Lecture 7 - Motion Perception (not MCQ)

Question 1

How do we detect motion?

Answer 1

Cells in V1 respond to lines/edges moving particular direction

Question 2

What is local motion?

Answer 2

Motion of individual (local) elements

Question 3

What is global motion?

Answer 3

Group motion of many individual elements to perceive complex pattern of global motion

To process: pool info from multiple local motion detectors

Question 4

What is motion coherence?

Answer 4

Set of moving dots (signal/noise), signal move in coherent direction whille noise distracts

Question 5

What is the motion coherence threshold?

Answer 5

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

Question 6

Diff between global/local motion detectors?

Answer 6

Local motion detectors: small receptive fields

Global motion detectors: large receptive fields

Question 7

Where does the brain process motion and how do we know?

Answer 7

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%

Question 8

What is optic flow? (OF)

Answer 8

Patterns of retinal motion produced when we move

Question 9

What is expansion? (OF)

Answer 9

Created by forward translation (moving forward)

Focus point of expansion: stationary while everything gets bigger on retina

Question 10

What is contraction? (OF)

Answer 10

Created by backwards translation (moving backwards)

Question 11

What is horizontal - constant speed/parallax? (OF)

Answer 11

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

Question 12

What is roll? (OF)

Answer 12

Created by eye/head/body roll

OF is complex, can have forward translation/head rotation/combined

Question 13

How did Smith image OF?

Answer 13

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

Question 14

Name 3 ways optic flow is used

Answer 14

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

Question 15

What is the flow-parsing hypothesis?

Answer 15

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

Question 16

What is biological motion?

Answer 16

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

Question 17

How is our ability for biological motion demonstrated?

Answer 17

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

Question 18

How do we process biological motion?

Answer 18

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

Question 19

What can we conclude about biological motion processing?

Answer 19

Conclude biological motion special type of complex motion processed in specialised area of brain – area STS