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.
Why does the size after effect occur?
Neurons of receptive fields of the associated width as (e.g.) fatter bars in the ‘adapt’ stimuli fire and are eventually inhibited, so lose sensitivity. When the standard test stimuli are then viewed, they will appear thinner, due to the decrease in response and contribution from neurons that have adapted.
What is the simultaneous tilt illusion?
The appearance that vertical bars are tilted when they are surrounded by oppositely tilting bars. E.g. bars tilted 20 degrees to the right would make vertical bars appear slightly tilted to the left.
What is the simultaneous size illusion?
The perception that one set of bars are thinner than another identical set of bars. Bars that are surrounded by fatter bars will look thinner and bars surrounded by thinner bars will look fatter.
What are titchener’s circles/Ebbinghaus illusion?
Two circles of same size surrounded by a series of larger or smaller circles, creating the illusion that they are different sizes. The circle surrounded by the larger circles looks smaller than the circle surrounded by the smaller ones.
Where in the brain do after-effects occur?
Areas close to V1, V2, V4, V5 and also higher visual areas such as the FFA.
How are simple cells constructed?
Combining the outputs of non-oriented LGN axons in the correct position and orientation.
What is the main difference between simple cells and LGN axons?
Simple cells are orientation selective/specific, while LGN axons are not.
How are complex cells constructed?
Wiring together the appropriately oriented simple cells. OR operations are then carried out for the complex cell to fire, as it uses the output of different simple cells depending on where the stimulus is in it’s receptive field.
What are the two main differences between simple and complex cells?
- Complex cells’ receptive fields do not have distinctive ON/OFF regions.
- Complex cells are phrase insensitive –> they give roughly the same response across their receptive field, unlike simple cells which may be inhibited, inactive or firing rapidly due to the position of the stimulus in their receptive field (they are phrase sensitive).
How is a hyper-complex cell constructed?
Combining several complex cells
What are the main differences between complex and hyper-complex cells?
- Hyper-complex cells are end-stopped. That is, they are sensitive to the length of lines in their receptive field like complex cells, but lines greater than optimal length and outside their receptive field will have an inhibitory effect. This is not the case with complex cells - their response remains at the optimum level.
Why might hyper-complex cells be end-stopped?
Could be that complex cells have inhibitory regions at either end of the receptive fields of hyper-complex cells.
What are trigger features?
The specific combination of characteristics of a stimulus which a corresponding neuron responds to.
What is the neural doctrine?
Just as physical stimuli directly cause receptors to initiate neural activity, so the active high-level cells directly and simply cause the elements of our perception.
