Lecture 7 - Motion perception

Question 1

What does motion perception do for us?

Answer 1

• it keeps updating us continuously on our environment
• important for grouping and segregation

Question 2

What are the ways to perceive motion?

Answer 2

1. real motion
   - a stimulus that actually moves
2. illusory motion: apparent motion
   -> perceived motion in a static stimulus
   -> is how film works
3. illusory motion: induced motion
   -> a moving frame of reference e.g. clouds can produce the illusion that a stationary object e.g. moon moves in the opposite direction
4. illusory motion: visual illusions
   -> e.g. peripheral drift illusion
   -> however only real motion produces retinal motion
   -> retinal motion = successive receptor stimulation by a moving stimulus in the visual field

Question 3

Physiological evidence of real motion?

Answer 3

• Motion-direction selective neurons in V1
• Motion perception takes place in the dorsal stream

Question 4

Newsome et al 1995?

Answer 4

• monkey experiment with moving dot display
• measured an MT neuron’s firing rate (single cell recording) depending on motion direction coherence
• found that with increasing motion direction coherence the monkeys judged the direction of motion more accurately and the MT (middle temporal) neuron fired more rapidly
• with 12.8% coherence (25 out of 200 dots), monkeys judged motion direction more accurately in almost every trial
  -> conclusion: MT neurons are highly specialised to detect motion direction

Question 5

What is the receptive field?

Answer 5

• the part of the visual field in which a stimulus can modify a neuron’s firing rate
• a single neurons receptive field only covers a part of the visual field

Question 6

What is the aperture problem?

Answer 6

• the direction of a moving stimulus through an aperture (such as a receptive field) is ambiguous
• the motion direction is unclear when the stimulus is larger than the receptive field
• the response of an individual direction-selective neuron does not provide sufficient information to indicate the direction of the movement

Question 7

Solutions?

Answer 7

1. Pooling responses of multiple neurons
2. End stopped cells signal the end of a stimulus (where the direction is unambiguous)
   - STS: receives projections from MT and IT; combines input about object form and motion direction

Question 8

Biological motion?

Answer 8

• this is self produced motion of a biological being
• we are experts in perceiving and recognising biological motion as we are human beings

Question 9

Collary discharge theory?

Answer 9

• assumes that motion perception depends on retinal motion and eye movements
• there are 4 muscles at the top, bottom, left and right of each eye and 2 wrapped around it

Question 10

What are the 3 signals that reveal whether or not real motion has occurred?

Answer 10

1. Image displacement signal (IDS): an image moves across the retina and successively stimulates the receptors (retinal motion)
2. Motor signal: a motion command is sent from the brain to the eye muscles
3. Corollary discharge signal: copy of the motor signal

Question 11

Evidence for corollary discharge theory?

Answer 11

• there is behavioural evidence: motion perception matches the model predictions
• also neuronal evidence by Galletti & Fattori: measured real motion neurons

Question 12

Evaluation of the corollary discharge theory?

Answer 12

it successfully explains why a moving stimulus in a static or moving eye generates motion perception while a static stimulus in a moving eye does not despite the fact that there is retinal motion