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PSYB51H3 (306)

psyb51-perception and cognition

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Matthias Niemeier

CHAPTER 4: PERCIEVING AND RECOGNIZING OBJECTS: - Middle (midlevel) Vision: loosely defined stage of visual processing that comes after basic features have been extracted from the image (early vision) and before object recognition and scene understanding (high-level vision) Middle Vision - goal of MV: organize the elements of a visual scene into groups that we can then recognize as objects Finding Edges: - trace the edge of the object with a finger must pass locations where there is no diff b/w the luminance of the object and luminance of background these points of the shape has no edge at all - lack of edge doesn’t bother visual system hard to see the gaps o visual system knows they are accidents of lighting and fills in the contour - Illusory Contours: contour that is perceived, een though nothing changes from one side of the contour to the other in the image Rules that make Contours: - Structuralism: school of thought  complex objects/perceptions could be understood by analysis of the components - Perceptions are the sum of atoms of sensationbits of colour, orientation, etc. - Structualist’s view perception is built up of an array of atoms - Gestalt: “form” in german; school of thought stresses that the perceptual could be greater than the apparent sum of the parts - Gestalt Grouping Rules: set of rules describing which elements in an image will appear to group tgt - Good continuation: Gestalt grouping rule stating that 2 elements will tend to group tgt if they seem to lie on the same contour Perceptual “Committees” - “all else being equal”; everyone gets tgt and voices opinions about how the stimulus ought ot be understood o Sometimes opinions collide results are unpredictable o Consensus view one single interpretation of the visual scene Occlusion: - Why does an edge sometime suddenly stop? o b/c something else gets in the way, hiding it from our view o notches in the circle represent contours can be extended  inference of an illusory contour Texture Segmentation and Grouping: - because the edge-finding members of the “image-parsing committee” are of little use o texture segmentation: carving an image into regions of common texture properties  closely related to Gestalt grouping principles  Similarity: G.G rule  tendency of two features to group tgt will increase as the similarity b/w them increases • Texture grouping can be based on similarity (color, size, orientN, aspects of form) o Proximity: G.G rule tendency of two features to group tgt will increase as the distance b/w them decreases - Weaker grouping principles: o Parallelism: rule for figure-ground assignment stating that parallel contours are likely to belong to the same figure o Symmetry: rule for figure-ground assignment stating that symmetrical regions are more likely to be seen as figure - Principle of Common Region: G.G rule stating that two features will tend to group tgt if they appear to be part of the same larger region - Connectedness: G.G rule stating that two items will tend to group tgt if they are connected Camouflage: - the same principle that is used to help us find objects in the world can also be exploited to hide them Perceptual Committees Revisited: - Low-level visual process deliver fairly str8fwd bits of information about a line here and a colour there o The collection of info needs to be interpreted before we know what we are seeing - Middle vision behaves like a collection of specialists, each with a specific area of expertise and indiv. Opinions about what the input might mean o Goal: have a single answer emerge out of this diversity of opinions Committee Rules: Honor Physics and Avoid Accidents: - Ambiguous Figure: visual stimulus that gives rise to 2 or more interpretations of its ID/structure - Necker Cube: outline that is perceptually bi-stable. Unlike the situation with most stimuli, two interpretation continually battle for perceptual dominance - Accidental Viewpoint: a viewing position that produces some regularity in the visual image that is not present in the world(EX: sides of two indep. Objects lining up perfectly) Figure and Ground: - Figure – Ground Assignment: process of determining that some regions of an image belong to the foreground object (figure) and other regions are part of the background (ground) o Principles that are at work in the assignment of regions to fig/ground o Surroundedness; size; symmetry; parallelism; extremal edges; relative motion (more info on pg 88) Complicated Business: - should speculate about what factors determine whether a circle is seen as a hole or a spot - imageobject: we need to understand something about how one object can occlude another - information about object recognition influences figure-ground assignment Dealing with Occlusion: - real world objects are partially hidden by other objects - Relatability: degree to which two line segments appear to be part of the contour o Visual system is unwilling to guess so elaborate a relation, so it concludes that lines are not actually related at all o Heuristic: mental shortcut
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