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 is closer to us, 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

What is presbyopia?

Answer 2

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

Question 3

What is myopia?

Answer 3

• when the eyeball is a bit bigger than normal causing a person to be short sighted
• when parallel light is shone from a distance the lens focuses it before it is able to hit the retina
• by the time the rays reach the back of the eye they are diverging causing a blurry image
• needs a concave (-) lens

Question 4

What is hyperopia?

Answer 4

• when the eyeball is a bit smaller than normal causing a person to be long-sighted
• when parallel light is shone onto the eye it will be focused after it has hit the back of the eye causing a blurry image at the retina
• needs a convex (+) lens

Question 5

Describe the cells in the eye that make up vision

Answer 5

Photoreceptor cells:

• rod (120 million) found around the periphery of the retina
• cones (6 million) more centrally in the macula peaking at the fovea

Rods cannot see colour as they only respond to one wavelength of light. They work best in dim light and are sensitive to small packets of light. Have lower resolution.

Cones have 3 types (blue, green and red) which respond to 3 different wavelengths of light so the brain can use this to generate colour vision. It works best in bright light and require less packets of light to hit the back of the eye

Question 6

Explain the path that light takes through the eye

Answer 6

• light enters through retina and passes to photoreceptor cells creating an excitation pathway
• a signal is sent to bipolar cells which excite ganglion cells and the axons of these cells run out the back of the eye forming the optic nerve

Question 7

Describe glaucoma

Answer 7

• eye condition where optic nerve becomes damaged
• causes cup to enlarge in proportion to disc (due to increased pressure of the eye and/or loss of blood flow to optic nerve causing fibres to die)
• also causes thinning of neuroretinal ring

Question 8

Describe how the pathway of the visual system works

Answer 8

• optic nerves join after exiting back of the eye at the chiasm where info crosses over
• information flows in optic tract to join the brain synapsing at lateral geniculate nucleus
• it then goes towards occipital lobe with info pathways splitting as it moves posterior
• half of info moves inferior to temporal lobes and half of info moves superiorly to parietal lobes
• all info meets again at occipital lobe which processes info and sends back to temporal and parietal lobes

Question 9

Describe visual development at the beginning of life

Answer 9

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