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Chapter 4

Chapter 4 - Study notes

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Matthias Niemeier

CHAPTER FOUR: PERCEIVING AND RECOGNIZING OBJECTS Heteronymous hemianopia? Wat is it ?which brain structure is likely damaged? Wat are the symptoms. half of the vision field is missing. Half relative to fovea, if it is the left visual field then left vision not working. one is in left visual field and the other eye has visual field defect in right visual field. In left eye, the left part is missing. In right eye the right part is missing. So hteronymous means it is different eye. Homonymous is both eyes same. The brain structure is the optic casism, it is the part where visual information is crossing over. wat is CSF-contrast sensitivity function. Can u sketch wat it usually looks like wat does that reflect in terms of perception? Humans limited to 60 cycles spatial frequency inverted U. On the vertical axis is contrast sensitivity function. Inverted U means ? middle (midlevel) vision loosely defined stage of visual processing that comes after basic features have been extracted from the image, early vision, and before objet recognition and scene understanding, high level vision MIDDLE VISION- FINDING EDGES -goal of middle vision is to organize elements of a visual scene into groups that we can then recognize as objects ie organizing the edges -illusory contours = a contour that is perceived, even though nothing changes from one side of the contour to the other in the image. Ex edges that are perceived despite lack of physical evidence for them -structuralism =believe that complex objects or perceptions could be understood by analysis of the components, so they disagree with illusory of contours cuz an extended edge is seen bridging a gap where no local atom of edgeness can be found -gestalt=’form’ in German. Perceptual whole could be greater than the apparent sum of the sensory parts. Gestalt grouping rules = set of rules describing which elements in an image will appear to group together. The original list of rules assembled by members of gestalt school of thought. Rules guide the visual system in its interpretation of the raw retinal image Good continuation a gestalt grouping rule, stating that 2 elements will tend to group together if they seem to lie on the same contour Perceptual committee = all the rules get together and voices opinions about how the stimulus ought to be understood and reach a consensus on how to interpret the visual scene An edge suddenly stops in an image according to the visual system is cuz there is another contour occluding (stopping) the edge TEXTURE SSEGMENTATION AND GROUPING www.notesolution.com - texture segmentation = craving an image into regions of common texture properties, this similar to the 2 principles below -similarity =a gestalt grouping rule stating that the tendency of 2 features to group together will increase as the similarity /b/ them increases . image chunks that are similar to each other will be more likely to group together ie colour, size, orientation, aspects of form. Combinations, conjunctions, of features do not work -proximity gestalt grouping rule stating that the tendency of 3 features to group together will increases as the distance /b/ them decreases, ie giving them a horizontally striped appearance 2 weaker grouping principles: parallelism (parallel contours are likely to belong to the same figure) and symmetry (symmetrical regions are more likely to be seen as figure) Common region gestalt grouping rule stating that 2 features will tend to group together if they appear to be part of the same larger region Connectedness gestalt grouping rule stating that 2 items will tend to group together if they are connected PERCEPTUAL COMMITTEES REVISTED -low level visual processes deliver fairly straightforward bits of info about a line here and a colour there. Middle vision behaves like a collection of specialists each interpreting a part of the puzzle and the goal is to have a single answer Oliver Selfridge’s committee of demons, feature, cognitive, decision demons. Physical substrate of a committee is probably a massively interconnected set of neurons Decision made by committee need not be final. Ambiguous figure visual stimulus that gives rise to 2 or more interpretations of itz ID or structure ex necker cube an outline that is perceptually bi- stable. Unlike the situation with most stimuli, 2 interpretations continually battle for perceptual dominance. Another ex is duck-rabbits, these are really exceptions that prove the rule, every image is, in theory, ambiguous, but middle vision perceptual committees almost always agree on a single interpretation Accidental viewpt a viewing position that produces some regularity in visual image that is not present in the world, ex the sides of 2 independent objects lining up perfectly, any shift in position will ruin the image. It is assumed to the visual system that most images will not consider all the accidental viewpts and it is also assumed that the visual system involves an implicit understanding of physics of the world, implicit meaning innate, we don’t have to verbalize it, we know that this rule ie human leg not gone when behind a table. Even animals know Images have no meaning until middle or high level visual processes dig into them FIGURE AND GROUND www.notesolution.com Figure-ground assignment process to distinguish figures (objects in foreground) from ground (surfaces or objects lying behind the figures) is a critical step on the path from image to object recognition Principles at work in figure ground assignment: surroundedness if one region is entirely surrounded by another, it is likely that the surrounded region is the figure; size: the smaller region is likely the figure; symmetry: symmetrical region is more likely to be seen as figure; parallelism: regions with parallel contours are more likely to be seen as figure; extremal edges: this cue is strong enough to overwhelm cues like surroundedness and size; relative motion: how surface details move relative to an edge (6 in total) DEALING WITH OCCULSION 3D objects often hide ie occuled themselves; a committee infers the presence of hidden pieces of the object Relatability =degree to which 2 line segments appear to be part of the same contour (similar to gestalt good continuation rule). This heuristic =mental shortcut is not infallible (perfect) as some objects that don’t seem to connect, do. Rules fail to hold true when we are viewing the scene from an accidental viewpt = nonaccidental features = a feature of an object that is not dependent on the exact or accidental viewing position for the observer ie a 3D object overlapping each other -global superiority effect properties of whole object take precedence over the properties of parts of the objects SUMMARIZING GOALS OF MIDDLE VISION -bring together that which should be brought together: Gestalt grouping principles (similarity, proximity, parallelism, symmetry, etc) and we have processes that complete contours and objects even when they are partially hidden behind occluders ie relatability heuristic -split into separate parts that which should be split: edge-finding processes that divide regions from each other. Figure ground mechanisms separate objects from the bg -use wat we know: 2D edge configurations are taken to indicate 3D corners or occlusion borders, and objects are divided into parts on the basis of an implicit knowledge of the physics of image formation -avoid accidents: avoid interpretations that require the assumptions of highly specific, accidental combinations of features or accidental viewpts -seek consensus and avoid ambiguity: the consensus answer serves as the input for processes that will recognization OBJECT RECOGNITION-TEMPLATES VS. STRUCTURAL DESCRIPTIONS www.notesolution.com - naive template theory idea that visual system recognizes objects by matching the neural representation of the image with a stored representation of the same ‘shape’ in the brain ie ‘lock and key’ metaphor Difficult with naive template model is that there are too many templates are required Structural description = description of an object in terms of the nature of itz constituent parts and relationships between those parts Geons= geometric ions in Biederman’s recognition model, which perceptual objects are built. They are a specified as collections of nonaccidental features, so visual system should be able to recognize a geon equally accurately and quickly, regardless of how t
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