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
