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Chapter 9

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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  1. Chapter 9 Action and the Perception of Events

  2. 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

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

  4. 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

  5. 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

  6. Biological motion

  7. 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

  8. Optic flow and visual expansion

  9. 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

  10. 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

  11. 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?

  12. 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

  13. 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 N

  14. 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 N

  15. 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

  16. 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 I

  17. 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 N

  18. 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

  19. 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

  20. 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 N

  21. 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 N

  22. Area MT N

  23. Correlated motion

  24. 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 N

  25. Motion Adaptation • Direction-sensitive neurons undergo neural adaptation • Neural adaptation to motion leads to illusory motion aftereffects (MAEs)

  26. Motion Adaptation

  27. 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 T

  28. 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 T

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