructions that enable
us to organize and interpret visual information in a reliable and
meaningful way. These principles are used involuntary and instantly
by the brain allowing for efficient interpretation and
understanding.
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There are three broad categories of these principles: 1.
Gestalt Principles- allow us to quickly interpret an image and
focus our attention on what is important. 2. Depth perception
principles- allow us to interpret the two dimensional image sent
from our eyes as three dimensional 3. Constancies- allow us to
perceive an object as remaining the same (constant) despite changes
to the image on the retina
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Suggest that we group individual elements together to see the
whole- we do not perceive the individual elements Gestalt
principles allow us to interpret and assign meaning to an image
more efficiently- but not always accurately. The whole is greater
than the sum of its parts There are four Gestalt principles:
1.Figure-ground 2.Closure 3.Proximity 4.Similarity
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The visual system simplifies a scene into a figure- the focus
of out attention- and the ground which is everything else- the
background. This helps us quickly identify the most important
information in a scene.
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When a figure is incomplete, our visual perceptual system will
complete the picture so that we see a whole.
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Objects close to each other are perceived as belonging to the
same group.
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Objects that are similar in appearance are perceived as
belonging to the same group.
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Refers to out ability to judge distance- remember that the
image projected on the retina is two dimensional but our world is
three dimensional. We use cues in the from both our body and from
our environment to judge how near or far an object is from us There
are two types of depth cues: 1.Primary Depth Cues 2.Secondary Depth
Cues
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Internal cues/signals provided by the body There are three
primary depth cues: 1. Retinal disparity 2. Convergence 3.
Accomodation
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Our eyes are positioned approximately 7 cm apart. Therefore
each eye receives a different image. The brain merges the two
images together and the amount of retinal disparity (difference)
acts as a depth cue. The more different the images, the closer the
object. The more similar the images, the further away the objects
are. Beyond 10 meters, retinal disparity does not act as a depth
cue because the images are virtually the same.
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The eyes generally turn in to focus an object on the retina.
Information on the position of the eye and muscle tension acts as
depth cues. The more the inward turning of the eye, the closer the
object. Convergence works for objects up to 7m from the eye. After
this, the eyes are parallel.
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Accommodation refers to the lens changing shape to focus on an
object. The muscles contract and the lens bulges to focus on near
objects. The muscles relax and the lens elongates to focus on far
objects. The information about the shape of the lens and the action
of the muscles is sent to the brain where is assists depth
perception Accommodation is effective for objects up to 3m from the
viewer. Accomodation is the only monocular (one eye only) depth
cue.
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Secondary depth cues are cues to depth that are provided by the
environment. There are five secondary depth cues: 1. Linear
Perspective 2. Interposition 3. Texture Gradient 4. Relative Size
5. Height in the Visual Field
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Two parallel lines merging together give the perception of
depth.
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Provides a cue for depth when two or more objects overlap each
other. The object behind is perceived as being further away
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Surfaces that are less detailed are perceived as being further
away than objects that include fine detail.
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An object that projects a large image on the retina is
perceived to be closer than an object that casts a small image on
the retina. Most effective with familiar objects
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Uses the horizon line as a cue for depth and distance. Objects
closer to the horizon are perceived as being more distant than
objects further from the horizon.
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The images projected onto the retina are dynamic and changing-
as an object moves, the image on the retina changes Even though
these images are changing we perceive the size, shape and
orientation of an object to be constant using the constancy
principles.
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An object has the same size regardless of the distance from
which it is viewed. We perceive the man in the background as being
normal height even though his image is smaller.
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The tendency to see an object as the same shape no matter what
angle it is viewed from The door remains a rectangle even though it
is moved and viewed from other angles.
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The tendency to visually perceive the true position of an
object in the environment even though the retinal image of the
object may be at a different orientation.
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Look closely at the image representing a seal act at a circus.
What do you see?
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If you were actually told that this image was of a costume
party what do you now see?
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The visual information in each diagram has not changed but your
expectation of what you will see does. Perceptual set is a
readiness to see something that we expect to see. How would you
interpret this scene? Your interpretation is influenced by your
perceptual set.
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Psychological factors influence how to perceive the world.
These factors include past experience, context, culture, motivation
and emotion. The way that we perceive and interpret events is
subjective and personal. We have a readiness to interpret stimuli
to meet our expectations. This is known as a perceptual set- a
mental predisposition to interpret stimuli in an expected and
predictable way.
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Perceptual set can assist visual perception when we actually
see what we expect to see. Perceptual set can also however mislead
us. We may expect to see something that does not occur causing us
to miss vital information in our environment. 'The Cat Sat on the
Map and Licked its Whiskers'
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Perceptual set may be influenced by many things including
context, past experience, culture, motivation and emotion. We will
now have a look at each of these in more detail.
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Past experience refers to the situations and events we have
encountered throughout our life prior to the present. Each
individuals past experiences differ and this contributes to our
individual uniqueness. Through experience we form ideas and
expectations about the world. Past experiences led to the formation
of the perceptual set and hence impact on our interpretation of the
world.
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Toch and Schulte (1961)
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Showed the below images simultaneously (one to each eye) to
participants who were: trained policemen, beginning police training
and university students with no police training.
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Toch and Schulte (1961) found that 52% of the policemen saw the
violent image but only 26% of the trainees and 22% of the
university students saw the violent image.
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The context is the situation or setting in which we view a
stimulus. The context in which we view a particular stimulus
influences our interpretation of the stimulus. The context provides
a cue to help your interpretation. With the aid of this cue we can
make quicker evaluations and interpretations of information.
However while correct on most occasions using cues based on context
can lead to mistakes in interpretation.
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Studying the images that you have been given for 1 minute.
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What could it be? Your answer would have been influenced by
your past experience on your perceptual set What could it be now?
Your answer shows the influence of context on the perceptual
set.
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Motivation leads our behaviour because we want to achieve a
particular goal. It also influences our perceptual set. Motivation
leads us to see what we want to see to aid the achievement of this
goal.
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Example: When you watch your favourite football team play you
are motivated to see them win. You therefore will explain the
events of the game with this motivation in mind. Someone who
supports the opposite team may explain the events of the game in
much different terms. You both see the same game but your
interpretation is influenced by your different motivations and
hence your perceptions are different.
Slide 70
Emotions can influence the way we perceive images of our
environment. For example, if you are scared of spiders you are more
likely to perceive an insect as a spider than any other
invertebrate. You are more likely to perceive an image as food if
you are hungry.
Slide 71
Culture includes things such as customs, traditions, beliefs,
values, attitudes, ethics, music and food. Culture influences our
perception of the world. E.g. When shown a black and white photo of
a dog, members of a Malawi tribe could not recognise the drawing
despite the fact that many owned dogs. The researches concluded
that this was because the members of the tribe has no experience
with photos or two dimensional images of dogs.
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A perceptual set is linked to top-down processing- observing
the whole image first and applying existing knowledge to attribute
meaning. When we proof read an assignment we may not notice simple
spelling mistakes because we are using top-down processing. This is
an efficient way of processing information but is not always
accurate.
Slide 73
For more accurate interpretations, we need to use bottom-up
processing. In bottom-up processing, the individual elements of the
stimulus are analyzed and after this a decision is made regarding
the nature of the stimuli. A developing child will use bottom-up
processing to learn about the world around them. We use bottom-up
processing to read a new word.
http://www.youtube.com/watch?v=0jkaSIOqUgY&f
eature=related
Slide 74
Visual perceptual principles are in place to help us to better
understand our world yet these mechanisms can also lead us to
misinterpret visual information When we misinterpret visual
information- our perception does not match reality- we see a visual
illusion Visual illusions are unavoidable and even when we know
they exist we still see them
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Psychologists have identified at least 200 visual illusions The
cause or explanation of why we see many of these is still unknown
but they have been able to help psychologists understand the
importance of the brain in perception.
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Which line appears to be longer? The Muller- Lyer Illusion is
one of the most researched visual illusions.
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The line with the feather tail is perceived as being longer
than the line with the arrow head. There are several explanations
of the Muller-Lyer Illusion We will explore them from each of the
four psychological perspectives
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Eye movement theory Because the feather at the end extends the
length of the line it has been proposed that the extra eye movement
interprets the line as being longer. However research has found
that even when we do not need to move the eyes we still see the
Muller-Lyer Illusion. Another theory proposes that we see the
illusion because we have inherited the ability in our genes.
Slide 89
Learning and past experience influence our perception of the
lengths of the lines. We see the line with the feather top as being
longer because it represents the inside corner of the further away
room. We see the line with the arrow head as smaller because it
represents the outside corner of a room closer up.
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When the two lines are presented together, your brain overrides
information from the retina and tells you that the line that is
further away (the feather tail line) must be longer.
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Cognitive psychologists suggest that we use the incorrect
mental strategies when processing the visual information We
perceive the smaller line as being closer and incorrectly use the
size constancy principle. (If the corner is closer and the lines
are equal then we must perceive the line further away to be longer
than the line closer.) Further we seen to be interpreting the lines
as what they represent and not what they actually are (room corner
example).
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It appears that our culture also plays a role in the
Muller-Lyer Illusion. Zulu people do not see the illusion- they see
the lines as equal in length. They live in round huts and are not
familiar with our angular world.
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Critics of the above theories have noted the fact that there
are many variations of the Muller- Lyer illusion. Psychologists
from all fields agree that there is no one single satisfactory
explanation of the Muller-Lyer illusion
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The person on the right appears to be much larger than the
person on the left despite the fact they are of equal size. This is
due to the construction of the room. The room which appears to be
rectangular is actually a trapezium. One side of the room is both
longer and higher than the other. Helping the illusion is the fact
that the room is viewed through a peep hole using only one eye
(monocular vision). This reduces some of our cues to depth.
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The back corners of the rooms produce an identical image on the
retina suggesting that the corners are the same size and distance.
However one corner is further away and higher. Therefore the person
in this corner is twice as far away and therefore looks a lot
smaller. Your brain appropriately applies the relative distance
principle but is tricked by the shape of the room into thinking the
room is rectangular and therefore each person is the perceived to
be the same distance from you. See Zimbardo youtube.
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