The human visual system

Question 1

What is the sclera and what is its function?

Answer 1

The white of the eye

Protective outer layer of collagen and elastin

Question 2

What does the choroid do?

Answer 2

Provide oxygen and nutrients to the outer retina, especially the fovea

Question 3

Where are cones concentrated?

Answer 3

The fovea (red and green, not blue)

Question 4

Describe horizontal cells

Answer 4

Interneurons that connect the photoreceptors laterally

The connections it makes are called plexiform layers

Question 5

What do bipolar cells do?

Answer 5

Transmit signals from photoreceptors to retinal ganglion cells via graded potentials rather than action potentials.

Question 6

What do amacrine cells connect?

Answer 6

Retinal ganglion cells (laterally)

Question 7

Describe the three types of retinal ganglion cells

Answer 7

M type: large receptive field, processes motion, not sensitive to colour
P type: small receptive field, detects fine features, colour sensitive
K type: large receptive field, function unknown

Question 8

What is the pathway for information to get from the eye to the visual cortex?

Answer 8

Eye -> Optic nerve -> Optic chiasm -> LGN -> Optic radiation -> Visual cortex

Question 9

What is the purpose of the lamina cribrosa?

Answer 9

Acts as a seal to maintain eye pressure

Question 10

What fibres do not cross over at the optic chiasm?

Answer 10

Temporal axons from retinal ganglion cells

Question 11

What does the parietal pathway do?

Answer 11

Determines where things are in space

Question 12

What is the LGN and what does it do?

Answer 12

The lateral geniculate nucleus acts as a relay centre between the eye and the visual cortex
It adds the signals from the two eyes to make a 3D representation

Question 13

Describe the three main features of the primary visual cortex

Answer 13

Occular dominance columns (information from the left and right eyes are kept separate)
Orientation columns (each column is sensitive to a certain orientation)
Blobs (detect colour)

Question 14

What are saccades?

Answer 14

Voluntary fast eye movements used to bring an image onto the fovea

Question 15

What is the purpose of fixational eye movements and what are the three features they are composed of?

Answer 15

Keeps the fovea on a fixed target of interest

Made of microsaccades, drifts and tremors

Question 16

What is the name of the eye movement used to keep track of a moving object?

Answer 16

Smooth pursuit

Uses predictive movement

Question 17

What is the fastest reflex in the body?

Answer 17

The vestibulo-ocular reflex

Question 18

Where are action potentials initiated?

Answer 18

Axon initial segment

Question 19

What are the different types of eye movement?

Answer 19

Sacades
Fixations
Smooth pursuit
Optokinetic nystagmus
Vestibulo-occular reflex
Vergence movements

Question 20

In what circumstances are smooth pursuit, optokinetic nystagmus and the vestibulo-occular reflex used?

Answer 20

Smooth pursuit = When a single object is moving
Optokinetic nystagmus = When the whole visual field is moving
Vestibulo-occular reflex = When the head/body is moving

Question 21

What is a method used to measure eye movement?

Answer 21

Video oculography

Question 22

Describe the vestibulo-occular reflex

Answer 22

Eye movements that compensate for head and body movements
Fastest reflex in the body
Involuntary, controlled at the level of the brainstem
Rotational VOR is mediated by the semi-circular canals
Translational VOR is mediated by the otolith organ

Question 23

What is a lack of a vestibulo-occular reflex called and how is it tested for?

Answer 23

Oscillopsia

Tested for using a Barany chair

Question 24

How do we view two things that are at different distances?

Answer 24

Vergence eye movements

There is no disparity between the eyes for items being focused on but there is for everything outside of the focal plane

Question 25

Describe infantile nystagmus

Answer 25

An involuntary movement of the eyes, usually horizontal
Often hereditary
Uses a foveation strategy
Can be idiopathic or caused by diseases that cause sensory deficits e.g. albinism