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Unit 3:
PERCEPTUAL PROCESS
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What is Perception?
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Perception is a means where information
obtained from the environment by means of
sensory organs, is transformed into experiences
of objects, events, sounds, taste, etc.
Perception is an important factor in human
behaviour
Perception ….A process by which individual
organize and interpret their sensory impressions in
order to give meaning to the environment.
Perception can be defined as the process of
receiving, selecting, organizing, interpreting,
checking, and reacting to sensory stimuli or data.
What is Perception?
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Perception is a process whereby, sensory
stimulus in the environment is converted to
mental representation (repeat).
 Refers
to the way the world looks (eyes),
sounds (ears), feels (skin), taste (tongue) or
smell (nose).
he first step in processing information
around us.
T
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Process in Perceptions
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Stimulus received from the environment provide
the raw data for experience  create an internal
representation of objects in our surrounding.
Sensation ( a process of transduction), where the
process of converting the energy of light or sound
(stimulus) into neural transmission
 i.e. The sensory information is then transformed,
elaborated and combined with memories to
create what we actually experience or PERCEIVE.
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The perceptual process
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The steps in this process are
arranged in a circle to
emphasize  the process is
dynamic and continually
changing.
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Blue arrows point to stimuli
Green to processing
Red to perceptual response.
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Arrows A, B, and C indicate
three important relationships
that researchers measure.
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The Perceptual Process
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Environmental stimuli - All available stimuli for an
observer
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Attended stimuli - Stimuli that are the point of focus
for the observer
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Stimulus on the receptors - “Image” of stimulus on
receptor cells
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Transduction - Change from environmental energy to
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electrical energy in the nervous system
Neural processing - Interconnected neurons that
propagate the electrical signal from receptor cells
throughout the brain
Figure 1.2 (a) We take the woods as the starting point for our description of the perceptual process.
Everything in the woods is the environmental stimulus. (b) Ellen focuses on the moth, which becomes the
attended stimulus.
Figure 1.3 (a) An image of the moth is formed on Ellen’s retina. (b) Transduction occurs when the
receptors create electrical energy in response to the light. (c) This electrical energy is processed through
networks of neurons.
Con’t…The Perceptual Process
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Perception - Conscious sensory experience
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Recognition - Ability to place objects in
categories that provide meaning
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Action - Motor activities that occur in reference to
the perceived and recognized object
Figure 1.4 (a) Ellen has conscious perception of the moth. (b) She recognizes the moth. (c) She takes
action by walking toward the tree to get a better view.
The Perceptual Process
Figure 1.1 The perceptual process. The steps in this process are arranged in a circle to emphasize the fact
that the process is dynamic and continually changing. Blue point to stimuli; green to processing; red to
perceptual responses. Arrows A, B, and C indicate three important relationships that researchers measure.
Two Interacting Aspects of Perception
a)
b)
Bottom-up processing
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Processing based on incoming stimuli
from the environment
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Also called data-based processing
Top-down processing
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Processing based on the perceiver’s
previous knowledge
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Also called knowledge-based processing
Figure 1.6 Perception is determined by an interaction between bottom-up processing, which starts with the
image of the receptors, and top-down processing, which brings the observer’s knowledge into play. In this
example, (a) the image of the moth on Ellen’s retina initiates bottom-up processing, and (b) her prior
knowledge of moths contributes to top-down processing.
WHY does perceptions vary from person to
person
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This could be due to:
Personal Variation
Physical Variations
Combinations of personal & physical variations
 different people perceive different things about
the same situation.
 We assign different meanings to what we
perceive  and the meanings might change for
a certain person.
• One might change one's perspective or simply make things
mean something else.
VISUAL SYSTEM -The Eye
visual perception
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Visual Perception
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The eyes play a vital role in transmitting
information surrounding objects to our brain.
The human eyes consists of about 7 million
cones and 125 millions rods
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Rods & cones function as receptors that would
accept information
The Eye
Lens – focuses
light on the retina
Ciliary muscles
alter the shape of
the lens as needed
Accommodation –
the process of
adjusting the lens
to bring images into
focus
Figure 6.4
The Basic sensory equipment
involved in human vision
Visual Perception
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Retina: The layer of the eye covered with the rods and
cones that initiate the process of visual sensation and
perception
 Parts of the retina: rods and cones, bipolar cells, and
ganglion cells.
The retina is in a sense “inside-out”
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Light passes through several cell layers before
reaching its receptors
LIGHT retinal ganglion cells
cells receptors cells
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Lateral communication
 Horizontal
cells
 Amacrine cells
 bipolar
Visual Perception (con’t.)
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Rods and Cones: These form the back layer of
neurons on the retina and are the first neurons
stimulated by light
Bipolar Cells: Patterns of neural firing from the
rods and cones are forwarded to the second layer of
neurons, the bipolar cells
Ganglion cells: Collected messages from the
bipolar cells are passed along to the third layer of
neurons, the ganglion cells.
Fovea: The highly sensitive region of the retina
responsible for precise, focused vision, composed
largely of cones
How do the eyes transmit visual information to
the brain?
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Each eye transmits visual information to the occipital
lobes of both hemispheres of the brain, to the region
known as the visual cortex
The Visual Pathway
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How accurate is our visual system?
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Vision - carries out complex activities
Our visual system can create an accurate,
richly detailed, 3 dimensional perception
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Through a tiny , distorted, upside-down, 2
dimensional retinal images projected upon the
visual receptors lining the back of our eyes.
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Eye Position and Binocular Disparity
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Convergence
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Binocular disparity
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eyes must turn slightly inward when objects are close
difference between images on the two retinas
Both are greater when objects are close
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provides brain with 3-D image and distance
information
The Visual System
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Functions of the Visual System:
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Answers two questions
 What
is it? (recognition)
 Where is it? (location)
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Transforms a distorted and upsidedown 2-D retinal image into the 3D world we perceive.
The Visual System
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Does the visual system create an exact copy
of the external world?
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No!
The visual system creates a perception of
reality.
Visual Illusions
Visual Illusions
Visual Illusions
Visual Illusions
Visual Illusions
Depth Perception
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What is Depth Perception?
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Depth perception is the visual ability to perceive the
world in three dimensions (3D).
Depth perception is a visual illusion.
The ability to judge how far away an object is as well
as how far away objects are from each other.
Depth perception refers to the phenomenon
describing the ability to perceive the environment in
three-dimensional form and to determine the spatial
distance among objects.
This phenomenon ascertains our ability to judge the
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inter-distance among two objects in space.
Why is depth perception important?
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Depth perception is crucially important for our
survival and hence interacting with the world.
Using depth perception as a significant factor we
may be able to provide response back to the
environmental stimuli.
Without it, we may become unable to navigate
around.
Depth perception enables us to estimate the
distance of an object from us.
Depth perception enables us to locate the position
of an object in space  hence we may become
able to either reach at it or avoid it.
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The Perception of 3-D space
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Perceiving 2-D objects is fairly simple because 2-D visual
 images are already constructed at the level of the
retina.
 Furthermore, our visual system is composed of
numerous areas containing Topographical Maps of
visual space.
The 3-D perception that we experience everyday results
from our visual system’s use of depth perception:
Depth perception can be divided into 3 cues:a) Oculomotor Cues
b) Binocular Cues
c) Monocular Cues
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Depth Perception
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Space/depth perception can be divided into 3 cues:Oculomotor Cues - Cues based on our ability to
sense the position of our eyes and the muscle tension
in our eyes muscles. Created by convergence (inward
movement of the eyes when we focus on nearby
objects) & accomodation (change in the shape of the
lens when we focus on objects at different distances)
 Binocular Cues - cues that are based on the receipt
of sensory information in three dimensions from both
eyes.
 Monocular Cues - cues that can be represented in
just two dimensions and observed with just one eye.
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a. Oculomotor Cues
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Oculomotor Cues is influence by 2 elements:(depend on the distance of the 0bject observed),
Convergence - Convergence can serve as a
cue for depth because the convergence of the
eyes depends on the distance of the objects we
fixate.
• The inward movement of the eyes that
occurs when we look at nearby objects.
 Accommodation - serve as a depth cue
because the shape of the lens depends on the
distance of the objects we focus on.
The change in the shape of the lens that occurs
when we focus on objects at various distances
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b. Binocular Cues: Retinal Disparity
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Binocular Cues – using both
eyes to look at an object
As binocular creatures, our two
eyes look onto the world from
slightly different position,
providing slightly different views
of the world.
Binocular Disparity grants us
important information about depth
relationships between objects.
Some neurons in the visual cortex
are able to detect retinal disparity
and act as depth detectors.
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Monocular Cues for Depth
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Eventhough Binocular disparity is a powerful cue for depth
perception  but we can perceive depth beyond our
binocular field of vision, i.e. and even with one eye closed.
Monocular Cues is cues for depth perception that come
from the image obtained from one single eye.
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Monocular Depth Cues - mostly pictorial cues that we use to
create an impression of depth on a flat canvas.
4 elements that influence monocular cues:
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Interposition
Linear Perspectives
Relative Size Texture Gradients
Light & Shadow
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Monocular Cues: Interposition
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When the boundary of an object
is interrupted by the presence of
another object, we use this
pattern of blocking as a cue to
determine the object as more
distant from us.
The near object is perceived as
interposed between the far
object and us.
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Monocular Cues: Linear Perspective
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Distant objects necessarily
produce a smaller retinal
image than nearby objects
of the same size.
This provides the basis for
the cue of Linear
Perspective.
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Monocular Cues: Relative Size
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That distant objects produce a
smaller retinal image than
nearby objects of the same size
also provides the basis for the
cue of Relative Size: i.e.
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the larger of two identical
objects tends to be perceived
as closer that the smaller one
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Framing Effect and Size Illusion
 An object that seems to fill a
bounding outline looks larger
 than the same object within a
larger outline.
Size Consistency
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Monocular Cues: Texture Gradients
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Uniformly textured
surfaces produce
Texture Gradients that
provide depth
information:
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as the surface
recedes, the texture
elements’ size
decreases and their
density increases.
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Monocular Cues: Light and Shadow
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The distribution of light
and shadow on a
objects is also a
powerful monocular cue
for depth provided by
the biologically correct
assumption that light
comes from above.
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The Perception of Movement
Where is it going?
Retinal Motion
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Our perception of movement arises from the
motion of images across the retina as detected
by visual neurons that either simply sensitive to
motion or specifically selective to the direction of
motion of visual stimuli.
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These neurons are well-suited to act as motion
detectors.
Apparent Movement
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Motion can be perceived even when there is no
motion of an image across the retina.
Appropriately timed change in position is
sufficient for the visual system to make
something appear as moving.
Illusion of Motion
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We not only detect
motion, we also
interpret it. Specifically,
the perceptual system
must solve the
Correspondence
Problem: determining
which elements of the
current view
corresponds with which
elements in the previous
view.
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Induced Motion
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The Perception of Form
What is it?
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The Perception of Form
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Form is our major avenue for
identifying what we see.
The question is how?
How do we recognize the various
forms and patterns taken by
different objects
 how do we recognize a form even if
its components parts are altered?
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The Perception of Form
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One solution to these issues was advanced by
Gestalt psychology, a school of psychology
whose adherents believed that organization is an
essential feature of visual perception.
They argued that a form is not perceived by
somehow summing up all its individual components,
but by considering is as a coherent, intact 
Gestalt, a whole that is different from the sum of its
parts.
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The Elements of Form
Perceiving a visual scene involves:
1) The detection of its features
2) The parsing of the scene so that figures can be
identified
from the background
3) The grouping of the figures’ parts into single objects
4) The recognition of the pattern, i.e., answering the
question: What is it?
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The Elements of Form
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The perception of form begins with the detection of
Primitive Features, the building blocks of visual
perception.
Various visual neurons in the brain act as feature
detectors, responding selectively to certain elements
of visual form.
Evidence suggests that these simple features (color,
orientation, curvature, ends of lines) form the basic
units in our perception of form.
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The Elements of Form (2)
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Once the we identified which features are present,
the next step is to organize the overall visual scene,
a process called Perceptual Segregation.
To make sense of a visual scene, the perceptual
system must somehow group the elements of the
scene appropriately.
The major questions are:
What goes with what? What is focal?
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Perceptual Segregation: What is it?
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Perceptual Segregation: What is focal?
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A crucial step in visual segregation is the separation
of the object from its setting, so that a the object is
seen as a coherent whole, separate from its
background.
This separation of Figure and Ground allows us to
recognize as focal both objects that we are familiar
with and objects without particular
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Figure and Ground
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The differentiation of Figure and Ground, like all
aspect of perceptual segregation, is contributed
by the perceiver.
It is not a property of the stimulus itself.
There can be more than one way to parse a
stimulus!
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Element of Form (3)
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The Perceptual Organization of the elements
within a visual scene is guided by some factors
that were described by Gestalt psychology and
regarded as principles.
These principles demonstrate that “the whole is
different than the sum of its parts”.
Once again, perceptual grouping is an
achievement of the perceiver and not a property
of the stimulus.
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Gestalt Principles: Prägnanz2
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Every stimulus pattern is seen in such a way that
the resulting structure is as simple as possible;
the simplest and most stable interpretations are
favored .
2 “goodness of form”
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Top Down Process
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