contrast sensitivity Flashcards
what is illuminance? state the units
a measure of light landing on a surface (lux)
what is luminance? state the units
Luminance is the combination of light that is being both reflected and emitted from a surface. (cd/m2)
what is contrast?
difference in luminance between target and background as proportion of average background luminance
what is the equation for contrast?
(Lt-Lb)/Lb where: Lt is target luminance and Lb is background luminance
expressed as either a decimal or percentage
how do you find increment luminance?
delta L = Lt-Lb
what is contrast threshold?
lowest contrast at which the target is detectable
what is the equation for contrast threshold and what constant is it equal to?
increment threshold (delta L)/ background luminance (Lb) = Weber’s constant
when is a target seen in terms of contrast and weber’s constant?
if the contrast exceeds Weber’s constant
what should contrast in photopic and scotopic conditions be?
-Contrast threshold in photopic conditions ~ 2%
-Contrast threshold in scotopic conditions ~ 14%
what are the two types of wave gratings for luminance?
-square wave grating
-sinewave grating
what is spatial frequency?
how fine grained a stimulus is so low SF has a few bars per degree so gross detail and then high SF has many bars per degree so fine detail
what are sine waves defined by?
-orientation
-contrast
-spatial frequency
what are sine wave gratings used for?
to investigate the sensitivity of the visual system to different spatial frequencies
what are the three parts of the contrast sensitivity graph?
-low SF roll off: lateral inhibition limits sensitivity
-peak sensitivity: lots of neurones are sensitive to medium SFs
-high SF cut off: optics of the eye limit visibility as human lens cannot transmit SFs above 60 cycles per degree (high)
what is the Michelson contrast for grating equation?
Michelson contrast = Lmax-Lmin / Lmax + Lmin
where L max and L min are maximum and minimum luminance
when can you plot a contrast sensitivity function?
when contrast sensitivity is measured for a range of spatial frequencies
What is the formula for sensitivity in terms of threshold?
sensitivity = 1/threshold
what’s another word for the human contrast sensitivity function?
a band pass function
what is the CSF in terms of the visual system?
a spatial modulation transfer function of the visual system
How do you measure contrast sensitivity function
- start with low SF sine wave grating at very low contrast
- increase the contrast until just seen to get your contrast threshold
- repeat with different spatial frequencies
what do the areas above and below a contrast sensitivity function graph show?
area above the graph is invisible and area below the graph is visible
be able to label contrast sensitvitiy function graph
check screenshots
what is the spatial modulation transfer functions?
how well an optical system/ lens transfers information
how can you work out the spatial modulation transfer functions of an object?
by plotting (image contrast/ object contrast) for different spatial frequencies and the image transfer is perfect when image contrast / object contrast = 1
how can you explain the high SF cut off of human contrast sensitivity function?
as graphically, SMTF of crystalline lens transfers low and med. S.F.s well, but attenuates high
spatial frequencies
what does the shape of the contrast sensitivity function suggest?
-the quantitiy and responsiveness of different sized receptive fields as RGCs and other neurones have receptive fields of various sizes and large receptive fields respond to low SFs and small receptive fields respond to high SFs
-means we can conclude Few neurones with receptive fields sensitive to low and high S.F.s
and no neurones sensitive to very low S.F.s, so see roll off at low S.F.s
what is Fourier analysis?
a breakdown of a stimulus into sinewaves of different spatial frequencies
give 2 pieced of evidence of spatal frequency channels
1) Selective-adaptation to specific spatial frequencies (Blakemore & Campbell, 1969)
2) Measurement of activity of simple cortical cells in response to different spatial frequencies as different simple cortical cells are tuned to respond maximally to different spatial frequencies (Maffei & Fiorentini, 1973).
how does an object or scene get perceived through Fourier analysis?
- the object/scene gets split up over several spatial frequency channels in Fourier analysis
- SF detectors receive the info via their corresponding channels as the detectors fire
- the frequencies then get combined in Fourier synthesis
- the object/scene is now perceived
what is lateral inhibition?
where the large bars in low SFs cover both the centre and surround of ganglion cells
what is centre surround antagonism?
where ganglion cells respond strongly to a stimulus which falls on the centre or surround
what does the contrast sensitivity function imply?
-Spatial contrast is vital for visual perception
-Visual acuity does not tell us about a person’s contrast sensitivity to low and middle spatial frequencies.
how is spatial contrast vital for visual perception?
– Low and middle spatial frequencies critically important for detecting and recognising objects.
– High spatial frequencies required for detecting detail.
– Visual acuity corresponds to high spatial frequency cut-off of CSF i.e. highest spatial frequency visible at max contrast
name 3 factors that affect the CSF
-age
-retinal eccentricity
-light levels
what is the relationship of CSF to Snellen acuity?
snellen 6/6 = 30 cycles per degree
how can young age affect CSF?
-infants can see little or nothing at frequencies above 2-3 cycles per degree so infants are only sensitive to low spatial frequencies
what is retinal eccentricity and explain how it affects the CSF?
-How far from the fovea images are so where images are in your peripheral vision
-this means increased eccentricity causes peak and cut off to shift towards lower SF due to:
-receptive fields getting larger with increased distance from the fovea
-decreased cortical representation of the periphery compared to the central retina due to cortical magnification
what is the relationship between eccentricity and contrast sensitivity?
increased eccentricity causes reduced contrast sensitivity
what is the relationship between retinal illuminance and contrast sensitivity?
reduced retinal illuminance causes reduced contrast sensitivity
how does light affect CSF?
-decrease in retinal illuminance causes Shift in peak sensitivity and high frequency cut-off to lower SF (shift from small cone system receptive fields to large rod system receptive fields
-and low spatial frequency roll off is reduced because centre-surround organisation breaks down
at very low luminances
what is the relationship between webers constant and luminance
Weber’s constant increases when luminance decreases when below the range of background luminance so contrast threshold ↑ and C.S.↓.
give 5 reasons why it’s good to take CSF measurements in clinic
- Screen for visual pathway disorders
- Explain symptoms of poor vision in people with good visual acuity
- Help justify referral of cataract patient with reasonable visual acuity
- Predict visual performance when changes are made to the refractive power of the eye
- Explain a poor response to an optical aid and help to fit a more suitable low
vision aid - they may have more of a CS issue than a VA issue
give daily problems patients with reduced contrast sensitivity may have
-mobility
-reading
-detecting
-low contrast objects
-recognising faces
what practical solutions can patients with reduced contrast sensitivity be given?
-good lighting
-improving contrast of everyday objects (pouring tea into a white mug rather than a black one)
-potentially using long wavelength pass filters (allows shorter wavelengths to be rejected)
name some optical issues that can cause CSF abnormalities
-uncorrected refractive error
-contact lenses
-refractive surgery
-keratoconus (low SFs are spared)
-iritis
-cataract
-AMD
-diabetic retinopathy
-glaucoma
-optic atrophy
-optic neuritis
-retinitis pigmentosa
-amblyopia
what 2 systemic diseases can affect CS?
-parkinsons disease causes general reduction in CS
-MS causes loss of CS for low spatial frequencies
what kind of SFs are affected by mild refractive error
high SFs
why can contact lenses caused reduces CS?
-deposits in the lens
-lens material ageing
-corneal oedema due to overwear
how can refractive surgery affect CSF?
-corneal haze causes all SFs to be affected
-aberrations and defocus cause high/ medium SFs to be affected
otherwise improves over time
how can iritis affect CSF?
light scatter causes reduced contrast sensitivity
in what 3 ways does cataract effect vision?
-causes a myopic shift
-changes spectral transmittance by causing greater short wavelength absorption
-increases in forward light scatter
in what 3 ways does cataract affect CSF?
-Myopia – if uncorrected, loss of contrast sensitivity for high SFs and attenuation of high SFs
-Absorption of light by the lens leads to reduced retinal illuminance (and reduced CS)
-Forward light scatter results in reduced retinal illuminance and reduced contrast on the retina
what are the 3 types of cataract and the SFs they effect?
-nuclear sclerosis: affects mid to high spatial frequencies first
-cortical cataracts: gradually affect mid to high spatial frequencies as they enroach on the visual axis
-posterior subcapsular cataracts affect CS at all SFs
which spatial frequencies does AMD affect?
mainly high (like normal age changes but just more pronounced)
how does diabetic retinopathy affect CSF?
-early background DR causes medium to high SFs to be affected
-pre proliferative causes severe CS loss
how does amblyopia have an affect on CSF?
-causes mid to high SF loss when strabismic
-also affects lower SFs when anismetropic
give 3 contrast sensitivity charts
-pelli robson chart = fixed spatial frequency at variable contrast
-low contrast bailey lovie chart = fixed contrast but low SF
-vistech = meausres CS at 5 spatial frequencies using 4 alternative forced choice procedure
what are the problems with clinical CS measurements
-patial CSF is sensitive to many disorders but does not have a high specificity to any disorder
-hard to diagnose a disease from just CSF measurement
