From Retina to the Brain 2

Question 1

What is meant by the extrastriate?

Answer 1

Areas in the visual cortex outside of the the area V1

Question 2

How can you explain the gold and white/ blue and black dress illusion?

Answer 2

When you think the dress is in the shadow, you interpret it as white and gold when you assume it is in full light, you interpret it as blue and black

Black and blue in sunlight emit exactly the same wavelengths as gold (yellow) and white in the shadow.

In this case your brain does nnit have sufficient information about the lighting conditions: is this bright sunlight (so that the dress is in the shade)? Is everything over exposed (so that the dress is in the sunlight)?

Question 3

Therefore is colour a wavelength?

Answer 3

No, it is simply how your brain interprets the coming wavelengths

Question 4

In the lecture, first three circles where shown which appeared green in colour. When the entire picture was revealed, it had a green overtone all over it but these circles now revealed the perception of red strawberries.

What phenomena did this demonstrate? Give an example

Answer 4

Colour constancy: we perceive the same colour despite differences in illumination. The wavelength of the light from the banana is very different in the morning versus the evening, yet it is perceived as yellow the whole day.

Perceived colours are not absolute wavelength, but are ‘computed’ by the brain depending on the wavelengths of the surround

Question 5

What did Victor then indicate was the primary function of colour vision

Answer 5

To judge the properties of the things you’re looking at, particularly food (ripeness, identifying different plants etc)

Question 6

Where in the visual cortex may colour constancy emerge? Give reasoning

Answer 6

Zeki: Colour constancy emerges at the level of V4. V4 receptive field is large enough to integrate numerous colour opponent signals and ‘discount’ the illuminant

Also colour is perceived on the level of V4 as what we consciously perceive, not just the wavelength emitted by it

However there are still a lot of discussions about this and it is not certain

Question 7

What do lesions to areas V4 and V8 result in?

Answer 7

Cortical colour blindness (achromatopsia)

Question 8

How is cortical colour blindness different to retinal colour blindness?

Answer 8

All colour is gone, rather than the perception of one wavelength. Can be pretty depressing.

Question 9

There is a theory that colour constancy already takes place at the retina. How would this work?

Answer 9

The horizontal cells which move across a range of receptive fields

Question 10

How is achromatopsia tested?

Answer 10

A colour is presented in a circle (eg red) and a ‘hue angle’ in present in degrees (eg red to yellow) and a patient is asked to match where the is on the gradient

(sometimes the achromatopsia may only be present in a certain quadrant of the visual field)

Question 11

Why are certain colours appealing to us?

Answer 11

Colour is brain code for tasty or nasty, thats why we have developed colour vision and colour constancy

Question 12

How has this fact affected marketing?

Answer 12

Some colours we do not find attractive. For example it is the trend for ‘light’ version product packaging to be blue. However blue is not an appetising colour and so these products seem less appealing to us.

Question 13

Why does the train illusion make you perceive the train as either moving forwards or backwards?

Answer 13

The high contrast contours of the train are moving in opposite directions

Question 14

When is apparent motion detected?

Answer 14

When a stimulus goes off in one place and goes on in another (bird in cage, Zoetrope movies, dots disappearing and reappearing)

Question 15

The lecture shows a ring of purple dots, and one appears before reappearing shortly after as the next one disappears. If you stare in the middle, the purple dots start to disappear and a green dot is perceived as going around in a circle.

What three effects does this display?

Answer 15

Colour aftereffect (green appearing)
Apparent motion (looking like the blank place is moving)
Troxler fading  (the purple dots disappearing)

Question 16

Describe the ‘Reichardt detector’ model for direction selectivity

Answer 16

Its a very simplified (almost naive) model for direction selectivity

The cell receives input from two cells that have spatially separate receptive fields. One of the cells has a delayed input. Only when the stimulus moves in the right direction, the cell receives simultaneous input from both cells and will fire

Question 17

What is particularly highly debated about this theory?

Answer 17

A plausible physiological mechanism for this delay is still highly debated

Question 18

What is another way of viewing motion?

Answer 18

A bar is moving to the right (x direction) can be seen as a diagonal beam in 3D space where position is X and Y and time (t) is the third dimension.

Looked at it this way, direction selectivity is the same as ‘orientation selectivity but now in the X-T plane.

Cells are tuned to particular speeds and directions of motion

Question 19

Where are these motion sensitive visual cells found?

Answer 19

Throughout the visual cortex but especially V1, V3 and MT (in some animals such as rabbits they are even found in the retina!)

Question 20

What is meant by the aperture problem?

Answer 20

Detecting motion through an aperture (gap) is ambiguous, many motion vectors can yield the same motion through an aperture. This is especially a problem in V1, as they only have small receptive fields

Question 21

How has this motion problem been studied?

Answer 21

By showing a circle with two gratings (diagonal perpendicular stripes) with orthogonal movement results in ‘plaid motion’ or ‘pattern motion’. your perception combines the two motions into a third, new motion direction (vector sum) of the plaid, and seeing the two ‘motion components’ is very difficult.

Cells that still encode the independent component motion vectors suffer from the aperture problem. Cells that combine the two motions and encode the plaid motion direction have solved the aperture problem.

Question 22

Describe what a component cells detect

Answer 22

This cell is tuned for movement upward (0 deg) when tested with a grating (open circles, solid line)

When tested with plaid motion patterns, it is no longer tuned for movement of the plaid upwards but it shows two peaks for each individual component (red or green) moving upward.

Hence this cell is not ‘seeing’ the pattern motion, but the movement of the cell components

Question 23

Describe what is detected by a ‘pattern cell’ during this test

Answer 23

This cell is also tuned for movement upwards (0 degrees) when tested with a grating (open circles, solid line)

When tested with plaid motion patterns, it is also tuned for movement of the plaid upwards (black arrow, grey circles).

Hence, this cell is ‘seeing’ the pattern motion, no longer the movement of the individual components.

This helps to solve the aperture problem

Question 24

Where are component and pattern cells in the visual cortex?

Answer 24

V1 cells are all component cells

MT about 50% are pattern cells

Question 25

What do MST neurons respond selectively to?

Answer 25

Particular motion flow fields. These are often caused by self motion (surroundings as you walk)

Question 26

How can these MST flow field selective neurons differ in basic ways? (3)

Answer 26

some are horizontally and vertically direction sensitive
Some are sensitive to rotation
some are sensitive regarding movement coming to and from you

Question 27

Name another more complex way in which patterns in motion are detected

Answer 27

Biological motion: recognising species, sex, mood etc from movement

Question 28

What brain area is particularly sensitive to biological

Answer 28

Superior Temporal Sulcus is selectively activated by biological compared to non-biological (scrambled) motion

Question 29

Name and describe a disorder associated with motion

Answer 29

Zihl’s Motion blind patient: Sees only stationary scenes, with objects flashing from one position to the next (Akinetopsia)

Question 30

What damage to the brain causes Akinetopsia? (3)

Answer 30

bilateral MT, MST, STS lesion

Question 31

How can one study the absence of motion perception in normal people?

Answer 31

By making a virtual lesion of MT (V5) using TMS. Although the disruption of movement is not as great as it is in patients with Akinetopsia and so stimuli is used that is already a bit vague. (using random dots that move in different directions). TMS is then applied when dots move in a coherent motion sequence which is usually perceived by people who do not have this temporary lesion.

It is only when TMS is applied to this areas V5 (MT) that this applies.

Question 32

Describe distributed hierarchal processing of visual features in regards to colour (2)

Answer 32

Wavelength (V1, V2) =>
Colour constancy (V4)

Question 33

Describe distributed hierarchal processing of visual features in regards to motion (3)

Answer 33

Component (V1, V3, MT) =>
Optic flow (MST) =>
Biological movement (STS)

Question 34

Some studies were conducted in which brain activity was measured when colour was shown, when motion was shown and when both were shown

What was determined?

Answer 34

That motion ‘wins’ from colour (and any other feature change)

e.g rotating colour wheel

Question 35

Which of the magnocellular and parvocellular pathways encode 1) motion and 2) colour. Also which pathways regarding their orientation in the brain are they related to?

Answer 35

Magnocellular- Motion (Dorsal pathway)

Parvocellular- Colour (Ventral pathway)

Question 36

What two broad ranges of ways can we perceive depth?

Answer 36

Monocular depth cues- using one eye

Binocular depth cues- using two eyes

Question 37

Name 8 monocular depth cues

Answer 37

• perspective
• Size constancy
• Occlusion
• Cast shadows
• Shading and contour
• Areal/ Atmospheric perspective
• Texture
• Motion parallax

Question 38

Name a binocular depth cue

Answer 38

Disparity

Question 39

Describe the monocular depth cue of linear perception

Answer 39

For example in train tracks, when we see the planks we assume are of equal length becoming progressively shorter we assume that they are going further away

Question 40

Describe size constancy as a depth cue

Answer 40

The relative size of an object serves as an important monocular cue for depth perception. It works like this: If two objects are roughly the same size, the object that looks the largest will be judged as being the closest to the observer.

Question 41

Describe a visual illusions that demonstrates linear perspective

Answer 41

Ames room: what appears a square room is actually a very abnormally shaped room making people at each end of the room appear as either very large or very short

Question 42

There was a paper bridge shown in the lecture in which little balls seemed to travel from the bottom to the higher level of the bridge and rest there. What illusion is this an example of and why is it significant that it was recorded with a camera?

Answer 42

This takes advantage of our linear perspective of depth. It is significant we see it on a screen because we would easily be able to spot the illusion in real life with binocular depth cues

Question 43

Describe occlusion as a depth cue

Answer 43

If one object is blocking another, we assume that it is in front of it

Question 44

How do cast shadows help us perceive depth?

Answer 44

Positioning of the shadow behind an object help us conclude how far an object is from the object behind us

Question 45

How does shading and contour help us perceive depth

Answer 45

Come on dawg, you did art in school you know dis brather

It gives an indication of where the light is coming from

Question 46

In an illusion in which a mask is seen to rotate, we seem to perceive each side as a face projecting outwards. In this instance, our shading and contour depth cues seem to fail us. Why is this?

Answer 46

This seems to due to the effect of knowledge. This is true for faces in which there is something special going on as it is very difficult for us to perceive a nose going inwards

Question 47

What is meant by aeriel of atmospheric perspective?

Answer 47

Vague, low contrast things are usually perceived as further away than more detailed, high contrast objects.

(often used as distant landscapes in painting)

Question 48

How is texture relevant to depth?

Answer 48

We look at how the texture of an object becomes more dense or the features of a texture (e.g bubbles in bubble wrap) become closer together and smaller (foreshortening) to perceive depth and folds in an object

Question 49

What is meant by motion parallax?

Answer 49

You can judge how for an object is from you by the speed at which it moves in comparison to the background or objects around it when you rotate around it. the object in from of you moves faster than the background

Question 50

What is peculiar about motion parallax in a train?

Answer 50

Since you are travelling fast in a linear fashion, the foreground moves much faster than the background