Explain visual pathway not lesion but pathway

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I now have all the information needed to give a thorough, well-illustrated explanation. Here is the complete visual pathway:

The Visual Pathway

The visual pathway carries light information from the retina to the primary visual cortex. It consists of the following sequential components:

Visual pathway from retina to calcarine cortex

Horizontal section showing the visual pathway. The right hemifield (shaded) is traced through both eyes to the left calcarine cortex.

Step 1 - The Retina (First-Order Neurons)

Light hits the retina and is converted to nerve signals through a three-neuron chain within the retina itself:

Photoreceptors (rods and cones) - detect light
Bipolar cells - relay signals from photoreceptors
Retinal ganglion cells (RGCs) - the output neurons whose axons form the optic nerve

There are two major ganglion cell types:

Midget (P) cells - small receptive fields, sensitive to fine detail and color; project to parvocellular layers of the LGN
Parasol (M) cells - larger receptive fields, sensitive to motion and low contrast; project to magnocellular layers of the LGN

The ganglion cell axons converge at the optic disc (papilla), which has no photoreceptors and creates the physiological blind spot. - Localization in Clinical Neurology, 8e

Step 2 - The Optic Nerve (CN II)

The optic nerve is formed by ~1.2 million ganglion cell axons. It is technically a CNS tract (not a peripheral nerve) because it is myelinated by oligodendrocytes and enclosed by cranial meninges.

The optic nerve has four portions:

Portion	Length	Notes
Intraocular (optic nerve head)	~1 mm	Axons become myelinated here
Intraorbital	~25 mm	S-shaped, allows eye movement
Intracanalicular	~9 mm	Travels through the optic canal
Intracranial	~4-16 mm	Between optic canal and chiasm

Localization in Clinical Neurology, 8e

Step 3 - The Optic Chiasm

The two optic nerves meet at the optic chiasm, located just anterior to the infundibular stalk of the pituitary.

The key event here is a partial decussation:

Fibers from the nasal (medial) retina of each eye cross to the opposite side
Fibers from the temporal (lateral) retina of each eye stay ipsilateral

Why this matters for visual fields:

The nasal retina of the left eye sees the right visual field
The temporal retina of the right eye also sees the right visual field
After the chiasm, ALL information about the right visual field travels in the left optic tract (and vice versa)

Rule: After the chiasm, the left hemisphere "sees" the right visual field; the right hemisphere "sees" the left visual field. - Neuroscience: Exploring the Brain, 5th ed.

Step 4 - The Optic Tract

Each optic tract carries fibers from both eyes representing the contralateral visual hemifield. The tracts course posterolaterally around the midbrain.

A small portion of fibers leave the optic tract here to go to:

Pretectal area and superior colliculus - for the pupillary light reflex and visual orienting
Hypothalamus (suprachiasmatic nucleus) - for circadian rhythm regulation

The majority continue to the LGN. - Gray's Anatomy for Students

Step 5 - The Lateral Geniculate Nucleus (LGN) of the Thalamus

The LGN is the main thalamic relay for conscious vision. It has 6 layers, numbered 1-6 from ventral to dorsal:

Layers	Cell Type	Input
1 and 2	Magnocellular	Motion, depth, low contrast
3, 4, 5, 6	Parvocellular	Color, fine detail
Between each	Koniocellular	Color (blue-yellow channel)

Eye input segregation in the LGN:

Contralateral eye projects to layers 1, 4, and 6
Ipsilateral eye projects to layers 2, 3, and 5

So both eyes remain segregated within the LGN - they are NOT yet combined at this stage. - Principles of Neural Science, Kandel 6th ed.

Step 6 - The Optic Radiation (Geniculocalcarine Tract)

Axons leaving the LGN form the optic radiation (also called the geniculocalcarine tract), which fans out toward the primary visual cortex. It has two divisions:

The retinofugal visual pathway from eye to primary visual cortex

Upper division (parietal / dorsal fibers)

Carries information from the superior retina (representing the inferior visual field)
Travels directly posteriorly through the parietal lobe
Terminates on the superior bank (cuneus) of the calcarine fissure

Lower division - Meyer's Loop (temporal fibers)

Carries information from the inferior retina (representing the superior visual field)
Loops anteriorly into the temporal lobe first, then swings back posteriorly (this loop is called Meyer's loop or Flechsig's loop)
Terminates on the inferior bank (lingual gyrus) of the calcarine fissure

Mnemonic: "PITS" - Parietal fibers carry Inferior visual field to the Top of calcarine; Temporal fibers (Meyer's loop) carry Superior visual field to the bottom. - Grainger & Allison's Diagnostic Radiology

Step 7 - The Primary Visual Cortex (V1 / Area 17 / Striate Cortex)

The optic radiation terminates in the primary visual cortex (V1), located in the occipital lobe along the banks of the calcarine fissure. It is also called:

Striate cortex - named after the Stria of Gennari, a thick myelinated band in layer 4 visible to the naked eye
Brodmann area 17

Key organizational features:

Each hemisphere's V1 receives input from the contralateral visual field
Information is projected inverted: superior retina → superior (dorsal) calcarine cortex; inferior retina → inferior (ventral) calcarine cortex
The macula (central vision) has disproportionately large representation at the occipital pole (cortical magnification)
LGN parvocellular neurons project to cortical layer IVCβ; magnocellular to layer IVCα; koniocellular to "blobs" in layers 2 and 3
Inputs from both eyes remain in separate ocular dominance columns within V1

Step 8 - Higher Visual Cortex (V2, V3, V4, V5)

From V1, processed information passes to visual association cortex (areas 18 and 19) and beyond, via two major streams:

Stream	Direction	Function
Dorsal "where?" stream	V1 → Parietal lobe	Motion, spatial location, visually guided action
Ventral "what?" stream	V1 → Temporal lobe	Form recognition, color, object identity, faces

Neuroanatomy through Clinical Cases, 3rd ed.

Summary Flowchart

Photoreceptors (rods/cones)
        ↓
Bipolar cells (retina)
        ↓
Retinal Ganglion Cells → axons form OPTIC NERVE (CN II)
        ↓
OPTIC CHIASM
  - Nasal fibers cross → contralateral optic tract
  - Temporal fibers stay → ipsilateral optic tract
        ↓
OPTIC TRACT (wraps around midbrain)
  - Side branch → pretectum/superior colliculus (pupillary reflex)
        ↓
LATERAL GENICULATE NUCLEUS (thalamus)
  - 6 layers; eye inputs remain segregated
        ↓
OPTIC RADIATION
  - Upper division (parietal): inferior visual field
  - Lower division (Meyer's loop, temporal): superior visual field
        ↓
PRIMARY VISUAL CORTEX (V1 / striate cortex)
  - Calcarine fissure, occipital lobe
        ↓
ASSOCIATION CORTEX (V2–V5)
  - Dorsal stream: "where?"
  - Ventral stream: "what?"

Sources: Gray's Anatomy for Students | Localization in Clinical Neurology, 8e | Neuroscience: Exploring the Brain, 5th ed. | Principles of Neural Science, Kandel 6th ed. | Grainger & Allison's Diagnostic Radiology | Neuroanatomy through Clinical Cases, 3rd ed.

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