Lecture 7 - Motion Perception (not MCQ) Flashcards
How do we detect motion?
Cells in V1 respond to lines/edges moving particular direction
What is local motion?
Motion of individual (local) elements
What is global motion?
Group motion of many individual elements to perceive complex pattern of global motion
To process: pool info from multiple local motion detectors
What is motion coherence?
Set of moving dots (signal/noise), signal move in coherent direction whille noise distracts
What is the motion coherence threshold?
Minimum proportion of signal dots needed to detect coherent motion, depends on proportion of signal to noise dots
Humans: 10% (5% when highly practiced) signal dots needed
Diff between global/local motion detectors?
Local motion detectors: small receptive fields
Global motion detectors: large receptive fields
Where does the brain process motion and how do we know?
Area MT (middle temporal)
Single cell recording: nearly all cells in area MT respond to motion + have preferred direction
Artificial Stimulation: Salzman et al. (1990) cells in monkey area MT that all had same preferred direction + artificial stimulation of cells led to motion judgements biased towards preferred direction
Imaging: Tootell et al. (1995) did fMRI study of motion after-effect, when we adapt to motion then view stationary test experience motion in opposite direction
Lesions: Newsome/Pare (1988) introduced small lesion to monkey MT
Undamaged threshold = 5%, damaged threshold = 80%
What is optic flow? (OF)
Patterns of retinal motion produced when we move
What is expansion? (OF)
Created by forward translation (moving forward)
Focus point of expansion: stationary while everything gets bigger on retina
What is contraction? (OF)
Created by backwards translation (moving backwards)
What is horizontal - constant speed/parallax? (OF)
Constant speed: created by eye/head/body rotation
All objects move at same speed across retina regardless of depth
Parallax: created by lateral (sideways) translation
Closer objects move faster on retina than further objects
What is roll? (OF)
Created by eye/head/body roll
OF is complex, can have forward translation/head rotation/combined
How did Smith image OF?
fMRI study to compare 2 cortical sub-regions (MT/MST)
Measured response of MT/MST to 5 types of motion (complex, expansion, rotation, translation, random), interested in diff in response to optic flow compared to random
Found diff in response to optic flow compared to random motion greater in MST than MT –> MST more specialised for processing optic flow motion than MT
Name 3 ways optic flow is used
Heading (tell us where we’re going) - not in driving though
Postural stability (balance relies on visual info too) - toddlers fell over in swinging room experiment, adults also swayed with room
Perception of object motion during self motion - self optic flow + object optic flow = total optic flow
What is the flow-parsing hypothesis?
Retinal motion due to self-motion subtracted + remaining motion attributed to object motion
Evidence: Warren/Rushton (2009) showed optic flow field influenced perceived trajectory of moving object even when flow in diff part of stimulus
What is biological motion?
Motion of another person’s body creates complex pattern of movement
Local motion signals need to be integrated to recover global pattern; we appear to be particularly adept at perceiving biological motion
How is our ability for biological motion demonstrated?
Point-light walker stimuli – small lights placed on person’s joints, biological motion can give info not only about what they’re doing but also gender/identity/affect
How do we process biological motion?
Grossman/Blake fMRI study: participants viewed biological motion vs scrambled biological motion, found Area STS more active for biological motion than scrambled
Grossman et al. TMS study: participants viewed biological/scrambled, noise dots added to make task difficult, find if stimulus biological motion/scrambled
TMS to STS caused significant decrease in ability to distinguish biological from scrambled, TMS to MT had no effect
What can we conclude about biological motion processing?
Conclude biological motion special type of complex motion processed in specialised area of brain – area STS
