Neural Representation

Question 0

Under what conditions are cones most sensitive?

Answer 0

They are most sensitive under light conditions - photopic conditions - bright light

Question 1

Under what conditions are rods most sensitive?

Answer 1

Under dim conditions - scotopic conditions

Well adapted for night vision

Question 2

Are the more rods or cones?

Answer 2

There are more rods than cones

Over 20 more times rods than cones

Question 3

Where is the fovea located and how does it achieve visual acuity?

Answer 3

The fovea is found at the central retina at the macula lutea - yellow spot.
Contains carotenoids which filter out blue light and reduce the chromatic aberration in the image being viewed.
The outer nuclear layers of the retina are displaced at the centre of the fovea forming a pit of exclusively cones which are closely packed together around the rim of the fovea making it the thickest part of the retina.

Question 4

Name the various types of retinal cells

Answer 4

Photoreceptors
Horizontal cells
Bipolar cells
Amacrine Los
Ganglion cells

Question 5

What happens when light falls onto photoreceptors?

Answer 5

In contrast to other sensory receptors, when light falls on the photopigments in the photoreceptors they are inhibited instead of excited and become more permeable to ions which causes their hyperpolarisation and the production of an action potential.
The light falling on the photopigment actually bleaches them and more photopigment has to be regenerated as the cell depolarises so the process can be repeated.

Question 6

What are the names of the pigments in rods and cones?

Answer 6

Rods - rhodopsin - composed of opsin and 11-cis-retinal

Cones - iodopsin composed of 11-cis-retinal and another molecule related to opsin

Question 7

What happens to 11-cis-retinal when is absorbs a photon of light?

Answer 7

It is converted into all-trans-retinal which is an isomer of the same molecule

Question 8

Describe the chain of events in transduction when a photon of light is absorbed by a photoreceptor?

Answer 8

The 11-cis-retinal is transformed to all-trans-retinal which causes a conformational change in the opsin molecules which are on the surface if the photoreceptor membrane
The photopigments become bleached
The G protein transducin is activated which releases cGMP (cyclic guanosine mono phosphate)
The release of this molecule closes the cGMP gated cation channel in the membrane
This stops the constant dark current which is the steady flow of ions through the membrane when there is no light being absorbed
The membrane therefore hyperpolarises when these channels are closed
Neurotransmitter release is REDUCED
All-trans-retinal is catalysed back into 11-cis-retinal by a chain of enzyme reactions and reattaches to an opsin molecule
The process is ready to start again

Question 9

What is 11-cis-retinal synthesised from?

Answer 9

Vitamin A
Without vitamin A we would lose our sight
Found in carotenes which are in green veg and carrots

Question 10

Describe what is meant by the phrase ‘neural image’?

Answer 10

A neural image is the preservation of an image through the transduction of a pattern of light which is converted into a corresponding map of neural response where the spatial relationships of the original imaged are preserved and transduced to the brain.

Question 11

What cells do photoreceptors make contact with in the retina?

Answer 11

Bipolar cells of which there are three types
Midget bipolar cells
Diffuse bipolar cells
Wide-field bipolar cells
These in turn link to the retinal ganglion cells

Bipolar cells can be on-centre and off-centre. On-centre bipolar cells react to light at the centre and are inhibited by light that is off centre
And vice versa for off-centre bipolar cells

Question 12

What is the name of the neurotransmitter which is released by photoreceptors?

Answer 12

Glutamate

Question 13

How do on-centre and off-centre bipolar cells react to changes in levels of the neurotransmitter glutamate?

Answer 13

When there is a light stimulus the cell becomes hyperpolarised and the amount of neurotransmitter is reduced.
When neurotransmitter levels drop it depolarises (stimulates) the on-centre bipolar cells and hyperpolarises (inhibits) the off-centre bipolar cells
And vice versa for an increased amount of neurotransmitter released

Question 14

Describe the receptive field and the behaviours of retinal ganglion cells?

Answer 14

Generate action potentials spontaneously and therefore have background firing
The stimulation or lack of stimulation therefore increases or decreases the firing rate
On centre stimulation increases firing rate whereas off centre stimulation decreases firing rate

Receptive field is organised in concentric circles

Question 15

How do retinal ganglion cells transmit information about colour vision?

Answer 15

Retinal ganglion cells use opponent-colour responses.
The cell can be excited by one wavelength of light and inhibited by another
There are four classes depending on their different opponent colour properties
1- red/green - the centre is sensitive to red light and the surround to green therefore +r-g
2- green/red which is e opposite to above +g-r
3- blue/yellow - the centre is sensitive to blue light and the surround to yellow light +b-y
4- yellow/blue - vice versa to above -b+y

Question 16

Describe how the retina adapts to changes in light and dark conditions?

Answer 16

The retina is able to adjust to lighting conditions through light and dark adaptation
Here is a change in sensitivity of he photopigment in the membrane sacks of the outer segment of the rods and cones. This photopigment is bleached in light conditions however in dark conditions it is regenerated (rapidly in cones and more slowly in rods) enabling them to respond to low levels of illumination
Neural adaptation also occurs which is he rapid reorganisation of the receptive fields of the ganglion cells so that the inhibitory regions are made less powerful. They are important for resolving spatial detail. During reorganisation they remain sensitive but less able to define spatial detail

Question 17

What are the three properties used to describe colour?

Answer 17

Brightness
Colourfulness
Hue

Question 18

What is spectral antagonism?

Answer 18

Cone excitations in response to a particular wavelength is combined with the responses of cones which are excited by other wavelengths with opposite sign.

Question 19

What is spatial antagonism?

Answer 19

where cone excitation from one area of the retina is combined with cone excitations from another area of the retina with opposite sign
An example is the on/off centre receptive fields

Question 20

What is opponent processing?

Answer 20

The combination of he three cone signals to generate 3 new channels that code for:

Luminance (receive combined info from L&M cones)
Redness-greenness (receive antagonistic/opposite signals rom L&M cones)
Yellowness-blueness (receives signals from s cones and combined antagonistic response from the other two L&M cones

Question 21

What are the two properties that distinguish an opponent response cell and where are they found?

Answer 21

Must have a spontaneous rate of firing

Must have an increased rate of firing in the presence of light in certain frequencies and a decreased rate of firing when exposed to light in the opposing spectral region.

These cells are found in the lateral geniculate nucleus LGN in the thalamus which is involved in visual processing

Question 22

Name the types of retinal ganglion cells?

Answer 22

Midget
Blue-yellow
Parasol

Each have off centre and on centre varieties

Question 23

Describe the function of midget ganglion cells

Answer 23

They are involved my colour vision and high acuity vision
Small receptive fields
Receive info from midget bipolar cells
Project into the parvocellular layer (P layer) in the LGN and are therefore called P cells
They have small axons
Red green colour opponent cells

Question 24

Describe the function of parasol ganglion cells

Answer 24

Involved in achromatic, low spatial vision, movement, luminance detection
Project into the magnocellular layer (M layer) of the LGN so are therefore known as M cells
The receive input from diffuse bipolar cells
Have larger axons than midget ganglion/P cells

Question 25

How is the retina divided in terms of colour vision?

Answer 25

It is divided I to two colour opponent channels
Red-green and blue-yellow
The project separately through the LGN and into the cortex

Question 26

What is tetrachromatic vision

Answer 26

Historic vision which involved four types of cone in vertebrates
Our visual systems have evolved to allow for colour vision
Tetrachromatic vision is still found in birds and fish
There is also dichromatic vision in cats and dogs, rabbits and some fish

Question 27

Name the two types of colour deficiency?

Answer 27

Protanopia (lack of L cones) and deuteranopia (lack of M cones)

Question 28

Name the five types of visual acuity?

Answer 28

Detection acuity - detecting a small object in the visual field
Resolution acuity - determining the separation between elements of a pattern
Recognition acuity - recognising objects
Localisation acuity - detecting whether two lines are laid end to end, continuous or offset
Dynamic acuity - detecting and locating a moving target

Question 29

What is the contrast sensitivity function (CSF)?

Answer 29

Gives a measure of the sensitivity of the eye at different spatial frequencies
Also see modulation transfer function in chapter 2 book 3

A low contrast threshold indicates high sensitivity
And vice versa