Temporal Vision: Motion Perception

Question 1

What is motion?

Answer 1

A continuous change in an object’s location as a function of time.

Question 2

How is motion experienced on a retinal level?

Answer 2

Motion is the change in retinal illuminance for that area of the retina that occurs at a rate which depends on the speed at which an object moves, and its spatial frequency composition.

Question 3

What is temporal frequency experienced by the retina equal to?

Answer 3

Temporal frequency (cycles / sec or Hz) experienced by area of retina = velocity (deg / sec) x spatial frequency (cycles /deg)

Question 4

True or False- Ability to detect motion is dependent on temporal resolution of visual system.

Answer 4

True

Question 5

When is temporal resolution at its highest?

Answer 5

At medium flicker frequency at high contrast.

Question 6

What are the benefits of motion perception/ purposes of motion perception?

Answer 6

Motion perception evolved early.

Associated with survival: detection of predators and prey in periphery (stops predators being able to sneak up on you.

Helps determine 3D shapes as we look around them.

Allows us to interact with environment and to co-ordinate our movement with that of moving objects.

Question 7

What are the 5 types of motion?

Answer 7

Real Motion

Apparent Motion

Induced Motion

Motion After Effect

Optic Flow

Question 8

Describe real motion

Answer 8

The physical stimulus is moving against its background e.g. a car is moving across a street.

We will still see this object as moving even if we fixate on the car (so that the image of the car is stationary on our retina) (because the background changing indicates it is moving).

Question 9

What is apparent motion?

What is an example of the use of apparent motion?

Answer 9

The illusion of motion produced by looking at a rapid succession of static images.

Basis of films, cartoons, TV, advertising.

Question 10

What are the two types of apparent motion?

Answer 10

Beta and Phi

Question 11

What is beta motion?

Answer 11

The illusion of smooth movement from one location to the other due to steady stimulus pace e.g. if you had two lights next to eachother and switched one on and off after each other it would appear as if the light jumps from right to left.

Question 12

What is phi motion?

Answer 12

The perception of flickering lights but no smooth movement between the two because of the rapid pace at which the lights flash e.g. it would be like having two lights next to eachother that you turn on and off one after the other but because you switch them on and off so fast the lights don’t appear to jump from one location to the other , rather they are just perceived as two flickering lights.

Question 13

What is induced motion?

Answer 13

Movement of one object (usually larger one) induces perception of motion of another object.

e.g. Trains on platform (feeling that your train is moving when it’s actually the neighbouring one)

Clouds against moon on windy night (these can make the moon also look like it is moving)

Question 14

What is the motion after effect?

Answer 14

If one stares consistently at movement in a particular direction, subsequently viewed stationary scenes briefly appear to move in the opposite direction.

[It is also called the ‘waterfall’ effect]

Question 15

What is optic flow?

Answer 15

As an observer moves through the world there is a gradual shift in the pattern of light (optic array) around them.

As we walk down the street optic array elements (light from environment) flow past on all sides.

This is Optic Flow. It is caused by relative movement between observer and environment.

Question 16

Using optic flow how can we determine whether we are moving towards or away from something?

Answer 16

Question 17

Which photoreceptor has the best temporal resolution?

Answer 17

Temporal resolution is better in cones than rods as (as cones undergo less temporal summation).

Question 18

Why does the periphery have a higher temporal resolution despite the fact that there are more rods in the periphery than cones?

Answer 18

Periphery has higher temporal resolution than fovea even though more rods/less cones as Magno- cells predominate in periphery & cones in periphery process faster than foveal cones.

Question 19

Which cells, magno or parvo, have better temporal resolution?

Answer 19

Magno- (parasol) ganglion cells have transient responses →which allows for good temporal resolution.

Parvo- (midget) ganglion cells have sustained responses (i.e. response holds for some time before stopping) →this accounts for poor temporal resolution

Question 20

What is the geniculo-cortical pathway?

Answer 20

Magno- ganglion cells projecting to magno-cellular layers of the LGN (layers 1 and 2)

Question 21

What is the retinotectal pathway?

Answer 21

A small proportion of fibres projecting from the retina to the superior colliculus (involved in eye movement control)

Question 22

What are the three types of receptive fields in V1?

Answer 22

• Simple cortical cells
• Complex cortical cells
• Simple and complex end-stopped cells (previously called ‘hypercomplex’)

Question 23

Describe the receptive field of a simple cortical cell?

When is the response of a simple cortical cell highest?

When is the response of a simple cortical cell at its lowest?

Is a simple cortical cell orientation specific?

Is a simple cortical cell direction sensitive?

Answer 23

Question 24

What does the receptive field of complex cortical cells respond to?

Answer 24

Complex cells respond to gratings and edges of a particular orientation, but may also respond to moving stimuli.

They are sensitive to:

• Orientation of stimulus
• Direction of movement
• (Some cells are also sensitivie to frequency as well)

Question 25

Describe the structure of end stopped cell receptive fields.

What are end stopped cell receptive fields sensitive to?

Answer 25

Question 26

Summarise the receptive fields of:

Retinal ganglion cells

LGN cells

Simple cortical Cells

Complex Cortical Cells

End Stopped Cells

Answer 26

Question 27

Describe the motion perception pathway of the extrastriate cortex

Answer 27

Directionally sensitive cells are concentrated in layer 4B of V1.

Magnocellular information is processed in thick stripes of V2.

Beyond V2 information is channelled into dorsal (‘where’) and ventral (‘what’) pathways.

Motion information travels in the dorsal pathway to V3, then to MT (middle temporal) area. 90% of cells in MT are direction selective, many are also speed selective.

From MT much information travels to the MST (medial superior temporal) area, concerned with ‘optic flow’

Question 28

What evidence suggests that MT (the Middle Temporal area) is important for motion perception?

Answer 28

Damage to MT can result in akinetopsia

Question 29

What is akinetopsia?

Answer 29

Motion Blindness

Question 30

What evidence do we have to suggest neurones are directionally sensitive?

Answer 30

Data from microstimulation experiments. - Recorded responses from cortical neurones in response to moving stimuli. Cells in MT and V1 are direction selective. Cells in retina and LGN are not (in cat and monkey).

Deprivation Experiments:

Pasternak (1990) reared kittens in stroboscopic environment where:

→vision consisted of still snapshots of environment

→this eliminated 90% of directionally sensitive neurones in V1

→and also eliminated cat’s ability to detect direction of movement.

The mechanism behind the motion after effect is evidence of directionally sensitive neurones

Question 31

What is the mechanism behind the motion after effect?

Answer 31

When we look at a stationary object…

all motion sensitive neurones will fire weakly (this is internal background noise).

Neurones sensitive to different directions of movement (←↑→↓) will all fire at about same rate.

→thus we perceive no motion.

When we stare at moving scene (e.g. waterfall)…

Neurones sensitive to downward movement adapt à firing rate of this group of neurones is reduced as they become fatigued thus baseline firing rate of upward sensitive neurones appears greater.

As a result we perceive the opposite direction to be occurring (i.e. in this case the cliffs to be moving upwards).

Question 32

What does interocular transfer of effect mean?

Answer 32

That despite soemthing being experienced/viewed only by one eye the effect will also be percieved/shown in the left eye

Question 33

Is there an interocular transfer of the motion after effect?

Answer 33

Yes - if you viewed a waterfall only by the right eye for a prolongued period of time and then covered the right eye and opened your left eye, the left eye would experience the motion after effect - suggesting the direction sensitive cells are cortical.

[The motion after effect would be weaker in the other eye suggesting that as well as it being a binocular response there are monocular components to the effect as well]

Question 34

What are the two mechanisms for encoding motion?

Answer 34

Direction Selective motion sensors - i.e. that we detect motion is occuring because a stimulus literally moves across from the receptive field of one cell into the receptive field of another cell.

Feature Tracking - i.e. that we detect motion is occuring based on the predicted movement of the object (i.e. when we assume a light jumps from right to left when two lights are flickering next to each other)

Question 35

Which mechanism for encoding motion perception is a high level process?

Answer 35

Feature tracking

Question 36

Which mechanism for encoding motion is a low level process?

Answer 36

Direction-selective motion sensors

Short-range motion is within the spatial/temporal limits of receptive fields of motion detectors.