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Chapter 4

Ch. 4 notes - Form Perception.doc

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Department
Psychology
Course
PSYCH 1XX3
Professor
Christopher Teeter
Semester
Winter

Description
Form Perception Gestalt Principles of Grouping • Gestalt Psychology – a theory to account for how we perceive the environment • “gestalt” = the way something is put together/configured (German) • Often report seeing more than just the sensory info that falls on our retina – visual system puts info together to make it easier to interpret – attempt to organize info into simple groups • “the whole is greater than the sum of its parts” • Ex. a group of 10 dots in a row is reported as a ‘line of dots’ instead of saying ’10 dots’ • Law of Pragnanz – we have a bias to organize things into the simplest organization • Ex. a group of 10 dots in a row is reported as a ‘line of dots’ instead of saying ‘dot’ 10x 6 Principles of Gestalt organization: 1. Figure-ground: When perceiving a visual scene, we organize the info as the central object of identification (figure), while the other objects fade into the background (ground) – object will be perceived as separate from its background 2. Proximity: things that are closer together will be perceived as members of the same group • Ex. 9 little squares spread out is just 9 squares but when they are all right beside each other (form 1 big square), we perceive them as 1 group 3. Closure: our brains not only process info from the visual scene, they actively interpret it – when we see an incomplete object, we fill in the gaps • Ex. WWF panda symbol is incomplete but we still see it as a panda 4. Similarity: things that share visual characteristics (ex. shape, size, colour, texture, value, orientation) will be seen as belonging together • Ex. squares form a plus sign in a grid of circles ad squares 5. Continuity: points that are connected by straight/curvy lines are seen in a way that follows the smoothest path – rather than seeing separate lines and angles, lines are seen as belonging together • Ex. 4 lines of dots are seen as forming an X pattern together 6. Common fate: elements tend to be perceived as grouped together if they move together • Ex. if you were walking through a park and there was a green snake lying in the grass, you probably wouldn’t notice it – as soon as the snake moves, you would notice it immediately – its movement causes your visual system to organize the figure against the background Role of Expectation in Perception of Patterns and Objects • When our interpretation emerges from the data (image that falls on retina), this is known a s data-driven/bottom-up processing • When our perception is influenced by sources beyond images in the retina (ex. knowledge/experience), higher-level cognitive processing is engaged to interpret the image; this is known as prediction /top-down processing • Influence of top-down processing has been shown through rat/man stimulus – subjects viewed this image after looking at pictures of animals/faces – depending on what they saw previously, they reported seeing a rat/man wearing glasses – they had been primed for one or the other interpretation of an ambiguous picture by being shown the previous slides – this is a form of top-down processing – expectations influence what we perceive Theories of Object Recognition in Humans • Studies focused on 2 questions: how are objects represented in the brain? and how do we match unlimited combinations of possible viewing conditions with our representation of each object? • Any complete theory must explain how humans are able to reliably identify specific objects from an unlimited number of orientations • Recognition-by-Components (RBC) – accounts for the successful identification of objects despite changes in the sixe/orientation of the image – also explains how moderately occluded/degraded images as well as novel ex. of objects are successfully recognized by the visual system • based on the assumption that just about any objects can be represented as an arrangement of 3 dimensional shapes (ex. cube, cylinder, cones, etc.) called geons • geons are a part of the object ‘alphabet’ and unique combinations from the equivalent of object ‘words’ that can be readily interpreted by the visual system • as we view an object, our visual system segments different geons and their relationship to one another • recognizing a geon involves recognizing the features that define it (feature analysis) • having identified the pieces out of which the object is composed and their configuration, one recognizes the objects the pattern composed from these pieces (a prototype comparison) • strength: people can typically recognize about 30,000 objects with the use of only 36 different geons • this model argues that we can see objects from unlimited viewpoints – different views of the same object should still lead to the same set of geon recognition – achieves view invariance • drawbacks: invariant features are difficult to extract in real images (ex. slight variations in lighting can completely change the unique features of a face that are used for individual identification)  geons are also poor at representing many natural objects that may/may not have simple parts-based descriptions (ex. many birds have tapered beaks and could be described by the same geon, but there is a lot of subtle variation in the features of bird beaks) – our knowledge of geons doesn’t help us understand how we distinguish between 2 birds with slightly different features • Template Matching Model – incoming sensory info is compared to copies (templates) stored in long-term memory – stored in the process of our past experiences and learning • Assumes that a retinal image is faithfully transmitted to the brain and that an attempt is made to compare it to a large number of literal copies (templates) in order to find a match against all templates • Works well when exact matches are expected (ex. computers (search for exact sequences of code)/barcode readers), but not with humans
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