Visual Pathways and Deficits

• Lecture notes from the Orbit, Part 2 CNS Components lecture from 7 February 2025 by Dan Selski, Ph.D.

Textbook Chapters & Pages:

• Haines' "Fundamental Neuroscience" chapter 20, pages 286-305 provides additional information.

Learning Objectives:

• List the name and function of connections from the retina to various areas of the brain.
• Describe and identify the layers, non-neural cells, and neurons within the retina and the differences across the surface of the retina.
• Describe and/or draw the topographic maps of the visual fields from the retina to primary visual cortex, noting the location of synaptic relays and crossing of fibers.
• Describe the processing of information in the retina that generates X/P and Y/M ganglion cells.
• Describe the location of the LGN and the function of P and M cells.
• Describe the columnar organization of primary visual cortex with reference to layers of cerebral cortex.
• Map the X/P and Y/M projections from primary visual cortex through the ventral and dorsal streams.
• Discuss the anatomical relevance of the clinical correlations discussed in lecture and in the assigned readings.

Overview of Visual Projections:

• Retinal ganglion cells are the only neurons sending axons away from the retina.
• The primary visual pathway goes from the retina to visual areas of the cerebral cortex, including the primary visual cortex.
• Connections of retinal ganglion cells extend to the superior colliculus in the midbrain
  • These connections are important for visual reflexes and subconscious movements of extraocular, neck, and axial trunk muscles.
  • These reflexive motions extensively coordinate with the vestibular system and cerebellum.
• Connections of retinal ganglion cells extend to the suprachiasmatic nucleus of the hypothalamus
  • This regulates diurnal rhythms.
• Connections of retinal ganglion cells extend to small midbrain nuclei (pretectal and accessory optic nuclei), which project bilaterally to the nucleus of Edinger-Westphal, controlling smooth muscle of the ciliary body and iris.
  • This bilateral projection is key to understanding the pupillary light reflex, where shining a light in one eye constricts both pupils.
  • Altered pupillary reflexes indicate damage to the optic nerve (CN II) or oculomotor nerve (CN III).
  • CN II lesions result in no reflex in either eye when light is shown to the lesioned eye.
  • CN III lesions result in no reflex only in the ipsilateral eye when light is shown to either eye.

Retina Histology:

• Focus initially on photoreceptors connecting to bipolar cells, which connect to retinal ganglion cells across the retina, then add detail to the other layers.
• The retina is a CNS division within the diencephalon.
• Müller cells are astrocyte-like glia cells spanning from the outer nuclear layer to the optic fiber layer.
• The outermost layer, the pigment epithelium, is non-neural and located between the choroid and neural retina.
  • It supports and nourishes the outer retinal layers.
  • Its pigment absorbs excess light.
  • Outer segments of photoreceptors are embedded in it, and it phagocytoses shed outer segments.
• Photoreceptors line the outer layer of the neural retina.
  • Rods predominate peripherally.
  • Three types of color-sensitive cones are centrally located.
  • Photoreceptors are specialized bipolar-like neurons with light-sensitive discs in their outer segments.
  • Many mitochondria reside in the inner segments.
  • Nuclei are in the outer nuclear layer.
  • Axons are in the inner plexiform layer.
• The outer plexiform layer is the synaptic area between photoreceptors and bipolar and horizontal cells of the inner nuclear layer.
• Bipolar cells transmit signals from the photoreceptors to retinal ganglion cells.
• Horizontal cells modify the signal from the photoreceptors onto bipolar cells.
• Amacrine cell bodies are in the inner nuclear layer, with dendrites in the inner plexiform layer that modify signals between bipolar and retinal ganglion cells.
• The inner nuclear layer is comprised of bipolar cell bodies, horizonal cells, and amacrine cells.
• The inner plexiform layer contains synapses between bipolar, amacrine, and retinal ganglion cells.
• The ganglion cell layer contains the cell bodies of the retinal ganglion cells.
• The optic fiber layer comprises axons from retinal ganglion cells coursing across the retina’s surface to form the optic nerve (CNII) at the optic disc.
• Histological differences exist across the retina from the peripheral edges to the macula.
  • The periphery of the retina is exclusively rod photoreceptors for high visual sensitivity to light and motion.
  • Gradually more cones exist closer to the macula.
  • The macula has almost exclusively cones and few rods.
  • The fovea centralis (or fovea) is exclusively cones for high visual acuity to detail.
  • The optic fiber layer gets progressively thicker from periphery to the optic nerve.
  • No neuronal cells exist at the origin of CN II, making the optic disc the location of the blind spot in each retina's visual field.

Light and Visual Pathway:

• Both eyes see almost identical visual fields.
• The visual field is treated as the same for both eyes and represented as a circle centered on vision (fovea).
• Each eye independently sees and carries visual field information to the brain.
• Light passes through the aqueous and vitreous humor, and all neural layers of the retina, to reach the photoreceptors’ outer segments.

Topographic Map of Visual Field onto the Retina:

• The visual field maps onto the retina as light refracts through the cornea and lens.
• The cornea refracts light just as much if not more than the lens.
• The upper half of the visual field maps onto the inferior retina.
• The lower half of the visual field maps onto the superior retina.
• The left visual field half maps onto the temporal (lateral) retina of the right eye and the nasal (medial) retina of the left eye.
• The right visual field half maps onto the nasal (medial) retina of the right eye and temporal (lateral) retina of the left eye.
• The surface of each retina is a 2-dimensional topographic map of the 2-dimensional visual field.

Visual Pathway:

• The topographic map splits at the optic chiasm and is sent to higher brain levels.
• Axons from the nasal half of each retina cross the midline to the contralateral side.
• Each optic tract has information from half of the visual field that went to both eyes, with information from each eye staying segregated but axons traveling together.
  • The right optic tract carries information from the left visual hemifield.
  • The left optic tract carries information from the right visual hemifield.
• Retinal ganglion cell axons in the lateral geniculate nucleus of the thalamus synapse onto neurons to maintain the topographic map.
• Neurons of the LGN send axons via the optic radiations to the ipsilateral primary visual cortex in the occipital lobe.
  • The topographic map of each visual hemifield is maintained within this white matter projection tract.
  • Meyer's loop is the part of the optic radiations for the contralateral visual quadrant (superior half of the visual hemifield mapped to the inferior retina).
    • These axons project into the temporal lobe's white matter on their way to the visual cortex.
• As the optic radiations from the LGN connect to the topographic primary visual cortex (V1):
  • The superior quadrant of the contralateral visual hemifield projects to the inferior bank of the calcarine sulcus on the lingual gyrus along Meyer's loop axons.
  • The contralateral visual hemifield's inferior quadrant projects to the superior bank of the calcarine sulcus on the cuneate gyrus.
  • Projections map from the calcarine sulcus to macula and fovea map to more posterior banks of the calcarine sulcus.

Lesions in the visual pathway result in specific, predictable deficits:

• Ophthalmic artery occlusion/compression of the optic nerve —> blindness in one eye.
• Pituitary/hypothalamus tumor compressing optic chiasm —> bitemporal hemianopia (tunnel vision with peripheral vision loss).
• Anterior choroidal artery occlusion supplying optic tract —> homonymous hemianopia (loss of visual field from both eyes).
• Vascular lesion to Meyer's loop (deep terminal branches of MCA) sparing other optic radiations (deep branches of PCA) —> left or right superior quadrantanopia.

Retina Processing:

• Photoreceptors are tonically active, chronically releasing inhibitory neurotransmitters onto synapses of bipolar and horizontal cells.
• Photon absorption leads to hyperpolarization and decreased inhibitory neurotransmitter release.
• Disinhibition essentially activates the bipolar cells.
• Rods have larger outer segments, more sensitivity to low-intensity light.
• Cones have less complex outer segments, and three types, each with a slight cone opsin variant, have maximal sensitivity to blue, green, or red illumination.
• Bipolar neurons compare information from adjacent photoreceptors aided by horizontal cells, which respond to and enhance the difference in signals.
• Lateral inhibition of adjacent bipolar cells.
• Center-surround bipolar cells are MOST responsive to a light point surrounded by no light.
• Bipolar cells receiving input from rods are sensitive to abrupt light intensity changes, translating to edge-detection and motion detection at downstream CNS processing regions.
• Bipolar cells receiving input from cones are most sensitive to color differences.
• This relatively simple information (center of light/color surrounded by no light/different color) is the main information processed by the retina and sent to the visual cortex.
• Ganglion cells and the LGN enhance this center-surround information without any additional layers.

Retinal Ganglion Cells:

• Most ganglion cells receive input from multiple bipolar cells, meaning a single ganglion cell samples a larger area of the retina对应 to a larger area of the visual field.
• Amacrine cells modify signals from bipolar cells to ganglion cells, enhancing contrast between the center and surround.
• Y/M - type ganglion cells (alpha ganglion cells):
  • Have a large receptive field and receive input from many bipolar cells.
  • Prevalent in the peripheral retina.
  • Mainly associated with rod photoreceptor input.
  • Ultimately transmits motion information (i.e.: WHERE) to the brain.
• X/P - type ganglion cells (beta ganglion cells):
  • Have a small receptive field and receive input from few or one bipolar cells.
  • Prevalent in the central macular and foveal retina.
  • Associated with cone photoreceptor input.
  • Ultimately transmits form (i.e.: WHAT) and color information to the brain.

Lateral Geniculate Body (LGN):

• The LGN is a tiny, highly structured nucleus of the posterior inferior lateral thalamus.
• The LGN is difficult to identify in gross dissection but hugely important.
• The LGN has 6 layers that keep information segregated from contralateral and ipsilateral eyes.
• The LGN processes and relays the information from the contralateral visual field FROM BOTH EYES.
• Layers are designated as parvocellular or magnocellular designating retinal ganglion cells carrying “what” or “where” information, respectively.
• Primarily, the LGN keeps the information from the retina organized and relay sensory information to the visual cortex with some center-surround processing/refinement .

Visual Cortex:

• A review of the 6 layers of cerebral cortex including cell type and function is needed.
• Thalamocortical projections and optic radiations synapse in layer IV, where stellate neurons relay information to layers II and III.
• Axonal connections relay info from layers II/III to deeper layers V and VI.
• Any area of V1 processes information from a defined visual field spot.
• The entire surface of V1 is the topographic map of the visual hemifield
• Tiny areas called hypercolumns can be broken down into smaller columns that process info separately.
  • Center-surround information from the retina combines into lines in specific orientations as orientation columns.
  • Some II/III and V neurons are sensitive to a simple line, while others are sensitive to a moving line.
  • Neurons combine input from multiple optic radiation axons to assemble more complex information.
  • Information from contralateral and ipsilateral eyes segregates as ocular dominance columns.
  • Comparison of information begins in V1 and elaborates in subsequent connections to other areas of the visual cortex (V2, V3, V4).
• A column of grey matter dedicated to processing and combining multiple information is the result.

Visual Cortex — Pathways to Conscious Perception:

Neurons in V1, mainly from layers II/III, send short association fibers to adjacent V2 and V3 regions.

• The columnar organization of the topographic map of the visual field is maintained.
• Columns process more complex types of information specific to the signal's origin from the retina.
  • X/P form and color information is elaborated in separate columns.
  • Y/M motion and location information is elaborated.
• From V2/V3, the information segregates into ventral (WHAT) and dorsal (WHERE) path ways across cerebral cortex.
  • X/P information for form/color projects sequentially via short association connections from V2 to V4 and the inferior temporal cortex (ventral brain surface). Small lesions to these areas can cause visual agnosias relating specifically to the recognition of specific objects.
• Y/M spatial location and motion information projects sequentially from V2/V3 to the middle temporal cortex (area MT/area 21) and the posterior parietal cortex (dorsal brain).
• As information passes through pathways, commissural connections across the corpus callosum splenium are prevalent.
• Conscious perception of visual stimuli initially appears at the ends of the dorsal and ventral pathways.
• Further connections from the inferior temporal/posterior parietal areas connect to multimodal association areas of the parietal and temporal cortex and prefrontal cortex.

Explore the intricacies of visual pathways, from optic tract function to the impact of lesions on visual fields. Understand the role of the lateral geniculate nucleus (LGN) and the effects of conditions like pituitary tumors and strokes on vision. Learn about photoreceptor function in visual processing.