Anatomy of the Lateral Geniculate Body (LGN) and Visual Cortex

1. The Lateral Geniculate Body (Nucleus)

Location and Gross Anatomy

• Superomedially: pulvinar of the thalamus
• Dorsomedially: auditory radiations from the medial geniculate body (on their way to Heschl's gyrus)
• Dorsolaterally: covered by the optic radiations

Six-Layer Organization

Three Cell Populations

• Large neurons receiving input from M-type retinal ganglion cells
• Rapidly conducting pathway
• Color blind - transmit only black-and-white information
• Poor spatial resolution (few M ganglion cells, wide dendritic spread)
• Project primarily to layer 4Cα of V1

• Small to medium neurons receiving input from P-type retinal ganglion cells
• Moderate conduction velocity
• Transmit color and accurate point-to-point spatial information
• Project primarily to layer 4Cβ of V1

• Small "dust-like" neurons
• Located between the principal layers
• Receive input from nonM-nonP (bistratified) ganglion cells
• Project primarily to layers 2 and 3 of V1 (the cytochrome oxidase "blobs")

Functions of the LGN

1. Relay function: Precise point-to-point transmission of visual information from optic tract to visual cortex via the optic radiation, maintaining high spatial fidelity throughout.
2. Gating function: Controls how much signal is allowed to pass to the cortex. Gating control arrives from:
   • Corticofugal fibers returning from primary visual cortex back to the LGN
   • Reticular areas of the mesencephalon Both sources are inhibitory and can selectively suppress transmission, helping highlight relevant visual information.

Optic Radiation (Geniculocalcarine Tract)

• (a) Upper bundle - from medial LGN, corresponding to superior retina; courses through deep parietal white matter and ends in the superior lip of the calcarine fissure
• (b) Central bundle - from medial LGN, serving the macular region; travels through posterotemporal and occipital white matter and ends in the posterior calcarine fissure on both lips
• (c) Lower bundle (Meyer's loop) - from the lateral LGN, corresponding to inferior retina; sweeps anteriorly into the temporal lobe before turning posteriorly to end in the inferior lip of the calcarine fissure

2. Primary Visual Cortex (V1)

Location

• Brodmann's area 17 in the occipital lobe
• Also called V1, striate cortex, or Visual Area I
• Located along the superior and inferior lips of the calcarine fissure on the medial aspect of the occipital lobe
• Extends approximately 1 cm around the posterolateral aspect of the occipital pole onto the lateral convexity

Why "Striate Cortex"?

Retinotopic Organization

• Macular (foveal) representation: Located at the posterior pole of the calcarine cortex (occipital pole). The fovea has several hundred times more cortical representation per retinal area than peripheral retina.
• Magnification factor: Central 10-15 degrees of vision occupies 50-60% of total V1 surface area
• Superior retina → superior lip of calcarine fissure
• Inferior retina → inferior lip of calcarine fissure
• Peripheral retina → anterior calcarine fissure (near junction with parietooccipital fissure)
• Each occipital lobe receives projections from the nasal half of the opposite eye and temporal half of the ipsilateral retina (i.e., both eyes representing the same contralateral hemifield)

Six-Layer Structure of V1

Inputs and Outputs of V1

• Main input: LGN via optic radiation → primarily layer 4C
• Intracortical connections: Radial (perpendicular) connections from layer 4 maintain retinotopy across layers; horizontal connections within layer 3 span different retinotopic locations
• Outputs from V1:
  • Layers 2/3 → higher visual areas (V2, V4, MT) via cortico-cortical connections
  • Layer 5 → superior colliculus, pulvinar, pons
  • Layer 6 → feedback to LGN (corticofugal)

Ocular Dominance Columns

Cytochrome Oxidase Blobs

3. Beyond Primary Visual Cortex

• Ventral stream ("what pathway"): V1 → V2 → V4 → IT - object identity, color, detail
• Dorsal stream ("where pathway"): V1 → V2 → V3 → MT → posterior parietal - spatial location, motion

Clinical Correlations

Keratometry

Definition and Principle

• Millimetres of radius of curvature (mm), or
• Dioptres (D) of refractive power

K (D) = (n - 1) / r = 0.3375 / r(m)

Instruments

1. Manual Keratometer (e.g., Javal-Schiotz, Bausch & Lomb)

• The Javal-Schiotz keratometer is a two-position instrument using two separate mires and rotating the instrument to measure each principal meridian.
• The Bausch & Lomb keratometer uses a single position with doubling prisms to measure both meridians simultaneously.
• Only a small central zone (~3-4 mm) of the cornea is sampled - the so-called "keratometric zone."

2. Automated Keratometers / Biometers

• Use two coaxial partially coherent low-energy laser beams to produce an interference pattern
• Simultaneously measure axial length, anterior chamber depth, corneal white-to-white distance, and keratometry
• Provide high reproducibility with less operator skill required

3. Corneal Topography (Videokeratography)

• Placido disc analysis: Projects concentric rings onto the anterior corneal surface and analyzes the reflected image. Provides information on anterior corneal power and regularity. Simulated K readings (SimK) can be generated.
• Rasterstereography: Projects a grid pattern rather than rings.
• Results are displayed as color-coded maps (warm colors = steeper/higher power; cool colors = flatter/lower power).

4. Corneal Tomography

• Rotating Scheimpflug photography (e.g., Pentacam): Images both anterior and posterior corneal surfaces plus corneal thickness throughout. Particularly useful for posterior corneal elevation mapping.
• Scanning slit (e.g., Orbscan): Similarly images the entire corneal profile.
• Anterior segment OCT: High-resolution cross-sectional imaging of cornea and anterior segment.

What Keratometry Measures

Clinical Applications

1. IOL Power Calculation (Biometry)

IOL Power = f(K readings, axial length, desired refraction, lens constants)

• SRK-T, Holladay 1 & 2, Hoffer Q, Haigis - each incorporating K and axial length
• For short eyes (AL < 22 mm): Hoffer Q, Haigis, Hill-RBF, and Kane formulae are preferred
• For long eyes (AL > 26 mm): BU II, Holladay, Haigis (optimized), and Kane formulae perform better
• Postoperative refraction is within 1.0 D of predicted outcome in ~90% of patients and within 0.5 D in two-thirds

• Contact lens-induced corneal distortion (remove soft CLs ≥ 1 week; rigid gas-permeable CLs ≥ 6 weeks before biometry)
• Previous corneal refractive surgery (LASIK/PRK alter the anterior corneal curvature, invalidating standard formulae - special adjustment methods such as the refractive history method or Haigis-L formula must be used)
• Irregular corneal surface
• Poor patient fixation

2. Refractive Surgery Screening and Planning

• Keratometry and topography are performed to exclude keratoconus and forme fruste keratoconus
• Contact lens-induced distortion must be allowed to resolve: soft CLs discontinued for ≥2 weeks, rigid gas-permeable CLs for ≥3 weeks (some surgeons recommend 1 week per year of wear)
• Post-ablation keratometry helps assess the achieved correction

3. Keratoconus Diagnosis and Monitoring

• Keratometry readings in keratoconus are characteristically steep (>47 D or higher)
• The keratometer mires appear irregular and distorted rather than the usual clean circular reflections
• Corneal topography is far more sensitive than keratometry alone, showing:
  • Inferior steepening
  • Asymmetric bow-tie pattern evolving to displaced steep cone
  • Progressive increase in K values over serial measurements

4. Contact Lens Fitting

5. Astigmatism Management

• Toric IOL selection (to neutralize corneal astigmatism at cataract surgery)
• Limbal relaxing incisions
• Toric contact lens prescription

6. Cataract Surgery - Unexplained Refractive Surprise

• Undiagnosed keratoconus (steep K readings)
• Undiagnosed previous laser refractive surgery (abnormally flat K values)
• Corneal thickness should also be measured

Limitations of Keratometry

Summary Table