Lecture 3: Visual System

Question 1

What are the 6 steps in the retino-geniculate-striate pathway

Answer 1

1. Eyes
2. Retina
3. Thalamus
4. Primary Visual Cortex (Occipital lobe)
5. Extrastriate cortex (Occipital lobe)
6. Extended Cortex (Temporal and Parietal lobe)

Question 2

What are the three main phases of seeing something?

Answer 2

1. Image formation
2. Transduction - image is transducted into signals
3. Visual process - understanding the image

Question 3

What is decussation?

Answer 3

Where the left visual field reaches the right cortex and the right visual field reaches the left cortex

Question 4

Bilateral visual fields are _____ and unilaterial visual fields are ______

Answer 4

Optic nerves are responsible for bilateral visual fields, optic tracts are responsible for unilateral visual fields

Question 5

40% of the optic nerves fibres cross at optic chiasm, T/F

Answer 5

False! 50% do

Question 6

Explain retinotopic

Answer 6

Retinotopic is the idea that adjacent points in the visual field map onto adjacent points on the retina, i.e., mapping of visual information from visual field to retina to brain. This mapping is maintained through visual processing

Question 7

Explain cortical magnification

Answer 7

More cortex is dedicated to processing the central visual field than the periphery, i.e., fovea disproportionately represented with high acuity, compared to periphery

Question 8

Explain what is occuring when a neuron fires with light in the centre and dark surround it, and when the same neuron suppresses when light surrounds dark spot

Answer 8

This process is termed receptive fields. The neuron is stimulated (excited) by this image due to its features and where the light lays (region), and is inhibited by the opposite of what stimulate it. This assists with mapping. The neuron describe in the example is a centre-surround cell.

Question 9

Where does refraction of light occur?

Answer 9

Cornea

Question 10

What is the role of the lens?

Answer 10

The lens focuses on a light through a process named accomodation reflex, that is, changing its shape to focus

Question 11

How does the iris and pupil work in conjunction?

Answer 11

The iris controls the size of the pupil, moderating the amount of light the eye receives

Question 12

Explain the process of how images are formed

Answer 12

An image is formed through light passing through the cornea, which is refracted. The light then passes through the pupil, in which the iris moderates the amount of light it receives by controlling the size of the pupil. Afterwards, the light reaches the lens which focuses the light onto the retina.

Question 13

Which part of the eye is responsible for transduction?

Answer 13

Retina. Receptors transduce light signals into neural signals

Question 14

What is the RGC?

Answer 14

Retinal ganglion cells, the final layer of axons to the brain

Question 15

Fovea is responsible for the small and low acquity central vision, t/f

Answer 15

False! Fovea is responsible for small and HIGH acuity of the central vision

Question 16

What is the optic disc?

Answer 16

Point of retina where RGC axons leave to become optic nerve

Question 17

Optic nerve is responsible for…

Answer 17

The neural transmission to the thalamus

Question 18

How does the fovea solve the backward wiring problem?

Answer 18

The fovea is a solution to the backward wiring problem because due to the fact that light has to pass many layers to reach photoreceptors, the fovea creates an area of high resolution by “pealing through the layers of photoreceptors”

Question 19

Name the 5 layers of different types of neurons and briefly describe their functions

Answer 19

1. Receptors, receives light and transduces
2. Horizontal cells, lateral communication
3. Bipolar cells, signals pass through and processes
4. Amacrine cells, lateral communication
5. Retinal ganglion cells (RGC), transmits signal to brain

Question 20

What is the difference between rods & cones

Answer 20

Cones: Responsible for colour vision and high positional acuity, due to their low convergence. They are low in sensitivity and thus require more well-lit scenes (photopic vision).
Rods: responsible for low position acuity, due to their high convergence (lots of rod cells converging into one area). And as well, due to their high sensitivity, they’re better suited for low light situations

Question 21

The retina is a ___ _______ ______, convergence is _____ _________, reducing axons to brain. Responsible for ____ detection & ______ detection

Answer 21

low processing centre, early processing. Edge & motion

Question 22

What is lateral inhibition? How does this occur and how does this create enhancement of edges?

Answer 22

Lateral inhibition is the proess of contract enhancement for edge detection. Each receptor inhibits its neighbours with inhibition being greater when there is increased intensity. If a dim light and an intense light are adjacent, the receptor A receive intense light will inhibit receptor B, and thus enhancing an edge

Question 23

Optic nerve transmit to….

Answer 23

Thalamus, (lateral geniculate nucleus)

Question 24

Describe the key processes of V1

Answer 24

The key function of V1 is to identify object boundaries, done by simple cells, and motion information, done by complex cells

Question 25

How do simple cells work?

Answer 25

Centre-surround cells (from layer 4) send signals to simple cells (in layer 3). They respond to the types of edges e.g., bars of light in dark field, orientation, and location. For example, simple cells can excite/fire when 1. light strikes (2) receptive field of simple cell, in a (3) particular orientation, however, without these, neurons can be inhibited

Question 26

Explain the process of contour integration

Answer 26

Contour integration is the process by which multiple simple cell signals are combined to detect and represent boundaries within visual map. Since simple cells respond to specific orientation and position of edges/lines, when combined, contours or edges pop out.

Question 27

Explain lateral facilitation in context of contour integration

Answer 27

Lateral facilitation is the process of neurons enhancing the activity of neighbouring neurons, in which permits an integration of local edge signals into a coherent contour representation.

Question 28

What is low and high spatial frequency? How is this detected?

Answer 28

Low spatial frequency refers to texture of an object. High spatial frequency refers to edge information. This is detected by simple the position of objects, e.g., wider fields detect texture, narrow fields detect edges

Question 29

Explain the role of complex cells

Answer 29

Complex cells are sensitive to more complex features such as motion and spatial frequency, that is, they respond to stimuli that move through receptive field in specific directions whilst also integrating information from multiple simple cells

Question 30

What is the columnar organisation of V1, columnly and across.

Answer 30

Column: - Same receptive field in general area of visual field
- same orientation preference
- same eye (monocular neuron) or same eye dominance (binocular neuron)
Across:
- dominance across shifts
- orientation slowly rotates with column
- receptive field locatiion shifts with column

Question 31

What is scotoma?

Answer 31

A damage in v1 that produces an area of blindness in contralateral field of both eyes (i.e., damage in left visual cortex, right visual field will be blind). Patients of this damage often have no conscious awareness due to completion

Question 32

What is blindsight and describe some instances of it at play

Answer 32

Seeing but without any conscious awareness. E.g., respond to visual stimuli in scotoma, see motion, etc.

Question 33

Describe the extrastriate’s role in processing visuals

Answer 33

Each area within the extrastriate cortex respond retionotopically and respond preferentially to stimuli, i.e., colour, movement, shape. The information is then distributed throughout.

Question 34

Where does the dorsal stream and ventral stream run to?

Answer 34

1. Posterior parietal cortex, 2. Inferior temporal cortex

Question 35

What is the role of the dorsal stream?

Answer 35

Dorsal stream (where):
- respond to spatial stimuli: object location or direction of motion
- superior longitudunal fasciculus
- large RF, mostly (60%) outside of fovea
- central to spatial attention and selective attention (enhanced processing at some location to select objects) and is highly connected to motor cortex (posterior frontal cortex)

Question 36

What is the role of the ventral stream?

Answer 36

Ventral stream (what):
- responds to characteristcs of objects; colour, shape, contours, surfaces, fields of motions assembled into objects
- attach semantic significance
2 stages of object recognition in 2 subdivisions:
- posterior: integration of visual features into objects
- anterior: association of object with knowledge of object
- large RF, all (including fovea)

Question 37

What is the akinetopsia (motion blindness)

Answer 37

Movement becomes snapshots