From Retina to the Brain 2 Flashcards
What is meant by the extrastriate?
Areas in the visual cortex outside of the the area V1
How can you explain the gold and white/ blue and black dress illusion?
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)?
Therefore is colour a wavelength?
No, it is simply how your brain interprets the coming wavelengths
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
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
What did Victor then indicate was the primary function of colour vision
To judge the properties of the things you’re looking at, particularly food (ripeness, identifying different plants etc)
Where in the visual cortex may colour constancy emerge? Give reasoning
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
What do lesions to areas V4 and V8 result in?
Cortical colour blindness (achromatopsia)
How is cortical colour blindness different to retinal colour blindness?
All colour is gone, rather than the perception of one wavelength. Can be pretty depressing.
There is a theory that colour constancy already takes place at the retina. How would this work?
The horizontal cells which move across a range of receptive fields
How is achromatopsia tested?
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)
Why are certain colours appealing to us?
Colour is brain code for tasty or nasty, thats why we have developed colour vision and colour constancy
How has this fact affected marketing?
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.
Why does the train illusion make you perceive the train as either moving forwards or backwards?
The high contrast contours of the train are moving in opposite directions
When is apparent motion detected?
When a stimulus goes off in one place and goes on in another (bird in cage, Zoetrope movies, dots disappearing and reappearing)
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?
Colour aftereffect (green appearing) Apparent motion (looking like the blank place is moving) Troxler fading (the purple dots disappearing)
Describe the ‘Reichardt detector’ model for direction selectivity
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
What is particularly highly debated about this theory?
A plausible physiological mechanism for this delay is still highly debated
What is another way of viewing motion?
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
Where are these motion sensitive visual cells found?
Throughout the visual cortex but especially V1, V3 and MT (in some animals such as rabbits they are even found in the retina!)
What is meant by the aperture problem?
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
How has this motion problem been studied?
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.
Describe what a component cells detect
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
Describe what is detected by a ‘pattern cell’ during this test
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
Where are component and pattern cells in the visual cortex?
V1 cells are all component cells
MT about 50% are pattern cells
What do MST neurons respond selectively to?
Particular motion flow fields. These are often caused by self motion (surroundings as you walk)
How can these MST flow field selective neurons differ in basic ways? (3)
some are horizontally and vertically direction sensitive
Some are sensitive to rotation
some are sensitive regarding movement coming to and from you
Name another more complex way in which patterns in motion are detected
Biological motion: recognising species, sex, mood etc from movement
What brain area is particularly sensitive to biological
Superior Temporal Sulcus is selectively activated by biological compared to non-biological (scrambled) motion
Name and describe a disorder associated with motion
Zihl’s Motion blind patient: Sees only stationary scenes, with objects flashing from one position to the next (Akinetopsia)
What damage to the brain causes Akinetopsia? (3)
bilateral MT, MST, STS lesion
How can one study the absence of motion perception in normal people?
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.
Describe distributed hierarchal processing of visual features in regards to colour (2)
Wavelength (V1, V2) => Colour constancy (V4)
Describe distributed hierarchal processing of visual features in regards to motion (3)
Component (V1, V3, MT) => Optic flow (MST) => Biological movement (STS)
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?
That motion ‘wins’ from colour (and any other feature change)
e.g rotating colour wheel
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?
Magnocellular- Motion (Dorsal pathway)
Parvocellular- Colour (Ventral pathway)
What two broad ranges of ways can we perceive depth?
Monocular depth cues- using one eye
Binocular depth cues- using two eyes
Name 8 monocular depth cues
- perspective
- Size constancy
- Occlusion
- Cast shadows
- Shading and contour
- Areal/ Atmospheric perspective
- Texture
- Motion parallax
Name a binocular depth cue
Disparity
Describe the monocular depth cue of linear perception
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
Describe size constancy as a depth cue
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.
Describe a visual illusions that demonstrates linear perspective
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
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?
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
Describe occlusion as a depth cue
If one object is blocking another, we assume that it is in front of it
How do cast shadows help us perceive depth?
Positioning of the shadow behind an object help us conclude how far an object is from the object behind us
How does shading and contour help us perceive depth
Come on dawg, you did art in school you know dis brather
It gives an indication of where the light is coming from
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?
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
What is meant by aeriel of atmospheric perspective?
Vague, low contrast things are usually perceived as further away than more detailed, high contrast objects.
(often used as distant landscapes in painting)
How is texture relevant to depth?
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
What is meant by motion parallax?
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
What is peculiar about motion parallax in a train?
Since you are travelling fast in a linear fashion, the foreground moves much faster than the background
