5. Spatial Perception

Question 1

If a cell responds strongly to a given stimulus, how will the cells’s response change following prolonged exposure to that stimulus?

Answer 1

the cell’s response will decrease

Question 2

What is a cell’s receptive field?

Answer 2

• A cell’s receptive field is the area on the retina which, when stimulated by light, elicits a change in the firing rate of the cell.
• The effect of the stimulus can be either excitatory or inhibitory.

Question 3

Describe Single Cell Recording

Answer 3

• An electrode, inserted into a neuron (e.g. V1) measures electrical activity
• Activity is that of a SINGLE neuron.

Question 4

What is the order in which the receptive fields of cells increase in complexity?

Answer 4

from most simple:

1. photoreceptors
2. ganglion cells
3. LGN cells
4. Simple cells
5. Complex Cells
6. Hypercomplex cells

Question 5

How do cells in V1 respond?

Answer 5

• V1 simple cells respond to oriented bars and edges

○ Respond to lines with particular orientation

Question 6

Describe the Tilt After-Effect

Answer 6

People perceive that the vertical pattern appears to be tilted in the opposite direction to the tilted pattern.

Question 7

Explain the 3 components of the Tilt After-Effect

Answer 7

1. Orientation tuned neurons respond best to preferred orientation but also respond to other similar orientations
2. Perceived orientation determined by distribution of responses across cells to find the degree of orientation. Cells have a preferred orientation
3. Adaptation – cell’s response decreases following prolonged activity.

Question 8

Describe the process of the Tilt After-Effect

Answer 8

1, Start of adaptation tilted line looks tilted

2. During adaptation tilted line continues to look tilted, but cells’ responses decrease
3. After adaptation vertical line looks tilted due to asymmetrical response distribution
4. Size of after-effect depends on difference between adapt
5. Wont get an after effect if you test at vertical and adapt at vertical

Question 9

What evidence does the Tilt After-Effect provide about Spatial Vision?

Answer 9

• Tilt after-effect provides evidence for orientation tuned cells in human visual system
• Shows we can use psychophysics to make inferences into what is happening in the visual pathway.

Question 10

What evidence does the Size After-Effect provide about Spatial Vision?

Answer 10

Size after-effect provides evidence for size-tuned cells in human visual system.

Question 11

Explain the Size After-Effect.

Answer 11

• Before adaptation size perceived veritically.
• Adapt to fatter bars
• After adaptation lines look thinner due to asymmetrical response distribution
• During adaptation cells’ response decreases

Question 12

Define spatial frequency.

Answer 12

= number of bars per unit distance (usually cycles per degree)
○ Fat bars = low spatial frequency
○ Thin bars = high spatial frequency

Question 13

In a natural images what do spatial frequencies show?

Answer 13

• High spatial freq = fine details

- Low spatial freq = corse info

Question 14

Define Contrast.

Answer 14

Contrast: difference in luminance between light areas and dark areas of the stimulus

Question 15

Describe the spatial contrast sensitivity function.

Answer 15

• Not equally sensitive to different spatial frequencies.
• Greater sensitivity to intermediate spatial frequencies = perceived at low contrasts
• Lower sensitivity to high and low spatial frequencies = need higher contrast to be perceived.

Question 16

Define Spatial frequency channels.

Answer 16

Collections of neurons tuned to the same spatial frequencies (i.e. respond to the same range of spatial frequencies)

Question 17

How do receptive field sizes change in the periphery?

what does this mean?

Answer 17

Receptive field sizes increase in the periphery.

= contrast sensitivity varies with eccentricity - we can’t see high spatial frequencies in the periphery.

Question 18

How does receptive field size effect sensitivity to spatial frequency?

Answer 18

• Neurons in our primary visual cortex that have different sizes of receptive fields
  ○ They are sensitive to different spatial frequencies
  Big RF = low spatial frequencies
• Therefore acuity is poorer in the periphery

Question 19

Define Resolution Limit.

Answer 19

Resolution limit = no longer visible.

There are no cells responding to that spatial frequency as no cells have small enough RF.

Question 20

What does Spatial frequency tell us about size?

Answer 20

• Spatial frequency tells us about size on the retina – it does NOT indicate real size in the world since the projected size depends on distance
• Any line on the retina could have been produced by infinite different lines in the world
• Not consciously aware that the size of the retinal image changes. We think that the thumb is getting closer, not getting bigger.

Question 21

How does distance and width of bars affect spatial frequency on the retina.

Answer 21

• Bars of different widths at different distances can project -to same SF on retina
• Bars with same width at different distances will project to different SF on retina

Question 22

Can we perceive both retinal and real size?

Answer 22

○ Stimuli 1 & 2 at different distances from participant
○ Discrimination experiment - which stimulus has thinner bars?
○ Discriminating retinal size – task very hard
○ Discriminating real size – task easy
○ Conclusion – conscious perception in terms of real size

Question 23

Define Size Constancy

Answer 23

Size constancy: we perceive an object’s real size in the world regardless of distance

Question 24

Define Orientation Constancy

Answer 24

Orientation constancy: we perceive an object’s orientation in the world regardless of the orientation on the retina

Question 25

Why is the drawing of a 3D object/ scene so difficult?

Answer 25

No conscious access to retinal image properties - explains why drawing a 3D object or scene can be such a challenge

Question 26

How can depth affect real size?

Answer 26

• We can’t ignore the depth information, so we perceive these balls as different sizes
• Size constancy ‘illusions’