Physiology of Vision Flashcards
What principle is used to calculate visual acuity and map the blind spot?
the principle of “similar triangles”
if two triangles have identical internal angles such that they are the same shape even though they may be different sizes, then any corresponding lengths between the two triangles are proportional
How could the theory of similar triangles be applied to this image?
What equation can be used?
the triangles are similar as they have the same internal angles, and therefore the same shape
the heights and widths of the triangles are all in proportion
the height of the blue triangle is 1.5x the height of the red triangle, so the width of the blue triangle is also 1.5x the width of the red triangle
the ratio of the heights is equal to the ratio of the widths
a/c = b/d
What is the purpose of the visual acuity experiment?
it is designed to test the visual acuity (clearness of vision) of the subject by determining the maximum distance at which the subject can distinguish between parallel lines
What are the first 3 stages in the visual acuity test?
- if subject wears glasses, keep them on for this experiment
- the subject and experimenter should stand a few metres apart, and the subject should close or cover one eye throughout the experiment
- a test card, printed with a number of black parallel lines, is held up by the experimenter
the experimenter asks the subject how many lines are visible
several observations with different cards (different numbers of black lines) should be made
What are the final 2 stages in the visual acuity test?
- if the subject can distinguish the individual lines on the cards, the subject and experimenter should move further apart and repeat the procedure
if the subject cannot distinguish individual lines on the card, the subject and experimenter should move closer together and repeat the procedure
- repeat until you find the distance at which the subject can only just distinguish the lines on the card
measure this distance between the subject and experimenter with a metre rule
What is represented by each distance a, b, c and d?
a = the distance at which the subject can only just distinguish individual lines on the test card
b = the distance between the lines on the test card
c = the distance from the eye’s lens to the retina (15 mm)
d = the distance that the test card lines were falling on the subject’s retina
you can use the principle of similar triangles to calculate distance d
What is a Snellen’s chart?
What is it used to assess?
it is used as a method of measuring degrees of impairment in visual acuity
the Snellen’s chart consists of square letters or numerals with block serifs, each stroke subtending an angle of one minute , while the entire character is usually contained in an angle of 5 minutes at the eye
With normal visual acuity, at what distance should a subject be able to read the largest and smallest lines of letters on a Snellen’s chart?
- they should be capable of reading the largest letter at a distance of 200 feet (61 metres)
- they should be able to read the smallest line of letters at a distance of 10 feet (3 metres)
- subjects usually stand 20 feet (6 metres) from the chart so should be able to read the line of letters with ‘20’ and ‘6’ next to it (third from bottom)
What is the “Snellen fraction”?
the subject’s actual visual acuity is given as the Snellen fraction:
visual acuity = [distance of subject from chart] / [number next to the smallest line on chart that can be read]
If a subject stands 6m away from a Snellen chart and can only read the top letter, what is their visual acuity?
the visual acuity (Snellen fraction) is 6/61
the subject is stood 6 metres away from the chart
the number next to the first letter is 61 (metres)
If a subject stands 6 m away from a Snellen chart and has normal vision, what line should they be able to read?
they should be able to read the third line from the bottom (with the ‘6’ next to it)
their visual acuity would be 6/6
if 6 metres is converted into feet, you get 20 feet
normal visual acuity in feet would be 20/20, which is where the phrase 20/20 vision comes from
this just means normal vision
What does it show if someone can read the bottom line of the Snellen chart whilst standing 6 metres away?
if they can read the very bottom line of the chart then their visual acuity is 6/3
this is better than the visual acuity of the normal eye, which is 6/6
What is meant by relative visual acuity and absolute visual acuity?
relative visual acuity:
- the visual acuity of an uncorrected eye
absolute visual acuity:
- the visual acuity of a corrected eye
- e.g. through using glasses or contact lenses
What are the first 3 stages in the procedure for testing visual acuity using a Snellen’s chart?
- subject stands a distance of 6m from the Snellen’s chart, as indicated by a line on the floor of the laboratory
- if the subject wears glasses, they can be removed to determine relative visual acuity
the test can be repeated with glasses on to measure absolute visual acuity
do not remove contact lenses
- the visual acuity of each eye should be tested separately, covering the eye that is not being tested
What are the final 2 stages involved in calculating visual acuity from a Snellen’s chart?
- the subject reads the letters from the top downwards
they must be able to read ALL the letters on a given line correctly
if they make a mistake on one letter, ask them to repeat the line, but do not indicate to them which letter in the line they got wrong
the subject stops reading when they cannot read all the letters in a line correctly
- note the number next to the smallest line of letters that the subject could read correctly (i.e. the distance at which someone with normal vision should be able to read that line) and calculate the visual acuity (Snellen fraction)
What is emmetropia?
Emmetropia refers to an eye that has no visual defects
Images formed on an emmetropic eye are perfectly focused, clear and precise (normal visual acuity)
Eyes that have emmetropia do not require vision correction
How does the lens of the eye refract light in someone with emmetropia?
with normal visual acuity (emmetropia), the eye’s lens refracts light so that the focal point is on the retina
this results in a sharp image
If a subject is far-sighted, how does the lens of the eye refract light?
What is the correct term for this condition?
hyperopia (hypermetropic)
the refractive power of the eye’s lens is too weak
the focal point of light is behind the retina, resulting in poor visual acuity
If a subject is short-sighted, how does the lens of the eye refract light?
What is the correct term for this?
myopia
the refractive power of the eye’s lens is too great
the focal point of light is in front of the retina, resulting in poor visual acuity
If Snellen’s test tells us an individual has poor visual acuity, then what test is used?
duochrome chart
this determines whether the poor visual acuity is due to hyperopia or myopia
this chart contains a series of concentric circles on red and green backgrounds
What is the composition of visible light like?
What happens to different colours as they pass through the lens?
visible (white) light is made up of a collection of component colours, each of which has a different wavelength
as white light passes through a lens, each colour is refracted by a different amount and white light is split into its component colours
What is refracted more, red or green light?
What is the consequence of this in someone with emmetropia?
green light has a shorter wavelength than red light, so it is refracted more by the eye’s lens
green light naturally focuses in front of red light
in someone with normal refraction of the eye (emmetropia), green light focuses slightly in front of the retina
red light focuses slightly behind the retina
How would someone short-sighted (myopia) see the duochrome chart?
Why?
the refractive power of the lens is too great so every wavelength of light is focussed in front of where it normally would be
green light is focussed far in front of the retina, while red light is focussed on the retina
the red part of the duochrome chart is seen most clearly
How would someone who is far-sighted (hyperopic) see the duochrome chart?
Why?
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
the green part of the duochrome chart is seen the most clearly
What type of lenses are used to correct hyperopia?
convex (“positive”) lenses
these increase the refractive power of the eye’s lens (which is too weak in hyperopia)
What type of lenses are used to correct myopia?
concave (“negative”) lenses
these decrease the refractive power of the eye’s lens (which is too great in myopia)
What are the first 4 stages in the blind spot test to measure the horizontal axis of the blind spot?
- subject stands at a distance of 1.0 m from a whiteboard
- the experimenter marks a small cross on the left-hand side of the whiteboard
- the subject covers their left eye and fixates on the cross with their right eye throughout the test
the experimenter moves a pointer horizontally from the cross towards the right side of the whiteboard
the subject reports when the tip of the pointer disappears from view, and the experimenter marks this point on the whiteboard
the experimenter continues the horizontal movement until the subject reports the reappearance of the pointer tip, and the experimenter marks this point on the whiteboard
- measure the distance between the two marks (measurement 1) - this represents the horizontal axis of the blind spot
How should the subject be positioned during the visual field test?
the subject rests the cheekbone under one eye on the pad of the visual field mapping apparatus, and fixates that eye on the dot in the middle of the central mirror
the subject should close or cover their other eye
it is important that fixation remains on the dot in the middle of the central mirror throughout the experiment and does not change while the visual field is mapped
How is the visual field initially mapped in black and white?
- the experimenter places on of the black discs in the disc holder of the visual field mapping apparatus, ensures the disc holder is at the very end of the apparatus arm, and rotates the apparatus arm so the arm is horizontal and the disc is to the right of the subject’s eye (apparatus arm with the disc is in the 0o position)
- experimenter slowly moves the disc along the arm until the subject first sees the disc in their peripheral vision
at this point, the experimenter reads from the apparatus arm the angle subtended by the disc
the subtended angle will be between 90o (the disc is at the very end of the apparatus arm) and 0o (the disc is moved all the way around to the central mirror)
How should the initial visual field in black and white be plotted?
as you were coming into the subject’s peripheral vision from the right (i.e. with apparatus arm in 0o position, you start from the right of the chart (marked with ‘0’ and ‘A’)
move from the outside of the chart towards the middle and mark a cross on the concentric circle corresponding to the subtended angle you read from the apparatus arm
After taking the first visual field measurement coming from the right of the peripheral vision, then what readings should be performed?
repeat the experiment three more times, each time with the disc coming from a different direction
- from directly above the subject’s eye (apparatus arm in 90o position)
- from the subject’s left (apparatus arm in 180o position)
- from directly underneath the subject’s eye (apparatus arm in 270o position)
each time, plot the measured subtended angle on the chart, making sure to mark the subtended angle against the corresponding concentraic circle in the correct direction on the chart
the shape of the visual field in black and white is then mapped by joining the various positions plotted on the chart with a continuous line
What are the three controls on the opthalmoscope?
- on / off switch (sometimes combined with a brightness control)
- aperture / filter controls
- focussing wheel
What should you do if the subject’s retina is not in focus?
turn the focussing wheel clockwise or anticlockwise, using trial and error until you obtain a sharp image
What does the Ishihara test consist of?
a series of “plates”, each containing a pattern of dots
within the dots are numbers (or lines) that the subjects must read
some plates are readable by those with both normal and abnormal colour vision
some plates are only readable by those with normal colour vision (or only by those with abnormalities in colour vision)
on some plate those with normal colour vision will read a different number to those with abnormalities in colour vision
with each set of plates, there is a key showing the experimenter whether the subject’s response to each plate indicates normal or abnormal colour vision
