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Online Lecture - Form Perception 1 and 2 (different version).doc

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McMaster University
Christopher Teeter

Form Perception Gestalt Principle • Gestalt Philosophy o Whole greater than sum of the parts o Reaction to structralists (breakdown to each invd. Pt) o Perceive continuous motion o Motion is an emergent property of the sequence of pictures o Gestalt Principle: laws that describe how we organize visual input (innate or quickly acquired) • Figure-ground o Ability to detemrin what aspect is object and what is background o Reversible figures • Proximity o Elements that are close in space tend to belong together • Closure o Gaps in contour of shape we tend to fill in the gaps • Similarity o Tendency to group physically similar elements • Continuity o Lets us perceive simple continues form rather than combination of awkward forms o Flowers vase stems • Common Fate o Things that change in the same way should be grouped together o Eg school of fish same direction o Eg. Moth on bark Pattern/object Recognition • What a person expects to see can influence what they do see • 1 figure our what’s foreground what’s background nd • 2 parts are grouped together • Bottom up processing o Objects recognition is guided by the features that are present in the stimulus o Analyzing individual features and comparing those features to things with similar feature you have in memory • Object recognition is both bottom up processing and top down processing • Top down o Own beliefs expectation are primary influence to determine what you see. (ambiguous stimuli) o Cant work alone need some stimulus input o Yet can’t solely be bottom up, b/c expectations can influence perceptions • Priming o Experimenter measures how fast a participant can read a ward that is flashed. If word presented beforehand recognition is faster • Bi-directional activation – processing occurs both directions at once • Features of objects in combination with our expectations and guides object recognition Stimuli cognition • Biederman’s Geon Theory o 36 different geons ( simple geometrical forms stored in memory) o Eg. Cone, sphere, sylinder o Possible to recognize 150 million different objects o Some stimuli hard to define by geons o Cons:some brain damage -> specific deficits  Cant recognize fruits but can recognize tools • Template o We store many templates in memory o Object is compared to all objects in memory o Match – able to use language area in brain to identify o New match – stored in memory • Prototype theory o Store most typical or best example of an object in memory o Flexible – don’t’ need exact match o We can recognize favorites so its likely that we have more than 1 type of representation for each object • Paralell processing o Diff brain systems process diff stimuli Perceptual Constancies • Define: our ability to perceive an object as unchanging even though the visual image roduced by the object is constantly changing • Shape constancy – perceive objects to have constant shape o Variation in our perspective change • Location constancy – stationary even though moving o Compensatin fro movement of eyes and body • Size constancy – size unchanging, front back further closer o Compensating for variation in distance • Brightness constancy – ability to know the brightness of objects don’t change even though object reflects more light or less depending on ambient lighting conditions o Compensating for differences in ambient light amplitude • Colour constancy – perceive objects to have constant colour even though light stimulus that reaches retina may change in different illumination conditions o Compensating for differences in ambient light wavelengths • Recognition b/c o Existing knowledge top down o Cues in scene – clues to perceive constancy in object  Depth cues – shape how we perceive friend Visual Illusions • Muller lier o Misapplying size constancy and inaccurately interpreting depth o Size constancy • Ames room o Trapezoid shape o One corner farther away from your point of view o Two people standing on each corner, one looks shorter than the other o Size constancy • Ponzo o Converging lines give a sense of depth Perception is an active process Same race – use holistic Form Perception 2 Feature detectors • Magno and Parvo cells o Transduce light to neural impulse o Magno – mainly periphery changes in brightness and motion and depth o Parvo – throughout retina, colour pattern form o Small receptive fields o Optic nerve > LGN > Primary visual cortex o Cells very specific in what activates them ( called feature detectors) • Hodgkin and Huxley o How to detect activity on indv. Neuron in squid o See how invd. Neuron responds to specific stimuli • Lettvin o Moving black dots – found a neuron • Hubel and Wiesel o Cells in cats and monkeys o Used more complex stimuli than flashes of light o Used lines of different orientations and thicknesses that moved in different directions, each neuron is specific about peak rate of firing o Needs a certain shape size position movement and this defines the receptive field for the cell • Simple cells o Respond maximally to bar of certain light and orientation in a particular reg
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