Comprehensive Physiology of Vision and Optics

Global Trends and Socioeconomic Impact of Visual Impairment

• Bourne et al. (2020) analyzed prevalence trends regarding blindness and vision impairment over 30 years (as of 2020 estimates):   - Moderate and severe vision impairment: $295 \text{ million}$ people (crude prevalence $32.7$ cases per $1000$).   - Mild vision impairment: $258 \text{ million}$ people.   - Visual impairment from uncorrected presbyopia: $510 \text{ million}$ people (crude prevalence $64.6$ cases per $1000$).
• Demographic Observations:   - A large proportion of the population is entering older age groups where presbyopia (gradual loss of accommodation) is common.   - Females account for approximately $55\%$ of cases across all categories, likely due to gender differences in exposure and access rather than biology.
• Leading Causes of Impairment Globally:   - Uncorrected refractive errors (the easiest to solve with lenses).   - Cataract: Opacification of the lens; accounts for $62\%$ of blindness in the Philippines.   - Age-related macular degeneration (AMD): Impacting older groups; related to high cholesterol.   - Glaucoma.   - Diabetic retinopathy: Mandatory baseline evaluation is required for diabetic patients.
• Pediatric Implications:   - Visual impairment in kindergarteners doubles by adolescence.   - Approximately $16\%$ of high schoolers have impairment; $90\%$ of these cases are myopia.
• Economic Costs:   - Annual global productivity losses: estimated at $\$411 \text{ billion}$.   - Philippines total cost (2021): approximately $\$2.143 \text{ million}$.   - Impact: Reduced workforce participation, lower productivity, and higher rates of depression and anxiety.

Fundamental Principles of Optics and Light

• Definition of Optics: The science concerned with the genesis and propagation of light, its changes, and associated phenomena.   - Branches: Physical optics and Geometrical optics.
• Light Characteristics:   - Light is the adequate stimulus for vision, existing as a part of the electromagnetic radiation spectrum with wave and particle (photon) properties.   - Photons: Massless particles traveling in wave-like patterns.   - Speed of light ($c$): $299,792 \text{ km/sec}$.   - Wavelength ($\lambda$): Distance between peaks or troughs of a wave.   - Frequency ($f$): Number of oscillations per unit time.   - Governing Equation: $\text{Frequency} \times \text{Wavelength} = \text{Speed of Light}$.   - Relationship: The longer the wavelength, the lesser the frequency.
• Electromagnetic Spectrum Hierarchy (Highest to Lowest Frequency):   - $\text{Gamma rays} > \text{Infrared photons} > \text{Visible light} > \text{UV} > \text{Microwave} > \text{Radio waves} > \text{AM waves}$.   - Higher frequency correlates to higher photon energy.
• Visible Light:   - Detected range: $400 \text{ nm}$ (violet) to $750 \text{ nm}$ (red).   - Breaking light through a prism yields ROYGBIV.

Behavior of Light: Absorption, Reflection, and Refraction

• Light travels in a straight line, but its speed changes when moving between media of different densities.
• Interaction with Opaque Objects:   - Absorption: Energy is transferred to particles on the object's surface, increasing internal energy.   - Reflection: Remaining wavelengths are scattered back to be detected by retinal photoreceptors.
• Refraction:   - Change in direction and speed of a wave due to density differences between two transparent media.   - Normal Line: An imaginary line drawn perpendicular ($90^{\circ}$) to the surface interface.
• Index of Refraction ($n$):   - Formula: $n = \frac{c}{v}$, where $c$ is speed in vacuum and $v$ is phase velocity in the medium.   - Refractive Ability: Indicates the ability of a medium to bend light.   - Air index: $1.00029$; Glass index: $1.52$.
• Light Behavior Rules:   - Low to High refractive index: Speed decreases; angle bends towards the normal.   - High to Low refractive index: Speed increases; angle bends away from the normal.

Spherical Lenses and Image Formation

• Convex Lenses ($+$):   - Bulge outward; known as converging lenses.   - Focal Point ($F$): Point where light rays cross.   - Focal Length ($f$): Distance from the lens center to the focal point.   - Power ($P$): $P = \frac{1}{f}$ (measured in diopters ($D$), with $f$ in meters).
• Concave Lenses ($-$):   - Curved inward; known as diverging lenses.   - Image characteristics: Always virtual, upright, and diminished; located between the object and the lens.
• Image Types in Converging Lenses:   - Object at infinity: Real, inverted, very small at $F$.   - Object between infinity and $2F$: Real, inverted, small between $F$ and $2F$.   - Object at $2F$: Real, inverted, equal size at $2F$.   - Object between $2F$ and $F$: Real, inverted, large beyond $2F$.   - Object between $F$ and lens: Virtual, upright, enlarged in front of lens.

Functional Anatomy and Dioptric Media of the Eye

• Outer Layer (Fibrous Coat): Transparent cornea and opaque sclera (tough connective tissue).
• Middle Layer (Vascular Coat):   - Iris: Contains circular smooth muscles (constriction/miosis via PNS) and radial muscles (dilation/mydriasis via SNS). Determines eye color via pigment levels.   - Choroid: Blood-vessel rich layer supporting the retina; contains pigments.
• Innermost Layer: Retina (functional posterior aspect).
• Muscles of the Eye:   - Ciliary Body Muscle: Controls lens shape for accommodation (PNS control).   - Pupillary Dilator: Increases pupil size (SNS control).   - Iris Sphincter: Decreases pupil size (PNS control).
• Specific Landmarks:   - Optic Disk: Site where axons leave; the "blind spot" (no photoreceptors).   - Geometric Optic Axis: Line through the nodal point to the retina between the fovea and optic disk.   - Visual Axis: Line from fixation point through the nodal point to the fovea.
• Dioptric Media (Passing Order):   - Cornea: Transparent dome; $2/3$ of refractive power ($40 \text{ D}$).   - Aqueous Humor: Fluid in anterior/posterior chambers.   - Vitreous Humor: Gelatinous material maintaining eyeball shape.   - Lens: Biconvex, elastic; adjusts refractive power ($20 \text{ D}$).

Aqueous Humor Dynamics and Glaucoma

• Formation: Active transport of $Na^+$ (followed by $Cl^-$ and $HCO_{3}^-$) by epithelial cells in ciliary processes. Osmotic pressure draws water into the space.
• Composition: Similar to protein-free plasma; contains glucose and amino acids.
• Drainage Mechanisms:   - Canal of Schlemm: Primary route via the trabecular meshwork. Miosis (iris moving away) increases outflow.   - Uveoscleral outflow: Resorption through ciliary muscle (increased when muscle relaxed).
• Intraocular Pressure (IOP):   - Normal range: $10-21 \text{ mmHg}$.   - Glaucoma: Decreased outflow leading to high IOP, which compresses the optic nerve and deprives fibers of nutrients, causing blindness.

Errors of Refraction and Physical Corrections

• Hyperopia (Farsightedness):   - Focal point is behind the retina.   - Causes: Short eyeball, flat cornea, or weak lens system.   - Correction: Convex (converging) lens to add power. (Note: Transcript mentions Dr. Rabe suggests biconcave in one instance, but establishes convex/converging as standard).
• Myopia (Nearsightedness):   - Focal point is in front of the retina.   - Causes: Long eyeball or excessively curved cornea.   - Correction: Concave (diverging) lens to neutralize excessive power ("Cave = dive").
• Astigmatism:   - Egg-shaped cornea/lens; meridians are unequal.   - Result: Multiple focal points and blurred/overlapping images.   - Correction: Cylindrical lens.
• Presbyopia:   - Age-related loss of lens flexibility limiting accommodation for near objects.   - Correction: Bifocal or progressive lenses.

Mechanism of Accommodation (Near Reflex Triple Response)

• Triple Response for Near Vision:   1. Accommodation: Ciliary muscle contraction reduces ligament tension, making the lens more spherical ($+15 \text{ D}$ power).   2. Convergence: Contraction of medial rectus muscles to project the image on the fovea.   3. Pupillary Constriction (Miosis): Sphincter pupillae contraction to increase depth of focus and eliminate peripheral divergent rays.
• Neural Pathway:   - Afferent: Nasal fibers cross at the optic chiasm to the lateral geniculate nucleus (LGN) and visual cortex.   - Efferent: Internuncial fibers from visual association cortices (Brodmann $18$, $19$) synapse on Edinger-Westphal nuclei of CN III.   - Action: CN III leads to medial rectus contraction, ciliary muscle contraction, and sphincter pupillae miosis.

Retinal Histology and Cell Circuitry

• Pigmented Epithelium: Outermost layer; prevents light scattering, provides nutrients, phagocytoses rod outer segments, and recycles visual pigments.
• Müller Cells: Retinal glial cells maintaining internal geometry; oriented radially.
• Three Nuclear Layers:   - Outer: Cell bodies of photoreceptors.   - Inner: Cell bodies of bipolar, horizontal, and amacrine cells.   - Ganglion Cell Layer: Form the optic nerve; the only cells producing action potentials.
• Circuitry Components:   - Vertical Flow: Photoreceptor to Bipolar to Ganglion cell.   - Lateral Flow: Horizontal cells (lateral inhibition between photoreceptor-bipolar synapses) and Amacrine cells (inhibitory modulation between bipolar-ganglion synapses).
• Receptive Fields:   - On-center Bipolar Cells: Depolarized by light in the center; uses metabotropic glutamate receptors (close in response to glutamate).   - Off-center Bipolar Cells: Depolarized in the dark; uses ionotropic glutamate receptors (open in response to glutamate).
• Ganglion Cell Types:   - P cells (Parvocellular): Small receptive fields; sensitive to color (Red-Green, Blue-Yellow) and detail; sustained tonic discharge.   - M cells (Magnocellular): Medium receptive fields; sensitive to movement and luminance (brightness); phasic bursts.   - W cells: Large receptive fields; influenced by amacrine cells.

Phototransduction and Dark Current

• Photoreceptors (Rods vs. Cones):   - Rods: High sensitivity (single photon), scotopic (night) vision, monochromatic, mostly peripheral.   - Cones: Lower sensitivity (hundreds of photons), photopic (day) vision, color, concentrated in the fovea.
• Visual Pigments:   - Opsin bound to $11\text{-cis retinal}$.   - Rods: Rhodopsin (absorbs best at $500 \text{ nm}$).   - Cones: Three opsins (Blue $419 \text{ nm}$, Green $533 \text{ nm}$, Red $564 \text{ nm}$).
• Phototransduction Process:   1. Light absorption causes isomerization of $11\text{-cis retinal}$ to $\text{all-trans-retinol}$.   2. Activated rhodopsin activates transducin (G protein).   3. Transducin activates cGMP phosphodiesterase.   4. cGMP converts to $5'\text{GMP}$, decreasing cGMP levels.   5. cGMP-gated $Na^+$ channels close.   6. Cell hyperpolarizes ($-70 \text{ mV}$), reducing glutamate release.
• Dark Current: In the dark, cGMP levels are high, $Na^+$ channels are open, and the cell is depolarized, releasing glutamate tonically.

Visual Acuity Testing

• Minimum Separable Distance: Rays must subtend an angle of $1 \text{ arcminute}$ ($1/60^{\circ}$).
• Foveal Discrimination: Can distinguish sources $25 \text{ arc seconds}$ apart.
• Snellen Chart (Far Vision):   - Conducted at $20 \text{ feet}$ ($6 \text{ meters}$).   - Result interpretation (e.g., $20/50$): The subject sees at $20 \text{ feet}$ what a normal eye sees at $50 \text{ feet}$.   - Pinhole Test: If vision improves, the cause is refractive error; if not, the cause is retinal.
• Jaeger Chart (Near Vision):   - Held at $14 \text{ inches}$.   - Paragraphs range from $J1$ to $J11$ ($J11$ is largest).
• Alternative Charts: Random E (Tumbling E) for non-readers; picture cards for children.

Visual Pathway and Cortical Processing

• The Pathway: Ganglion cells $\rightarrow$ Optic nerve $\rightarrow$ Optic chiasm $\rightarrow$ Optic tract $\rightarrow$ Lateral Geniculate Body (LGB/LGN) $\rightarrow$ Optic radiation $\rightarrow$ Visual cortex (Area $17$).
• Optic Chiasm Decussation: Nasal fibers cross; the left visual field is represented in the right hemisphere.
• Lateral Geniculate Nucleus Structure:   - Six layers; Retinotopic map.   - Layers $1-2$: Magnocellular (M cells).   - Layers $3-6$: Parvocellular (P cells).
• Primary Visual Cortex (Brodmann $17$ / Striate Cortex):   - Processing: Color, stereopsis (depth), contrast (simple/complex cells).   - Macular representation: Largest and most posterior part.
• Secondary Visual Cortex (Association Area):   - Processes form, $3D$ position, motion (M cells), and meaning (reading, color detail).
• Visual Agnosias:   - Prosopagnosia: Inability to identify faces; bilateral inferior occipital lesions.   - Apperceptive Agnosia: Can name objects by touch but cannot draw them or recognize by sight; occipital lobe damage.

Lesions and Visual Field Defects

• Definitions:   - Scotoma: Small defect.   - Anopia: Large defect.   - Hemianopia: Half of field lost.   - Homonymous: Same side in both eyes.   - Heteronymous: Opposite sides (e.g., Bitemporal).
• Specific Lesion Locations:   - Optic Nerve: Total blindness in one eye.   - Optic Chiasm: Bitemporal hemianopia (loss of peripheral fields).   - Optic Tract: Contralateral homonymous hemianopia.   - Optic Radiation: Homonymous hemianopia with macular sparing (due to dual blood supply from middle and posterior cerebral arteries).

Depth Perception and Color Vision Theories

• Monocular Depth Cues (7 types):   1. Relative Size: Familiar objects that are larger are perceived as closer.   2. Interposition: Overlapping objects (covering object is closer).   3. Linear Perspective: Parallel lines converging at a horizon.   4. Aerial Perspective: Distant objects appear hazier/bluer.   5. Light and Shade: Shadows provide dimension.   6. Texture Gradient: Closer objects show more detail than smooth distant ones.   7. Motion Parallax: Closer objects appear to move faster than distant ones.
• Binocular Depth Perception (Stereopsis):   - Retinal Disparity: Eyes are $6.5 \text{ cm}$ apart; more disparity indicates the object is closer. Useful up to $60 \text{ meters}$.
• Color Vision Theories:   - Trichromacy Theory (Young-Helmholtz): Perception via activation of three cone types (Red, Green, Blue).   - Opponent-Process Theory (Hering): Colors are linked in antagonistic pairs (Red-Green, Blue-Yellow, Black-White). Explains after-images through neural fatigue.
• Color Blindness (X-linked hereditary):   - Protanopia: Loss of red cones.   - Deuteranopia: Loss of green cones.   - Tritanopia: Loss of blue cones.   - Test: Ishihara Chart (color plates); X-Rite Color Challenge.

Eye Movement Control

• Functional Classes:   - Gaze Stabilization: Vestibulo-ocular and Optokinetic systems.   - Gaze Shifting: Saccades (sudden jerky movements), Pursuit (smooth voluntary tracking), and Vergence (altering angle for near objects).
• Extraocular Muscles and Innervation:   - CN III (Oculomotor): Superior, Inferior, Medial Recti; Inferior Oblique.   - CN IV (Trochlear): Superior Oblique.   - CN VI (Abducens): Lateral Rectus.
• Fixation Areas:   - Voluntary: Frontal lobe (Frontal eye field).   - Involuntary: Visual association area (near the primary visual cortex).

Questions & Discussion

• Case Summary: Congenital Cataract   - Red reflex: Result of light reflecting off retinal blood vessels. Absence indicates blockage (leukocoria).
• Case Summary: Glaucoma   - Patient statistics: $61$-year-old with IOP of $21 \text{ mmHg}$ (OD) and $23 \text{ mmHg}$ (OS). Result of trabecular meshwork blockage.
• Case Summary: Vitamin A Deficiency   - Symptoms: Nyctalopia (night blindness) following small intestine resection. Retinol levels $< 0.06 \text{ mg/L}$.
• Case Summary: AMD   - Features: "Black spot" in central vision; presence of Drusen (lipid/protein deposits).
• Concept Checkpoint - Triple Response Answer: Accommodation, convergence, and pupillary constriction.
• Concept Checkpoint - Fovea Answer: Central macula for high-acuity, contains only cones.
• Quiz Question: "Photoreceptors have depolarizing action potentials when activated by light." - False (they hyperpolarize).
• Quiz Question: "Retinal disparity is a binocular cue while motion parallax is a monocular cue." - True.