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Chapter 9. Action and the Perception of Events. The role of motion in perception. Motion is important in object detection, figure/ground segmentation, guidance of visual attention, and object identification Individuals with damage to certain areas of the brain are unable to perceive motion.

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chapter 9

Chapter 9

Action and the

Perception of Events

the role of motion in perception
The role of motion in perception
  • Motion is important in object detection, figure/ground segmentation, guidance of visual attention, and object identification
  • Individuals with damage to certain areas of the brain are unable to perceive motion

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structure from motion
Structure from motion
  • Structure from motion (SFM) refers to our ability to derive information about 3-dimensional shape from motion
structure from motion1
Structure from motion
  • Biological motion is a special type of SFM that allows us to distinguish between animate and inanimate objects
  • Biological motion enables us to distinguish living creatures from other moving objects
biological motion
Biological motion
  • The perception of biological motion from just a few points of moving light is called point-light motion
  • Identification of biological motion is more than the detection of non-random motion
visual guidance of locomotion
Visual guidance of locomotion
  • Visual expansion is a type of optic flow (changing pattern of stimulation) that signals the approach of an object
  • The relationship between the rate of retinal image expansion and time of impact with an object enables us to avoid collision
visual guidance of locomotion1
Visual guidance of locomotion
  • With our eyes closed, we can navigate around obstacles and reach for objects
  • Vision is required for more precise motion
  • Optic ataxia is a condition characterized by an inability to make precise movements

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effects of eye movements
Effects of eye movements
  • Saccades are rapid, jerky eye movements that occur between fixations
  • Saccadic suppression is the dampening of vision that occurs during a saccade
effects of eye movements1
Effects of eye movements
  • The rapid movement of a saccade is not seen because of visual masking
  • The visual world remains still, even though saccades constantly shift the retinal image
  • How is this possible?
effects of eye movements2
Effects of eye movements
  • The visual system tracks command signals going to extraocular muscles
  • The brain uses this information to update its representation of space
effects of eye movements3
Effects of eye movements
  • Unlike saccades, pursuit (smooth) eye movements are not jerky or ballistic
  • Signals going to the oculomotor muscles are updated constantly to keep the object’s image focused on the fovea

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space time receptive fields
Space-time receptive fields
  • Direction-selective cells are sensitive to relative changes in light within adjacent retinal regions
  • Motion defined by luminance variations over space/time is called first-order motion

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perceptual errors and accidents
Perceptual errors and accidents
  • Why do so many collisions occur at railroad crossings?
  • The size of an object and its apparent speed are inversely related, so we tend to underestimate the speed of large objects
perceptual errors and accidents1
Perceptual errors and accidents
  • Perceptual errors can also be useful in preventing accidents
  • For example, closely-spaced stripes painted on a rotary create a speed illusion that slows motorists down

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apparent motion
Apparent motion
  • We experience apparent motion when the visual system takes discrete inputs and makes them continuous
  • Motion perception (real and apparent) involves direction-sensitive neurons

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motion perception
Motion perception
  • How does the visual system register that an object seen at one moment corresponds to the same object seen at another moment?
  • The perception of group movement versus element movement depends on conditions such as the interval between displays
the aperture problem
The aperture problem
  • Because it responds only to what is happening within its own receptive field, a DS neuron generates ambiguous signals
  • This is resolved by integrating local measurements to produce a global response
resolving visual ambiguity
Resolving visual ambiguity
  • Ambiguous early responses are channeled to a second stage of visual processing involving higher-order neurons
  • An array of spatially distributed V1 neurons contribute to individual MT receptive fields

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area mt s role in vision
Area MT’s role in vision
  • MT neurons contribute motion information that is qualitatively different from the information provided by V1 neurons
  • Area MT is important in the detection of correlated motion

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ds neuron interactions
DS neuron interactions
  • Direction-sensitive neurons may inhibit each other (compete), or amplify each other (cooperate)
  • Cooperative-competitive interactions underlie the perception of global motion

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motion adaptation
Motion Adaptation
  • Direction-sensitive neurons undergo neural adaptation
  • Neural adaptation to motion leads to illusory motion aftereffects (MAEs)
higher order effects in motion perception
Higher-order effects in motion perception
  • Stimuli are more difficult to detect when our expectations are uncertain
  • Selective adaptation occurs under conditions of certainty; observer is less sensitive to a target after prolonged exposure to that target

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higher order effects in motion perception1
Higher-order effects in motion perception
  • In multiple object tracking, an observer may group the random movement of targets by tracking a virtual object
  • Multiple object tracking is mediated by frontal and parietal regions, and area MT

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