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Chapter 3

PSY270 Chapter 3.docx

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University of Toronto St. George
Kristie Dukewich

Chapter 3: Perception The Nature of Perception Perception: experiences resulting from stimulation of the senses Perceptions can… 1)…change based on added information (ex: Crystal‟s view became better as she got closer to the umbrella) 2)…involve a process similar to reasoning or problem solving (ex: Crystal figured out what the object was based partially on remembering having seen that umbrella the day before) In most cases, perception occurs so rapidly and effortlessly that it appears to be automatic. Perception occurs in conjunction with action (ex: running and perceiving simultaneously). Perception involves dynamic processes that accompany and support our actions. Perception Starts at the Receptors: Bottom-Up Processing Bottom-up processing: processing that begins with stimulation of the receptors Bottom-Up Processing: Physiological Stimulation of receptors triggers a series of events in which electrical signals are transmitted from the receptors toward the brain. Perception occurs after the electrical signals have reached the brain. (Neurons in the cortex that respond best to simple shapes like lines or bars with specific orientations are called feature detectors because they respond to simple features) The feature detectors‟ response is the first step in the brain‟s response to objects. Ex: when looking at an object like a tree, neurons in the visual cortex that respond to specific orientations fire to features of the tree, such as the trunk and branches. Bottom-Up Processing: Behavioural Recognition-by-components (RBC) theory (Irving Biederman): proposes that we perceive objects by perceiving elementary features called geons, which are perceptual building blocks that can be combined to create objects According to RBC… 1)…we can recognize an object if we are able to perceive just a few of its geons 2)…we can perceive objects even if portions of the geons are obscured  Principle of componential recovery: if we can recover (see) an object‟s geons, we can identify the object; when the object‟s geons are covered, we cannot recognize the object RBC provides an example of bottom-up processing because its basic unit – the geon – is simple and because perceiving simple geometric objects can be related to patterns of stimulation on the retina Beyond Bottom-Up Processing If perception were determined solely by bottom-up processing, then we could understand perception by considering only the information presented to the receptors. But perception depends on information in addition to that falling on the receptors, including knowledge that a person brings to the situation. Perception Depends on Additional Information Same geons can be combined to create different objects, so we are able to recognize these different objects based on the arrangement of their geons and to give these objects names because of knowledge we bring to the situation. Top-down processing: processing that begins with a person‟s prior knowledge or expectations Top-down processing is also involved in our ability to recognize objects based on just a few geons or when large portions of the object are obscured As signals from the receptors (that provide info about an object‟s basic features) travel to the brain, other signals become involved as well. Some signals provide info about other parts of the scene (context of object). Feedback signals: travel down from higher centers to influence incoming signals; are associated with a person‟s knowledge and expectations Perceptual examples can be used to demonstrate how the perceptual system takes additional info into account. Perceiving Size: Taking Distance into Account A large amount of research has shown that if two objects are perceived to be at different distances but case the same-sized image on the retina, the perceptual system takes distance of the farther object into account, so it is perceived as its true, larger size. In additional to depth, the perceptual system could also be taking into account the size of the object relative to other objects in the environment. Size constancy: we tend to perceive objects as remaining the same size even when they move to different distances Conclusion: perception of the size of an object does not depend solely on the size of the object‟s image on the receptors in the retina (farther objects = smaller images) Perceiving Odor Intensity: Taking Sniffing into Account The perceptual system takes sniff intensity into account when a person is perceiving odor intensity. Using Knowledge: Top-Down Processing The sound signal for speech is generally continuous, and when there are breaks in the sound, they do not necessarily occur between words. Speech segmentation: the ability/inability to tell when one word ends and the next one begins based on one‟s knowledge (or lack thereof) of a language Helmholtz’s Theory of Unconscious Interference Theory of unconscious interference: states that some of our perceptions are the result of unconscious assumptions that we make about the environment Likelihood principle: we perceive the object that is most likely to have caused the pattern of stimuli we have received Helmholtz described the process of perception as being similar to the process involved in solving a problem – the problem is to determine which object caused a particular pattern of stimulation, and this problem is solved by a process in which the observer applies his/her knowledge of the environment in order to infer what the object might be (usually unconscious). The Gestalt Laws of Organization 30 years after Helmholtz‟s theory, a group called the Gestalt psychologists proposed another approach. They were concerned with perceptual organization – the way elements are grouped together to create larger objects. Laws of perceptual organization indicate how elements in the environment are organized or grouped together. Starting point: things that usually occur in the environment Law of good continuation: points that, when connected, result in straight or smoothly curving lines are seen as belonging together, and the lines tend to be seen in such a way as to follow the smoothest path. Also, objects that are overlapped by other objects aare perceived as continuing behind the overlapping object Other Gestalt laws:  Pragnanz o “Good figure” from German o Law of pragnanz/good figure/simplicity: every stimulus pattern is seen in such a way that the resulting structure is as simple as possible  Similarity o Law of similarity: similar things appear to be grouped together  Ex: if two shapes are of the same colour, they are perceived to be part of the same object  Meaningfulness or Familiarity o Law of familiarity: things that form patterns that are familiar or meaningful are likely to be grouped together  It is then very hard to not perceive them as such The Gestalt “Laws” are “Heuristics” The Gestalt “laws” do not always predict what is in the environment. Because of the fact that the Gestalt laws can sometimes lead to incorrect perceptions, it is more accurate to call them heuristics – rules of thumb that provide a best-guess solution to a problem. Algorithm: procedure that is guaranteed to solve a problem (ex: procedures learned for solving addition, subtraction, and long division) Heuristics are faster than algorithms. Taking Regularities in the Environment into Account Regularities in the environment: characteristics of the environment that occur frequently We can distinguish two types of regularities:  Physical regularities o Regularly occurring physical properties of the environment o Oblique effect: people can perceive horizontals and verticals more easily than other orientations (because those occur more frequently in the environment) o Light-from-above heuristic: the assumption that light is coming from above; made because most of the light in our environment comes from the above  Light from above = bulging out circles  Light from below = indented circles  Semantic regularities o Applied to perceiving scenes, semantics refers to the meaning of a scene. o Semantic regularities: the characteristics associated with the functions carried out in different types of scenes (ex: in an office, in an airport, etc.) Neurons and Knowledge About the Environment There is neural activity behind every behaviour, and research has demonstrated connections between neural activity, the nature of the environment, and perception by showing that there are neurons that are tuned to respond best to things that occur regularly
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