Physiology Lab - Vision Flashcards
What is visual acuity?
The ability of the eye to see sharply and clearly
Resolving power of the eye
What do clinical tests of visual acuity indicate?
Function of the fovea
Assume remainder of the visual system is normal
What is a standard test of visual acuity?
Ability to read small letters (Snellen chart) at a distance of 4 or 6 metres
What does 6/6 visual acuity mean?
Can see at 6m the letters visible to a normal eye
What does a 6/9 vision mean?
At 6m, an individual can see only those letters that should still be visible at 9m
What does the numerator in the Snellen chart exam denote?
Distance in metres at which the test is conducted
What does the denominator of a vision test describe.
Distance at which the smallest letters can be seen
The greater the denominator the poorer the vision
What test is used to test vision in young children and how does it work?
Illiterate E test
Series of Es randomly orientated in one of 4 directions - child is asked to point the fingers of his or her hand in the same direction as the E
Is visual acuity better tested with isolated or many symbols?
Isolated symbols
How do we overcome the defect of isolated symbols?
Surrounding the symbol on all four sides with an adjacent black bar (confusion bar)
What is a useful test for infants and how is it done?
Letter matching test
Able to recognise the shape of a letter shown individually on a card by pointing to the same letter on a key card bearing all five or seven letters used in the test
What do we call lazy eyes and how is this condition fixed?
Amblyopia
If detected early in life, it can be corrected/treated with occlusion (patching)
What is near vision testing used for?
Test the power of accommodation
How do we test near vision?
Examples of print of different sizes is used
Patient is asked to read the text and the smallest text which is able to be read is noted
Documented as N followed by the number of print size e.g. N5 to N48
What is emmetropia?
Normal condition of the eye in which with no accommodation, parallel light is focused on the retina
What is refractive error/ametropia?
Any optical departure from emmetropia
Name the three major types of refractive error.
Myopia
Hypermetropia
Astigmatism
What is myopia?
Eye is too long for its refracting system or the refracting system is too powerful for its length
Rays of light come to focus in front of the retina when the eye is at rest
What is hypermetropia?
Eye is too short or the refraction is too weak
Rays of light come to focus behind the retina
What is astigmatism?
Curvature of some component of the optical system of the eye is not spherical.
Refraction in different meridians of the eye is not the same = preventing light rays from coming to a single focus (point focus) on the retina
What is the most common form of colour blindness?
Red-green
More common in males
What causes congenital colour vision defects?
Deficiency/absence of one or more of the cone pigments
What chart is used to test colour vision?
Ishihara chart - consists of numbers/symbols on a slightly different background
The numbers/symbols are made up of differently coloured spots
What is perimetry?
Assessed by confrontation - patient’s fields are compared to those of the examiner
Accurate instrumental measure of visual fields
What is the Bjerrum Screen?
Perimetry measurement
Subject sits in front of screen with one eye occluded. Focus on a white spot in the centre of the screen - target brought in from peripheral field until they can see it.
Entire mid peripheral field can be mapped out
What is the Goldmann Perimeter test?
Equipment
Subject to detect light flashes shone randomly around their visual field
Entire visual field can be mapped
