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Perception 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1. Brain Mechanisms of Visual Perception 2. Perception of Form 3. Perception of Space and Motion
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7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

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7 - 3 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Brain Mechanisms of Visual Perception  Primary Visual Cortex  Visual Association Cortex  Effects of Brain Damage on Visual Perception
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Page 1: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Perception

1. Brain Mechanisms of Visual Perception

2. Perception of Form

3. Perception of Space and Motion

Page 2: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 2 © 2000 Pearson Education Canada Inc., Toronto, Ontario

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 meaning (coherence) from many different

inputs.

Page 3: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 3 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Brain Mechanisms of Visual Perception

Primary Visual Cortex

Visual Association Cortex

Effects of Brain Damage on Visual Perception

Page 4: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 4 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Primary Visual CortexF 7.1Action 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:

•form or shape•movement•colour

Thus the processing of visual information is hierarchical

Page 5: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 5 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Primary Visual Cortex

F 7.2

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

Page 6: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 6 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Results

Neuron fires most when bar is moved to 50 degree angle and then moved

Stimulus Action Potentials generated by neuron

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

Page 7: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 7 © 2000 Pearson Education Canada Inc., Toronto, Ontario

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.

Page 8: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 8 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Visual Association CortexThe 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 • an area concerned with the perception of shape• 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

Page 9: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 9 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Visual Association CortexF 7.3

Page 10: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 10 © 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.

Page 11: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 11 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Effects of Brain Damage on Visual Perception

achromatopsia exampleF 7.5

Right visual field is missing colour

Revealing damage to what side of the brain?

Answer: The left

Page 12: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 12 © 2000 Pearson Education Canada Inc., Toronto, Ontario

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?

Page 13: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 13 © 2000 Pearson Education Canada Inc., Toronto, Ontario

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

Page 14: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 14 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Figure and GroundF 7.6

Page 15: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 15 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Figure and Ground

F 7.7

Page 16: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 16 © 2000 Pearson Education Canada Inc., Toronto, Ontario

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.

Page 17: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 17 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Gestalt Laws of Grouping

F 7.8Here we view a pattern of dots and make a larger shape from them

Page 18: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 18 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Gestalt Laws of Grouping

F 7.9

This pattern “fools” us into seeing and inverted apparently lighter white triangle on top of another

Page 19: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 19 © 2000 Pearson Education Canada Inc., Toronto, Ontario

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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Perception 7 - 20 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Gestalt Laws of Grouping

F 7.11

Law of similarity: similar elements are perceived as belonging to the same figure

Page 21: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 21 © 2000 Pearson Education Canada Inc., Toronto, Ontario

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”.

Page 22: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 22 © 2000 Pearson Education Canada Inc., Toronto, Ontario

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

Page 23: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 23 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Evaluating Scientific Issues: Does the Brain Work Like a Serial Computer?

F 7.19The 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

Page 24: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 24 © 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

Page 25: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 25 © 2000 Pearson Education Canada Inc., Toronto, Ontario

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 7 - 26 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Perception of Space and Motion

Depth Perception

Perception of Motion

Page 27: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 27 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Depth PerceptionF 7.26

The apple is perceived in our visual field in the retina in different places and so it is interpreted as distanceConvergence: each eye is fixated and moves together to focus on the same pointThis works withRetinal 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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Perception 7 - 28 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Depth Perception

Principle of good form - The objects could be:(a) two identical rectangles as shown in (b) or(c) a rectangle and an L-shaped object

F 7.28

Page 29: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 29 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Depth Perception

F 7.29

Linear perspective: This does not involve binocular vision and as such it is a learned perception

Page 30: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 30 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Depth PerceptionThe 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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Perception 7 - 31 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Depth Perception

F 7.38

Form constancy:This figure can be perceived as a trapezoid….

Page 32: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 32 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Depth Perception

F 7.38

Form constancy:but because we recognize the figure as a window, we perceive its shape as rectangular.

Page 33: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 33 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Depth Perception

F 7.39

Page 34: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 34 © 2000 Pearson Education Canada Inc., Toronto, Ontario

Perception of MotionF 7.40

Page 35: 7 - 1 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 1.Brain Mechanisms of Visual Perception 2.Perception of Form 3.Perception of Space.

Perception 7 - 35 © 2000 Pearson Education Canada Inc., Toronto, Ontario

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

Movement is called saccades while brief stops are called fixations