visual pathways

Anatomy of Visual Awareness of the Environment 

The visual pathway transmits information about the environment—defined as the 'visual field'—from the receptors in the retina to the cerebral cortex. 

Initial Processing and Retinal Structure 

1. Visual Field to Retina: Light is refracted as it travels through the cornea and the lens, causing the image of the visual field projected onto the retina to be inverted and reversed. Each eye sees only part of the full visual field, and these fields overlap. 

2. Receptors and Neurons: The pathway begins with the photoreceptors (rods and cones). 

    ◦ The 1st order neuron is the bipolar cell (considered a special sense neuron). 

    ◦ The 2nd order neuron is the ganglion cell. 

3. Optic Nerve: Axons of the ganglion cells form the optic nerve, which carries information from the ipsilateral eye. 

Central Pathway and Lateralization 

1. Optic Chiasm (Lateralization): Lateralisation of the visual field occurs at the optic chiasm. 

    ◦ The temporal visual field projects onto the medial (nasal) retinae. 

    ◦ The medial visual field projects onto the temporal retinae. 

    ◦ Axons from the temporal retinae remain ipsilateral. 

    ◦ Axons from the nasal retinae decussate (cross the midline) at the optic chiasm. 

2. Optic Tract: After the chiasm, axons from both eyes travel through each optic tract. Crucially, the optic tract carries information originating from the contralateral visual field. 

3. Thalamic Synapse: The axons synapse in the Lateral Geniculate Nucleus (LGN), which is located in the thalamus. 

    ◦ Midbrain Involvement: Some axons and collateral branches also synapse in the midbrain, specifically in the superior colliculus and the pretectal area. 

Cortical Projection 

1. Optic Radiations: The 3rd order neuron travels via the optic radiations from the LGN to the cortex. 

    ◦ The optic radiations exhibit superior–inferior retinotopy. 

    ◦ Fibres carrying information regarding the upper visual field travel via the Meyer loop (which is the inferior fibre bundle of the optic radiation). 

2. Visual Cortex: The optic radiations terminate in the cortex surrounding the calcarine sulcus. This area is the primary visual cortex, located in the occipital lobe. The remainder of the occipital lobe constitutes the secondary visual cortex. 

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Anatomy of Visual Field Loss 

Lesions along the optic pathway result in visual field loss, referred to as ‘-anopia’. If the pattern of loss is the same in both eyes, it is termed ‘homonymous’. Lesions are named according to the specific visual field area lost. Visual field loss is recorded using two circles representing the field seen by each eye, typically testing one eye at a time, with the dark shaded area indicating the loss. 

Anatomical Sites of Lesion and Vascular Supply 

Visual field loss can occur due to damage at various points along the pathway, often related to specific vascular territories: