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Chapter 3

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McGill University
PSYC 213
Jelena Ristic

CHAPTER 3: PERCEPTION 3.1 Introduction  Illusion of clarity – an example: o A grid placed over a blocky image makes the image appear clearer than the same image without the grid. But a grid placed on a high-resolution image decreases clarity.  Visual Agnosia: The eyes are fine but unable to recognize what is seen. o But visual recognition is not always completely impaired. o Impaired recognition only applies to vision. Patients can identify objects through other sense like touch.  Associative Agnosia: A form of visual agnosia. Patients can‟t recognize objects even though they can reproduce the objects by drawing them.  Time Spaces: The perceptual experience of time units such as days of the week or months as occupying spatial locations outside of the body. o This experience is automatic, goes with them wherever they go, and cannot be consciously inhibited.  Perception: Processing sensory information such that it produces a meaningful understanding of the information.  Stimulus: An entity in the external environment that can be perceived by an observer. 3.2 Perception as a Function of the Environment  Theory of Ecological Optics – Gibson: o Perception involves directly absorbing the visual information present in the environment.  Ambient Optical Array (AOA) (Gibson): o All the visual information that is present at a particular point of view. o Gibson said that every different viewing point has a unique pattern of light that enters the eyes. o Gibson thought that perception was accomplished mostly by the sensory organs.  Texture Gradients: Gradual changes in the pattern of a surface that is normally assumed to be uniform, which provides information about the surface characteristics such as whether the surface is receding or curved.  Topological Breakage: The discontinuity created by the intersection of two texture gradients. o A useful indicator for the edges of objects.  Scatter-reflection: The degree to which light scatters when reflected from a surface. o How light scatters from objects tells us a lot about the nature of the object‟s surface.  Gibson thinks that the problem with classical theories of perception is that they relied on assumptions of a fixed, monocular perspective. But, with illusions as an example, the moment you allow an observer to move about, the illusion vanishes.  Transformation: The change of optical information hitting the eye when the observer moves.  Optic Flow Field: The movement of objects or the observer through the environment produces changes in what is seen. o When you look out the front windshield while driving, the objects straight in front of you appear stationary but those at the edge move faster. 3.3 Pattern Recognition  Differs from Gibson‟s theories in 2 ways: o Theories of pattern recognition do not consider the complex array of light information reflecting from all surfaces and objects. o They focus on how it is that we build internal representation of objects during the process of recognition.  Pattern Recognition: The ability to recognize an event as an instance of a particular category of event.  Percept: Meaningful interpretation of sensory information.  Recognizing a configuration involves contact between the emerging percept and memory.  Memory Trace: The trace that an experience leaves in the brain.  To recognize a letter “a,” your emerging perception of “a” must make contact with the memory trace of “a.” o This process is called the Hoffding function: When an experience makes contact with a memory trace, resulting in recognition. Template Matching  Template: A model against which a stimulus is compared to determine whether it belongs to a particular category.  Prototype: A model that possesses all the typical characteristics of its class.  Template matching theory: Stimulus is compared to template and if they match the stimulus belongs to that category. o Problem: How to specify a template can match not only patterns that are identical to it, but also patterns that are similar enough to it.  Multiple-trace memory model - Hintzman: Traces of each individual experience are recorded in memory. No matter how often a particular kind of event is experienced, a memory trace of the event is recorded each time. o This approach distinguishes between primary and secondary memory:  Primary memory: what we experience at any point in time.  Secondary memory: all the memory traces created out of all the experiences we‟ve had.  Primary memory → probe → secondary memory → activated memory trace → echo back to primary memory → echo leaves memory trace in secondary memory  Experiment with prototypical patterns showed that: o Prototypical patterns were quite well classified, even though they had never been seen before. o Sometimes participants falsely recognized the prototype as a pattern they had seen before, even though they had previously only seen distortions of the prototype.  Hintzman‟s explanations for these results: o Memory traces of the set of distorted patterns produce an echo which contains what the different distortions have in common and the prototype is recognized. Feature Detection  Feature Detection Theory – Selfridge: o Detecting patterns on the basis of their individual features.  Pandemonium: A model of pattern recognition consisting of three levels: data (e.g. image and feature) cognitive demons, and decision demons! o Data level: features such as size, color, shape… o Cognitive demons: Feature detectors that decide whether the stimulus matches its pattern.  The more similar the pattern in the image is to the one that the demon is looking for, the louder this demon shouts. o Decision demon: selects the cognitive demon that is shouting the loudest and this choice constitutes the pattern that is recognized.  Black letters on a white background have more contrast energy than grey letters on white background. o To identify a word that is flashed on a screen for 200 milliseconds, greater contrast energy is needed when there are more letters in the word. o Letters with low contrast energy are weak signals that the visual system may “squelch.”  Squelching: Nervous system inhibits processing of unclear features. Recognition by Components  Recognition by Components (RBC) - Biederman: A model of perception based on subdividing objects into a basic set of geometric shapes.  Geons: The basic geometric shapes that compromise objects. o There are 36 geons.  The theory says that once objects are reduced to their constituent geons, then these geons are compared with existing geon configurations stored in memory.  Object recognition should be a function of the number of geons available to perception.  When people were shown 2/3 geons, accuracy of recognition was 80%.  Recognition improved with the addition of more geons.  More complex objects were recognized more efficiently than simpler ones.  Recoverability: The degree to which geons can be made out in a degraded image of an object. o Obscuring geons by making them unrecoverable reduces object recognition. 3.4 Context & Knowledge  Bottom-up = Data-Driven Processing: When perception results from the combination of individual pieces of sensory information. o Not enough to explain illusions.  Top-Down = User-Driven Processing: When perception is driven
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