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Lecture

Psychology Notes - Chapter 7.docx


Department
Psychology
Course Code
PSYA01H3
Professor
Steve Joordens

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Psychology Notes
Chapter 7 Perception
- Primary function of the sense organs is to provide information to guide behaviour
- When our bodies or our eyes move, exposing the photoreceptors to new patterns of visual
information, our perception of the scene before us does not change
- Our brain compensates for the constantly changing patterns of neural firing that these
movements cause
- Perception is the process by which we recognize what is represented by the information
provided to our sensory organs
- It is an unconscious process, not a deliberate one by which we puzzle out the meaning of what
we see
- We do not first see an object and then perceive it; we simply perceive the object
- If we look at a scene carefully, we can describe the elementary sensations that are present, but
we do not become aware of the elements before we perceive the objects and the background of
which they are a part of
- Visual perception comes after it is complete; we are presented with a finished product, not the
details of the process
- When we look at a scene carefully, we can describe the elementary sensations that are present,
but we do not become aware of the elements before we perceive the objects and the
background of which they are part.
- Visual perception presents us with a finished product, not the details of the process
Brain Mechanisms of Visual Perception
- Perception takes place in the brain
- The optic nerve sends visual info to the thalamus, which relays the info to the primary visual
cortex, located in the occipital lobe at the back of the brain
- Neurons in the primary visual cortex send visual info to two successive levels of the visual
association cortex
- The first level, in the occipital lobe, surrounds the primary visual cortex
- The second level if divided into two parts, one in the middle of the parietal lobe and one in the
lower part of the temporal lobe
- Visual perception = the hierarchy of information processing
- Circuits of neurons analyze particular aspects of visual info and send the results of their analysis
to another circuit, which performs further analysis
- Eventually the perception leads to the perception of the scene and the objects in it
Primary Visual Cortex
- Microelectrodes are extremely thin wires with sharp points that are inserted into regions of
visions systems of cats, monkeys to detect action potentials.
- After microelectrode is close to a neuron, stimuli is presented on a large screen infront of the
anaesthetized animal (unconscious animal)
- Moved stimuli on screen to find point in which it had the largest effect on the electrical activity
of the neuron
- Geography of the visual field is found in the primary visual cortex
- The surface of the retina is mapped on the surface of the PVC
o Largest amount of area given to the centre of the visual field
- Each module in the visual cortex receives information from the small region of the retina

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o It receives info from a small region of the visual field the scene that the eye is viewing
- Neural circuits within each module analyzed various characteristics of their own particular part
of the visual field receptive field
- Circuits detect the presence of lines passing through the region and signalled the orientation of
these lines
o others detect thickness, movement, direction and colour
- orientation sensitive neuron responds when a line oriented at 50 degrees to the vertical is
placed in this location especially when the line is moving through the receptive field
o response is specific to that orientation
- other neurons share the same receptive field but respond to different orientations
- orientation is signalled by an increased rate of firing of particular neuron in the cluster
The Visual Association Cortex
- first level of VAC is broken down into subdivisions each containing a map of the visual scene
- each receives info from different parts of the neural circuits within modules of PVC
- one subdivision receives info about the orientation and widths of lines and edges
- one subdivision receives info about perception of shapes
- one subdivision receives info about movement of objects
- one subdivision receives info about colour
- second level puts together info that is gathered and processed by sub regions of first level
- 3D form perception takes place here
- The visual association cortex in the parietal lobe is responsible for perception of objects
o It is the part of the system for object-directed behaviors
Effects of Brain Damage on Visual Perception
- When the primary visual cortex is damaged, a person becomes blind in some portion of the
visual field
o The location depends on where the brain damage is
- Even if a person loses much of their site they can still be able to perceive objects and their
backgrounds
- Damage to part of the visual association cortex can cause disrupted color vision
o Condition known as achromatopsia
o Everything looks like its in black and white
o Damage to only one side produces achromaopsia to only on of the visual fields
- Damage to another subregion of the visual association cortex can make it difficult for a person
to perceive motion and keep track of moving objects
o Ex. Being unable to cross the street without traffic lights because could not tell at what
speed the cars were moving
o Cars jump “upwards and downwards”
- If the visual cortex in the parital lobe is damaged disruption in the ability to keep track of the
location of objects in the visual scene
o Balint’s syndrome
o Cant see where they are located but can visualize objects
o The scene is a jumble of individual objects
- Damage to the visual cortex of the temporal lobe
o Cant recognize objects visual agnosia
- Prosopagosia
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o Can recognize a catagories of objects but cannot differentiate between the different
stimuli of the objects
Ex. Being unable to distinguish between the different types of cars
A farmer cant tell his cows apart
Perception of Form
Figure and Ground
- Objects are things having particular shape and particular locations of space known as figure
- Backgrounds are formless and serve mostly to judge the location of objects we see in front of
them known as ground
- The classification of objects as a figure or ground is dependant on the behaviour of the observer
o Ex. If you watch bird, they are the figures and the clouds and the sky are ground but if
your watching the clouds then the birds become the ground
- If the visual field contains a sharp and distinct change in brightness, colour or texture, we
perceive it as an edge
- If the edge forms a continuous boundary, we perceive the space enclosed by it as a figure
Organization of Elements: Gestalt Laws of Grouping
- Illusionary contours lines that do not exist
- Gestalt Psychology the task of perception was to recognize objects in the environment
according to the organization of their elements
o The whole is more then the sum of its parts
o Depends of the relationships of the elements between one another not just the
elements that make up a scene
- We can perceive forms that are fuzzy and incomplete because our visual system groups and
combines elements
o Ex. Backgrounds can be the same colour as objects and the lines of the objects can be
not distinct
- The law of proximity states that elements that are closest together will be perceived as
belonging together
- The law of similarity states that elements that look similar will be perceived as part of the same
form diamond inside of a square
- Good continuation predictability or stability
o A line is predicted to keep moving straight rather then make a sharp bent turn
- The law of closure states that our visual system often supplies missing information and “closes”
the outline of an incomplete figure
o Gaps in triangles, and when you fill the gaps it undeniably looks like one
- Law of common fate states that elements that move in the same direction will be perceived as
belonging together and forming a figure
o Animals that blend in to the background but when they move you are able to then
perceive the animal
o Visual objects stand out from their background to the extent that they exhibit contrasts
in colour or brightness
Templates and Prototypes
- Templates are special kinds of memories used by the visual system
- A type of pattern used to manufacture a series of objects
o A cookie cutter is a type of objects used to cut out cookie shapes out of dough
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