Week 7 - The Functiontal Architecture of the Visual System

Question 1

Name the Photo-receptors of the eye, and their properties (use, location, numbers, temporal response)?

Identify photo receptor shows higher levels of Neural Convergence, and why?

Answer 1

Rods, used in low-light condition, sensitive to light intensity and motion, 100 million in the eye, most common in retina, not in the Fovea, slow temporal response
Cones, used in bright conditions, sensitive to colour and fine detail, 4 million in the eye, concentrated in the Fovea, fast temporal response

Neural convergence allows for the rod system (with its higher number) to be more sensitive to low levels of light, because of more convergence. This impacts on the acuity of the rods, where cones have a higher acuity.

Question 2

What occurs when light travels towards to retina?

Answer 2

(1) Light travels to the back of the retina (through the initial layers of cells - ganglion and bipolar), hitting the photo-receptors (rods and cones), releasing neurotransmitters using pigments.
(2) These cause synapse of the bipolar cells, towards to ganglion cells.
(3) Axons of the retinas ganglion cells form optic nerves, carrying information from the retina to the brain.

Question 3

What are the four main components of the eye and their function?

Answer 3

The cornea (fixed curvature to bend light towards to retina), pupil (controls amount of light), lens (focuses the light), which focus light (accommodation) onto the retina, which contain photo-receptors to tranduce light/images into neural signals .

Question 4

What is the pathway of vision from the Retina’s ganglion cells to the LGN?

Answer 4

Action potentials in the optic nerves (ganglion cells axons) transmit information towards the Optic Chiasm, where the optic nerves split again. Here, the nasal side of the retina (inner visual field) crosses over to the other side of the brain (contralateral), and the temporal side (outer visual field) does not (ipsilateral). From here, most axons will terminate in the Lateral Geniculate Nucleus, some passing to the Superior Colliculus.

Question 5

What is the visual field?

Answer 5

The whole area you can see without moving your head or eyes.

Question 6

How many layers are in the LGN, and what and is the composition of these layers, and why?

Answer 6

6 layers: 2 magnocellular layers (primarily rods), and 4 parvocellular layers (primarily cones, more layers because of more complex details (colour and fine detail).

Question 7

Which side of the brain does the visual field go?

Answer 7

The right temporal visual field will send to the right brain, and right nasal projection/visual field will go to the left. and vice versa.

Question 8

How is the visual information stored in the LGN with regards to L and R visual information?

Answer 8

Although a cross-over of visual tracts occurs, the L/R information is still stored separately in the LGN.

Question 9

What is the main path from LGN to cortex? Which cortex does information from the right visual field go to?

Answer 9

The information from the LGN is transmitted to the striate cortex (primary visual cortex), via optic radiations. The left primary visual cortex get information from both eyes, but only from the right visual field.

Question 10

Why does a blind spot occur?

Answer 10

At the blind spot (optic disc), no photo-receptors to capture an image exist, as it is completely made up of optic nerves.

Question 11

What does photoreceptors (pigments) do to physical light energy?

Answer 11

Convert the light energy (visual light waves) into neural signals/neurotransmitters using chemical properties of pigments

Question 12

what pigments does cones and rods have, and what happens when light hits them?

Answer 12

Cones have three kinds of cone opsins (for the 3 basic colours), and rods have Rhodopsin. When hit by light, release neurotransmitters in a complex exchange by closing sodium channels (blocking sodium ions - Na). Thus starting the process of transmitting info to the brain.

Question 13

In summary, what is the pathway from light energy to the brain?

Answer 13

Visual information from light is focused by the eye on the retina, which sends information using optic nerves to the straite cortex (via the LGN) or directly to the Superior Colliculus.

Question 14

In detail, What is the pathway from LGN to Cortex (with reference to parietal and temporal areas).

Answer 14

The LGN send information to V1 (striate cortex), then to V2 (extrastiate cortex). From here, Parvo/Magnocellular systems are sent down two different streams. Dorsal stream (Where - V3 (dynamic form) and V5 (motion) sends Magnocellular ganglion cells to the parietal lobe. Ventral stream V3a (form) and V4 (colour) what - more sophisticated) send Parvocellular to the temporal lobe.

Question 15

Describe the function of the what and where streams?

Answer 15

The what stream is the ventral stream from the striate cortex to the temporal lobe, used to identify oritentation, colour and shape of objects in high resolution. The where stream is the dorsal stream from the striate cortex to the parietal lobe, used to working out the spatial aspects of objects and their motion - magnocellular.

Question 16

What are parvocellular and mangocellular ganglion cells?

Answer 16

The two retinal visual systems, parvo refers to ganglion cells connecting primarily to cones, and magno with rods.

Question 17

What is retinotopic encoding?

Answer 17

Left VF - Right M1

The orderly mapping of the visual field (retinas image) on area M1 of the brain. Adjacent regions of the visual field (left) are processed by adjacent region of the cortex (in right hemisphere).

This allow neuron information to interact together easily.

In retinotopic coding, the fovea is over-represented in the cortex, compared with periphery (located more anteriorly)

Question 18

In terms of retinotopic encoding, what does the fovea region do in the cortex.

Answer 18

Because the Fovea processes complex information with high acuity and detail, it uses a large portion of the cortex (over-represented, compared with peripheral vision).

Question 19

Why do we have centre/surround antagonism? what happens when light overs the whole receptive field?

Answer 19

Without it we could not detect changes in light, allowing us to detect boundaries, edges, and textures of the physical world. If the surround/centre field is fully covered in light or dark, ganglion cells are NOT active.

Question 20

What happens when Photo-receptors are hit with light?

Answer 20

They will always fire action potentials/become excited, releasing neurotransmitters (glutamate) to excited/inhibit (depolarise/hyperpolarise) particular bipolar cells, which stimulate ganglion cells (causing action potentials to fire).

Question 21

What happens to bipolar cells when photorecepters become hyperpolarized (excited)?

Answer 21

Photorecepters release decreased Glutamate (neurotransmitter) which depolarise (excites) on-centre bipolar cells, and hyperpolarize off-centre bipolar cells.

Question 22

What does hyperpolarising of cells cause?

Answer 22

Decrease in the release of neurotransmitter (Glutamate), which has opposite effects for on-centre and off-centre bipolar cells.

Question 23

Explain Retinal Ganglion cells receptive fields and their antagonistic behaviour?

Answer 23

On-centre/off-surround ganglion cells are excited when light hits the centre, increasing the rate of firing, and when light hits the surround, it creates an inhibitory effect, which cancels out the excitation. Vice versa for off-centre/on-surround cells.

Question 24

Explain what phenomenon happens in the eyes for the Hermann grid?

Answer 24

Different activity occurs at the intersection and non-intersection. The centre of receptive field receives the same amount of excitation in both, although at the intersection a greater inhibitory effect occurs because their is a lot of white in the surround. Same excitation, different levels of inhibition.

Question 25

What happens to visual projection on their way to V1 (part of the visual cortex)? and what does this allow us to see?

Answer 25

Ganglion cell signals will remain separate from the Retina to the LGN, but from the LGN to V1 (primary visual cortex) they will converge into a single neuron (allowing depth and other binocular vision).

Question 26

How does the retina and V1 relate?

Answer 26

The Retina is a 2D map of the visual world, which is combined and repeated within the V1 area of the brain - or, topographic projection of the retina.

Question 27

What is a receptive field?

Answer 27

A stimulus sensitive region within a cell that produces action potentials.

Question 28

What are the two types of ganglion cell retinal receptive fields?

Answer 28

On-centre/off-surround, and off-centre/on-surround.

Question 29

How do we map receptive fields? (experiments in the past)

Answer 29

Observing the behaviour of individual neurons, For example, using a cat: Stimuli hits the photoreceptors in the retina of the cat which fire action potentials, where Micro-electrodes are hooked up to the cats neurons so they can be amplified and recorded on a screen (oscilloscope).

Question 30

Name the three types of Cortical receptive fields in area V1?

Answer 30

Simple, complex, and hypercomplex cortical cells.

Question 31

What is the Cortical receptive fields? and how do they become more complex?

Answer 31

the receptive field of V1 that has neurons that respond to orientated lines and edges. Higher neuron receptive fields (more complex) are created by adding together lower neurons receptive fields.

Question 32

What are simple cortical cells?

Answer 32

Simple cortical cells respond to bar shapes with particular width and orientation (long, flat light will make max rate of firing/spikes).

Question 33

What are complex and hypercomplex cortical cells?

Answer 33

Complex cortical cells are similar, but prefer a moving stimuli with particular orientation and direction of movement
Hypercomplex is the addition of complex cells (combination of moving stimulus in a direction of movement), and are concerned with length in the receptive field.

Question 34

How are receptive fields organised in the V1 area? and why are they organised this way?

Answer 34

Orientation of neuron receptive fields in V1 are organised in an orderly fashon. Neurons in the same column (stacked on top of each other) have same orientation, whereas adjacent columns will slightly change as you get further away to form an array of 180 degree.

This allows us to see objects and their various orientated lines/edges in space.

Question 35

What are receptive fields in the extrastriate visual areas used for?

Answer 35

Processing faces and objects.

Question 36

How does receptive fields in the extrastriate visual areas process information?

Answer 36

Neurons in the ventral stream (what stream - temporal lobe) form columns that respond to particular shapes/stimuli.

Question 37

Explain the tilt aftereffect?

Answer 37

The effect when neurons get fatigued from firing a lot in a certain orientation (10 degrees), so when realigning vision to 0 degree, the midpoint has moved (appears bent).

Question 38

What is Distributed Coding?

Answer 38

A Pattern of activity in area VI of orientation selective cells (pattern of activity is centred at 0)