W2 Presentation Notes

Topics covered and key learning outcomes

• Topics: Visual acuity, contrast sensitivity, visual field testing, colour vision.

• Learning outcomes:

  • Explain the physiological basis of vision: roles of rods and cones, and the retinal-to-brain pathways.

  • Define and measure visual acuity: interpretation of Snellen and logMAR notation; distance and near acuity testing.

  • Interpret contrast sensitivity and its clinical relevance: contrast sensitivity vs visual acuity; CSF shape; early signs of ocular/neurological disease.

  • Understand visual field testing and perimetry: hill of vision concept; differences between kinetic and static perimetry.

  • Recognise colour vision mechanisms and testing methods: trichromatic theory, opponent processing, common colour vision deficiencies, and tests (Ishihara, HRR, Farnsworth D-15).

Fundamentals of vision and light

• Light as visible electromagnetic radiation; wavelength range approx. 380–750 nm.

• Light is measured in radiometric and photometric units.

• Electromagnetic radiation is emitted as photons; photons are detected by photoreceptors in the retina.

• Visual detection involves signal detection amidst noise; phototransduction converts photons to neural signals; final signals travel via photoreceptors → bipolar cells → retinal ganglion cells → optic nerve → visual cortex.

Rods, cones, and photopigments

• Retina contains rods and cones:

  • Cones mediate colour and high acuity vision; three cone classes in normal humans (trichromacy):

  • L-cones (long wavelength) peak ~560 nm

  • M-cones (medium) peak ~530 nm

  • S-cones (short) peak ~420 nm

  • Rods mediate scotopic (low-light) vision.

• Dichromats have only two cone classes; many mammals are dichromatic.

• Photopigments consist of opsin bound to 11-cis-retinal (derived from vitamin A).

• Absorption of photons triggers photoreceptor responses, leading to hyperpolarisation of photoreceptors.

• Rod distribution: ~120 million rods; absent at the central retina; density peaks ~160k–190k cells at 20–30° eccentricity.

• Cones: ~6 million per eye (≈5% of rod count); peak density at the fovea; density declines with eccentricity.

• Peak photopigment absorption: rhodopsin (rods) and cone photopigments (S, M, L).

• Photopigments are cyclically regenerated; overall colour perception arises from complementary activity of cone types.

Visual acuity (VA)

• VA definition: the spatial resolving capacity of the visual system; affected by optical quality (diffraction, aberrations) and neural factors (photoreceptor density/spacing).

• VA tasks include: detection, recognition (most clinical charts), resolution, and hyperacuity localisation.

• Standard notation and concepts:

  • Snellen notation: VA = D'/D, where D' is the standard viewing distance (usually 6 m) and D is the distance at which each letter subtends 5 arcminutes; each letter stroke subtends 1 arcminute.

  • MAR (Minimum Angle of Resolution): MAR is the smallest angular separation the visual system can distinguish; in testing, MAR corresponds to the angle subtended by the critical detail of the optotype.

  • logMAR notation: logMAR = log10(MAR) where MAR is expressed in the test’s ratio form (see below) — provides equal-interval scaling.

  • Decimal notation: used in some countries; e.g., 6/6 ≈ 1.0 in decimal terms.

  • Conversion between notations (per lecture slides):

  • 6/x Snellen corresponds to MAR = D'/D = 6/x. For 6/6, MAR = 1; for 6/60, MAR = 10.

  • LogMAR: logMAR = log10(MAR). Example: 6/60 (MAR = 10) → logMAR = log10(10) = 1.0.

  • Snellen to logMAR example: 6/60 → logMAR ≈ 1.0; 6/6 → logMAR = 0.0.

• Snellen line interpretation: “D’/D” where D’ is standard viewing distance and D is distance at which letter subtends 5 arcmin.

• Key relationships:

  • VA examples from slides: Snellen 6/60 has MAR = 10; logMAR = 1.0; decimal ≈ 0.10.

  • 6/6 has MAR = 1; logMAR = 0; decimal = 1.00.

• Why measure VA?

  • Quantifies resolving power of the eye; reflects ocular health; monitors disease progression and treatment effects; informs refractive prescriptions.

• Near VA:

  • Measured with reading-style charts; use ‘N’ notation; held at reading distance with habitual correction; assesses accommodative/working distance demands.

Contrast sensitivity (CS) and CSF

• CS quantifies the ability to detect differences in luminance between an object and its background; CS = 1 / contrast threshold.

• Contrast sensitivity function (CSF): sensitivity across spatial frequencies (SF); human CSF is band-pass, peaking at ~2–5 cycles/degree (c/deg) and declining at low and high SFs.

• Real-world visuals are more complex than high-contrast VA charts; CS adds functional insight beyond VA.

• Terminology:

  • Contrast: difference in brightness between target and background relative to average luminance.

  • Michelson contrast (for sinusoidal gratings): CM = (Lmax − Lmin) / (Lmax + L_min).

  • Weber contrast (for near-uniform backgrounds): CW = ΔL / Lbackground.

• CS measurement tools/environment:

  • Gratings (sine-wave and square-wave) as standard CS measures.

  • Spatial frequency represented as cycles per degree (cpd).

  • CSF plotted with SF on x-axis (log scale often used) and sensitivity (1/threshold) on y-axis.

• Why measure CS?

  • CS is a key parameter of functional vision; can reveal early signs of ocular or neurological disease not detected by VA alone.

• CS assessment methods:

  • Pelli-Robson chart (contrast-based letter chart): 3/30 letters per triplet at 3 m; 16 triplets; fixed spatial frequency; threshold determined by last triplet with two correct letters; scoring per triplet steps (0.15–2.25 logCS). The chart is rapid, reliable, and compares CS across individuals.

  • CSF measurement across SF using grating methods yields a CSF curve.

• CSF factors affecting shape:

  • Conditions: uncorrected refractive error, amblyopia, cataracts, diabetes, MS, Parkinson’s, Alzheimer's, AMD.

  • Testing conditions: lighting, stimulus size, temporal elements (motion), background, color.

• CSF implications:

  • Different ocular pathologies affect CSF in characteristic ways (e.g., MS vs cataract with high refractive error) and cannot be inferred from VA alone.

• Practical notes:

  • Lighting, stimulus size, and temporal aspects influence CS thresholds; CS testing complements VA in clinical assessments.

Visual field and perimetry

• Visual field definition: the area seen by the eye when fixating a point; conceptualized as a 3D hill of vision (HoV).

• Normal monocular field extents: ~60° superior, ~60° nasal, ~80° inferior, up to ~100° temporal; binocular overlap ~120°; physiological blind spot at ~15° temporal to fixation.

• Perimetry measures retinal sensitivity across the field using light stimuli; results expressed in decibels (dB). 0 dB = brightest stimulus; 50 dB = dimmest detectable stimulus.

• Kinetic perimetry

  • Moving target of constant luminance (e.g., Goldmann perimeter).

  • Maps isopters (contours of equal sensitivity).

• Static automated perimetry (SAP)

  • Stationary stimuli of varying intensities; threshold algorithms (e.g., 4-2-1 staircase, SITA standards) determine differential light sensitivity at multiple retinal points.

• Key 3D representation: The Hill of Vision as a 3D map of sensitivity around fixation; isopters form rings; axes show distance from fovea and sensitivity.

• Data representations in perimetry:

  • Raw threshold data: patient-specific sensitivity values per tested location (in dB).

  • Grey scale: visual interpretation of sensitivity; not a substitute for numerical data.

  • Total Deviation Plot: compares patient points to age-matched normal means (numeric deviation and probability plots).

  • Pattern Deviation Plot: adjusts for generalized depression/elevation (e.g., media opacities, refractive error) to reveal localized defects (e.g., glaucoma).

• Practical interpretation:

  • Central vision vs peripheral sensitivity, relative to normal values.

  • Localised defects suggest focal pathology along visual pathways.

  • Cross-sectional/side profile helps interpret how patient sensitivity deviates from normal.

Modulation Transfer Function (MTF)

• Purpose: Measures optical image quality by assessing how well an optical system preserves contrast at different spatial frequencies.

• Method: Project sinusoidal gratings (alternating dark/light stripes) of known contrast through the optical system.

• Parameters of a sinewave grating:

  • Spatial frequency: number of cycles (stripe pairs) per degree of visual angle.

  • Contrast: ratio of luminance difference to the sum of luminances.

  • Phase: positional alignment of peaks/troughs.

• Key property: A degraded optical system reduces amplitude (contrast) but preserves the sinewave form.

• MTF calculation: MTF(f) = (contrast in image) / (contrast in object) as a function of spatial frequency f.

Spectral sensitivity, color vision, and chromaticity

• Spectral sensitivity: human eye sensitivity to wavelengths approximately 380–780 nm; peak sensitivities of photoreceptors influence color perception.

• Colour basics:

  • Hue: type of color (e.g., red, green, blue).

  • Brightness (luminance): perceived lightness.

  • Saturation (chroma): vividness of color.

• Cone-based colour vision (L, M, S cones) underpins trichromacy.

• Chromaticity vs luminance:

  • Chromaticity describes color irrespective of luminance; color matching can be represented on the CIE diagram.

• CIE Chromaticity Diagram and MacAdam ellipses:

  • Chromaticity coordinates (e.g., u', x, y) map color independent of luminance.

  • MacAdam ellipses describe regions of color that look the same to normal observers; confusion loci indicate colors indistinguishable to certain color vision deficiencies.

• Colour gamut and spectral locus:

  • The spectral locus marks pure monochromatic colors; white lies toward the centre.

  • Gamut defines the subset of colors representable by a given system or set of primaries.

• Cone photopigments and wavelength sensitivities:

  • L-cones peak ~560 nm; M-cones ~530 nm; S-cones ~420 nm.

  • Rod pigment (rhodopsin) and melanopsin contribute to non-image-forming light responses.

Inheritance and types of colour vision defects

• Colour vision defects are often X-linked recessive.

• Classification:

  • Dichromats: missing one photopigment (two cone classes present).

  • Anomalous trichromats: all three photopigments present but one has altered spectral sensitivity.

  • Protanomaly/Protanopia: long-wavelength pigment missing or altered (red deficiency).

  • Deuternomaly/Deutanopia: medium-wavelength pigment missing or altered (green deficiency).

  • Tritanomaly/ Tritanopia: short-wavelength pigment missing or altered (blue deficiency).

  • Monochromacy: absence of functional cones; extremely reduced color perception.

• Inheritance patterns depicted by pedigrees:

  • X-linked inheritance leads to higher prevalence in males; female carriers may show mild symptoms due to X-inactivation skewing.

Colour vision testing methods and interpretation

• Types of tests:

  • Pseudoisochromatic plates: plates with a figure formed by dots of colors that blend into the background for people with normal color vision; used for screening protan/deutan defects.

  • Sorting (arrangement) tests.

  • Matching tests (anomaloscopy).

  • Naming/occupational tests.

• Ishihara plates (most common screening tool):

  • Purpose: rapid screening for protan and deutan defects; sensitivity ~98%, specificity ~94%.

  • Administration: tested at 75 cm under daylight with color temperature near 6740 K; allow 3 seconds for responses; response is the number seen on each plate.

  • VA requirement: At least 6/60 corrected vision.

  • Plate set: 24-plate edition (in many cases); 6/60 equivalence and plate-level details.

  • Plate categories in 24-plate edition: Introduction (No. 1), Transformation (No. 2–7), Vanishing (No. 8–13), Hidden (No. 14–15), Classification (No. 16–17), Tracing (No. 18–24).

• How Ishihara works:

  • Target colors differ from background while maintaining similar luminance; helps avoid non-color clues.

  • Normal color vision sees orange and pink as similar; protans/deutans misperceive due to confusion lines on CIE diagrams.

• Chromaticity diagrams and confusion lines:

  • Confusion loci join chromaticity points that appear the same to a given type of colour-vision deficiency (e.g., protan/deutan).

  • Test plates are chosen from color space regions where confusion lines differ for protans vs deutans.

• Inheritance visuals:

  • X-linked patterns show carrier and affected individuals across generations; most affected are males; some female carriers may show mild effects.

Practical considerations and testing conditions

• Testing conditions for color vision and vision screening:

  • Lighting, background luminance, and color temperature affect test results.

  • Correct refractive errors and adequate pupil dilation (if applicable) improve accuracy.

• Ethical and educational context:

  • Cultural sensitivity and inclusion in vision science education; acknowledgment of Indigenous land and communities in academic settings.

Quick references and key readings

• Adler’s Physiology of the Eye: Chapter 32 (visual processing and VA/CS concepts).

• Pelli DG, Bex P. Measuring contrast sensitivity. Vision Research. 2013;90:10-4.

• Phu J, et al. The value of visual field testing in the era of advanced imaging: clinical and psychophysical perspectives. Clin Exp Optom. 2017;100(4):313-332.

• Cole BL. Assessment of inherited colour vision defects in clinical practice. Clin Exp Optom. 2007;90:157-175.

Summary of key numerical relationships and formulas

• Visual acuity notation and relationships:

  • VA = D'/D, with D' = standard viewing distance (usually 6 m) and D = distance at which the letter subtends 5 arcmin.

  • For 6/60: MAR = D'/D = 6/60 = 10; logMAR = log10(10) = 1.0; Decimal ≈ 0.10.

  • For 6/6: MAR = 1; logMAR = 0; Decimal = 1.00.

• Minimum Angle of Resolution (MAR) and its log:

  • MAR is the smallest angular separation resolvable by the visual system; for 6/6 letters it corresponds to ~1 arcmin, per testing context.

  • LogMAR = log10(MAR) (with MAR expressed in the test’s ratio form, e.g., 6/x where MAR = 6/x).

• Spatial frequency and cycles per degree:

  • SF (cpd) describes the number of cycles per degree of visual angle.

  • 6/6 letter corresponds to SF ≈ 30 cpd; 30 cpd is equal to CS ≈ 1 on a logCS scale (NB: CS and VA are related but measure different aspects).

  • 1 degree = 60 arcminutes; conversion between Snellen and SF is used to interpret cpd values.

• Contrast sensitivity function (CSF):

  • Peak sensitivity at ~2–5 cpd; CSF falls at low and high SFs.

  • CS = 1 / (contrast threshold); higher CS means better sensitivity.

• Decibels (dB) in perimetry:

  • dB represents retinal sensitivity; higher dB means higher sensitivity; 0 dB = brightest detectable stimulus; higher numbers indicate dimmer stimuli detectable.

• Ishihara plate interpretation basics:

  • Detects protan/deutan defects; accuracy depends on appropriate testing conditions and patient response time.