lecture 8- objects and motion

Question 1

4 steps from sensation to perception

Answer 1

1) Raw input
○ Photoreceptors (rods and cones) collecting light, colour and intensity
○ There is then retinal processing where there is contrast detection.
○ The optic nerve then relays signals.
2) Early visual processing (V1-3)
○ Primary visual cortex detects edges, orientation and motion direction
○ Includes retinotopic mapping so the spatial layout is preserved.
3) Specialised pathways:
○ Dorsal (where and how) pathway identifies motion, spatial location and gives action guidance
○ The MT/V5 areas look at motion, direction, speed and optic flow.
○ Ventral stream (what): object, identity form and colour
○ Area V4: shape, object recognition and colour constancy
4) Perception:
○ Object constancy
○ Motion perception
Integration with memory, attention and expectation.

Question 2

Cognitive and cortical specialisation:
object agnosia, human evidence and animal models

Answer 2

Object agnosia: patients can see but cannot recognise objects
- Associative agnosia is where patients can recognise and perceive shapes and reproduce them but cannot link to knowledge of what the object may be.
Human evidence: FMRI finds the lateral occipital cortex is activated when viewing both objects and scrambled controls (Malach et al 1995)
- Showing it responds to object shape, independent of texture or colour.
Animal models: single unit recordings in monkey’s Inferotemporal cortex:
- Cells are selective for complex shapes and even entire objects, cells excited by different, specific shapes or objects (Gross 1972)
Some neurons respond to specific object views, invariant to size or position

Question 3

4 theories of object perception

Answer 3

constructivism, ecological perception and integrative perception approaches

Question 4

Constructivism object perception theory

Answer 4

Sensory input is ambiguous and so we understand using hypotheses, and expectations based on previous experiences help to understand what the object is.
- For example the Muller Lyer where the brain inappropriately use size constancy scaling
The duck rabbit illusion where people see different things at first and can shift perception based on cues (around easter or following bird discussion)

Question 5

ecological perception object perception theory

Answer 5

Visual perception is based on rich sensory input available, where insufficient information is available, perception will fail
- Perception is direct and action orientated, the environment provides rich and structured information that is immediately available.
○ Expanding central radial flow gives cues about direction, depth and speed
- Gibson and Walk 1960: visual cliff experiment:
○ Babies able to pick up cues about the environment in an action orientated way only using environmental information
○ Perception is directly related to affordances (what the environment provides (steep cliff affords falling), and affordances are dependent on properties of the environment and abilities of the perceiver.

Question 6

integrative perception approaches: recognition by components (Biederman)

Answer 6

• Objects are recognised as combinations of geometric objects, known as GEONS.
  - The geon structure is the compared to concepts stored in long term memory

Question 7

Integrative perception approaches: feature integration theory (Treisman)

Answer 7

• Perception relies on extraction of basic visual features, which requires attention to be bound into a coherent whole
  - Bottom up: extraction of features
  - Top down: attention needed for binding
  § During visual search, single feature differences make for an efficient and simple search regardless of number of distractors. Where there are multiple differences- conjunction search, more labour intensive search

Question 8

integrative perception approaches: mental imagery/rotation

Answer 8

• Shepard and Metzler 1971: same/different decisions: Reaction time linearly related to angle of rotation
  - Processing is suggestive of cognitive analogues to real life representations (mental vs physical manipulation)
  - Recognition can be harder when objects are rotated from a familiar viewpoint as the brain stores multiple views or uses transformation mechanisms to match input to stored representations.
  § This contrasts with view invariant theories (Biederman) which suggest abstract representations don’t require rotation.

Question 9

Object blindness: agnosia:

Answer 9

• Some people are unable to recognise common objects despite intact cognitive functioning. Affecting naming, describing and demonstrating functions
  
  • 2 types:
    1) Apperceptive agnosia: difficulty forming coherent perceptual representation of an object leading to inability to recognise it.
    Associative agnosia: able to perceive object but unable to link visual percept to meaning or name

Question 10

Types of motion

Answer 10

1) Real motion: actual movement of object across the retina
2) Apparent motion: perceived motion without real displacement like static images appear in sequence with short intervals like in animated movies
3) Induced motion: stationary object appears to move because of motion in surrounding field (like clouds moving make it seem like sun is moving in opposite direction)
Biological motion: perception of movement from minimal cues suggesting a living agent (motion patterns of living organisms we can rapidly recognise)

Question 11

3 motion perception theories

Answer 11

Ecological/optic flow model
Energy/ computational model
Bayesian/ predictive coding model

Question 12

Ecological/optic flow model motion perception

Answer 12

• Perception of motion is directly specified by global flow patterns where no heavy internal reconstruction is needed.
  ○ Ano object getting larger on the retina seems to be approaching but no reconstruction is needed
  ○ Walking also includes making sense of optic flow cues, the world is perceived through a set of systematic changes of luminance on the retina
  
  • Effective explanation for self-motion and understanding navigation in space and objects in relation to self
  • But not for motion of other objects

Question 13

Energy/computational model: motion perception theory

Answer 13

• Motion is computed by spatiotemporal correlation detection
  ○ Supporting perception of direction based on local changes in luminance
  
  • Neural explanation is that linear filters are tuned to different directions and speeds
  • Different motion neurons act like pattern detectors in space and time

Question 14

Bayesian/predictive coding model: motion perception theory

Answer 14

• Perception = likelihood x prior
  
  • The visual input is underspecified, it could be caused by multiple real world motions
    ○ The brain must infer which 3d motion is most likely
  • Where priors are not well defined, perception is bistable
    ○ We can influence by shading, adding depth cues etc
  • The brain infers motion by combining ambiguous sensory data with prior expectations, without depth cues, the brain oscillates between equally predictable directions of motion
  • Motion illusions like the aperture problem where there is the same local motion but different global interpretations due to different priors
  • Stable perception in noise is achieved as priors help to fill in gaps
  • Prior experience or context can bias motion interpretation

Question 15

VR motion sickness

Answer 15

• Typically result of sensory conflict between visual motion signals and absence of vestibular feedback input
  
  • Resolved by:
    ○ Reducing latency of delays
    ○ Tuning of field-of-view: narrowing peripheral motion reduces nausea
    ○ Predictive rendering: stabilise apparent world motion, more congruence between movement and visual field

Question 16

Film industry:

Answer 16

• Alternating contrast polarity makes motion appear smooth and apparent
  Studios can use this to increase perceived fluidity of motion without higher frame rates

Question 17

Key points for motion perception theories

Answer 17

Energy models detect local motion not object tracking, does not explain how objects can move across the retina. Shows how neurons are tuned to capture local motion signals.
Ecological model does not mean there is no brain processing, we receive rich enough visual information to specify movement directly from the optic glow but the brain is not passive, not need for internal interpretations/ rebuilding
Bayesian mechanism suggests perception is computed but where the prior is not specific, we cannot tell what assumptions the visual system is bringing, making the model unfalsifiable. So requires labelled priors.

Question 18

Motion blindness: Akinetopsia

Answer 18

• Very rare neurological disorder with total inability to perceive motion.
  
  • The world appears to be a series of static images and presents challenges like pouring a drink and crossing the road
  • Often associated with damage to motion specific area in V5

Question 19

Motion perception failures

Answer 19

Neglect: patients with unilateral spatial neglect may fail to attend to motion in their contralesional field but can sometimes detect static stimuli
Parkinsons: dopaminergic loss affects motion integration and speed perception, resulting in underestimated of velocity, impairments in motion-based depth cues and deficits in biological motion recognition
Schizophrenia: sometimes show reduced motion coherence sensitivity.