Chapter 8: Perceiving Depth and Size. Overview of Questions. How can we see far into the distance based on the flat image of the retina? Why do we see depth better with two eyes than with one eye? Why don’t people appear to shrink in size when they walk away?. Cue Approach to Depth Perception.
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Overview of Questions • How can we see far into the distance based on the flat image of the retina? • Why do we see depth better with two eyes than with one eye? • Why don’t people appear to shrink in size when they walk away?
Cue Approach to Depth Perception • Oculomotor - cues based on sensing the position of the eyes and muscle tension • Convergence - inward movement of the eyes when we focus on nearby objects • Accommodation - change in the shape of the lens when we focus on objects at different distances
Cue Approach to Depth Perception - continued • Monocular - cues that come from one eye • Pictorial cues - sources of depth information that come from 2-D images, such as pictures • Occlusion - when one object partially covers another • Relative height - objects that are higher in the field of vision are more distant • Relative size - when objects are equal size, the closer one will take up more of your visual field • Perspective convergence - parallel lines appear to come together in the distance
Pictorial Cues (continued) • Familiar size - distance information based on our knowledge of object size • Atmospheric perspective - distance objects are fuzzy and have a blue tint • Texture gradient - equally spaced elements are more closely packed as distance increases • Shadows - indicate where objects are located
Figure 8.3 A scene in Tucson, Arizona containing a number of depth cues: occlusion (the cactus occludes the hill, which occludes the mountain); perspective convergence (the sides of the road converge in the distance); relative size (the far motorcycle is smaller than the near one); and relative height (the far motorcycle is higher in the field of view; the far cloud is lower).
Figure 8.5 A scene along the coast of California that illustrates atmospheric perspective.
Motion-Produced Cues • Motion parallax - close objects in direction of movement glide rapidly past but objects in the distance appear to move slowly • Deletion and accretion - objects are covered or uncovered as we move relative to them • Also called occlusion-in-motion
Figure 8.8 One eye, moving from left to right, showing how the images of a nearby tree and a faraway house change their position on the retina because of this movement. The image of the tree moves farther on the retina than the image of the house.
Binocular Depth Information • Binocular disparity - difference in images between the two eyes • Difference can be described by examining corresponding points on the retina that connect to same places in the cortex • The horopter - imaginary circle that passes through the point of focus • Objects on the horopter fall on corresponding points on the retina
Binocular Depth Information - continued • Objects that do not fall on the horopter fall on noncorresponding points • These points made disparate images • The angle between these points is the angle of disparity • Objects located in front the horopter have crossed disparity • Objects located beyond the horopter have uncrossed disparity
Figure 8.11 Location of images on the retina for the “Two Eyes: Two Viewpoints” demonstration. (a) Both images are on the fovea when the left eye is open. (b) The images are on different places on the retina when the right eye is open.
Figure 8.12 Corresponding points on the two retinas. To determine corresponding points, imagine that one eye is slid on top of the other one.
Figure 8.13 (a) When the lifeguard looks at Frieda, the image of Frieda, Susan, and Harry fall on corresponding points on the lifeguard’s retinas, and the images of the other swimmers fall on noncorresponding points. (b) The locations of the images of Susan, Frieda, and Harry on the lifeguard’s retina.
Figure 8.14 The location of the images of Carol and Lee in the lifeguard’s eyes. Because Carol and Lee are not located on the horopter, their images fall on noncorresponding points.
Figure 8.15 Crossed disparity occurs for objects in front of the horopter; uncrossed disparity occurs for objects behind the horopter. The arrows indicate that the retinal images move inward, toward the nose, as the object moves further away.
Binocular Depth Information - continued • Stereopsis - depth information provided by binocular disparity • Stereoscope uses two pictures from slightly different viewpoints • 3-D movies use the same principle and viewers wear glasses to see the effect • Random-dot stereogram has two identical patterns with one shifted to the right
Figure 8.19 Top: a random-dot stereogram. Bottom: the principle for constructing the stereogram. See text for an explanation.
Physiology of Depth Perception - continued • Neurons have been found that respond best to binocular disparity • Called binocular depth cells or disparity selective cells • These cells respond best to a specific degree of disparity between images on the right and left retinas
Figure 8.21 Disparity tuning curve for a disparity-sensitive neuron. This curve indicates the neural response that occurs when stimuli presented the left and right eyes create different amounts of disparity. (From Uka & DeAngelis, 2003.)
Figure 8.22 Where images of Frieda and Lee fall on the retina. See text for explanation.
Connecting Binocular Disparity and Depth Perception • Experiment by Blake and Hirsch • Cats were reared by alternating vision between two eyes • Results showed that they: • Had few binocular neurons • Were unable to use binocular disparity to perceive depth
Size Perception • Distance and size perception are interrelated • Experiment by Holway and Boring • Observer was at the intersection of two hallways • A luminous test circle was in the right hallway placed from 10 to 120 feet away • A luminous comparison circle was in the left hallway at 10 feet away
Figure 8.24 Setup of Holway and Boring’s (1941) experiment. The observer changes the diameter of the comparison circle to match his or her perception of the size of the text circle. Each of the test circles has a visual angle of 1 degree. This diagram is not drawn to scale. The actual distance of the test circle was 100 feet.
Figure 8.25 (a) The visual angle depends on the size of the stimulus (the woman in this example) and its distance from the observer. (b) When the woman moves closer to the observer, the visual angle and the size of the image on the retina increases. This example shows how halving the distance between the stimulus and observer doubles the size of the image on the retina.
Experiment by Holway and Boring - continued • Part 1 of the experiment provided observers with depth cues • Judgments of size were based on physical size • Part 2 of the experiment provided no depth information • Judgments of size were based on size of the retinal images
Figure 8.29 The moon’s disk almost exactly covers the sun during an eclipse because the sun and the moon have the same visual angles.
Size Constancy • Perception of an object’s size remains relatively constant • This effect remains even if the size of the object on the retina changes • Size-distance scaling equation • S = K (R X D) • Changes in distance and retinal size balance each other
Figure 8.33 Two cylinders resting on a texture gradient. The fact that the bases of both cylinders cover the same number of units on the gradient indicates that the bases of the two cylinders are the same size.
Visual Illusions • Nonveridical perception occurs during visual illusions • Müller-Lyer illusion: • Straight lines with inward fins appear shorter than straight lines with outward fins • Lines are actually the same length
Figure 8.34 The Müller-Lyer illusion. Both lines are actually the same length.
Ponzo Illusion • Horizontal rectangular objects are placed over railroad tracks in a picture • Far rectangle appears larger than closer rectangle but both are the same size • One possible explanation is misapplied size-constancy scaling
Figure 8.39 The Ponzo (or railroad track) illusion. The two horizontal rectangles are the same length on the page (measure them), but the far one appears larger.
Adelbert Ames Room (1946) • Two people of equal size appear very different in size in this room • The room is constructed so that: • Shape looks like normal room when viewed with one eye • Actual shape has left corner twice as far away as right corner
Figure 8.41 The Ames room, showing its true shape. The woman on the left is actually almost twice as far away from the observer as the woman on the right; however, when the room is viewed through the peephole, this difference in distance is not seen. In order for the room to look normal when viewed through the peephole, it is necessary to enlarge the left side of the room.
Moon Illusion • Moon appears larger on horizon than when it is higher in the sky • One possible explanation: • Apparent-distance theory - horizon moon is surrounded by depth cues while moon higher in the sky has none • Angular size-contrast theory - moon appears smaller when surrounded by larger objects
Figure 8.42 An artist’s conception of the moon illusion showing the moon on the horizon and high in the sky simultaneously.
Figure 8.43 When observers are asked to consider that the sky is a surface and are asked to compare the distance to the horizon (H) and the distance to the top of the sky on a clear moonless night, they usually say that the horizon appears farther away. This results in the “flattened heavens” shown above.
Effects of Person’s Ability to Take Action on Distance Perception • Distance perception can also be affected by the perception of ability to take action • Experiment by Proffitt et al. • Participants made distance judgments with or without a backpack • Those with the backpack increased their estimates, even though they did not have to walk the distance
Effects of Person’s Ability to Take Action on Distance Perception - continued • Experiment by Witt et al. • Phase 1: • Participants threw balls to targets 4 to 10 meters away • They used either a light or heavy ball • Distance estimates were larger after throwing the heavy ball
Effects of Person’s Ability to Take Action on Distance Perception - continued • Phase 2: • Participants were divided into two groups: • One group was told they would have to throw the balls while blindfolded • Other group was told that would have to walk to targets while blindfolded • Group that was told they would be throwing balls increased their estimates