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PSYC2030 - Sensation and Perception Chapter 4

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York University
PSYC 2220
Jennifer Steeves

Ben Kim PSYC2030 – Sensation & Perception Page 1 of 7 Lecture 3 Perceiving and Recognizing Objects – Chapter 4 Gestalt Approach • Phenomenology – why do things look as they do • Groups of stimuli acquire a pattern quality which is greater than the sum of their parts o Figure/ground o Vision • Helps us interact (vision for action) with the environment by providing information about o What o Where • Uses prior knowledge to guide current perception Bottom-Up Processing • Information is built up through the visual processing stream o Increasingly elaborated Challenges of Perception • The input is not enough information to explain our perception at any single moment • There is too much sensory input to incorporate everything into a coherent percept at any single moment • The problem of object recognition: o The pictures were just a bunch of pixels on a screen, but in each case you perceived a house  How does your visual system move from point of light, like pixels, to whole entities in the world, like houses? • “Middle vision”: A loosely defined stage of visual processing that comes after basic features have been extracted from the image and before object recognition and scene understanding o Involves the perception of edges and surfaces o Determines which regions of an image should be grouped together into object Extracting Edges and Surfaces, and Grouping them • The initial information the visual system is extremely impoverished o This is the input o You end up with the # of objects, their sizes, shapes, distances, textures, motions o An edge is a sudden discontinuity in intensity Ben Kim PSYC2030 – Sensation & Perception Page 2 of 7 Computation of Edges Why edges? • Edges frequently correspond to the boundaries of objects; a map of edges is a good start to identifying objects • Edges are invariant to lighting conditions Middle Vision What about textures? - Why don’t you see a million objects when you see a hate with many “edges” (Herringbone pattern)? • Assess at more than one scale • Computer-based edge detectors are not as good as humans o Sometimes computers don’t find enough edge or find too many • Rules that make contours o Gestalt: In German, “form” or “whole” o Gestalt psychology: “The whole is greater than the sum of its parts.”  Opposed to other schools of thought, such as structuralism, that emphasize the basic elements of perception o Gestalt grouping rules: A set of rules that describe when elements in an image will appear to group together • Rules that make contours o Good continuation: A Gestalt grouping rule stating that two elements will tend to group together if they lie on the same contour o Some contours in an image will group because of good continuation o Can you find the shape embedded within the field of lines at right? • Texture segmentation and grouping o Texture segmentation: Carving an image into regions of common texture properties o Gestalt grouping rules:  Similarity  Proximity o Parallelism: Parallel contours are likely to belong to the same figure o Symmetry: Symmetrical regions are more likely to be seen as figure o Common region: Two features will group if they appear to be part of the same larger region o Connectedness: Two items will tend to group if they are connected o Dynamic grouping principals  Common fate: elements that move in the same direction tend to group together  Synchrony: Elements that change at the same speed tend to be grouped together • Figure and ground Ben Kim PSYC2030 – Sensation & Perception Page 3 of 7 o Figure-ground assignment: The process of determining that some regions of an image belong to a foreground object (figure) and other regions are part of the background (ground) o An image cannot be a figure and a ground at the same time o The brain has to choose between one of two images, it can go back and forth but it cannot make a definite image of something that looks ambiguous • Gestalt figure-ground assignment principals o External edges: if edges of an object are shaded such that they seem to recede in the distance, they tend to be seen as figure o Relative motion: if one region moves in front of another, then the closer region is figure • Dealing with occlusion o Reliability: the degree to which two line segments appear to be part of the same contour • Parts and wholes o Global superiority effect: the properties of the whole object take precedence over the properties of parts of the object • Nonaccidental feature: a feature of an object that is not dependent on the extract (or accidental) viewing position of the observer o T junctions: indicate occlusion. Top T is in front and stem of T is in back o Y junctions: indicate corners facing the observer o Arrow junctions: indicate corners facing away from the observer o • Committee rules: honor physics and avoid accidents o Ambiguous figure: a visual stimulus that gives rise to two or more interpretations of its identity or structure  Perceptual committees tend to obey the laws of physics o Accidental viewpoint: A viewing position that produces some regularity in the visual image that is not present in the world  Perceptual committees assume viewpoints are not accidental • Summarizing middle vision o Goals of middle vision:  Bring together that which should be brought together  Split asunder that which should be split asunder  Use what you know Ben Kim PSYC2030 – Sensation & Perception Page 4 of 7  Avoid acc
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