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Chapter 5

Sensation and Perception Psych 367 Chapter 5.docx

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University of Alberta
Douglas Wylie

Chapter 5: perceiving objects and scenes Why is it so difficult to design a perceiving machine The stimulus on the receptors is ambiguous - The image cast on your retina is ambiguous - Inverse projection problem: the fact that an image on your retina can be created by many different objects - Information from a single view of an object can be ambiguous o Humans solve this problem by moving to different viewpoints o And by making use of knowledge they have gained from past experiences Objects can be hidden or blurred - People easily understand that the part of an object that is covered continues to exist - People can detect objects that are not in sharp focus, such as faces Objects look different from different viewpoints - Another problem for a perception machine is objects are often viewing from different angles - This mean that the images of objects are continually changing depending on the angle from which they are viewed - Viewpoint invariance: ability to recognize objects seen from different viewpoints - Gestalt: a whole configuration that cannot be described merely as the sum of its parts The gestalt approach to object perception - Wundt’s approach was called structuralism o Perceptions are created by combining elements called sensations, - Apparent motion: when two stimuli that are slightly different positions are flashed one after another with the correct timing, movement is perceived between the two stimuli - Illusory contours: perceive faint corner that represent the edges of a cube o Covering the circles or seeing the cube through the holes causes the illusory contours to vanish - structuralists have a hard time explaining illusory contours because there is no actual contour, so there cant be any sensations where the contour is perceived - bistable figures: like a cube that can switch back and forth as they are viewed gestalt laws of perceptual organization - perceptual organization: involves the grouping of elements in an image to create larger objects good continuation - law of good continuation: points that connect, result in straight or smoothly curving lines are seen as belonging together, o follows the smoothest past proximity - law of proximity: things that are near each other appear to be group togethercommon region - principle of common region: elements that are within the same region of space appear to be grouped together uniform connectedness - principle of uniform connectedness: a connected region of visual properties, such as lightness, is perceived as a single unit synchrony - principle of synchrony: visual events that occur at the same time are perceived as belonging together - occurs without movement common fate - law of common fate: things that are moving together in the same direction appear to be grouped together meaningfulness or familiarity - law of familiarity: things that form patterns that are familiar or meaningful are likely to become grouped together perceptual segregation: how objects are separated from the background - perceptual segregation: of one objects from another - figure ground segregation: what causes perceptual segregation - figure: the object that we see - ground: the figure stands out from the background what are the properties of figure and ground - reversible figure ground: it can be perceived alternately either as two blue faces looking at each other, - properties of the figure and ground are o figure is more thing like, and more memorable then the ground o figure is seen as being in front of the ground o ground is seen as unformed material and seems to extend behind figure o the contour that separates the figure from the ground belong to the figure what factors determine which area is figure - regions in the lower part of a display are more likely to be perceived as figure than regions in the upper part - vercera o flashed stimuli and ask observers to indicate which are they saw as the figure o there is no left or right preference - 4 factors that help determine the figure o symmetry is seen as the figure o the smaller area is more a figure o vertical-horizontal cross is figure o meaningfulness is the figure the gestalt laws as heuristics - heuristics: rules of thumb that provide a best guess solution to a problem o may not me right every time o usually faster - algorithm: procedure that is guaranteed to solve a problem recognition by components - our recognition of objects is based on features called geons - 36 different geons - Non accidental properties (NAPs): properties of edges in the retinal image that correspond to the properties of edges in the 3D environment - Property of curvature o NAP because the only time it doesn’t occur is when you view at a certain angle o Accidental viewpoint: when you cant see the NAP - Discriminability: Each type of geon has a unique set of NAPs - Viewpoint invariance: geons can be identified from a number of views - Principle of componential recovery: ability to identify an object if we can identify its geons - Our ability to identify objects depends on our ability to identify the objects geons - We can recognize objects based on relatively small number of geons - RBC cant explain because the theory is not meant to explain everything about object perception - It doesn’t deal with the processes involved in grouping objects Perceiving scenes and objects in scenes - Scene: view of a real world environment that contains o Background elements o Multiple objects that are organized in a meaningful way relative to each other - Objects are compact and acted upon - Scenes are extended in space and are acted within Perceiving the gist of a scene - Gist of a scene: Despite the size and complexity, you can identify most scenes after viewing them for only a fraction of a second - Possible to perceive the gist of a scene within a fraction of a second - Mary potter: o Showed observers a target picture and asked them to indicate whether they saw that picture as they viewed a sequence of 16 pictures o Almost 100% accurate - Li Fei fei: o Presented picture of scenes for times ranging from 27ms to 500ms and asked observers to write a description of what they saw o Masking: to ensure observer saw the picture for the exact desired duration o 67ms they could identify large objects o 500ms they could identify small objects - olive and torralba o global image features: which can be perceived rapidly and are associated with specific types of scenes  degree of naturalness: natural scenes, such as the beach • compared to man made scenes  degree of openness: open scenes such as the beach have a visible horizon line and few objects  degree of roughness: smoothness  degree of expansion: convergence of parallel lines, like what you see when you look down railroad tracks • dependent on the obser
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