Visual system (3)

Question 1

Name four key characteristics of light.

Answer 1

1. A quantum of light energy is called a photon.
2. The greater the number of photons, the brighter the light appears.
3. Light is made up of electric and magnetic energy.
4. Light is measured in nanometres.

Question 2

Describe the process of light entering and leaving the eye (4).

Answer 2

• Light enters the eye through the cornea.
• Light is focused by the lens onto the retina.
• Photoreceptors and ganglion cells in the retina convert light energy into an electrical signal.
• This electrical signal leaves the eye via the optic nerve.

Question 3

Describe three features of rods.

Answer 3

1. Work best in dim light.
2. Densely packed in the peripheral retina.
3. Primarily black and white detectors.

Question 4

Describe three features of cones.

Answer 4

1. Work best in bright light.
2. Packed in the central retina (fovea).
3. Colour receptors.

Question 5

Describe two key points of the relationship between colour vision and cones.

Answer 5

• Normal human colour vision is trichromatic.
• Cone types each have different photopigments that determine the wavelength of light that the cell will respond to.

Question 6

What wavelength corresponds with what colour?

Answer 6

Short –> blue.
Medium –> green.
Long –> red.

Question 7

What do photoreceptors produce and what effect does it have?

Answer 7

• They produce a graded response.
• This alters the release of neurotransmitters in response to an increase or decrease of light energy absorbed.

Question 8

What do bipolar cells and ganglion cells produce? Action potentials or graded responses?

Answer 8

Bipolar cells = graded response.
Ganglion cells = action potential.

Question 9

What acts as the interneurons in the retina and what is their function?

Answer 9

• The bipolar cells.
• They provide a connection between the input and output cells.

Question 10

What is the synaptic relationship between bipolar cells, rods and cones and ganglion cells?

Answer 10

Rods and cones synapse with bipolar cells and bipolar cells synapse with ganglion cells.

Question 11

What are the input and output cells in the retina?

Answer 11

Input = photoreceptors.
Output = ganglion cells.

Question 12

What is the purpose of the neurons base rate of firing?

Answer 12

Allows the neuron to be either activated, inhibited or resting.

Question 13

What is the receptive field of the neuron?

Answer 13

It reflects the features of a stimulus that the neuron will respond to.

Question 14

What sort of features can vary per stimuli for a receptive field?

Answer 14

• Shape.
• Colour.
• Movement.
• Location in visual field.

Question 15

What does the response of ganglion cells depend on?

Answer 15

They cannot respond to objects larger than their receptive field.

Question 16

What is the opponent process theory?

Answer 16

The opponent process theory suggests responses from three colour channels (red/green/blue) are compared, to allow colour vision.

Question 17

What sort of receptive field does a retinal ganglion cell have?

Answer 17

Retinal ganglion cells have single colour opponent receptive fields with an antagonistic centre/surround organisation.

Question 18

Describe two classes of ganglion cells in relation to object recognition.

Answer 18

1. Magnocellular – respond to large objects. Analysis of features of the stimulus.
2. Parvocellular – respond to small objects. Colour vision and fine detail.

Question 19

Where does each neuron in the LGN receive its input from?

Answer 19

Only a very few ganglion cell axons.

Question 20

Describe the receptive field of ganglion and LGN receptive fields.

Answer 20

Antagonistic and concentric.

Question 21

What is the optic chiasm?

Answer 21

The optic chiasm is the part of the brain where some optic nerve fibres cross.

Question 22

Which retinal fibres cross over in the optic chiasm and which remain on the same side?

Answer 22

Nasal retinal fibres switch.
Temporal retinal fibres remain.

Question 23

Where does sensory information travel from the optic chiasm?

Answer 23

From the optic chiasm, information travels along the optic tract and to the lateral geniculate nucleus (LGN).

Question 24

How is motion detected in the retina?

Answer 24

If a pattern of light falls on one area of the retina, then the exact same pattern of light is detected milliseconds later in another adjacent part of the retina, the motion of an object can be assumed.

Question 25

Describe the three classes of ganglion cells in relation to output and colour processing.

Answer 25

1. Magnocellular – send output to layers 1 and 2 of LGN. No colour processing.
2. Parvocellular – sends output to layers 3, 4, 5 and 6 of LGN. Red and green colour processing.
3. Koniocellular – blue colour processing.

Question 26

Where does information travel from the LGN?

Answer 26

From the LGN, information travels to optic radiations and then into the visual cortex.

Question 27

What is V1 responsible for?

Answer 27

For processing low-level visual information and passing it on to higher levels.

Question 28

What layer of V1 does info from the LGN arrive in?

Answer 28

Layer 4.

Question 29

Describe the pathway of magnocellular neurons in and out of V1.

Answer 29

Magnocellular cells send their neurons into 4Cα. It then travels to 4B, then out to V2.

Question 30

Describe the pathway of parvocellular neurons in V1.

Answer 30

Parvocellular cells send their neurons in 4Cβ. Parvocellular blob information is sent to the blobs in layers 2 and 3.
The parvocellular inter blob information is sent to the inter-blob area in layers 2 and 3.

Question 31

Describe the difference between simple and complex cells in V1.

Answer 31

• Simple cells respond best to bars of light with a particular location in the visual field.
• Complex cells respond best to bars of light with a particular direction of motion.

Question 32

What type of receptive fields do simple cells in V1 have and how are they created?

Answer 32

• Antagonistic receptive fields (square).
• Created through connections with LGN cells.

Question 33

How many stimuli do V1 neurons have in their receptive fields? What are they? And what is this called?

Answer 33

• Two.
• Light colour and light wavelength.
• Double colour opponent receptive fields.

Question 34

How is V1 organised?

Answer 34

In two kinds of columns:
- Orientation columns.
- Ocular dominance columns.

Question 35

How is V2 organised?

Answer 35

In three kind of stripes. Thin stripes, thick stripes and inter-stripes.

Question 36

Where does V2 receive input from and what is said input?

Answer 36

• Magnocellular and parvocellular cells.
• Spatial location and form and colour.

Question 37

What kind of processing is V3 involved in?

Answer 37

Dynamic form and 3D structure from motion.

Question 38

What kind of processing is V4 involved in?

Answer 38

Processing colour constancy and object recognition.

Question 39

What happens in the inferotemporal (IT) cortex and how is it organised?

Answer 39

• Complex object recognition.
• Cells in the IT cortex respond to complex stimuli that involves motion or texture.
• Organised in columns.

Question 40

What is another name for V5?

Answer 40

Middle temporal cortex.

Question 41

What does V5 process and how is it different from V1?

Answer 41

• Processes an objects position, direction and speed of motion.
• Different because it has a larger receptive field than V1.

Question 42

What two streams does the two-streams hypothesis include?

Answer 42

1. The ‘what’ object identification pathway.
2. The ‘where’ motion processing pathway.

Question 43

Where does the ‘what’ pathway end and what cortexes does it involve?

Answer 43

• Temporal cortex.
• V1, V2, V4 and inferotemporal.

Question 44

Where does the ‘where’ pathway end and what cortexes does it involve?

Answer 44

• Parietal cortex.
• V1, V2, V3, V5 and parietal.