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Chapter 5: Object Perception

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Psychology 2115A/B
Christine Tsang

Chapter 5 Review: Object Perception By the end of this section, you should know: - Why “the whole differs from the sum of its parts” - How “rules of thumb” help us - How we distinguish objects from their background - Why even the most sophisticated computers are unable to match a person‟s ability to perceive objects  We usually think about vision is as though it works as a camera – we have a static visual image of the world that comes in on the retina and then the retinal image is processed  But in fact, this camera analogy breaks down once you get past the retina  Human vision is actually a 3D view of the world  One of the keys to human vision is invariance – our ability to see the world as it actually is vs. how it is represented in an image  Our visual world in human vision is invariant to a wide range of changes that span changes in levels of light, changes in size, and changes in shape  Not simply recording images, our vision is actually transforming the simple property of light into mental constructs of a 3D world in which we all live  Doing this transformation of what is physically present on the retina and what is mentally represented in our mind, this is the trick and this is what we cannot get computers to do  Our minds are transformational and creative (fills in missing pieces)  Need to think of vision as holistic, not separate pieces The Challenge of Object Perception  Stimulus is ambiguous on the receptors o Lots of different objects can create the same image on the retina o Different objects from the same viewpoint are ambiguous o The way we resolve the ambiguity is to try to manipulate the object and observe it from lots of different viewpoints  Objects can be hidden or blurry o We can recognize objects even if part of them is hidden or obscure o We still recognize the object as what it is even though a large part of the object is obscure or hidden o We use our knowledge of what objects are supposed to look like – computers have not been able to do this o Are able to recognize objects that are not in good focus – computers find this a relatively difficult task – computers looking for certain points on the face whereas humans do much more  shows the difficulty of the visual task that we deal with every day  Objects look different from different viewpoints o The basic problem is that images of the same object are continually changing as we change our viewing angles o Suggests we are not deconstructing the visual world and then putting it back together like our physiology suggests Gestalt Psychology  Gestalt = a configuration or idea o Before Gestalt psychology, it was thought that all of our sensations that we experience sum up to create perception  structuralism: the idea we can break down our perception of the world into its constituent bits and pieces and then we can put all those pieces back together again to create perception  Strategies of perceptual organization o Despite the fact that objects are hidden, blurry, changing from different viewpoints, etc. this suggests that it can‟t all be about sensation, it has to be something more than this o Gestalt school was formed – believed that perception that creates our understanding of form is not a property of the object itself but rather it represents some sort of organization that we impose upon those sensations in the brain o Suggesting that the representation of the 3D form is really created by organizing our sensations into stable patterns of perception  perceptual constancies o In order to create these, our brain has to make specific assumptions about how the world or have certain expectations about our perceptual experience – partially derived from how our brain is organized physiologically (rate of neural firing) but also from our experiences form the everyday world (we learn about how the world is supposed to be) o „The 5 phenomenon‟/„apparent motion phenomenon‟: dots can appear to move even though there is no actual movement  therefore, it can‟t just be about sensations since there is no sensation between the dots but we still perceive them as actually moving Wertheimer vs. Structuralism  The whole differs from the sum of its parts o It must be more than a summation of all the senses that are going on, but it must be some sort of organization what we are adding to it as well  Apparent motion  Illusory contours: contours that are not present in the stimulus but are still perceived o Imposing lines that don‟t actually exist o It must be more then sensation on our receptors because the receptors aren‟t receiving stimuli from contours Laws of Perceptual Organization  Pragnanz, “good figure” or “simplicity” o Every pattern that we encounter should be seen in such a way so that the resulting pattern is the most simple one possible o Classic example: the Olympic rings – simpler to view them as five circular rings intertwined them all the separate bits of objects stuck together  Similarity: similar things tend to be grouped together  Continuation: lines tend to be seen in such a way that they follow the smoothest path  Proximity: we tend to perceive dots that are closer together as belonging to the same object o Things that are near each other are grouped together  Common fate: objects that have a similar end outcome are likely to be grouped together  Familiarity: things that are familiar tend to be grouped together o Using previous knowledge of what objects ought to be like as a means of recognizing what the object is Figure and Ground  Winner-take-all strategy: o “Our eyes are accustomed to fixing on specific objects. The moment this happens everything around is reduced to background… The human eye and mind cannot be busy with two things at the same moment, so there must be a quick and continual jumping from one side to another” – M.C. Escher o One object is the one that we focus on and that is the object that becomes the „figure‟ of the image and the rest of the picture becomes the „background‟ or „ground‟ o We can‟t attend to two objects at the same time – only one have one figure at a time Properties of Figure and Ground  The figure is more “thing-like” and more memorable than ground  The figure is seen in front of the ground  The ground is more uniform and extends behind figure  The contour separating figure form ground belongs to the figure (border ownership) Factors that Determine Which Area is Figure 1. Elements located in the lower part of displays 2. Units that are symmetrical 3. Elements that are small 4. Units that are oriented vertically 5. Elements that have meaning  Interplay between how we like to perceive objects and also which object we attend to at any given time Are Gestalt Laws Really “Laws”?  Heuristics vs. algorithm o When we use the word „law‟ we tend to think of something that is unchangeable o More so heuristics than laws o Heuristics sometimes work but laws are algorithms that always work o Gestalt laws derive partially form experience, sometimes they can fool us which is why these illusions works  therefore are heuristics and not laws o Algorithms are rules that always work – you will always come to the answer sooner or later o Gestalt laws usually work because the world is usually organized using these patterns Recognition by Components Theory  While Gestalt psychologists gave us interesting ideas of perception, it is difficult to come by hard evidence to test it  Objects are recognized by volumetric features called geons o Theory proposes there are 36 geons that combine to make all 3D objects o Geons include cylinders, rectangular solids, and pyramids Properties of Geons  Proposed that geons have specific properties  View-invariant properties: aspects of the object that remain visible from different viewpoints o We know what these objects look like from all
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