Topic 4&5 The eye and receptive fields

Question 1

What are the major parts of the eye and what are their functions?

Answer 1

1) Cornea: does most of the refraction
2) Pupil: partially regulates how much light enters the eye
3) Lens: focuses incoming light (does less refraction but is adjustable)
4) Retina: location of photoreceptors (cones and rods)
5) Optic disc & the blind spot

Question 2

Are we actually blind to objects that cast their image onto the blind spot, and if so, what does this suggest about how the visual system processes the retinal image?

Answer 2

There is a point where image is to fall on the optic disc. There will be no photoreceptors to perceive that image, so that is a blind spot.
We are not actually blind. Our visual system can predict the image according to the near regions so that we can see them.

Question 3

What is the process of accommodation and why does it need to occur?

Answer 3

Accommodation is the mechanism by which the eye changes refractive power by altering the shape of lens in order to maintain a clear image or focus objects at variable viewing distances.

Question 4

What are some of the abnormalities that can occur with the accommodation process?

Answer 4

1) Myopia
2) Hyperopia
3) Presbyopia

Question 5

What are the two major types of photoreceptors and what are their properties

Answer 5

Rods: high luminance sensitivity and rapid saturation
Cones: low luminance sensitivity and less rapid saturation, as well as mediate colour vision and ability to see fine detail

Question 6

What is meant by scotopic and photopic vision and what are the characteristics of each?

Answer 6

1. Scotopic vision: 
The vision under low light conditions with Rods only (cones not responding)
Characteristics of Scotopic:
1)	No colour vision 
2)	Poor ability to see fine detail
3)	Blind to images falling in the fovea 

2. Photopic vision: 
The vision under well-lit conditions with Cones only (rod response has saturated)
Characteristics of Photopic:
1)	Colour vision
2)	Good spatial acuity
3)	Best spatial acuity in the fovea

Question 7

Describe, qualitatively, how the density of rods and cones varies across the retina.

Answer 7

The fovea is rod-free and has the highest density of cones. The density of cones falls off rapidly to a constant level at some degree from the fovea. The density of rods first increases (reaches a maximum at some degree from the fovea) but then declines as moving away from fovea to peripheral. The blind spot has no receptors.

Question 8

If you wanted to view a dim object at night with the naked eye, would you look directly at it? Why?

Answer 8

No.

Rods are most sensitive and there are no rods in the fovea, therefore we need to look slightly away from it.

Question 9

What is meant by dark and light adaptation? What changes occur during this process?

Answer 9

The ability of the eye to adjust to various levels of darkness and light.
Change in sensitivity: The eye increase sensitivity in the dark; the eye decrease sensitivity in the light.
Change in rods and cones system.
Change in pupil.

Question 10

10) During dark adaptation, the ability to detect a small dot of light improves for about 10 minutes and then stays the same, whereas sensitivity to a larger dot of light keeps on improving for about 30 minutes. Why does this occur?

Answer 10

?

The small dot falls into fovea that contains only cone system.

Question 11

If you wanted to read a map at night but still maintain good night vision, what coloured light would you use and why?

Answer 11

Red light. As red wavelength is far enough away from the rods that it doesn’t activate the rods, therefore doesn’t ruin the night adaptation (night vision).

Question 12

If you wanted to read a map at night and use the least amount of light, what coloured light would you use and why?

Answer 12

Green light. Cones are most sensitive to the green colour wavelength (i.e. the peak in cone sensitivity is at the wavelength corresponding to green), so it requires less light to activate.

Question 13

What are the three ways that the visual systems maintain its sensitivity to a wide range of luminance intensities?

Answer 13

1) Variable pupil size
2) Two types of photoreceptors
3) Dark and light adaptation

Question 14

What is the receptive field of a visual cell?

Answer 14

Region in space where a stimulation leads to a response in the visual cell.

Question 15

What happens to the average size of receptive fields as the distance from the fovea increases?

Answer 15

Size increases with distance. Cells in fovea have small RF, while cells far from fovea have larger RF.

Question 16

What is meant by retinotopic and topographic mapping of visual cells in area V1?

Answer 16

Retinotopic mapping means that the retina is mapped onto Vi.

Topographic mapping means that the outside world is mapped onto V1.

Question 17

With respect to receptive-field structure, what is meant by spatial opponency and what are the functional consequences of it?

Answer 17

Have balanced excitatory and inhibitory regions (i.e. different spatial regions of the RF have opposing effects on the cell).
Concerned about change, not regions of uniform light.

Question 18

What is the receptive-field structure of cells at the retinal-ganglion and LGN levels in the visual system, and, consequently, to what sort of stimuli are they sensitive to?

Answer 18

The centre-surround structure.
An on-centre cell detects when a bright dot is placed in its centre.
An off-centre cell detects when a dark dot is placed in its centre.

Question 19

The visual system maintains sensitivity to light increments and decrements by having separate On and Off cells. How could a single On system encode both light increments and decrements and what would be the disadvantages of such a single system, compared to separate On and Off system?

Answer 19

By having a high maintained-discharge level.
Drawbacks with a single system:
1. High metabolic cost,
2. Much activity noise,
3. Reduced effective dynamic range of the cell

Question 20

What is the Herman-Herring grid illusion and explain why it occurs. (Note: two aspects to the illusion, the appearance of grey dots and their location in the visual field –i.e. only see them in peripheral vision).

Answer 20

A sensory illusion characterized by grey dots perceived at the intersections of a white grid on a black background. The grey dots disappear when looking directly at an intersection.

It occurs because of lateral inhibition processing.
The crossover point falls into the excitatory region of RF in the centre, while the lines fall into inhibitory region, causing a general high inhibition.
Since a crossover point is surrounded by more intensity than a point at the middle of a line, the intersection appears darker due to the increased inhibition.
The illusion only occurs in the peripheral vision because of relationship with size of the RF and size of the grid. In fovea, there are lots of smaller receptive fields (less inhibition) which allow us to get the correct resolution, while RFs in peripheral vision is larger.

Question 21

What are the main types of cells found in area V1 and what are their receptive-field properties? Consequently, what sort of stimuli are they sensitive to?

Answer 21

1) Simple cells (linear RF): sensitive to orientation and width, as well as luminance polarity and position
2) Complex cells (non-linear RF): sensitive to orientation and width, as well as motion
3) Hyper-complex cells (end stopped): length selective
4) Concentric cells: colour selective

Question 22

One model of brain anatomy states that complex cells are made by combining the output of simple cells? If the entire output of simple cells was pooled in this manner, i.e. if the visual system couldn’t process the information of a single simple-cell, what information would be lost and/or degraded?

Answer 22

The position and luminance polarity.

Question 23

What is meant by the feature-detection model of visual processing (i.e. grandmother cells)?

Answer 23

Moving up the visual system, cells are tuned to more complex features.

Question 24

What are the drawbacks of the feature-detection model?

Answer 24

1. Numbers of cells are required
2. Problem with new objects
3. Cortical cells respond to many stimulus properties