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Chapter 4c - Form and Perception Video Lecture Psych 1XX3

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McMaster University
Joe Kim

Chapter 4: Sensory Systems Form Perception I Gestalt Principles  Gestalt Psychologists o 1920's and 1930’s, German Psychologists o “The whole is greater than the sum of its parts” o Believed that people perceive the whole stimulus rather than each individual part o A reaction to the structuralism approach  Motion is an emergent property of the sequences of pictures o Perceive continuous movement in a rapid sequence of still pictures, even though there isn’t continuous movement in or across any of the pictures  Gestalt Principles – laws that describe how we organize visual input o Thought that they are innate or acquired rapidly 1. Figure-Ground – the ability to determine what aspect of a visual scene is part of the object itself and what is part of the background o In simplest scenario – small, enclosed figure with distinct borders or edges that is surrounded by a large region (the background)  Edges and borders give figure a perceptible form – perceived as being in front of the background  Background normally formless or made up of multiple forms o Reversible Figures – makes determining what is figure and what is background difficult 2. Proximity – elements that are close together in space tend to belong together o Eg/ XX XX XX XX XX  You group the close X’s together 3. Closure – refers to the fact that if there are gaps in the contours of a shape, we tend to fill in those gaps and perceive a whole object o Eg/ Dashed lines – know it’s a rectangle 4. Similarity – the tendency for us to group together elements that are physically similar o Eg/ Drive by rows of corn and rows of sunflowers – able to group together the vegetation of the same type o Eg/ X O X O X O X Able to group the rows of X’s and rows of O’s X O X O X O X X O X O X O X X O X O X O X 5. Continuity – lets us perceive simple, continuous form rather than a combination of awkward forms o Eg/ X is perceived as two continuous lines (/ and \) as opposed to a combination of dots (. .) o Eg/ Flowers in a vase; stems may cross but can perceive individual stems as a continuous lines 6. Common Fate – things that change in the same way should be grouped together o Tend to group elements that are moving together in the same direction at the same time – strong enough to perceive a group of elements as an object on its own o Eg/ School of fish o Eg/ Moth on bark – hard to distinguish when moth isn’t moving; once it moves, elements within the moth pattern that are moving together in the same direction at the same time allow the contour of the moth to be perceived Pattern/Object Recognition  Expectation – what a person expects to see can influence what they do see o Eg/ Costume ball vs. Marine land picture o Eg/ Two Faces vs. Vase picture  The preliminary steps in object recognition involve identifying what aspect of the scene is the figure and what is the background o The parts of the figure are then identified and grouped together into a single object  Bottom-up Processing – object recognition is guided by the features that are present in the stimulus o Eg/ Recognizing a cow; four legs, goes “moo”, has an udder, a big nose, two long ears on the side of its head and two big eyes – recognize what you see by analyzing individual features and comparing them to things with similar features that you have in memory  Top-down Processing – object recognition is guided by your own beliefs or expectations o Eg/ A, 13, C vs. 11, 12, 13  B and 13 are made up of the same structure o Eg/ Priming  Experiment measures how fast a participant can read a word that is flashed on a screen  If you tell a participant the next word is going to be an animal – they process words like dog and duck faster than log and luck  Shows that processing of a word is more efficient if the participant is primed o expect a word from a certain category  Bidirectional activation – where processing occurs in both directions at once o Both Bottom-Up and Top-Down Processing involved o The features of the object in combination with our expectations guides object recognition  Theories of Object Recognition o Biederman’s Geon Theory  Suggests that we have 36 different geons (simple geometric forms) stored in memory  Using just these 36 geons, it is possible to recognize over 150 million different objects  Eg/ Ice cream come – cone with sphere  Problems  Objects for which it is difficult to determine which geon would be used, yet able to recognize o Faces o Crumpled Paper  In brain damaged patients lose ability to recognize different types of fruit, but can name different tools – if geons were involved in object recognition, deficits in recognizing all types of objects based on their shapes and not a specific category of objects would be expected o It is possible that geons are processed at a different level of processing separate from the area of brain damage o Template Theory  We store many different templates in memory  When we come across an object, we compare that object to all templates in memory  If match is found – familiar object; can name it by activating connections to other language areas in the brain  If no match is found – unfamiliar object; new template stored in memory  Problems  Would have to store an incredible number of different templates to recognize all of the different objects that we encounter o Prototype Theory  Store the most typical or ideal example of an object in memory  More flexible than template theory – don’t need an exact match between observed object and object stored in memory  Able to recognize common objects we haven’t seen before  Eg/ New dog or coffee mug  Able to recognize specific individual objects  Eg/ Pet dog, or favorite coffee mug  Importance of Parallel Processing o Different brain systems process different components of the visual signal simultaneously Perceptual Constancies  Five Perceptual Constancies o Perceptual Constancy – our ability to perceive an object as unchanging even though the visual image produced by the object is constantly changing 1. Shape Constancy – the fact that we perceive objects to have a constant shape, even though the actual retinal image of the shape would change as your point of view changes or as the object changes position o Eg/ Door – perceived as rectangular; only produces rectangular retinal image when you look at it straight on, when door opens, it is not perfectly rectangular – still perceived as a rectangle 2. Location Constancy – despite constant movement, we perceive objects around us as stationary o Eg/ Driving – entire scene is moving fast in retinas, but we don’t perceive the objects in the scene to be moving 3. Size Constancy – tend to see the size of objects around us as unchanging, even though as they vary in distance the size of the retinal image produced can vary o Eg/ Person walks away – even though the retinal image gets smaller, they don’t appear to be shrinking – just getting farther away 4. Brightness Constancy – ability to know that the brightness of objects around us does not change even though the object may reflect more of less light depending on the ambient lighting conditions o Eg/ Perceive coffee to be the same brightness whether we see it outside in the sun, or inside  Black looks black and white looks white regardless of whether we are inside under low illumination or outside on a bright day – yet black object outside reflects more light than white objects inside o Eg/ Checkerboard with two squares same color example 5. Color Constancy – the way we perceive objects around us to have a constant color even though the light stimulus that reaches the retina may change with different illumination conditions o Eg/ White dog under a red light – you still know the color of your dog  Explaining Perception Constancies 1. Existing Knowledge  Depends on knowledge about objects – can provide top-down influence on how we see those objects  Eg/ Know our friend is a constant size  Eg/ Our coffee mug is a particular brightness  Eg/ Our dog is a certain color  Eg/ Buildings don’t move,  Eg/ Doors don’t morph into different shapes as we open the, 2. Cues in scene  Perception constancy occurs because we know that certain properties of objects do not change and our perceptual system automatically factors in other cues in the environment that gives us information about the object of interest  Eg/ Might use depth cues to determine that our friend is far away and to shape how we perceive our friend in the context  Eg/ Under red light, everything takes on reddish cast and our brain can use information from the rest of the scene to fine tune our perception of objects in the scene  Eg/ As you drive toward a stationary bus you know the bus isn’t moving toward you – brain integrates the motion of the elements in the scene  If the bus was moving toward you, the scene would remain stationary  When everything in the scene is moving toward you, the brain determines that the movement is yours and adjusts how you perceive the scene accordingly Visual Illusions  Brain is tuned to recognize a variety of objects in different situations – sometimes makes mistakes  Visual Illusions – ambiguous or partial information; we use perceptual strategies which work most of the time but don’t belong in these situations  Eg/ Dog with reddish cast in scene where nothing else has a reddish cast –fail to recognize own dog  Muller-Lyer Illusion – which line looks longer?  both same length o One explanation - example of misapplying size constancy and inaccurately interpreting depth  Angled lines on top of the vertical lines look like a corner – one pointed toward and one pointed away from you  Since the two lines give the same retinal image but one is closer to you than the other, the closer one is perceived as shorter o People from cultures that are not surrounded by right angles are less susceptible to the Muller-Lyer illusion  illusion is partly due to cultural and experience dependent processes  Ames Room o Room looks like a normal rectangle, but it is trapezoidal in shape – one corner is much farther away from point of view than the other corner o If you have two people of equal height standing in each corner, the one standing in the farther corner looks smaller than the person standing closer to you o Since you believe the room to be a normal rectangular room of normal height – you interpret the scene as though each person is the same distance from you o Normally leads to size constancy – can be tricked by the cues normally used for distance constancy  Ponzo Illusion – which line is longer?  same length o Top line looks longer o Two vertical lines are converging, gives a sense of depth o Tend to perceive the top of the lines, where they have converged the most, as farther away than the bottom of the lines where they’re further apart o Using depth cues to gauge size Form Perception II Feature Detectors  Magno and parvo cells in the retina transduce the light stimulus into a neural impulse o Magno cells  Found mainly in the periphery of the retina  Used for detecting changes in brightness, motion and depth o Parvo cells  Found throughout the retina  Important for detecting color, pattern and form  From the retina, the axons of these cells exit the eye via the optic nerve, travel to the LGN and end up in the primary visual cortex in the occipital lobe  Feature Detectors – cells in the primary visual cortex; are particular about what make them fire  Hodgkin and Huxley – 1952 o Recorded the electrical activity in an individual neuron of the squid – paved the way for other researchers to use this technology to see how individual neurons respond to specific stimuli  Lettvin et al. – 1959 o Discovered a neuron in the optic nerve of a frog that responded only to moving black dots – called theses cells “bug detectors”  Hubel and Wiesel – earned Nobel Prize in 1981 o Studied cells in the visual cortex of cats and monkeys o 1962 – began exploration of the visual cortex by trying to learn what type of stimuli the individual cortical cells responded to  Put microelectrodes in the cortex of a cat to record the electrical activity of individual neurons as the cat was shown different types of visual stimuli, such as flashes of light  Problem – not getting much response from the neurons  Presented cat with a slide that had a crack – line projected from crack moved across the cats visual field  neurons started firing  Realized that neurons must respond to stimuli that are more complex than diffuse flashed of light  Began using lines of different orientations and thickness that moved in different directions – found that each neuron is very specific about what will make it fire the most  Cells fire maximally to stimuli of a certain shape, size, position and movement – defines the receptive field for the cell  Simple Cell – responds maximally to a bar of a certain length and orientation in a particular region of the retina  This simple cell responds the most to a horizontal bar  If bar is moved outside region, or changes orientation – cell is inhibited and fires less than baseline  The receptive field for the simple cell is
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