What is single cell recording?
An electrode is inserted into a neuron (e.g. V1) and measures the electrical activity of a single neuron
What do V1 simple cells respond to?
Oriented bars and edges
How did Hubel and Wiesel find oriented bar detectors in the V1 of cats?
By using single cell recording
Do we have a visual system similar to cats?
Yes
What is the explanation for the tilt after effect?
- Orientation tuned neurons respond best to preferred orientation but also respond other similar orientations
- the perceived orientation is determined by the distribution of responses across cells (the visual system finds peak of distribution of responses across all the cells)
- Adaptation. The cell’s response decreases following prolonged activity (after staring at the vertical image for a minute the response will decrease)
What is the tilt after effect?
When we adapt to a particular orientation when we look at vertical stimuli we will perceive the lines to tilt in the opposite direction
What do the size of receptive fields in V1 vary in terms of?
Size and orientation of the receptive field
What is the size after effect?
After adapting to larger bars we will perceive test bars as smaller and vice versa
What is the size after effect explanation?
- Before adaptation size is perceived vertically
- all cells are tuned to vertical lines of different width
- they adapt to fatter bars
- during adaptation the cells’ response decreases
- after adaptation the line looks thinner due to asymmetrical response distribution
- no longer symmetrical about a specific cell (e.g. cell 3) because cell 4 was adapted but cell 2 wasn’t
- so the peak is shifted to narrower bars
What do the size and tilt after effect provide evidence for?
- tilt after effect provides evidence for orientation tuned cells in the human visual system
- the size after effect provides evidence for size tuned cells in the human visual system
What is the spatial frequency?
The number of bars per unit distance (usually cycles per degree)
What is a cycle?
White to black
Do fat bars have a high or low spatial frequency?
Low
Do high spatial frequency or low spatial frequency show fine details?
High spatial frequency?
Does high spatial frequency of low spatial frequency show coarse information?
Low spatial frequency
What is contrast and what does it mean if something is high contrast?
Contrast is the difference in luminance between lighter and darker areas
High contrast means that there is a big difference between the lightest area and darkest area
How are spatial frequency and contrast related?
With high contrast lines we can perceive them at all the different spatial frequencies When the stimulus is low contrast it’s harder to see low and high spatial frequencies (see the bars). You can see intermediate spatial frequencies.
What is the spatial contrast sensitivity function?
- we have greater sensitivity to intermediate spatial frequencies. They can be perceived at low contrasts
- we have lower sensitivity to high and low spatial frequencies. They need higher contrast to be perceived
What are the groups of cells that respond to different spatial frequencies referred to as?
Channels
Why is it hard to recognise faces that are far away?
The fine details become really high spatial frequency and our channels are no longer sensitive
What happens in the periphery in terms of spatial contrast sensitivity function?
- receptive field sizes increase in the periphery
- this means contrast sensitivity varies with eccentricity - we can’t see high spatial frequencies in the periphery so acuity is poorer
Does spatial frequency tell us about size on the retina or real world size?
Size on the retina. Not real size in the world since the projected size depends on distance
- bars of different widths at different distances can project to the same spatial frequency (SF) on the retina
What provides evidence that our conscious perception is in terms of real size not size on the retina?
Burbeck 1987:
- stimuli 1 and 2 are at different distances from the participant
- discrimination experiment: ask which stimulus has thinner bars
- discriminating retinal size - task is very hard
- discriminating real size - task is easy
What is size constancy?
We perceive an object’s real size in the world regardless of distance
