L4 - Spatial Vision Flashcards
Who discovered receptive fields of cells?
Hubel and Wiesel (1950)
Define textons
The simplest defining properties of textures
Describe the features of textons
- elongated
- line endings
- line crossings
What did Nothdurft (1990) find about orientation?
It’s not just orientation that’s important, orientation close to the boundary is very important too.
What is local orientation contrast?
Difference in orientation close to the boundary.
Are orientation doubly opponent cells sensitive to the orientation of the line?
No - it will respond to any orientation of the line within the cell’s receptive field.
What is the tilt after-effect?
When neurons adapt to lines of a certain orientation and then vertical lines are looked at, the result is that the vertical lines no longer look (in this case) vertical. Further to this, the initially vertical lines will now look like they are the opposite orientation to the lines you adapted to.
What are the implications of the tilt after effect?
We have orientation selective neurons
What is the distance paradox?
The finding that the biggest effect demonstrating the tilt after effect is found a little away (but not too far from the test pattern
What is the resolution limit?
The highest spatial frequency that we can see
What is the spatial contrast sensitivity function?
A graph showing how sensitive we are to contrasts at different spatial frequencies
What is spatial frequency?
Number of bars that would fit in a certain distance.
What does the contrast sensitivity function describe?
Our window of visibility - anything above the curve is invisible to us.
What are channels?
Collection of neurons carrying out the same function
What ability do we lose as we move into our peripheral vision?
Ability to see high spatial frequencies
Why does visual acuity/ability to see high spatial frequencies decrease the further we go into our peripheral vision?
We have larger receptive fields in our peripheral vision
What creates the impression of extra height, horizontal or vertical lines?
Horizontal
What creates the impression of extra width, horizontal lines or vertical lines?
Vertical (contrary to popular belief)
What does the Oppel-Kundt Illussion show?
Illusions of filled extent - filled spaces look bigger than unfilled spaces.
Why do horizontal stripes on a dress make the wearer look taller and narrower?
Because the horizontal stripes fill more space than vertical stripes
Why can our visual system not perceive orientation of lines as a result of the neurons which respond most intensely?
Because then we would only be able to respond to as many orientations of lines that we have corresponding neurons for. A line between these orientations would be misperceived.
Explain why the tilt-after effect occurs.
When a particular neuron responds to a line that is of it’s preferred orientation, neurons that prefer slightly different orientations also fire, but not as much. Neurons that have significantly different orientations do not fire at all. Therefore, when we look at a vertical line after we have adapted to lines at a specific orientation, half of the neurons that are typically activated, do not respond as much as they normally do. This means that neurons to one side of the vertical line fire more, meaning we no longer perceive it as a vertical line, for a brief amount of time.
What is the tilt-specific threshold elevation?
The effect seen when we adapt to bars/lines at a specific orientation. As neurons associated with these orientations begin to lose sensitivity, stimuli will require higher (elevated) contrasts for the same neurons to detect it and fire in response.
What is the size after-effect?
The change in perceived thickness of bars when we adapt to thicker/thinner bars.