The Eye and Vision

Question 1

Describe how vision is created

Answer 1

When we look at something, the light is diverging as it hits the cornea and needs to converge quickly before it hits the back of the eye to be seen clearly.

If the object we are trying to focus on it closer to us then the lens of our eye will change shape to be more convex, increasing its power to focus light on the back of the eye (accomodation).

Question 2

Describe refractive error

Answer 2

Many types:
* presbyopia: the gradual decrease in our abilities to adapt the shape of the eye lens to accommodate for focusing on nearby objects, consequence of ageing

• myopia: when the eye is bigger than normal it can cause a person to be short-sighted meaning that when parallel light is shone from a distance the lens focuses on it before it is able to hit the retina so by the time it reaches the back of the eye the light rays are diverging resulting in a blurry image (- concave lens needed)
• hyperopia: caused by a smaller eye causing the light shone into the eye to converge after it hits the back of the eye so the rays have not yet met at a single point as it hits the retina resulting in a blurry image (+ convex lens)

Question 3

Describe the cells in the eye that make up vision

Answer 3

Cells are at the bottom of the retina:

• rod cells (120 million): around the periphery of the retina. Cannot see colour as they only respond to one wavelength of light. Work best in dim light and are sensitive to small packets of light. Has a lower resolution.
• cone cells (6 million): found centrally in the macula peaking at the fovea. Responds to 3 wavelengths of light (blue, green, red) so can generate colour vision. Works best in bright light and requires less packets of light to hit the back of the eye.

Sensitivities of the cone cells receptors are different:

• 2 peak close together at the red-green area meaning the eyes are good at differentiating between red and green
• the other peaks further away in the blue area meaning that the eyes are not as good at differentiating between green and blue colours

Question 4

Explain the path that light takes through the eye

Answer 4

Light comes in through the retina to pass to photoreceptor cells to create an excitation pathway. Once photons excite photoreceptor cells it sends a signal to bipolar cells which excite ganglion cells and the axons run out the back of the eye forming the optic nerve

Question 5

Describe glaucoma

Answer 5

An eye condition where the optic nerve becomes damaged.

Characterised by cup enlargement in proportion to the optic disc (due to increased pressure in the eye and/or loss of blood flow to the optic nerve causing nerve fibres to die). Also causes thinning of the neuroretinal ring.

Question 6

Describe how the pathway of the visual system works

Answer 6

The optic nerves join soon after exiting the back of the eyes at the chiasm where all the information from the nasal half of the retina sees the temple half of vision and crosses over.

The information continues in the optic tract and joins the brain synapsing in the LGN. It then goes towards the occipital lobe with information pathways splitting as it moves posteriorly with half the information going inferiorly to temporal lobes and half going superiorly to parietal lobes.

All information then comes together in the occipital lobe which processes the information to pass back to the temporal and parietal lobes.

Question 7

Describe the physiology behind different types of visual field loss

Answer 7

• left optic nerve lesion = complete loss of vision in left eye
• compression of optic chiasm = bitemporal hemianopia
• disease of left optic tract = contralateral right homonymous hemianopia
• interruption of left temporal visual paths = contralateral right superior quadrantanopia
• interruption of left parietal visual paths = contralateral right inferior quadrantanopia
• left occipital lobe damage = homonymous hemianopia

Question 8

Describe visual development at the beginning of life

Answer 8

• birth-3months: cannot perceive colour, poor contrast sensitivity, will squeeze eyes shut to light, can fixate on faces within close range, poor switching fixation
• 3-6months: can fix on faces further back, starts to recognise objects ad familiar vs unfamiliar faces, development of stereo vision, development of switching vision between objects
• 6-12months: can fix on faces over 2m away, increased interest in more abstract forms of books and TV, motor and vision more strongly integrated
• 12-18months: 6/18 visual acuity, can point directly at pictures in books and look for hidden toys

Question 9

Name the functions of different brain lobes in relation to the visual system

Answer 9

occipital lobe = primary visual cortex

temporal lobe = visual library

parietal lobe = attention