Transcript of Visual Perception. Sensation Stimuli from the outside world is detected by sensory receptors found...
- Slide 1
- Visual Perception
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- Sensation 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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- Selective Attention 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
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- Perception While the sense organs gather the information
(sensation), perception is the name given to the process during
which the brain organises and interprets the information from the
sense organs. Perception givens meaning to the sensations.
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- Sensation and Perception Sensation is the same for everyone
(except the visually impaired). Perception is not the same for
everyone. List the following: 1.Your favourite colour 2.Your
favourite vegetable 3.Your favourite muscial instrument 4.Your
favourite perfume/cologne 5.The temperature at which you like to
eat soup
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- Thresholds 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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- The Visual Perceptual System 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 electromagentic spectrum 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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- Wavelength, frequency and amplitude Electromagnetic energy
travels in waves and therefore has different wavelengths,
frequecies and amplitudes. We see different wavelengths/freuencies
as different colours. We see different amplitudes as different
intensities of light.
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- Structures of the Human Eye
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- More on the lens 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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- More on the lens 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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- More on the retina 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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- Find your blind spot
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- Rods 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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- Cones 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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- Structure of the Retina
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- From eye to brain- the visual perceptual process
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- The Visual Perceptual Process 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 Visual perceptual principles guide
the way in which visual information is organized, grouped together
and given meaning- 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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- Visual Perceptual Principles There are three broad categories
of these principles: Gestalt Principles Depth perception principles
Constancies
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- Gestalt Principles Suggest that we group individual elements
together to see the whole- we do not perceive the individual
elements 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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- Figure- ground 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 scene.
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- Closure When a figure is incomplete, our visual perceptual
system will complete the picture so that we see a whole.
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- Proximity Objects close to each other are perceived as
belonging to the same group.
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- Similarity Objects that are similar in appearance are perceived
as belonging to the same group.
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- Gestalt Principles and Advertising
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- Depth Perception Principles 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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- Primary Depth Cues Internal cues/signals provided by the body
There are three primary depth cues: 1.Retinal disparity
2.Convergence 3.Accomodation
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- 1. Retinal Disparity 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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- 2. Convergence 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. If more 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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- 3. Accomodation 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
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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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- 1. Linear Perspective Two parallel lines merging together give
the perception of depth.
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- 2. Interposition Provides a cue for depth when two or more
obejcts overlap each other. The object behind is perceived as being
further away
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- 3. Texture Gradient Surfaces that are less detailed are
perceived as being further away than objects that include fine
detail.
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- 4. Relative Size 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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- 5. Height in the Visual Field Uses the horizon line as a cue
for depth and distance. Objects drawn closer to the horizon are
perceived as being more distant than objects further from the
horizon.
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- Constancy Principles 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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- Size Constancy 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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- Shape Constancy 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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- Orientation Constancy 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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- The Perceptual Set 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. How would you interpret this scene?
You interpretation is influenced by your perceptual set.
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- Processing 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. 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.
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- Past Experience 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 a perceptual set and hence impact on our
interpretation of the world.
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- Context 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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- 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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- Visual Illusions
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- Muller-Lyer Illusion
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- Explanation of the Muller-Lyer Illusion- Apparent distance
theory
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- The Ames Room
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- Explanation of the Ames Room