Chapter 7. Biological Motion

7.1 Title slide

Demonstration 7.6
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Biological Motion

Interact with animated point-light walker displays where you can control different aspects of the motion in the display.

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The characteristic motion of a galloping horse would be instantly recognizable even if the motion were seen apart from the horse, in a point-light walker display. (This video consists of photos taken by Eadweard Muybridge, a nineteenth-century English photographer who did pioneering work on animal locomotion. © Library of Congress, Prints & Photographs Division, LC-USZ62-52703)

Does Meaningfulness Influence Figure-Ground Organization?

Motion can be an extremely powerful cue to the 3-D form of an organism, but to understand why this is so, we need to be able to look at just the organism's motion, without seeing its body. The point-light walker display makes this possible. Small lights (point-lights) are placed at critical locations on the organism's body—for example, at a person's shoulders, elbows, wrists, hips, knees, and ankles, so videos can be shot of the person moving in complete darkness, with only the point-lights visible.

Any one frame of such a video looks like randomly scattered dots. But as soon as the video starts to play and the dots begin to move, the person's form becomes apparent—the visual system is so attuned to the perception of biological motion that it almost immediately understands the complex pattern of moving dots as the motion of a person engaging in some specific activity. (In the figure, the panels on the left show where point-lights are placed on the person's body, and the panels on the right show still images from the display.)

Indeed, not only is it possible to look at a point-light walker display and recognize whether you're seeing a runner, a dancer, or a walking dog, but you can also tell if a person is a man or a woman. What's behind this ability? As a person moves, the distance between the elbow and the wrist, for example, remains constant, as does the distance between the knee and the ankle, while the distance between the wrist and the ankle may change dramatically. But even the distance between, say, the knee and the ankle may seem to change because of foreshortening if the leg tilts backward or forward in depth. Apparently, in conjunction with knowledge about the structure of the human body, the visual system is able to pick up on these regularities to correctly perceive the invisible person in a point-light walker display.

7.2 Custom activity

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The locations of the dots below may look random, but they actually represent points on the body of an organism. Click PLAY to see the motion of the dots when the
organism moves.

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7.3 Custom activity

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Click PLAY to put this point-light walker display into motion. Then drag the sliders to adjust the attributes of the person in the display. Can you tell which attribute is controlled by each slider? Click SHOW ATTRIBUTES to see the answers.

ATTRIBUTE 1

FEMALE

MALE

ATTRIBUTE 2

HEAVY

LIGHT

ATTRIBUTE 3

SAD

HAPPY

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7.4 Explain

explain_text

How Do We Use Biological Motion as a Cue to the Form of the Moving Organism?

Motion can be an extremely powerful cue to the 3-D form of an organism, but to understand why this is so, we need to be able to look at just the organism's motion, without seeing its body. The point-light walker display makes this possible. Small lights (point-lights) are placed at critical locations on the organism's body—for example, at a person's shoulders, elbows, wrists, hips, knees, and ankles, so videos can be shot of the person moving in complete darkness, with only the point-lights visible.

Any one frame of such a video looks like randomly scattered dots. But as soon as the video starts to play and the dots begin to move, the person's form becomes apparent—the visual system is so attuned to the perception of biological motion that it almost immediately understands the complex pattern of moving dots as the motion of a person engaging in some specific activity. (In the figure, the panels on the left show where point-lights are placed on the person's body, and the panels on the right show still images from the display.)

Indeed, not only is it possible to look at a point-light walker display and recognize whether you're seeing a runner, a dancer, or a walking dog, but you can also tell if a person is a man or a woman. What's behind this ability? As a person moves, the distance between the elbow and the wrist, for example, remains constant, as does the distance between the knee and the ankle, while the distance between the wrist and the ankle may change dramatically. But even the distance between, say, the knee and the ankle may seem to change because of foreshortening if the leg tilts backward or forward in depth. Apparently, in conjunction with knowledge about the structure of the human body, the visual system is able to pick up on these regularities to correctly perceive the invisible person in a point-light walker display.

7.5 Test - single choice

You must select an answer and then click SUBMIT before you can proceed to the next screen.
test_single_choice

Select your answer to the question below. Then click SUBMIT.

Question

XfofimiSVvqk6KatUwOl8R95ZTpU2chREMBOfvjL/SmuQX1WF+4S/y2BMBYZO6nemR9y8QxiEi9l5RJmbQ4YDzw7WcrqnSJUfZfOosaG7JZBH/YafpCtD6D0MSJvA/DGbeGyoSa3+mB4z1qzS3pg8uk3Fvk6eaNPc8Wdt9OzDGgV7YwCZDZCEnQ2Tmoo2S0Aehd0X4GMNMQg8cQNmMJNYS/FqgoUfZNj6wce7366tiStlYUp3trzo/x3eAExAl1/TR6pFhdbEWmjfMmqUBthe0iaT2Ly6rNY58J81StScmapHrFE1wOHUFCKW16R2fyI31MN3lfSeqL42A6mHetk47HD5wXZAqclB4A+wPM9i6TRNXPCGGGsA9TN6MuPf4IpDXTXS9Q6HNnFcwF9JSc3o91qGQlqm0UaKa3oXlGlVLCJ8Q4SaFrLrL4MucE=
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Incorrect.
The correct answer is C.
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7.6 Test - single choice

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test_single_choice

Select your answer to the question below. Then click SUBMIT.

Question

6o4CTcCYcsrO3G/a0BtCm0KsxNEDZZNhxrWjy0iL66h9R5pp6uc9MXTY0SZjLW2s6X5jdtFopm061yHTwJsXyzCTO/Qn/vUfCkzFrlJPEfF6zrsW29HjHcL4/5Rey0JBemdVnjkhrhtdhBsmlO8nRMgFIr+qtXgTgCrJtKQ2OsIXMEriA82hoVuszvw4qXA94A9474maWdPw9EQYT3v18o3HsrN46i2c0p4xipRsmb9zKmNl85F1+3GwtSLXxOxNTroSH6u2zBKRwqHGTA3fJBmywKbmbwr5k96JlMfvERcVfACN1XCCmKQwkCPRNkg0G6/yjANv4JXzYO8zcKbbXxkOpJY2NH+q2jYHkWaKGcC9iwGNk7UYJPXm5feKueMdjJzNK0xrm7sH/VyO+Km/efo+0DOSX81/dYuPBEzrz6MKgxuYHEUQQoVYJ8B1TkbJNpvGXuhkdj3Knqy45je0KwrgSAStDu4KyoRPqzcOSaugu5rH11pQ0iqFwiG6r3vgRgA1lxdYCJB4pjMnrw1dCcDzEotD158XbaQtDP0nA/YW66r8SBV6Nl5wdkzXdjPFgiNH4+cvs4O7J9IJgMQ6cS6fO16jNdJrQdK0H3NcOAwsIlV6A5eCgn2UWvS2jtnfRYTIJHzFwuvnuPJd8P0NZ6LRIlr/xNIl3yzZ/g==
Correct! Click EXPLAIN if you want to review this topic.
Incorrect.
The correct answer is D.
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7.7 Test - single choice

You must select an answer and then click SUBMIT before you can proceed to the next screen.
test_single_choice

Select your answer to the question below. Then click SUBMIT.

Question

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Incorrect.
The correct answer is D.
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7.8 Activity completed

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