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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


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


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


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


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


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


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


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


Area MT


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


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


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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