Visual Field Projection and Retinal Organization

Visual Field and Retinal Projection

• The visual field is divided into 4 quadrants: nasal and temporal halves, and superior and inferior halves.
• The temporal visual field is projected to the nasal hemiretina, and the nasal visual field is projected to the temporal hemiretina.
• The upper visual field is reflected to the lower hemiretina, and the lower visual field is reflected to the upper hemiretina.
• The visual field is projected to the retina in an inverted (upside down) and reversed (left to right) manner.

Overview of the Eye

• Reflective rays of light from an object first strike the eye for vision to occur.
• The optic tract consists of nerve fibers that run directly without interruption posterior to the optic chiasm.
• The optic tract terminates in the lateral geniculate body, superior colliculi, or pretectal area.

Lateral Geniculate Body (LGB)

• The LGB is an oval swelling projecting from the pulvinar of the thalamus.
• The majority of optic tract fibers terminate in the LGB, which receives input from the retina representing the contralateral visual field.
• Each LGB contains a 6-layered neuron, with each layer receiving input from only one eye.
• There are no lateral geniculate cells with binocular receptive fields.

Optic Radiations/ Geniculocalcarine Tract

• The optic radiations are fibers from the axons of the LGB reaching the cortex.
• Fibers from the lateral part of the LGB are directed downward and forward, then bend backward in a sharp loop (Meyer's Loop) passing through the temporal lobe and sweep posteriorly to the occipital lobe.
• Medial fibers are more direct, with a non-looping course.
• Both fibers terminate in the cortical layer 4 of the upper and lower lips of the calcarine fissure in the occipital lobe.

Oculomotor Nerve (CN III)

• The oculomotor nerve nucleus is located at the level of the superior colliculus of the midbrain.
• The oculomotor nerve consists of different nuclear groups, including the lateral nucleus, Edinger-Westphal nuclei, and midline nuclei.
• The lateral nucleus supplies the superior, medial, and inferior recti, and inferior oblique muscles.
• The Edinger-Westphal nuclei supply the ciliary muscle and sphincter pupillae muscles.

Trochlear Nerve (CN IV)

• The trochlear nerve is located at the inferior colliculus of the midbrain.
• The trochlear nerve supplies the superior oblique muscle.

Pupillary Light Reflex

• The pupillary light reflex is a reflexive response to light that involves the constriction of the pupil.
• The reflex pathway includes the retina, optic nerve, optic chiasm, optic tract, and superior colliculus.
• The examination of pupil function should include assessment of pupil size, reactivity to light, and direct and consensual response.

Supranuclear, Nuclear, and Infranuclear Levels

• The supranuclear level is above the nucleus and involves the cerebral hemispheres.
• The nuclear level is at the pons and contains the nucleus of the nerves (CN III, IV, VI) innervating the muscles.
• The infranuclear level involves the nerves (CN III, IV, VI) and their involvement in muscle movement.

Pathway of Eye Movements

• The pathway of eye movements involves the yoke muscles, which are contralaterally paired EOMs that work synergistically to direct gaze in a given direction.
• The medial longitudinal fasciculus (MLF) controls and coordinates eye movements, acting as a bridge between the two sides of the brainstem.