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is What Perception? The ability to organize, recognize, and make sense of the world from the sensory input. Perceptual processes extract information from the arrangement of individual stimulus elements in space and time. Perception Perception--Outline I. II. Psychophysics Perceptual Organization A. Gestalt Psychology B. 3-D Form and Depth Perception C. Perceptual Constancies III. Bottom-up and Top-down Processing I. II. Perception--Outline Psychophysics Perceptual Organization A. Gestalt Psychology B. 3-D Form and Depth Perception C. Perceptual Constancies III. Bottom-up and Top-down Processing Psychophysics Psychophysics--the study of the relationships between physical stimuli & psychological responses. Structuralists Example: How dim a light can be detected? Detection "Is there a stimulus present?" Absolute threshold: smallest amount of stimulus energy that can be detected (faintest detectable stimulus). subthreshold < threshold < suprathreshold 1 Thresholds Difference Threshold Difference Threshold--How much must you change a stimulus in order for a difference to be detected? Procedure: first present standard stim (always the same intensity) then comparison stim (different intensities). Absolute Threshold Intensity of a stimulus (e.g., light) that is sufficient for detection 50% of the time. threshold Subject asked to report if comparison stimulus is same or different than the standard. Psychophysical function Discrimination "Are the stimuli different?" Difference Threshold --Results look like Absolute Threshold except y-axis = % detection of difference and x-axis = size of difference between stimuli. --Difference Threshold is the size of the difference which can be detected 50% of the time. Discrimination threshold: smallest difference in stimulus energy that can be discriminated between two stimuli. also known as the difference threshold or just-noticeable-difference (jnd) Thresholds Detection % detection of difference % detection Difference Threshold The jnd is smaller for less intense standard stimuli and larger for more intense standard stimuli. Weber's law--the jnd between stimuli is a constant proportion of the magnitude of the standard stimulus: jnd = k I. (where k = Weber Fraction and I = stimulus intensity) Is different for different sensory tasks: smaller k = greater sensitivity to stimulus differences Discrimination threshold threshold stimulus intensity difference in intensity 2 Difference Threshold Example: jnd = k I k = 0.1 for line length judgments if I = 10 mm jnd = ? if I = 100 mm jnd = ? Difference Threshold Example: jnd = k I k = 0.1 for line length judgments if I = 10 mm jnd = 1 mm if I = 100 mm jnd = 10 mm Thus, a typical subject can barely discriminate between a 10 and 11 mm line or between a 100 and 110 mm line. The smallest detectable difference between two stimuli may be measured with the difference threshold method Difference Threshold Perception--Outline I. II. Psychophysics Perceptual Organization--how the perceiver organizes the world. Gestalt Psychology Describes various ways our perceptual system tends to organize or group elements into whole objects. Gestalt Psychology was a reaction against Structuralists. Emphasized that our visual system actively organizes world into objects vs. just registering the brightness levels and color of stimuli. "the whole is different from the sum of the parts" => e.g., phi-phenomenon (demo) A. Gestalt Psychology B. 3-D Form and Depth Perception C. Perceptual Constancies III. Bottom-up and Top-down Processing 3 Figure-Ground: automatic tendency to see some parts of a scene as objects and other parts as background. Figure = the parts of the scene that stand out from other parts as sharply delineated, distinct shapes. --Figure is usually the smaller, surrounded field (located in front of the ground). --Contour seems to belong to the figure. Ground = usually the larger, enclosing field. Extends behind the figure. Figure-Ground Figure-Ground: edges belong to the figure Reversible Figures (figure-ground alternation) Escher's "Day and Night" Grouping Principles: simple rules by which we perceive shapes and forms Proximity Similarity Closure Good Continuation Common Movement/Fate Good Form 4 Grouping Principles Proximity--tend to see stimulus elements that are near each other as part of the same object. Similarity--elements that are similar tend to be grouped together. Grouping Principles Closure--We tend to supply missing elements to close/complete a familiar figure. Good Continuation--We tend to see elements in ways that produce continuous lines with minimal change of direction. Common Movement/Fate -- When elements are moving in the same direction at the same rate, we tend to see them as part of a single object. Good Form or Simplicity--perceptual system strives for the simplest interpretation. Seen as 2 objects Seen as 1 object because of symmetry Seen as a flat pattern of lines because of symmetry Seen as a 3-D cube. Perception--Outline Psychophysics Perceptual Organization A. Gestalt Psychology B. 3-D Form and Depth Perception C. Perceptual Constancies III. Bottom-up and Top-down Processing I. II. What grouping principle does camouflage try to defeat? 5 3-D Form and Depth Role of vision is to register the 3-D spatial arrangement of objects and surfaces in the environment. The 2-D array of light contains cues to 3-D depth and distance. Going from a 2-D retinal image to a 3-D representation of the world is not always straightforward -- sometimes different 3-D arrangements in the world can give rise to the same 2-D image. Thus, there may be multiple valid 3-D interpretations of the retinal image. Sidewalk art! These are 2-D pictures... really! http://www.ebaumsworld.com/sidewalkdrawings.html 3-D Form and Depth Role of vision is to register the 3-D spatial arrangement of objects and surfaces in the environment. The 2-D array of light contains cues to 3-D depth and distance. Going from a 2-D retinal image to a 3-D representation of the world is not always straightforward -- sometimes different 3-D arrangements in the world can give rise to the same 2-D image. Thus, there may be multiple valid 3-D interpretations of the retinal image. 3-D Form and Depth: 3-D ambiguity The Necker Cube: multiple 3-D interpretations Let the rabbits help you see different 3-D interpretations 6 3-D Form and Depth Binocular Cues: require 2 eyes. Works for objects less than ~30 feet away. includes: eye convergence; retinal disparity Binocular Cues--Convergence Convergence--the inward turning of the eyes that occurs when you look at an object that is close to you. If you look at a distant object, the eyes point straight ahead. The positions of your eyes, therefore, indicates how near something is. Monocular Cues: can be used with a single eye. Includes the "pictorial cues". includes: motion parallax; accommodation; pictorial cues => shape-from-contour; shading/lighting; linear perspective & texture gradients; relative image size; elevation; aerial/atmospheric perspective Binocular Cues--Convergence Binocular Cues--retinal disparity Retinal disparity--Since the eyes are in different positions, they get different views of the world. disparity = depth: --the fixation point is projected to the foveae the of 2 eyes. --all other points fall on disparate (different) parts of each retina. --the size of the disparity for a single point corresponds with the 3-D depth between this point and the fixation point. -- from eye muscles Binocular Cues--retinal disparity Binocular Cues--retinal disparity Retinal disparity--Since the eyes are in different positions, they get different views of the world. disparity = depth: --the fixation point is projected to the foveae of the 2 eyes. --all other points fall on disparate (different) parts of each retina. --the size of the disparity for a single point corresponds with the 3-D depth between this point and the fixation point. 7 Binocular Cues--retinal disparity Monocular Depth Cues Fixation point fovea Or How can pirates (or Cyclops's) drive? Monocular Cues--Motion Parallax Motion Parallax--one of the most important cues for depth perception. The relative motion of elements in a display specifies depth and structure. Example 1: When observer moves head, objects located at different distances move at different velocities. These relative velocities specify depth. Close elements move fast and in a direction opposite to the observer's movement, etc. Monocular Cues--Motion Parallax (demo) Monocular Cues--Motion Parallax Optic Flow Example 2: "Optic Flow"--when move toward or away from objects, get radial pattern of motion. Specifies direction of heading. Example 3: structure-from-motion Flow pattern for a pilot landing an airplane (demo) Flow pattern for a person looking out of the back of a train 8 Optic Flow is important for maintaining balance Monocular Cues--Motion Parallax Swinging room: (a) moving the room forward creates the same optic flow pattern as when a person sways backward, as in (b). (c) subjects often compensate for this apparent sway by swaying forward, often losing their balance. Example 2: "Optic Flow"--when move toward or away from objects, get radial pattern of motion. Specifies direction of heading. Example 3: structure-from-motion (point-lightwalkers) Structure-from-Motion: (demo) Monocular Cues Accommodation--bring image to focus on retina using muscles that change the shape of the lens. Sensory activity from muscle low intraocular muscles high Neural information is from intraocular muscles. Brain interprets low sensory activity as "far away", and high sensory activity as "nearby". Monocular Cues -- Shape from Contour Shape from Contour--the way that contours intersect with each other provides cues to local shape and depth. T-junction: T suggests occlusion or interposition cross: + suggests 2 wires lying over each other fork: Y suggests a corner shape Each intersection imposes constraints on organization of adjacent surfaces. Recover 3-D structure by simultaneously solving the constraints. Monocular Cues -- Shape from Contour 9 Shape from Contour "impossible" figures Impossible scene: Escher's "The Waterfall" Escher's "Ascending & Descending" Monocular--Shading/lighting Shading/Lighting--surfaces nearest and oriented perpendicular to light source are brightest. --Surfaces with orientations slanted away from light source are less bright and may be covered by a shadow. --Interpret pattern of shading as shape. Usually assume light source as coming from above. Shading/lighting: assume light source is from above Monocular Linear Perspective and Texture Gradients-systematic decrease in size of more distant elements and the spacing separating them. Linear Perspective--gradients of lines. E.g., apparent convergence at a distance of parallel lines (e.g., railroad tracks). Texture Gradients--refinement of size, shape and spacing of surface elements with distance 10 Linear Perspective Monocular Linear Perspective and Texture Gradients-systematic decrease in size of more distant elements and the spacing separating them. Linear Perspective--gradients of lines. E.g., apparent convergence at a distance of parallel lines (e.g., railroad tracks). Texture Gradients--refinement of size and spacing of surface elements with distance. Texture Gradients Monocular Relative Image Size--2 objects of the same shape but one is smaller will be judged to be at different depths. Monocular Elevation (proximity to the horizon)--objects closer to the horizon in the visual field (b&c) are usually seen as located at a greater distance than objects lower/higher (a&d) in the visual field. Monocular Aerial/Atmospheric Perspective-atmospheric particles reduce contrast/clarity of distant objects. 11 Depth Cues in Renaissance Painting Perception--Outline I. II. Psychophysics Perceptual Organization A. Gestalt Psychology B. 3-D Form and Depth Perception C. Perceptual Constancies III. Bottom-up and Top-down Processing Perceptual constancies We extract constant properties (lightness, shape, & size) of distal stimuli (stimuli in the world) despite vagaries of (changes in) the proximal stimulus (retinal image). Lightness Constancy--the apparent lightness of an object is ~constant despite sometimes drastic changes in illumination. Example: a piece of black coal in the sunlight might reflect more light to the eye than a piece of white paper in a dark room, yet coal still looks black & the paper still looks white. Lightness constancy--take shadows into account Lightness constancy--take shadows into account Shape Constancy-- the perceived shape of an object is the same regardless of viewing angle 12 Shape Constancy take into account viewing angle to determine perceived shape Size Constancy Size Constancy--perceived object size is the same regardless of object distance. The tops of the 2 tables are of identical size and shape in the plane of the picture. perceived size = retinal image size x perceived distance Must take into account both retinal image size & distance in order to accurately estimate actual physical size. Some distance/size illusions: --Ames Room --Moon illusion --M ller-Lyer Illusion and Ponzo Illusion --Shepard Figure Ames Room--why do the 2 people look so different in size? Some distance/size illusions: Ames Room--why do the 2 people look so different in size? Distort perspective cues to make end wall seem at same distance to viewer. Ames Room 13 Some distance/size illusions: Moon illusion--moon looks much larger at horizon vs. zenith. Perceive sky as a flattened dome: Moon on the horizon is perceived as being further away due to presence of depth cues (texture fields, superposition, perspective) If two objects produce the same retinal image, then the further one must be larger! The Moon Illusion -- moon looks much larger at horizon vs. zenith Some distance/size illusions: M ller-Lyer Illusion and Ponzo Illusion-identical lines appear to be different in length. Ho: Same length lines appear to be at different depths. The M ller-Lyer & Ponzo Illusion (demo) Explanation for the M ller-Lyer Illusion Explanation for the M ller-Lyer Illusion 14 Perception--Outline --Drawing by R. Shepard --The 2 monsters appear to be very different in size but are identical in size in the picture plane (i.e., have the same retinal image sizes). Psychophysics Perceptual Organization A. Gestalt Psychology B. 3-D Form and Depth Perception C. Perceptual Constancies III. Bottom-up and Top-down Processing I. II. The End! 15

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