OP1.1 | Physiology of Vision, Ocular Optics and Visual Pathway Anatomy — Summary & Reflection

KEY TAKEAWAYS

Key takeaways — Physiology of Vision, Ocular Optics and Visual Pathway Anatomy:

• The total refracting power of the emmetropic eye is ~60 D: the cornea contributes ~43 D (fixed) and the crystalline lens ~17–20 D (variable by accommodation).

• Rods (scotopic, rhodopsin, peripheral, convergent) mediate dim-light vision; cones (photopic, foveal, 1:1:1 ratio at fovea, three types L/M/S) mediate daylight and colour vision. Phototransduction causes photoreceptor hyperpolarisation via cGMP reduction.

• The visual pathway: retina → optic nerve → optic chiasm (nasal fibres cross) → optic tract → LGN (M layers 1–2; P layers 3–6) → optic radiations (Meyer's loop through temporal lobe for superior visual field) → primary visual cortex (V1, area 17, calcarine sulcus).

• Lesion localisation by field defect:
• Optic nerve: monocular loss
• Chiasm: bitemporal hemianopia
• Optic tract: contralateral incongruous homonymous hemianopia
• Temporal radiation (Meyer's loop): superior quadrantanopia (pie-in-the-sky)
• Parietal radiation: inferior quadrantanopia (pie-on-the-floor)
• Occipital cortex: congruous homonymous hemianopia with macular sparing

• Pinhole test: improvement = refractive cause; no improvement = organic (pathway/retinal/cortical).

• Monocular vs binocular field loss determines anterior vs post-chiasmal localisation.

REFLECT

Reflect on the following using Kolb's cycle of experiential learning:

Concrete experience: You encounter a 40-year-old software engineer who complains of gradual blurring of vision in his right eye over 3 weeks, associated with mild eye ache on lateral gaze. His visual acuity is 6/18 in the right eye and 6/6 in the left. Right RAPD is present.

Reflective observation: What features make this presentation worrying? How does the RAPD fit with what you know about the afferent arc of the pupillary light reflex?

Abstract conceptualisation: Using the anatomy from this module, construct a list of diagnoses from optic nerve head to optic chiasm that could explain monocular visual loss with an RAPD. What investigations would you prioritise?

Active experimentation: If the MRI shows a white matter lesion in the right optic nerve consistent with demyelination, what would you counsel the patient about regarding the risk of multiple sclerosis and available treatments? How would your counselling change if the RAPD were absent and the visual field showed an inferior altitudinal defect (suggesting AION)?