Week 7 Lecture 7 - motion perception Flashcards
What is local motion?
Motion of individual (local) elements
What is global motion?
We can group the motion of many individual elements to perceive a complex pattern of global motion
To process global motion, what do we need?
we need to pool information from multiple local motion detectors
What is the motion coherence threshold?
minimum proportion of signal dots needed to detect coherent motion
What is humans motion coherence threshold?
Humans = 10% (5% when highly practiced)
What does the motion coherence threshold depend on?
proportion of signal to noise dots
What size receptive fields do local motion detectors and global motion detectors have?
Local motion detectors – small receptive fields
Global motion detectors – large receptive fields
What area of the brain processes motion?
area MT = middle temporal
How do we know MT is an important area for motion processing?
Single cell recording:
- Nearly all cells in area MT respond to motion – and they have a preferred direction
Artificial Stimulation:
- Salzman et al (1990) – identified cells in monkey area MT that all had same preferred direction
- Artificial stimulation of cells led to motion judgements being biased towards preferred direction
Imaging:
- Tootell et al (1995): fMRI study of the motion after-effect
- When we adapt to motion, then view stationary test, we experience motion in the opposite direction
- Results: MT remains active during period of after-effect (i.e. no direct stimulation, but motion still perceived
Lesions:
- Newsome and Paré (1988) introduced small lesion to monkey MT
- Undamaged coherence thresholds = 5%
- Damaged coherence threshold = 80%
How are different movements created in optic flow?
- Patterns of retinal motion produced when we move
- Expansion: created by forward translation
- Contraction: created by backwards translation
- Horizontal (constant speed): created by eye, head or body rotation
- Horizontal (parallax): created by lateral (sideways) translation
- Roll: Created by eye, head or body roll
- Complex motions: forward translation, head rotation, combined
Smith et al. (2006) comducted an fMRI study to compare 2 cortical sub-regions: MT, MST
Measure response of MT and MST to 5 types of motion:
- Complex
- Expansion
- Rotation (Roll)
- Translation
- Random
What were they interested in?
-Interested in the difference in response to optic flow compared to random motion
Smith et al. (2006) comducted an fMRI study to compare 2 cortical sub-regions: MT, MST
Measure response of MT and MST to 5 types of motion:
- Complex
- Expansion
- Rotation (Roll)
- Translation
- Random
What was found?
- The difference in response to optic flow compared to random motion is greater in MST than MT
- Suggests MST is more specialised for processing optic flow motion than MT
What is optic flow used for?
Heading:
- Gibson proposed we use optic flow to tell us where we are heading and to control locomotion
Postural stability:
- Balance relies on vestibular and proprioceptive information but visual information also important
- Optic flow provides information on how our posture is changing
Perception of object motion during self-motion:
- Retinal motion due to 2 sources –>Self motion, Object motion
- Flow-parsing hypothesis
What is the Flow-parsing hypothesis?
- Retinal motion due to self-motion subtracted
- Remaining motion attributed to object motion
Apart from optic flow, what other information may be used for heading?
- Driving – Land & Lee (1994) showed we don’t look at FOE when driving, we look at other parts of the scene e.g. When on a curved path we look at the tangent
What evidence is there for optic flow’s role in postural stability?
The swinging room experiment (Lee & Aronson, 1974):
- Moving walls and ceiling but floor fixed
- Simulates the optic flow that would be experienced by a person swaying
- Participants were 13- 16 month old toddlers
- When room swings towards toddler it creates pattern of expansion
- Toddler compensates by swaying backwards
- When room swings away – toddler sways forwards
- 26% swayed, 23% staggered and 33% fell over!
- Adults also found to sway in phase with movements of the room
Conclusion – Optic flow important source of information for balance and can override other sources of balance information
What is total optic flow?
Total optic flow = Retinal motion (object motion + self motion)
What evidence is there for flow parsing?
Warren & Rushton (2009):
- an optic flow field influenced the perceived trajectory of a moving object even when the flow was in a different part of the stimulus
- This provides strong evidence for global processing of optic flow
What is biological motion?
- The motion of another person’s body creates a complex pattern of movement
- Local motion signals need to be integrated to recover the global pattern of motion
- We appear to be particularly adept at perceiving biological motion
What can biological motion provide information about?
Biological motion can provide information not only about what someone is doing but also:
- gender
- identity
- affect
Grossman & Blake (2001) conducted an fMRI study where participants viewed biological motion vs. scrambled biological motion
What were the results?
Area STS (superior temporal sulcus) more active for biological motion compared to scrambled
Grossman et al (2005) conducted a TMS experiment where participants viewed biological motion vs scrambled
Noise dots added to make task more difficult
What was the task?
Task –is the stimulus biological motion or scrambled?
Grossman et al (2005) conducted a TMS experiment where participants viewed biological motion vs scrambled
Noise dots added to make task more difficult
Where was TMS applied?
Applied TMS to STS and MT
Grossman et al (2005) conducted a TMS experiment where participants viewed biological motion vs scrambled
Noise dots added to make task more difficult
What were the results and conclusion?
- TMS to STS caused significant decrease in ability to distinguish biological motion from scrambled
- TMS to MT had no effect on biological motion perception
- Biological motion is a special type of complex motion processed in a specialized area of the brain – area STS
