Practicals (Michaelmas)

Question 1

What equipment is used to measure the shape of the visual field?

Answer 1

Perimeter

Question 2

What are the advantages and disadvantages of forward-facing eyes?

Answer 2

• Advantages: Allows for greater degree of binocular overlap (for distance and depth perception)
• Disadvantages: Large blindspot behind the head

Question 3

What are the advantages and disadvantages of side-facing eyes?

Answer 3

• Advantages: Elimination of blinds-spot at back of head
• Disadvantages: Almost no overlay of visual fields and depth perception

Question 4

What is the fovea?

Answer 4

Pit in the thickest part of the retina

Question 5

What is the diameter of the fovea?

Answer 5

1 - 1.5 mm

Question 6

What is the foveola?

Answer 6

Centre of the fovea containing no rods (only cones) and mediating highest acuity vision

Question 7

What is the diameter of the foveola?

Answer 7

~0.25 mm

Question 8

What is the macula?

Answer 8

Part of the retina containing the fovea and covered by yellow pigment

Question 9

What is the diameter of the macula?

Answer 9

6 mm

Question 10

What is the relation between macula, fovea and foveola?

Answer 10

Question 11

What is the distribution of the cones in the retina?

Answer 11

Throughout the whole of the retina

Question 12

What is the function of vision outside fovea?

Answer 12

1. Directing gaze
2. Maintaining posture and balance
3. Estimating walking speed and direction
   - Avoiding obstacles while walking

Question 13

What are the useful apporximations to bear in mind when measuring visual acuity?

Answer 13

1. An object 1 cm wide viewed from distance of 57 cm subtends an angle of ~1^o on retina
2. 1^o subtends ~300 μm on the retina
3. The thinnest foveal cone has diameter of ~2.3 μm

Question 14

What is the equation to calculate distance subtended on retina from angle subtended?

Answer 14

Distance (in μm) = 17000 x tan(angle in degrees)

Question 15

What is the diameter of the eyeballs?

Answer 15

~20 mm

Question 16

What is the thickness of the thickest parts of retina?

Answer 16

~400 μm

Question 17

What is the thickness of the thinnest part of the retina?

Answer 17

~30 μm

Question 18

What is the diameter of a cone tip?

Answer 18

~2.3 μm

Question 19

What is the ideal visual acuity of person under ideal conditions?

Answer 19

0.5 minutes

Question 20

What is the position and diameter of the blindspot?

Answer 20

• Position: 5-6mm from fovea (or ~15^o from fixation)
• Diameter: 1.5mm

Question 21

What are the limits of the monocular visual field?

Answer 21

• Temporal = ~90^o
• Nasal = ~60^o
• Vertical = ~70^o (both sides)

Question 22

How can the visual acuity from FrACT test be interpreted?

Answer 22

Acuity (minutes) = 1/[FrACT output]

Question 23

How many axon fibres are located in the auditory nerve?

Answer 23

30000

Question 24

What are the relative percentages innervating different hair cells?

Answer 24

IHC = 95%

OHC = 5%

Question 25

What are the limits of the scala media?

Answer 25

1. Reissner’s membrane
2. Basilar membrane
3. Spiral ligament

Question 26

What is the organ of Corti?

Answer 26

Epithelium containing hair cells involved in sound transduction

Question 27

How many inner hair cells are in the cochlea?

Answer 27

~3000

Question 28

How many outer hair cells are in the cochlea?

Answer 28

~9000