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3.5 - Form Perception 1.pdf

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McMaster University
Joe Kim

Arnav Agarwal 2011 Form Perception 1 Module 1: Introduction to Form Perception Introduction to the Perception of Form - Visual system needs to figure out: o what is part of the background and what is part of the object of interest o what each object is, and what each component of the visual stimuli belongs to - One can recognize picture of vase against flower background within one second, despite many components being present to the visual stimuli and mixed together - Form perception is the basis of this ability of our visual system Module 2: Gestalt Principles The Gestalt Philosophy - 1920s, 1930s: German psychologists termed “Gestalt psychologists” began studying how people perceive the world around them o Believed that “THE WHOLE IS DIFFERENT THAN THE SUM OF ITS PARTS” o Believed that people tended to perceive the whole stimulus rather than just putting together a collection of the stimulus’ discrete parts - Gestalt movement: partly a reaction to the structuralist approach in vogue at the time o Structuralist argument: everything can be reduced to basic elements - eg: watching a movie -> slightly different static pictures flashed every second, and continuous movement perceived in watching the rapid sequence of them o note: no continuous movement in/across frames o perception of entire movie (including complex movement) -> more than collection of thousands of still photographs (perceiving the still pictures individually doesn’t give same perception as watching movie) - motion: emergent property of sequence of pictures - Gestalt Principles: laws that describe how we organize visual input, grouping them in certain ways o Thought to be innate or acquired rapidly Six Gestalt Principles: Figure-Ground - Figure-ground: Ability to determine which aspects of a visual scene are the object and the background - eg: viewing vase of flowers against flowery wallpaper -> figure/ground problem o vase of flowers = determined to be object o flowery wallpaper = determined to be background Arnav Agarwal 2011 - Simplest scenario: o Small, enclosed region (object) surrounded completely by larger region (background) o Figure would have distinct borders/edges, giving it perceptible form o Perceived as being in front of the background o Background: typically formless/made up of multiple forms  Not one distinct form like the object/figure - Distinguishing figure and object is an automatic process - Reversible figures: cues used to make figure-ground decisions aren’t clear - Reversible figures: (refer to pictures below) [pictures with two interpretations?] o eg: two faces about to kiss? or vase? o eg:  if black is background, white arrows pointing upwards and downwards  if white is background, men running down the stairs Arnav Agarwal 2011 Six Gestalt Principles: Proximity - Elements that are close together in space tend to belong together - eg: field of daisies o daisies not uniformly spaced out, but are clustered close together in some areas and fewer in numbers in others o high daisy density areas will be seen as a group of daisies more than some daisies from different clusters, because of their proximity to each other - eg: rox of X’s varying in spacing between o one is more likely to group together X’s that are close together than those far apart Arnav Agarwal 2011 Six Gestalt Principles: Closure - If there are gaps in the contours of the shape, we tend to fill in those gaps and perceive a whole object Closure - eg: truck with pole coming out of it; part of truck isn’t visible, but visible parts still perceived as one truck, not two separate objects (refer to picture) o missing part automatically filled in to perceive entire object as one - eg: rectangle with small gaps will still be perceived as rectangle o eyes fill in gaps and perceive object as whole - tendency to fill in gaps may be so strong that one might even see faint lines across the gaps Arnav Agarwal 2011 Six Gestalt Principles: Similarity - Tendency to group elements that are physically similar Similarity - eg: driving by farmer’s field o even though distance between rows might be same or less than distance between plants within a row, this isn’t defining factor of one’s perception o one tends to group vegetation of the same type instead; looks for similarity Arnav Agarwal 2011 - eg: grid of alternating X’s and O’s o one tends to see columns of same elements (X’s or O’s), rather than rows of both together, alternating; the similarity in each column influences one’s perception Six Gestalt Principles: Continuity - Perceiving a simple, continuous form rather than a combination of awkward forms Continuity - eg: “X” seen as two continuous diagonal lines crossing in the middle; not two V’s in opposite directions joined at their tip o V’s are not continuous; diagonal lines are Arnav Agarwal 2011 vs. - eg: vase of flowers - stems o stem perceived as continuous line despite being criss-crossed by other stems o top of two stems not seen as one form; each stem seen as a form as it isn’t broken Six Gestalt Principles: Common Fate - Things that change in the same way should be grouped together Common Fate - Tendency to group objects moving in the same direction as being together - eg: school of fish o moving together, same direction Arnav Agarwal 2011 o grouped together as a result o tendency leads to perception of group of elements to be seen as one object itself - eg: camouflaged animal when it moves (eg: moth against bark of tree) o when stationary: almost impossible to see where wings of the moth and bark begin o when it moves: elements within the moth’s pattern move together in the same direction at the same time; contour of the moth’s shape is perceived; moth appears against tree Concept Check: 1) Which of the following are not Gestalt Principles? a. Figure-Ground b. Proximity c. Continuity d. Closure e. Common Fate f. Similarity g. Interposition 2) Objects that change in the same way should be grouped together, this idea illustrates which of the following Gestalt Principles? a. Common Fate b. Similarity c. Closure d. Proximity e. Figure-ground f. Continuity Module 3: Pattern/Object Recognition Arnav Agarwal 2011 Expectations - What a person expects to see can influence what a person actually sees Processes of Object Recognition - Preliminary steps in object recognition: o identifying figure and background in object o identifying parts of the figure and grouping them into a single object o (recognizing object) -> two processes Bottom-Up Processing - Features that are present in the stimulus itself guides object recognition - Recognition is based on analyzing individual features and comparing those features to things with similar features that one has in memory o eg: recognizing a cow -> four legs, big nose, udder, “moo”, two long ears, two big eyes Top-Down Processing - Own beliefs/expectations are the primary influence for determining what one recognizes the object to be - eg: ambiguous stimuli examples seen before (reversible figures?) - eg: second letter in both words is the same, but context influences what one reads it as (below) Arnav Agarwal 2011 - eg: B/13 series (below) nd th o 2 and 10 symbols are physically identical, but read as “B” and “13” respectively - eg: priming experiments o experimenter measures how fast reader can read word presented on screen o if experimenter indicates next word is an animal, animal names (eg: dog/duck) are recognized and read a lot faster than other words (eg: log/puck) o conclusion: processing of a word is more efficient if the participant is primed to expect a word from a certain characteristic - Both processes must be involved in object recognition o Top-down processing can’t work alone; some input from stimulus needed before expectations of that stimulus can influence one’s recognition of it o Bottom-up processing can’t explain everything alone; expectations certainly do influence our perceptions - Bi-directional activation: processing occurs in both directions at once Arnav Agarwal 2011 o Result: features of the object in combination with our expectations guides object recognition o Two components:  object features  viewer’s expectations Theories of Object Recognition - Many theories have been put forth that rely on varying degrees of bottom-up and top-down processing Biederman’s Geon Theory - We have 36 different geons (simple geometric forms) stored in memory o eg: cone, sphere, cylinder - Possible to recognize over 150 million different objects using these - eg: ice cream cone -> cone + sphere - eg: garbage can/glass -> cylinder - Problems: o stimuli like faces and crumpled paper are hard cases to determine which geons would be used in  yet, we are able to easily distinguish these stimuli o some forms of brain damage lead to very specific deficits Arnav Agarwal 2011  eg: not being able to identify fruits but being able to identify tools  if geons were involved, deficits should be in recognizing all types of objects based on their shapes, not a specific category of objects  however: it is also possible that geons could be processed at a different level of processing separate from the area of brain damage Template Theory - we store many different templates in memory - compare visual stimuli we receive to all the templates - if a match is found, then object is familiar and person can name it by activating connections to other language areas in the brain - if not: unfamiliar object; new template is stored in memory for it --> - Problems:
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