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Chapter 4

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Matthias Niemeier

Chapter 4 Perceiving and Recognizing Objects Middle (midlevel) vision: loosely defined stage of visual processing that comes after basic features have been extracted from image (early vision) and before object recognition and scene understanding (high level vision) Act of recognition must involve matching what we perceive now to a memory of something we perceive in past Middle Vision Goal of middle vision is to organize elements of visual scene into groups that we can then recognize as objects Finding Edges o Because objects abut and overlap other objects, simple connectedness wont work o Occasional lack of edge doesnt seem to bother visual system at all o Kanizsa figure named after Gaetano Kanizsa an Italian psychologist who spent many years investigating stimuli o Illusory contours: contour that is perceived, even though nothing changes from one side of contour to other in image Rules That Make Contours o Tendency of visual system to go beyond info given was problematic for one of earliest psychologists structuralists o Structuralism: school of thought believing that complex objects or perceptions could be understood by analysis of components o Structuralists like Wundt argued that perceptions are sum of atoms of sensation its of color, orientation and so forth Perception is built up of local sensations the way crystal might be built of array of atoms o Illusory contour challenges view because extended edge is seen bridging gap where no local atom of edgeness can be found o Gestalt: in German, literally form. In perception, name of school of thought stressing that perceptual whole could be greater than apparent sum of parts o Most enduring contribution of Gestalt was to begin description of set of organizing principles Gestalt grouping rules: set of rules describing which elements in an image will appear to group together. Original list was assembled by members of Gestalt school of thought o Good continuation: Gestalt grouping rule stating that 2 elements will tend to group together if they seem to lie on same contour Perceptual Committees o all else being equal phase o Everyone gets together and voices opinions about how stimulus ought to be understood opinions collide, and results are somewhat unpredictable o Consensus view almost always quickly emerges and settle on single interpretation of visual scene Occlusion o Why does an edge suddenly stop? o Guess might be that it stops because something else gets in the way, hiding it from view o Answer that visual system seems to come up with is that theres another contour occluding vertical line, with occluding edge oriented perpendicularly to occluded edge Texture Segmentation and Grouping o Connecting little pieces of line segments will get only so far in dividing raw image into objects o Texture segmentation: carving image into regions of common texture properties Closely related to Gestalt grouping principles o 2 strongest principles Similarity: Gestalt grouping rule stating that tendency of 2 features to group together will increase as distance between them decreases Texture grouping can be based on similarity in limited number of features color, size, orientation, and form Combinations of features dont work well Proximity: Gestalt grouping rule stating that tendency of 2 features to group together will increase as distance between them decreases o Weaker principles Parallelism: rule for figure ground assignment stating that parallel contours are likely to belong to same figure Symmetry: rule for figure ground assignment stating that symmetrical regions are more likely to be seen as figure o Common region: Gestalt grouping rule stating 2 features will tend to group together if they appear to be part of same larger region o Connectedness: Gestalt grouping rule stating that 2 items will tend to group together if theyre connected Camouflage o Same principles that are normally used to help find objects in world can be exploited to hide them o Art f camouflage is art of getting features to group with features of environment so as to persuade observer that features dont form perceptual group of their own Perceptual Committees Revisited o Low level visual processes deliver fairly straightforward bits of info about line here and color there o Middle vision behaves like collection of specialists, each with specific area of expertise and individual
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