testPerception
Perception of Form
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What is Perception?
It is the process by which we interpret sensations provided to us by our sense organs (eyes, ear, Capacinian corpuscles)
It is a rapid “automatic” process that extracts:
useful information from non-useful
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Brain Mechanisms of Visual Perception
Primary Visual Cortex
Visual Association Cortex
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Primary Visual Cortex
F 7.1
Action potentials from the eyes are first processed in CNS at the level of the thalamus after decussating in the optic chasm
Afterwards the information is sent to the primary visual cortex where it is processed by groups of neurons which are connected in such a way as to favour the reaction (generate action potentials) to certain kinds or patterns of light.
This specialization can be:
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Primary Visual Cortex
Experimenter moves black bar within the visual field of cat
Microelectrode is implanted in the primary visual cortex of the cat whose gaze is fixed on a spot on a blank screen
Fixed point
Neuron does not respond if bar is outside (hard wired) visual field
The purpose of this experiment is to define how the neuron whose activity is being recorded responds to the bar being moved in the defined visual field
Cat views this screen
Results
Neuron fires most when bar is moved to 50 degree angle and then moved
Stimulus
Movement starts Movement finishes
So the experiment shows that this neurone is activated when an object tilted at 50 degrees moves in this particular part of the visual field
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Now Imagine that……
There are thousand of neurones each with their own sensitivities that are activated by other objects in other orientations in many different visual fields.
AND
There are thousand of neurones which tend to be inhibited by objects moving in a unique part of the visual field.
Visual field is divided up at the level of the retina in modules (0.5 X 0.7 mm of tissue containing about 150,000 neurones) which are then connected to primary visual cortex (2500 modules).
THEN…
Our perception of visual stimuli can be conceptualized as a mosaic of neuron firing patterns which together define the shape and movement of an object.
The same principles are used in colour vision certain neurones are sensitive to red, green or blue.
The only difference is that the perception of colour depends on the colour of the background around as well.
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Visual Association Cortex
The mosaic of images is then passed on to the next level of processing.
Different kinds of information are passed on to an area of the cortex that surrounds the primary visual cortex, it is called to visual association cortex.
Three different subdivisions exist
another concerned with detecting movement
and finally one concerned with colour
Also information from both eyes is integrated at this level
Finally information from these three areas are combined in the secondary visual cortex located in the temporal lobes
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Visual Association Cortex
© 2000 Pearson Education Canada Inc., Toronto, Ontario
Effects of Brain Damage on Visual Perception
Balint’s syndrome: due to damage of the visual association cortex results in the inability to place objects in space. So an object can be identified when placed directly in front of the person but it is not possible to keep track of it when moved
Visual Agnosia: temporal lobe damage results in the inability to determine what an object is based solely on its shape.
A particularly interesting form of agnosia is prosopagnosia which involves the inability to recognise complex shapes like people’s faces.
Achromatopisa: colour blind ie. No colour vision which is different from red/green colour blind which is the inability to distinguish between red and green.
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achromatopsia example
F 7.5
Revealing damage to what side of the brain?
Answer: The left
Perception of Form
Figure and Ground
Gestalt Laws of Grouping
Models of Pattern Perception
Evaluating Scientific Issues: Does the Brain Work Like a Serial Computer?
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Perception of Form
Figure: an object that perceived as having form against a perceived background
Ground: the backdrop that is de-emphasized to provide visual context
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Figure and Ground
Figure and Ground
How do we differentiate between these two “points of view”?
Gestalt psychology
Gestalt is German word for “form” and as such this theory recognizes we have a learned ability to view visual stimuli and organise them into a perception that permits us to interpret the original stimulus.
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Gestalt Laws of Grouping
F 7.8
Here we view a pattern of dots and make a larger shape from them
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Gestalt Laws of Grouping
F 7.9
This pattern “fools” us into seeing and inverted apparently lighter white triangle on top of another
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Gestalt “Laws” of Grouping
F 7.10
Here arranging dots differently makes us organize the dots into either columns or rows.
Law of proximity: elements closest to one another tend to be perceived as belonging to the same group
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Gestalt Laws of Grouping
F 7.11
Law of similarity: similar elements are perceived as belonging to the same figure
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Gestalt Laws of Grouping
But also our innate sense of wanting to simplify input makes us perceive patterns
Good continuation: given a choice the simpler form is perceived as “true”.
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Models of Pattern Perception
Most important we also learn to decipher varied stimuli into a commonality
This pattern of recognition relies on our learned perception of what “N” looks like. Thus, we often perceive the world in what is termed as PROTOTYPES
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© 2000 Pearson Education Canada Inc., Toronto, Ontario
Evaluating Scientific Issues: Does the Brain Work Like a Serial Computer?
F 7.19
The answer appears to be “NO”
We recognize complicated images about as fast as simple ones thus the mosaic is processed in parallel and not in series steps
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© 2000 Pearson Education Canada Inc., Toronto, Ontario
Evaluating Scientific Issues: Does the Brain Work Like a Serial Computer?
F 7.20
So although we get a series of inputs the brain processes these inputs at all the same time but each is interconnected to another
Thus the output arrives and it will reflect the complexity of the initial input independent of the processing complexity
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Other tricks of the mind
We also use context to judge and define an object in the visual field
This means that an incongruous object will tend to slow our perception of it while one that belongs is immediately recognized
Fig 7.22 in text is a good example of this.
This is referred to as top-down processing
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Perception of Space and Motion
Depth Perception
Depth Perception
F 7.26
The apple is perceived in our visual field in the retina in different places and so it is interpreted as distance
Convergence: each eye is fixated and moves together to focus on the same point
This works with
Retinal disparity: the object is located in different spots in the retina but as the image conveyed to the cortex is represented differently in each half of the cortex the brain can tell us where it is in space.
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Depth Perception
(a) two identical rectangles as shown in (b) or
(c) a rectangle and an L-shaped object
F 7.28
Depth Perception
F 7.29
Linear perspective:
This does not involve binocular vision and as such it is a learned perception
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Depth Perception
The way light hits an object is also used to provide information
Here a series of dots are shown in such as way as to create the illusion of depth
The confusion arises as we we do not know where the light is coming from thus we can invert the image at will.
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Depth Perception
F 7.38
Form constancy:
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Depth Perception
F 7.38
Form constancy:
but because we recognize the figure as a window, we perceive its shape as rectangular.
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Depth Perception
F 7.39
Perception of Motion
Perception of Motion
Motion is perceived by repeated comparisons of visual stimuli through the movement of the eyes, it is involuntary
But this movement is not smooth in fact it is series of stops and starts.
When stopped it gathers detailed information but when moving the relative changes in the visual filed are compared