pl3103-week2-perception-handout.pdf

PL3103 Cognitive Psychology Dr. Cynthia Siew Today’s Topic: Perception (Week 2) After today’s lecture, you will learn about: 1. The difference between sensation and perception. 2. Fundamental principles of perceptual organization from the Gestalt psychologists. 3....

PL3103 Cognitive Psychology Dr. Cynthia Siew Today’s Topic: Perception (Week 2) After today’s lecture, you will learn about: 1. The difference between sensation and perception. 2. Fundamental principles of perceptual organization from the Gestalt psychologists. 3. Theoretical frameworks and experimental findings that explain the processes involved in pattern recognition, ob- ject recognition, and face recognition. You will also gain an appreciation for: 1. Your own object and face recognition abilities, in contrast to those with perception disorders. 2. Why we find visual illusions and ambiguous images com- pelling, surprising, and funny. Sensation vs. Perception Sensation: The intake of information by receptors, and the translation of this information into signals that the brain can process as images, sounds, smells, tastes, etc. Perception: The interpretation and understanding of these sensations. 1 In this lecture we focus on visual perception, as vision is our most dominant sense. What is shown in this image? 1. ask students to enter their guesses into pollev 2. use marker to outline the image there is a similar example of the cowboy in the textbook. Visual illusion #1 Video of dog face that is actually a cat. Visual illusion #2 How do we transition from sensation to perception? Usually this is such a fast and automatic process that we do not even notice it! 2  Ambiguous or degraded stimuli, or visual illusions, like the previous example, illustrate that perception and sen- sation are not the same process. Laws of Perceptual Organization The Gestalt Psychologists’ key contribution to psychology is the law of Pragnanz, which states that: what we perceive is the simplest possible organization of the visual environment. The percept (“figure”) is more than the sum of sensations. “Gestalt” is German for “figure”. The Gestalt Laws (a) law of proximity (b) law of similarity (c) law of continuation (d) law of closure 3 law of continuation, and closure law of similarity (colors) Figure-ground segregation The visual environment is separated into the figure, which has a distinct form, and the ground, which lacks form. The figure is also perceived to be “more important”, and to be in front of the ground. 4 5 6 Figure-ground segregation vs. Object recognition Gestalt psychologists argued that figure-ground segrega- tion (early process) occurred before object recognition (late process). Recent studies suggest that this is not necessarily the case: If you recognize the object, knowledge of the object helps you with figure-ground segmentation! For an example, think back to the black and white image on Slide 5. Did knowledge of the dog better help you segment the image into figure and ground? Evaluation of Gestalt psychologists Positive aspects 7 8  Discovered important underlying principles of perceptual organization Negative aspects De-emphasized contribution of experience and top-down knowledge in perception Not able to explain underlying processes of perception What can we learn from visual illusions? Pick one of the illusions shown previously and write down a few sentences describing what you think we can specif- ically learn from it. Discuss your answers with the person next to you. (specific students) Paste your answers in the Poll Every- where link. Pattern recognition Pattern recognition refers to the identification of two- dimensional patterns. This is a necessary step prior to object recognition. Pattern recognition requires the matching of the stimulus to a category of objects stored in memory. 9  This is challenging because the stimulus likely does not fully match with what’s in our memory! Theories of pattern recognition Template theories a pattern is recognized when it closely matches with a template stored in memory a template is a form or pattern that is stored in long-term memory Feature theories a pattern consists of a set of features or attributes patterns are matched when they share the same set of features Theories of pattern recognition Limitations of template theories unrealistic; especially when the stimulus can take on dif- ferent forms, or viewed from different perspectives Limitations of feature theories incorrectly assumed that local processing occurs before global processing (see Navon (1977) study) Hubel and Wiesel’s discovery of feature detectors if we process the basic features of a visual stimuli, we should be able to detect brain cells that process these basic features Hubel and Wiesel discovered the presence of simple and complex cells in the occipital cortex 10 11  simple cells: absence of presence of very simple features like horizontal or vertical lines at a particular location in the visual field complex cells: responsive to features independent of lo- cation and to combinations of features transition from features to objects Post-video reflection questions https://www.youtube.com/watch? v=RPv0a9ftu6Y Were you fooled by the illusions? (be honest!) The presenter argues that your visual experiences and environment shape your perception of illusions. What are the implications of this claim? [pollev] Contribution of top-down processes Object superiority effect: The finding that a feature is easier to process when it is part of a meaningful object (than when it is part of an unknown form). 12 Why does this occur? Knowledge of the object provides useful context for interpreting specific features. for these illusions, the key point to make is that when you know what the image is about - that helps you interpret specific parts of the face as the trumpet, and vice versa. 13 14 Summary of pattern recognition Both bottom up and top down processes matter and interact together for pattern recognition to occur. Top-down effects become more apparent when the stimuli is ambigious. Template or feature theory? Template theories provide an explanation for the fast recogni- tion of familiar stimuli, but too inflexible. Feature theories assume that processing of features occurs be- fore holistic processing, but de-emphasizes top-down or envi- ronmental influences. Object recognition Perceptual organization and pattern recognition work together to lead to object recognition, which occurs when a mental representation is activated strongly enough to be selected as the most likely interpretation of the stimulus. A key theory of object recognition is Biederman’s (1987) recognition-by-components theory. Biederman’s Geons: Recognition-by-components According to Biederman, edges are extracted from the stim- ulus (i.e., patterns) which are combined into basic shapes or components called geons (geometric ions). Geons are object primitives, the building blocks of object recognition. Object recognition relies on the identification of geons. Examples include: blocks, cylinders, spheres, etc. 15 What is this object? 16 17 What about this object? Experiment to remove contours to disrupt identification of geons object recognition is harder when parts of the contour providing information about concavities were omitted than when other parts of the contour were deleted. in the two cup examples, the same percentage of information was deleted, but where the information was deleted made a difference. Counter-evidence: What if objects do not have geons? Examples: Clouds, fire, only contours or silhouettes. Ambigu- ous or degraded images rely more on top-down knowledge and expectations. Foster & Gilson (2002): Object recognition depends on the “view” a. SAME (different viewpoint) 18 19  b. DIFFERENT (different object, same viewpoint) c. DIFFERENT (different object, different viewpoint) RT_c is slower than RT_b Evaluation of Recognition-by-components theory The classic theory assumes that invariant geon-like components are involved in object recognition and hence viewpoint-invariant Counter-evidence from Foster and Gilson (2002): we use both components and viewpoint information for object recognition – depends on the familiarity of the object (mediate top-down influence) Theory may not work for objects that do not have “geons” (like smoke or sand) Face recognition Good for familiar face, bad for unfamiliar More holistic than object recognition, evidence from: – 1.Part whole effect memory study – 2. Composite face effect - lecture example – 3. Face inversion - lecture example Composite face illusion is the top half of the face same or different? when matched with a different bottom half, the top half seem different evidence of holistic processing of faces, very hard to separate out parts of the face 20 21 Face inversion effect Face inversion effect Are you a super recognizer, or…? Face blindness, also known as prosopagnosia, is a condition where recognition of faces is severely impaired, but with little to no impairment of object recognition. Brad Pitt claims to be face blind (link) Famous Faces https://testmybrain.org/ Complete the Famous Faces test + report your results on PollEv. Relect on the Famous Faces test Write a few sentences reflecting on your experiences completing the Famous Faces test. Some helpful prompts: 22  were your results better or worse than expected? do you think the use of famous faces (of a particular cul- ture) is a good test of face recognition ability? if someone is unfamiliar with celebrity faces, how would you test for their face recognition ability instead? [pollev] Prof’s results https: //www.faceblind.org/research/ Disorders of perception Visual agnosia - problems recognizing objects (visually) Apperceptive agnosia: object recognition is impaired due to deficits in perceptual processing Associative agnosia: perceptual processes are intact but problems accessing knowledge about objects from mem- ory early vs late stages of object recognition Visual agnosia - Example 1 Visual agnosia - Example 2 https://www.youtube.com/watch? v=T1qnPxwalhw Questions to think about https://www.youtube.com/watch? v=dG8JGg-d2Pk Which patient has apperceptive agnosia and which pa- tient has associative agnosia? Explain your answers. How are the lives of these patients impacted by their vi- sual agnosia? 23 Today’s Topic: Perception (Week 2) After today’s lecture, you have learned about: 1. The difference between sensation and perception. 2. Fundamental principles of perceptual organization from the Gestalt psychologists. 3. Theoretical frameworks and experimental findings that explain the processes involved in pattern recognition, ob- ject recognition, and face recognition. You have also gained an appreciation for: 1. Your own object and face recognition abilities, in contrast to those with perception disorders. 2. Why we find visual illusions and ambiguous images com- pelling, surprising, and funny. See you next week for the lecture on “Attention”! 24