opto1001 week 2

testing visual function

what are rods

photoreceptors that mediate scotopic vision

where are rods located

peak density at the mid peripheral retina 20-30 degrees eccentricity

what are cones

photoreceptors that mediate photopic vision, colour and high-acuity vision

where are cones located

peak density at fovea and declines with eccentricity

what are the three cone classes (trichromacy)

L-cones, M-cones, and S-cones

what is the colour and wavelength of L-cones

red and 560nm

what is the colour and wavelength of M-cones

green and 530nm

what is the colour and wavelength of S-cones

blue and 420nm

what is visual acuity testing

a test that measures the eye’s ability to distinguish fine details at various distances

what is snellen chart

chart with rows of decreasing letter size to measure distance vision

what is the snellen fraction (D’/D)

D’ is the testing distance and D is the distance at which the letter should be legible to someone with normal vision

what is the minimum angle of resolution

smallest angular separation the eye can distinguish

what is the length for near vision testing held at

40cm

what is contrast sensitivity testing

measures the eye’s ability to see in low-contrast environments

what is the two types of contrast and what are they used for

Michelson contrast is used for sinusoidal gratings whereas Weber contrast is used for square-wave gratings

what is our peak contrast sensitivity threshold

2-6 cpd (3cpd for 6/60)

what does low contrast sensitivity mean

higher contrast needed to detect a target

what does high contrast sensitivity mean

lower contrast needed to detect a target

what does contrast sensitivity function mean

describes sensitivity across spatial frequencies

what is the Pelli-robson chart

3/30 letter chart where the letters fade from black to light grey to test contrast sensitivity

what are the testing conditions that affect CSF

lighting, stimulus size, temporal elements, motion, colour

what is the modulation transfer function

assess how well an optical system preserves contrast across spatial frequencies

what is the visual field

area of space visible while the eye is fixed at a point

where is the physiological blind spot

15 degrees temporal to the point of fixation

what is perimetry

test to see how well the retina can detect light in different parts of the visual field

what does high dB mean in perimetry

higher sensitivity where dimmer stimulus was seen

what does low dB mean in perimetry

lower sensitivity where brighter stimulus was seen

what does congenital defects mean

colour vision defect that are present from birth and affects both eyes equally

what does acquired defects mean

colour vision defect that develops later in life and affects one eye more than the other

what wavelength does the eye responds to

380-780nm

what are dichromats

two photopigments present, one missing

what are trichromats

all three photopigments present

what are anomalous trichromats

three photopigments present but one photopigment has a different sensitivity

what are protans

missing or altered L-cones (red)

what are deutans

missing or altered M-cones (green)

what are tritans

missing or altered S-cones (blue)

what are the MacAdam Ellipses

regions or colour that appear identical to normal observers

what are the confusion loci

lines of chromaticity diagram connected colours that appear the same to dichromats

how many confusion loci for protanopes

17

how many confusion loci for deuteranopes

27

what are the sensitivity and specificity of the Ishihara plates

98% and 94%

what does the Ishihara plates detect

proton and deutan defects because they see orange and olive colours as more similar