Chapter 4perception.pdf

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Matthias Niemeier

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Chapter 4 – Perceiving and Recognizing Objects What an Where Pathways • Extrastriate cortex o A set of visual areas (V2, V3, etc.) that lie just outside the primary visual cortex • Where pathway (focused on later in course) o Heads up into the parietal lobe o Processes information relating to the location of objects in space and the actions required to interact with them • What pathway o Heads down into the temporal lobe o Object recognition o Inferotemporal cortex (IT) cortex ▪ Maintains close connections with parts of the brain involved in memory formation – hippocampus ▪ Some of the cells have very specific tastes • Agnosia – “psychic blindness” o A failure to recognize objects in spite of the ability to see them o Can be quite specific • Hierarchal model of visual perception o Small receptive fields and simple features of the visual cortex are com- bined with ever-greater complexity as one moves from the striate cortex to IT cortex eventually culminating in a cell that may fire when you see your grandmother ▪ Grandmother cell • Seems to be selectively responsive to one specific object • Propagnosia o Inability to recognize faces ▪ Processed holistically unlike other objects which may be analyzed in parts • Must be possible to do some rough object recognition on the basis of the first wave of activity as it moves from retina to striate cortex to extrastriate cortex and beyond o Feed-forward process The Problems of Perceiving and Recognizing Objects • Must have processes that successfully combine features into objects o Middle vision ▪ A stage of visual processing that comes after basic features have been extracted from the image and before object recognition and scene understanding • The act of recognition must involve matching what we perceive now to a mem- ory of something we perceived in the past Middle Vision • Goal of middle vision is to organize the elements of a visual scene into groups that we can then recognize as objects Finding Edges • The occasional lack of an edge does not bother our visual systems o Different bits of information are then combined to make the system’s best guess about the preference of a contour o Ex. Kanizsa figure – illusory contour • Rules of Evidence o Tendency of the visual system to make inferential leaps ▪ Problem for structuralists like Wilhelm Wundt and Edward-Brad ford Titchener • Argued that perceptions are the sum of atoms of sensation o Gestalt School ▪ Coined by Max Wertheimer, Wolfgang Kohler, and Kurt Koffka ▪ The perceptual whole is more than just the sum of its sensory parts o Gestalt grouping rules ▪ Good continuation • Two elements will tend to group together if they seem to lie on the same contour (orientation, facing same direction) • Perceptual“Committees” • Occlusion o Why does an edge suddenly stop in an image? ▪ Maybe something is in the way, or hiding it from our view (oc- cludes) Texture Segmentation and Grouping • Texture segmentation o Carving an image into regions of common texture properties ▯ ▯ • Two of the strongest principles of Gestalt grouping: o Similarity ▪ Image chunks that are similar to each other (in a number of fea- tures: coulour, size, aspects of form) will be more likely to group together o Proximity ▪ Items near each other are more likely to group together • Two of the somewhat weaker principles of Gestalt grouping: o Parallelism ▪ Parallel contours are likely to belong to the same figure o Symmetry ▪ Symmetrical regions are more likely to be seen as the same figure • Camouflage o Getting your features to group with the features of the environment so as to persuade an observer that your features do not form a perceptual group of their own Perceptual Committees Revisited • Perceptual committees in middle vision o Behaves like a collection of specialists o Receives straightforward bits of information from low-level visual pro- cesses • Oliver Selfridge’s Pandemonium model o Feature demons ▪ Found vertical lines, acute angles, etc. o Cognitive demons ▪ One for each letter ▪ Looked at the work of the feature demons and made noise propor- tional to the evidence for its letter o Decision demon ▪ Identifies letter based on the loudest yell • Committee Rules: Honour Physics and Avoid Accidents o Necker cube ▪ An outline that is perceptually bi-stable • Two interpretations continually battle for perc- eptual domi nance o Ambiguous figure ▪ Generates two or more plausible interpretations ▪ Every image is ambiguous, but the perceptual committees almost always agree on a single interpretation ▯ o Accidental viewpoint ▪ A viewing position that produces some regularity in the visual im- age that is not present in the world o Perceptual committees know about accidental viewpoints and know not to bet on them o Implicit understanding that objects block light o Opaque objects occlude other objects behind them to generate plausible interpretations of image elements like the notched circles and dead-end edges o Another committee considers all possibilities and devalues any that in- volve accidental viewpoints Figure and Ground • Figure-ground assignment o Determining which regions of an image are the background and fore- ground • Edgar Rubin o Vase/face o Rare cases in which a perceptual committee has a difficult time reaching a consensus • Principles at work in the assignment of regions to figure-ground: o Surroundedness ▪ If one region is entirely surrounded by another it
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