Chapter 9
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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

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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

C


Structure from motion

  • Structure from motion (SFM) refers to our ability to derive information about 3-dimensional shape from motion


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

  • 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


Biological motion


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


Optic flow and visual expansion


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

C


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 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 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 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

  • 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

  • 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 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

  • 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

  • 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

  • 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

  • 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

  • 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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Area MT

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Correlated motion


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

  • Direction-sensitive neurons undergo neural adaptation

  • Neural adaptation to motion leads to illusory motion aftereffects (MAEs)


Motion Adaptation


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 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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