Physiology of Vision

Question 1

What chart is used as a method of measuring degrees of impairment in visual acuity?

Answer 1

Snellen’s Chart

Question 2

In normal visual acuity;

a) at what distance should you be capable of reading the largest letter on the Snellen’s Chart?
b) at what distance should you be capable of reading the smallest line of letters on the Snellen’s Chart?

Answer 2

a) at a distance of 200 feet (or 61 metres)
b) at a distance of 10 ft (3 m)

Question 3

How is a ‘Snellen fraction’ calculated?

Answer 3

Question 4

What does a Snellen fraction of 6/61 mean?

Answer 4

For example, if the subject stands 6 m away from the chart above and can only read the top letter, their visual acuity (Snellen fraction) would be 6/61.

Question 5

What is the difference between relative and absolute visual acuity?

Answer 5

Relative visual acuity is the visual acuity of an uncorrected eye; absolute visual acuity is the visual acuity of a corrected eye (e.g. using glasses or contact lenses).

Question 6

What is ‘emmetropia’?

Answer 6

normal visual acuity

Question 7

How does the lens refract light in ‘emmetropia’?

Answer 7

the eye’s lens refracts light so that the focal point is on the retina, and this results in a sharp image

Question 8

What is hyperopia/hypermetropia?

Answer 8

Far-sighted

Question 9

Describe the refractive power of the lens in hyperopia

Answer 9

The refractive power of the eye’s lens is too weak, and the focal point of light is behind the retina, resulting in poor visual acuity

Question 10

Describe the refractive power of the lens in myopia

Answer 10

The refractive power of the eye’s lens is too great, and the focal point of light is in front of the retina, resulting in poor visual acuity.

Question 11

If the Snellen’s test tells us an individual has poor visual acuity, which chart can determine whether that poor visual acuity is due to hyperopia or myopia?

Answer 11

Duochrome chart

Question 12

Describe the wavelength of green vs red light. How does ths affect it as it passes through the lens?

Answer 12

• Green light has a shorter wavelength than red light, it is refracted more than red light by the eye’s lens.
• As such, green light naturally focusses in front of red light

Question 13

How would a duochrome chart appear to an individual with emmetropia?

Answer 13

would see both parts of the duochrome chart (red and green) ever so slightly out of focus, but because the effect is small, and red and green are equally out of focus, it may not be noticeable:

Question 14

How would a duochrome chart appear to an individual with myopia?

Answer 14

The refractive power of the lens is too great so every wavelength of light is focussed in front of where it normally would be: because of this, green light is now focussed far in front of the retina, while red light now focusses on the retina (see the left-hand panel in the figure above), and so the red part of the duochrome chart will be seen most clearly.

Question 15

How would a duochrome chart appear to an individual with hyperopia?

Answer 15

The refractive power of the lens is too weak so every wavelength of light is focussed behind where it normally would be: green light now focusses on the retina, while red light focusses way behind the retina, and so the green part of the duochrome chart will be seen most clearly.

Question 16

If the red chart is clearer, what would this tell you about the subject’s visual acuity?

Answer 16

The subject is short-sighted (myopic)

Question 17

Which lens is hyperopia corrected with?

Answer 17

convex (“positive”) lenses

Question 18

Function of convex (“positive”) lenses?

Answer 18

increase the refractive power of the eye’s lens (which is too weak in hyperopia)

Question 19

Which lens is myopia corrected with?

Answer 19

Concave ‘negative’ lenses

Question 20

Functino of concave ‘negative’ lenses?

Answer 20

Decrease the refractive power of the eye’s lens (which is too great in myopia):

Question 21

What is astigmatism?

Answer 21

A visual acuity defect that affects just one meridian (axis) of the eye, e.g. from top to bottom, due to defects in the shape of the eye’s cornea or lens. As with overall visual acuity defects, astigmatism can be myopic or hyperopic.

Question 22

Which lenses is astigmatism corrected with?

Answer 22

Cylindrical lens

Question 23

What is a ‘toric lens’?

Answer 23

combined spherical and cylindrical lens (to correct for an overall myopia/hyperopia with an additional astigmatism) is known as a toric lens.

Question 24

Why is the blind spot “blind”?

Answer 24

It is the area of the retina, known as the optic disc, where blood vessels enter and leave and where the optic axons exit the eye. There are no photoreceptors (rods or cones) on the optic disc.

Question 25

Is there any difference between the fields of vision noted for the black disc and for the coloured discs? If so, why is this?

Answer 25

The colour visual field is typically smaller than the black & white visual field. This is because cones (the photoreceptors used to sense colour) are concentrated in the centre of the retina, in the fovea, whereas rods (photoreceptors which cannot sense colour) are spread throughout the whole retina.

Question 26

What test is used to demonstrate abnormalities in colour vision?

Answer 26

Ishihara tests

Question 27

What specific deficit leads to red-green colour-blindness? Why are males more likely to be colour blind than females?

Answer 27

It is a sex-linked recessive disorder. The genes for red and green colour receptors are located on the X chromosome of the 23rd chromosome pair (i.e. X-linked recessive inheritance).