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PSYC2030 - Sensation and Perception Chapter 6

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York University
PSYC 2220
Jennifer Steeves

Lecture 6 Space Perception and Binocular Vision • “Were it not for our own personal experiences, it would be temping to conclude that the visual perception of 3D shape is mathematical impossibility” (Todd, 2004) Euclidian Geometry • Parallel lines remain parallel as they are extended in space • Internal angles of a triangle always add to 108 degrees, etc. • Notice that images projected into he retina are non-Euclidian • The combination (or “summation”) of signals from each eye in ways that make performance on many tasks better with both eyes than with either eye alone • The two retinal images of a three-dimensional world are not the same Monocular Depth Cues • Occlusion: a cue to relative depth order in which, for example, one object obstructs the view of part of another object o Most important monocular depth cue • Linear Perspective: Lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image as they extend into the distance o E.g. looking straight down a railroad • Vanishing point: The apparent point at which parallel lines receding in depth converge • Size: A comparison of size between items without knowing the absolute size of either one o Relative Size: If two objects are known to be the same size (e.g., two trees) but their absolute size is unknown, relative size cues can provide information about the relative depth of the two objects. If one subtends a larger visual angle on the retina than the other, the object which subtends the larger visual angle appears closer. o Familiar Size: A cue based on knowledge of the typical size of objects. Since the visual angle of an object projected onto the retina decreases with distance, this information can be combined with previous knowledge of the object's size to determine the absolute depth of the object. For example, people are generally familiar with the size of an average automobile. This prior knowledge can be combined with information about the angle it subtends on the retina to determine the absolute depth of an automobile in a scene.  Can be a cue for relative distance • Relative Height Cues: the object closer to the horizon is perceived as farther away, and the object further from the horizon is perceived as closer • Texture gradient o Based on the geometric fact that items of the same size from smaller images when they are farther away o Texture gradients result from a combination of the cues of relative size and relative height • Aerial (or Atmospheric) perspective: A depth cue based on the implicit understanding that light is scattered by the atmosphere o More light is scattered when we look through more atmosphere o Thus, more distant objects are subject to more scatter and appear fainter, and less distinct • Shading: Light diminished in intensity as a result of the interception of the rays; partial darkness. Pictorial Depth Cue • Pictorial depth cue: A cue to distance or depth used by artists to depict three- dimensional depth in two-dimensional pictures • Anamorphosis (or anamorphic projection): Use of the rules of linear perspective to create a two-dimensional image so distorted that it looks correct only when viewed from a special angle or with a mirror that counters the distortion Motion Cues • Motion parallax: Images closer to the observer appear to move faster across the visual field than images farther away o The brain uses this information to calculate the distances of objects
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