5. Spatial Vision Flashcards
What is spatial vision
Ability to resolve detail and process lines and edges
What do V1 simple cells respond to
oriented bars and edges (lines)
Why is there a bell shaped distribution for orientation tuned neurons
Neurons have a preferred orientation they fire to, but will also fire (less) to other similar orientations
What happens if you see a line stimulus which doesnt perfectly match and neurons preferred orientation?
There will be a distribution of responses and the visual system will use the peak to perceive an angle somewhere in between
What is adaptation in response to a stimulus?
The cell will start to respond less following long periods of being active
Explain the tilt after effect if the test stimulus are vertical lines and the adaptation stimulus is diagonal lines to the left
- before adaptation, cell 3 that prefers vertical lines is responding most to the test. 2 and 4 are slightly active.
- during adaptation, 2 is responding most but after a while will adapt and respond less
- when test presented again, there will be less response from 2 and 3, but 4 will respond larger relative to the others
- will percieve tilted lines in the opposite direction
Explain the tilt after effect if the test stimulus are vertical lines and the adaptation stimulus is diagonal lines to the left
- before adaptation, cell 3 that prefers vertical lines is responding most to the test. 2 and 4 are slightly active.
- during adaptation, 2 is responding most but after a while will adapt and respond less
- when test presented again, there will be less response from 2 and 3, but 4 will respond larger relative to the others
- will percieve tilted lines in the opposite direction
What influences the size of the tilt after-effect
the difference between the adaptation stimulus and the test stimulus
You will get a smaller tilt after-effect if there is a _ difference between test and adaptation (bigger or smaller)
bigger
If you make the adaptation and test stimuli more similar, you get a _ tilt after-effect (larger, smaller)
Larger
What is the peak degree difference between stimuli for a strong tilt-aftereffect
10-20 degree difference
The tilt after effect provides evidence for:
orientation tuned cells in the human V1
Explanation of the size after-effect if the test lines are intermediate thickness (Cell 3), and adapt stimulus are fat lines (cell 4)
The test stimulus will activate cell 3 most, and 2 and 4 slightly. Then when a fatter adapt stimulus presented, cell 4 would be most active, and 3 and 5 slightly active. Eventually cell 4 would adapt
When test stimulus presented again, cells 2 and 3 would respond more strongly relative to cell 4, making the lines appear thinner.
Size after-effect provides evidence for:
size tuned cells in V1
‘Fat’ bars have a __ spatial frequency (high or low)
Low - less fit in given space on retina
‘Slim’ bars have a __ spatial frequency
High
Spatial frequency is:
the number of bars per unit distance
What is used to define the size of bars on the retina
Spatial frequency
High spatial frequency in natural images corresponds to:
Finer details of the image ie contours of a face
High contrast
there is a large difference between light and dark areas
Low contrast
there is a low difference in light/dark regions
How does contrast sensitivity affect spatial frequency?
Can see a larger range of SFs at higher contrast (bell shaped distribution)
We have the lowest sensitivity to these types of spatial frequency (high, low, intermediate)?
High and low - need higher contrast to see
We are most sensitive to this spatial frequency (high, low, intermediate)
Intermediate - can see at lower contrasts
Why is it harder to recognise faces from far away?
The fine details have a much higher spatial frequency that becomes invisible to us as there aren’t cells with small enough RFs
Collections of neurons tuned to the same SFs are known as:
Spatial frequency channels
In V1, cells with smaller receptive fields are correspond to which part of the retina:
The fovea
Why do V1 cells corresponding to the fovea have smaller RFs?
To enable us to see finer detail (higher SFs)
Size constancy
we perceive an objects real size in the world regardless of distance and the size on the retina
if you adapt neurons to a line that is 10 degrees anticlockwise to vertical, and then see a vertical line, how will it appear?
will appear to be less than 10 degrees clockwise from vertical
