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Chapter 6


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University of Toronto Scarborough
Steve Joordens

CHAPTER 6: Perception Sense organs are there to provide information to guide behavior, but it doesnt do it by itself Ex. our vision: the brain receives information from approx 1 million axons in each of the optic nerves and it organizes everything into a perception of a scene - and even when our bodies or eyes move, so the photoreceptors are exposed to entirely new patterns of visual information, our perception of the scene stays the same, we see a stable world Ex. when we hear someone talking, we locate the person using the sensations of Chapter 5, but even when we move our head, change our position, it doesnt change what we know about the speakers location So even when sensation can change, perception could stay the same Perception: a rapid, automatic, unconscious process by which we recognize what is represented by the information provided by our sense organs - the process of perception gives unity and coherence to the input - it is not deliberate or effortful, we dont have to puzzle out the meaning of what we see Its not that we see the object, then perceive it, we just perceive the object Once in awhile, what we see may require us to reflect on what it might be, or get more evidence to decide what it is, but this has to do more with problem solving than perception If we look at a scene, we can describe some of the elements that are present We dont necessarily become aware of the elements first, then perceive the objects and the background Ex. when you see a tall, cylindrical object on a countertop, you immediately perceive: a glass - you then perceive the smudges, the lettering, the beverage, etc. Note: our awareness of the process of visual perception comes only after it is complete The line between sensation and perception is not too clear For some sensory systems like pain and vestibular sense, the distinction is arbitrary since they help us to react rather than to provide a representation of the world around us Brain Mechanisms of Visual Perception Visual perception is often looked at as a hierarchy of information processing Circuits of neurons analyze particular aspects of visual information and send the results of the analysis to another circuit which performs even more analysis More and more complex features are analyzed during the process Eventually, the process leads to perception of the scene and all the objects in it The higher levels of perceptual process interact with memories: we can recognize familiar objects, and learn the appearance of new unfamiliar objects The Primary Visual Cortex Hubel and Wiesel: inserted microelectrodes into various regions of the visual systems of cats and monkeys and looked at the APS produced by individual neurons After inserting a microelectrode, they would present various stimuli on a screen in front of the open-eyed unconscious animal (neurons in the visual system still respond) The stimulus was moved around on the screen and found the point where the neuron would be most affected - they also presented stimuli of different shapes and saw which shape affected the neuron the most Conclusion: the geography of the visual field is retained in the primary visual cortex; the surface of the retina is mapped on the surface of the primary visual cortex - the map is distorted, the largest amount of area is given to the centre of the visual field where our vision is most precise The map is like a mosaic with many tiles, and each tile is called a module Module: a block of cortical tissue that receives information from the same group of receptor cells - all the neurons within a module receive information from the same small region of the retina - the primary visual cortex has about 2500 modules Each module receives information only from a small region of one retina, and so information from only a small region on the visual field (the scene projected on that part of the retina) Neural circuits within each module analyze their own particular part of the visual field, their receptive field Receptive field: that portion of the visual field in which the presentation of visual stimuli will produce an alternation in the firing rate of a particular neuron ex. some circuits detected the presence of lines passing through the field, and also signaled the orientation of these lines or the width or the movement or the direction of the movements or the colours A certain cluster of neurons receives information from a small portion of the visual field One of the neurons respond to lines oriented at 50 degrees to the vertical Other neurons in this cluster that share the same receptive field respond to lines of different orientations Conclusion: the orientation of lines that pass through this receptive field is signaled by an increased rate of firing particular neurons the cluster The Visual Association Cortex An individual module of the primary visual cortex receives very little information So for us to perceive objects and entire visual scenes, the information from all the individual modules must be combine, and this happens in the visual association cortex Two Streams of Visual Analysis Visual information analyzed by the primary visual cortex goes to be analyzed even further in the visual association cortex We found more than 2 dozen distinct regions and sub-regions of the visual cortex of the rhesus monkey - these regions are arranged hierarchically and it starts with the primary visual cortex Information is sent from one circuit of neurons to the next, where more and more complex features are analyzed Only a few milliseconds later, there is perception of the scene and the objects in it So information goes from the primary visual cortex to the visual association cortex The visual association cortex divides into two pathways: the ventral stream and the dorsal stream Ventral stream: the flow of information from the primary visual cortex to the visual association area in the lower temporal lobe; used to form the perception of an objects shape, colour, and orientation (the what system) - what is the object? What form does it have? What colour? Dorsal stream: the flow of information from the primary visual cortex to the visual association area in the parietal lobe; used to form the perception of an objects location in 3D space (the where system) - where is the object located? Is it moving? The Ventral Stream: Perception of Form Animals studies: The recognition of visual patterns and identification of particular objects takes place in the inferior temporal cortex, at the end of the ventral stream This is where analyses of form and colour are put together and perception of 3D images emerge Functional-imaging studies and study of people with damage to the visual association cortex confirm these things Brain damage can cause a group of deficits called visual agnosia visual agnosia: the inability of a person who is not blind to recognize the identity of an object visually; caused by damage to the visual association cortex - agnosia = failure to know - ex. you cant identify common objects by sight even though they have normal vision - when you hold the object, for ex., you could recognize it by touch A common symptom of visual agnosia is prosopagnosis prosopagnosia: a form of visual agnosia characterized by difficulty in the recognition of peoples faces; caused by damage to the visual association cortex - they see eyes, noses and mouths, but cant recognize the particular configuration that identifies a specific persons face - they still remember who they are, just cant recognize them by just their face Face-recognizing circuits are found in the fusiform face area (FFA) - fusiform face area (FFA): a region of the ventral stream of the visual system that contains face-recognizing circuits - located at the base of the brain Much evidence: face-recognition circuits develop as a result of experience with seeing peoples faces ex. brain lesions that produce prosopagnosia can also impair the ability of a farmer to recognize his cows, or the ability of a driver to recognize their own car So the failure of recognition is not confined to faces Experiment: bird or car experts viewed pictures of birds or cars and the FFA was activated - the FFA was not activated when non- experts looked at the pictures Suggestion: the FFA can be actually called the flexible fusiform area, since it has to do with the visual recognition of diverse objects People with autistic disorders cant develop normal social relations and in severe cases, they give no indication that other people exist They showed a deficit in the ability to recognize faces and looking at faces didnt activate the FFA It may be that other brain abnormalities associated with autistic disorder may result in a lack of interest in other people and therefore the failure to acquire face recognition during childhood Functional-imaging studies: there are several additional regions of the ventral stream that respond differently to particular categories of visual stimuli ex. the extrastriate body area (EBA) Extrastriate body area (EBA): a region of the occipital cortex, next to the primary visual cortex, that responds to forms resembling the human body - activated by photographs, si
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