Spatial Vision

Question 1

What is single cell recording?

Answer 1

An electrode is inserted into a neuron (e.g. V1) and measures the electrical activity of a single neuron

Question 2

What do V1 simple cells respond to?

Answer 2

Oriented bars and edges

Question 3

How did Hubel and Wiesel find oriented bar detectors in the V1 of cats?

Answer 3

By using single cell recording

Question 4

Do we have a visual system similar to cats?

Answer 4

Yes

Question 5

What is the explanation for the tilt after effect?

Answer 5

1. Orientation tuned neurons respond best to preferred orientation but also respond other similar orientations
2. 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)
3. Adaptation. The cell’s response decreases following prolonged activity (after staring at the vertical image for a minute the response will decrease)

Question 6

What is the tilt after effect?

Answer 6

When we adapt to a particular orientation when we look at vertical stimuli we will perceive the lines to tilt in the opposite direction

Question 7

What do the size of receptive fields in V1 vary in terms of?

Answer 7

Size and orientation of the receptive field

Question 8

What is the size after effect?

Answer 8

After adapting to larger bars we will perceive test bars as smaller and vice versa

Question 9

What is the size after effect explanation?

Answer 9

• 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

Question 10

What do the size and tilt after effect provide evidence for?

Answer 10

• 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

Question 11

What is the spatial frequency?

Answer 11

The number of bars per unit distance (usually cycles per degree)

Question 12

What is a cycle?

Answer 12

White to black

Question 13

Do fat bars have a high or low spatial frequency?

Answer 13

Low

Question 14

Do high spatial frequency or low spatial frequency show fine details?

Answer 14

High spatial frequency?

Question 15

Does high spatial frequency of low spatial frequency show coarse information?

Answer 15

Low spatial frequency

Question 16

What is contrast and what does it mean if something is high contrast?

Answer 16

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

Question 17

How are spatial frequency and contrast related?

Answer 17

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.

Question 18

What is the spatial contrast sensitivity function?

Answer 18

• 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

Question 19

What are the groups of cells that respond to different spatial frequencies referred to as?

Answer 19

Channels

Question 20

Why is it hard to recognise faces that are far away?

Answer 20

The fine details become really high spatial frequency and our channels are no longer sensitive

Question 21

What happens in the periphery in terms of spatial contrast sensitivity function?

Answer 21

• 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

Question 22

Does spatial frequency tell us about size on the retina or real world size?

Answer 22

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

Question 23

What provides evidence that our conscious perception is in terms of real size not size on the retina?

Answer 23

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

Question 24

What is size constancy?

Answer 24

We perceive an object’s real size in the world regardless of distance

Question 25

What is orientation constancy?

Answer 25

We perceive an object’s orientation in the world regardless of the orientation on the retina