Chapter 4 Perception, motion and action 121-154
1. Direct perception
The dominant approach until 30 years ago was that the central function of visual perception is to
allow us to identify and recognize objects in the world around us. This often involves extensive
cognitive processing, including relating information extracted from the visual environment to our
stored knowledge of objects.
James Gibson (1950, 1966, 1979):
Argued that this approach is of limited relevance to our visual perception in the real world.
- Keeping in touch with the environment.
- Relate to ‘vision-for-action’ system à There is an intimate relationship between action and
perception.
- = ecological approach: perception facilitates interactions between the individual and his
environment.
1. The pattern of light reaching the eye is an optic array: it contains all the visual information
from the environment striking the eye.
Optic array = the pattern of light falling on the retina.
Optic flow = the changes in the pattern of light reaching an observer when there is movement of
the observer and/or aspects of the environment.
- Forward movement: outflow from point of expansion.
- Backward movement: inflow from point of expansion.
Focus of expansion = the point towards which someone in motion is moving; it does not appear
to move but the surrounding visual environment apparently moves away from it.
2. The optic array provides unambiguous or invariant information about the layout of objects.
This information comes in many forms, including texture gradients, optic-flow patterns and
affordances.
Gibson argued that certain higher-order characteristics of the visual array (=invariants) remain
unaltered as observers move around their environment.
à The focus of expansion is an invariant feature of the optic array.
- Optic flow provides unambiguous information about: direction, speed, altitude.
- How this information is combined to yield successful perception remains underspecified in
Gibson’s theory.
Invariants = properties of the optic array that remain constant even though other aspects vary
(part of Gibson’s theory).
Affordances
Gibson claimed that potential uses of objects (affordances) are directly perceivable.
‘Affordances are opportunities for an action that exist in the environment and do not depend on
the animal’s mind, they do not cause behaviour but simply make it possible’.
à Present images of objects
- Pappas& Mack: each object’s main affordance produced motor priming.
- Wolf à affordance of graspability triggers rapid activity in the motor system.
- Bars Alou: motor system is typically involved when we understand meaning of object.
, Strengths Limitations
Emphasized ecological validity Processes involved in perception are much
- Noted the informational richness contained more complicated than implied by Gibson.
within visual stimuli.
Noted the importance of changes in optic array. The failure to assume the existence of
internal representations to understand
perception is seriously flawed.
Captured the dorsal system before it became Underestimated the importance of top-down
important in other theories. processes.
Also predated “embodied cognition”: seeing Largely ignored vision-for-perception
an object activates the motor areas in the brain system.
that are active when using object.
2. Planning-control model (3)
How do we use visual information when we want to perform an action with respect to some
object?
1. Planning system
Planning depends on a visual representation located in the inferior parietal lobule together with
motor processes in the frontal lobes and basal ganglia. The inferior parietal lobe is involved in
integrating information about object identification and context with motor planning to permit tool
and object use.
2. Control system
Proprioception = a form of sensation making the individual aware of the position and orientation
of parts of his/her body.
Control depends on a visual representation located in the superior parietal lobe combined with
motor processes in the cerebellum.
,3. Visually-guided action (2)
Heading and steering
Global radial outflow hypothesis: if we are not moving directly to our goal, we use the focus of
expansion and optic flow to bring our heading into alignment with our goal.
- Which the overall or global outflow pattern specifies an observer’s heading.
- Works well for straight moving, not for non-straight moving.
Retinal flow field = changes in the pattern of light on the retina; is determined by two factors:
- Linear flow containing a focus of expansion
- Rotary flow produced by following a curved path and by eye and head movements.
Efference copy = an internal copy of a motor command; can be used to compare actual with
desired movement.
Heading: future path
Wilkie & Wann: Heading judgments are not very relevant for movement along a curved path.
Future path strategy = fixate some point along their future path when it is curved.
Tangent point strategy = the point on the inside edge of the road at which its direction appears to
reverse.
- It is easy to identify and to track.
- Road curvature can readily be worked out by considering the angle between the direction of
heading and that of the tangent point.
- NOT: should be very hard to use tangent-point information when the inside road edge is
degraded or removed.
à Drivers use both: future path in centre and outside, tangent point in the inside of road.
Time to contact
Lee (1976, 2009)
- We do not need to work out the distance or speed of an approaching object to work out the
time to contact.
- If constant velocity
o We can use TAU = size of an object’s retinal image // rate of expansion. It specifies the
time to contact with an approaching object, faster rate of expansion = less time.
- When driving
o We can use TAU-dot = indicates whether there is sufficient braking time to stop at the
target.
Observers can work out time to contact from variables measurable directly by the eye.
Strengths Limitations
The tau hypothesis is appealingly Tresilian (1999)
simple. - Only works with objects of constant velocity;
ignores acceleration.
- Only provides information about time to contact of
eyes.
- Objects need to be spherically symmetrical.
Tau is often used to assist in making The image size and expansion of the object must
decisions about time to contact. both be detectable.
Tau can be computed based on variables Estimates are often based on previous
measured directly from the eye. knowledge. & other information also used.
Only work well in uncluttered environments.
Ignores the emotional content of approaching.
, 4. Perception of human motion
Biological motion in point-light displays (Johansson 1975)
Based on gestalt principle of common fate.
Observers can even determine the gender of the moving person (Gutting,
1978)
Ability to perceive biological motion is: spontaneous, automatic and innate
(Johansson, 1975)
- There is development in perception of biological motion towards more
specialization for human motion.
- Top-down processes can influence the detection of biological motion.
Recognition of human action à superior temporal sulcus.
Work out goals of observed actions à inferior frontal gyrus.
Grossman applied rTMS to the superior temporal sulcus to
produce a “temporary lesion”. This caused a substantial
reduction in observer’s sensitivity to biological motion.
Processes involved in perceiving biological motion differs from those in perceiving object motion:
- Saygin found in stroke patients with lesions in superior temporal and premotor frontal areas
that their perception of biological motion was more impaired than that of non-biological
motion.
- Vaina: patient who performed poorly on basic motion task but reasonably good at detecting
biological motions from point-light displays.
Why is biological motion perception special?
1. Biological motion is the only type of visual motions humans can produce as well as
perceive.
o Serino studied patients with hemiplegia of one arm.
§ When the animated arm corresponded to their paralyzed arm, their ability to
identify arm movements was much worse than when it was the other arm.
2. Most people spend more time perceiving and trying to make sense of other people’s
motion than any other form of visual motion.
o Jacobs: It is easier to identify walkers from point-light displays if they had previously
been observed > 20 hours, than if that was <5 hours.
3. Other people’s movements are very rich source of social and emotional information.
o Atkinson: Ability to identify emotion is partly determined by speed of movement.
Emotional state can be established from biological motion.
Strengths Limitations
An impressive ability Relatively little is known about how bottom-up
Specific brain regions and top-down processes interact.
Behavioural and patients’ studies indicate that The shared and differential components of
biological motion is processed differently to biological and non-biological motion
non-biological motion. processing are not well understood.
1. Direct perception
The dominant approach until 30 years ago was that the central function of visual perception is to
allow us to identify and recognize objects in the world around us. This often involves extensive
cognitive processing, including relating information extracted from the visual environment to our
stored knowledge of objects.
James Gibson (1950, 1966, 1979):
Argued that this approach is of limited relevance to our visual perception in the real world.
- Keeping in touch with the environment.
- Relate to ‘vision-for-action’ system à There is an intimate relationship between action and
perception.
- = ecological approach: perception facilitates interactions between the individual and his
environment.
1. The pattern of light reaching the eye is an optic array: it contains all the visual information
from the environment striking the eye.
Optic array = the pattern of light falling on the retina.
Optic flow = the changes in the pattern of light reaching an observer when there is movement of
the observer and/or aspects of the environment.
- Forward movement: outflow from point of expansion.
- Backward movement: inflow from point of expansion.
Focus of expansion = the point towards which someone in motion is moving; it does not appear
to move but the surrounding visual environment apparently moves away from it.
2. The optic array provides unambiguous or invariant information about the layout of objects.
This information comes in many forms, including texture gradients, optic-flow patterns and
affordances.
Gibson argued that certain higher-order characteristics of the visual array (=invariants) remain
unaltered as observers move around their environment.
à The focus of expansion is an invariant feature of the optic array.
- Optic flow provides unambiguous information about: direction, speed, altitude.
- How this information is combined to yield successful perception remains underspecified in
Gibson’s theory.
Invariants = properties of the optic array that remain constant even though other aspects vary
(part of Gibson’s theory).
Affordances
Gibson claimed that potential uses of objects (affordances) are directly perceivable.
‘Affordances are opportunities for an action that exist in the environment and do not depend on
the animal’s mind, they do not cause behaviour but simply make it possible’.
à Present images of objects
- Pappas& Mack: each object’s main affordance produced motor priming.
- Wolf à affordance of graspability triggers rapid activity in the motor system.
- Bars Alou: motor system is typically involved when we understand meaning of object.
, Strengths Limitations
Emphasized ecological validity Processes involved in perception are much
- Noted the informational richness contained more complicated than implied by Gibson.
within visual stimuli.
Noted the importance of changes in optic array. The failure to assume the existence of
internal representations to understand
perception is seriously flawed.
Captured the dorsal system before it became Underestimated the importance of top-down
important in other theories. processes.
Also predated “embodied cognition”: seeing Largely ignored vision-for-perception
an object activates the motor areas in the brain system.
that are active when using object.
2. Planning-control model (3)
How do we use visual information when we want to perform an action with respect to some
object?
1. Planning system
Planning depends on a visual representation located in the inferior parietal lobule together with
motor processes in the frontal lobes and basal ganglia. The inferior parietal lobe is involved in
integrating information about object identification and context with motor planning to permit tool
and object use.
2. Control system
Proprioception = a form of sensation making the individual aware of the position and orientation
of parts of his/her body.
Control depends on a visual representation located in the superior parietal lobe combined with
motor processes in the cerebellum.
,3. Visually-guided action (2)
Heading and steering
Global radial outflow hypothesis: if we are not moving directly to our goal, we use the focus of
expansion and optic flow to bring our heading into alignment with our goal.
- Which the overall or global outflow pattern specifies an observer’s heading.
- Works well for straight moving, not for non-straight moving.
Retinal flow field = changes in the pattern of light on the retina; is determined by two factors:
- Linear flow containing a focus of expansion
- Rotary flow produced by following a curved path and by eye and head movements.
Efference copy = an internal copy of a motor command; can be used to compare actual with
desired movement.
Heading: future path
Wilkie & Wann: Heading judgments are not very relevant for movement along a curved path.
Future path strategy = fixate some point along their future path when it is curved.
Tangent point strategy = the point on the inside edge of the road at which its direction appears to
reverse.
- It is easy to identify and to track.
- Road curvature can readily be worked out by considering the angle between the direction of
heading and that of the tangent point.
- NOT: should be very hard to use tangent-point information when the inside road edge is
degraded or removed.
à Drivers use both: future path in centre and outside, tangent point in the inside of road.
Time to contact
Lee (1976, 2009)
- We do not need to work out the distance or speed of an approaching object to work out the
time to contact.
- If constant velocity
o We can use TAU = size of an object’s retinal image // rate of expansion. It specifies the
time to contact with an approaching object, faster rate of expansion = less time.
- When driving
o We can use TAU-dot = indicates whether there is sufficient braking time to stop at the
target.
Observers can work out time to contact from variables measurable directly by the eye.
Strengths Limitations
The tau hypothesis is appealingly Tresilian (1999)
simple. - Only works with objects of constant velocity;
ignores acceleration.
- Only provides information about time to contact of
eyes.
- Objects need to be spherically symmetrical.
Tau is often used to assist in making The image size and expansion of the object must
decisions about time to contact. both be detectable.
Tau can be computed based on variables Estimates are often based on previous
measured directly from the eye. knowledge. & other information also used.
Only work well in uncluttered environments.
Ignores the emotional content of approaching.
, 4. Perception of human motion
Biological motion in point-light displays (Johansson 1975)
Based on gestalt principle of common fate.
Observers can even determine the gender of the moving person (Gutting,
1978)
Ability to perceive biological motion is: spontaneous, automatic and innate
(Johansson, 1975)
- There is development in perception of biological motion towards more
specialization for human motion.
- Top-down processes can influence the detection of biological motion.
Recognition of human action à superior temporal sulcus.
Work out goals of observed actions à inferior frontal gyrus.
Grossman applied rTMS to the superior temporal sulcus to
produce a “temporary lesion”. This caused a substantial
reduction in observer’s sensitivity to biological motion.
Processes involved in perceiving biological motion differs from those in perceiving object motion:
- Saygin found in stroke patients with lesions in superior temporal and premotor frontal areas
that their perception of biological motion was more impaired than that of non-biological
motion.
- Vaina: patient who performed poorly on basic motion task but reasonably good at detecting
biological motions from point-light displays.
Why is biological motion perception special?
1. Biological motion is the only type of visual motions humans can produce as well as
perceive.
o Serino studied patients with hemiplegia of one arm.
§ When the animated arm corresponded to their paralyzed arm, their ability to
identify arm movements was much worse than when it was the other arm.
2. Most people spend more time perceiving and trying to make sense of other people’s
motion than any other form of visual motion.
o Jacobs: It is easier to identify walkers from point-light displays if they had previously
been observed > 20 hours, than if that was <5 hours.
3. Other people’s movements are very rich source of social and emotional information.
o Atkinson: Ability to identify emotion is partly determined by speed of movement.
Emotional state can be established from biological motion.
Strengths Limitations
An impressive ability Relatively little is known about how bottom-up
Specific brain regions and top-down processes interact.
Behavioural and patients’ studies indicate that The shared and differential components of
biological motion is processed differently to biological and non-biological motion
non-biological motion. processing are not well understood.
