Unit 3: PERCEPTUAL PROCESS

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What is Perception? 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? 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).

The first step in processing information around us.

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Process in Perceptions 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 steps in this process are arranged in a circle to emphasize the process is dynamic and continually changing.

Blue arrows point to stimuli Green to processing Red to perceptual response.

Arrows A, B, and C indicate three important relationships that researchers measure.

The perceptual process

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The Perceptual Process

1. Environmental stimuli - All available stimuli for an observer

2. Attended stimuli - Stimuli that are the point of focus for the observer

3. Stimulus on the receptors - “Image” of stimulus on receptor cells

4. Transduction - Change from environmental energy to electrical energy in the nervous system

5. 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

6. Perception - Conscious sensory experience

7. Recognition - Ability to place objects in categories that provide meaning

8. 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.

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.

Th

e Percep

tual P

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Two Interacting Aspects of Perception

a) Bottom-up processing Processing based on incoming stimuli

from the environment Also called data-based processing

b) Top-down processing Processing based on the perceiver’s

previous knowledge 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

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 EyeThe Eyevisual perceptionvisual perception

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Visual Perception

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 Rods & cones function as receptors that would

accept information

Figure 6.4

The EyeThe Eye

LensLens – focuses – focuses light on the light on the retinaretina

Ciliary muscles Ciliary muscles alter the shape of alter the shape of the lens as needed the lens as needed

AccommodationAccommodation – – the process of the process of adjusting the lens adjusting the lens to bring images to bring images into focusinto focus

The Basic sensory equipment involved in human vision

Visual Perception 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”

Light passes through several cell layers before reaching its receptors

LIGHT retinal ganglion cells bipolar cells receptors cells Lateral communication

Horizontal cells Amacrine cells

Visual Perception (con’t.) 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?

Each eye transmits visual information to the occipital lobes of both hemispheres of the brain, to the region known as the visual cortex

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The Visual Pathway

How accurate is our visual system?

Vision - carries out complex activities Our visual system can create an accurate,

richly detailed, 3 dimensional perception 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

Convergence eyes must turn slightly inward when objects are close

Binocular disparity difference between images on the two retinas

Both are greater when objects are close provides brain with 3-D image and distance

information

The Visual System Functions of the Visual System:

Answers two questions What is it? (recognition) Where is it? (location)

Transforms a distorted and upside-down 2-D retinal image into the 3-D world we perceive.

The Visual System Does the visual system create an exact copy

of the external world? 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? 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 inter-distance among two objects in space.

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Why is depth perception important? 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

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 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 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 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

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. 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:-

Interposition Linear Perspectives Relative Size Texture Gradients Light & Shadow

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Monocular Cues: Interposition 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

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 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. 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.

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Size Consistency

Monocular Cues: Texture Gradients

Uniformly textured surfaces produce Texture Gradients thatprovide depth information: as the surface

recedes, the textureelements’ size decreases and their density increases.

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Monocular Cues: Light and Shadow

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 MovementWhere is it going?

Retinal Motion

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.

These neurons are well-suited to act as motion detectors.

Apparent Movement 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 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 FormWhat is it?

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The Perception of Form

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 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 FormPerceiving 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 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)

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?

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 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)

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 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 .

632 “goodness of form”

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Top Down Process

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