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

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

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

7 - 4 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 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: form or shape movement colour Thus the processing of visual information is hierarchical

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

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

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

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

7 - 9 © 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Visual Association Cortex F 7.3

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

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Effects of Brain Damage on Visual Perception achromatopsia example F 7.5 Right visual field is missing colour Revealing damage to what side of the brain? Answer: The left

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 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?

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

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Figure and Ground F 7.6

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Figure and Ground F 7.7

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception  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.

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Gestalt Laws of Grouping F 7.8 Here we view a pattern of dots and make a larger shape from them

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Gestalt Laws of Grouping F 7.9 This pattern “fools” us into seeing and inverted apparently lighter white triangle on top of another

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

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Gestalt Laws of Grouping F 7.11 Law of similarity: similar elements are perceived as belonging to the same figure

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception 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”.

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

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

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

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

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Perception of Space and Motion  Depth Perception  Perception of Motion

© 2000 Pearson Education Canada Inc., Toronto, Ontario 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.

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

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Depth Perception F 7.29 Linear perspective: This does not involve binocular vision and as such it is a learned perception

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

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Depth Perception F 7.38 Form constancy: This figure can be perceived as a trapezoid….

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Depth Perception F 7.38 Form constancy: but because we recognize the figure as a window, we perceive its shape as rectangular.

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Depth Perception F 7.39

© 2000 Pearson Education Canada Inc., Toronto, Ontario Perception Perception of Motion F 7.40

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