Visual System

Retina Organization

• The retina converts light into neural signals for vision.
• The retinal pigment epithelium (RPE) is the outermost layer, which supports photoreceptors by absorbing excess light, recycling visual pigments, and removing photoreceptor cell outer segments.
• Rods (120 million) are responsible for low-light (scotopic) vision but do not detect color.
• Cones (6 million) are responsible for color (photopic) vision and high-resolution vision.
• The outer plexiform layer is where rods and cones synapse with bipolar and horizontal cells.
• Horizontal cells help in contrast enhancement by integrating signals from multiple photoreceptors to adjust sensitivity.
• Bipolar cells transmit electrical signals from photoreceptors to ganglion cells, with some receiving input from amacrine cells.
• The inner plexiform layer is where bipolar cells synapse with ganglion and amacrine cells.
• Amacrine cells enhance contrast through lateral inhibition and help in motion detection and temporal filtering.
• Ganglion cells are the final output neurons of the retina, and their axons form the optic nerve.
• M cells (Magnocellular) process motion and contrast.
• P cells (Parvocellular) are involved in color vision and fine detail.
• The nerve fiber layer contains ganglion cell axons that form the optic nerve.
• The optic disc is the blind spot because it lacks photoreceptors.
• The macula, near the retina's center, contains a high concentration of cones and is specialized for central vision.
• The fovea, in the center of the macula, has the highest cone density for sharpest vision, with no rods.
• The peripheral retina has more rod cells, which provide night vision and motion detection in low light conditions.
• The central retinal artery supplies the inner layers, while the choroidal circulation supplies the outer layers like photoreceptors.

Photon Transduction and Action Potential in the Optic Nerve

• Photoreceptors (rods and cones) contain photopigments that absorb light, triggering a chemical change (photoisomerization).
• In phototransduction, light energy is converted into electrical signals
• Rhodopsin absorbs a photon to activate transducin in rods.
• Activated transducin activates phosphodiesterase (PDE), which breaks down cGMP.
• Decreased cGMP closes Na⁺ and Ca²⁺ channels, hyperpolarizing the photoreceptor.
• Cones use a similar process with different opsins for different light wavelengths (red, green, or blue).
• Photoreceptor hyperpolarization decreases glutamate release.
• On-center bipolar cells are excited by decreased glutamate, while off-center bipolar cells are inhibited.
• Bipolar cells stimulate ganglion cells, which generate action potentials if sufficiently depolarized.
• Action potentials travel down ganglion cell axons, forming the optic nerve, and send visual signals to the brain.
• Photopigment regeneration is needed to prepare for the next photon, and this requires retinaldehyde dehydrogenase to convert active form back to its original state.
• Guanylate cyclase regenerates cGMP, reopening cation channels to return the photoreceptor to its dark state.
• Bipolar cells return to baseline activity to prepare for the next light input.
• Ion pumps, like Na⁺/K⁺ ATPase, re-establish ionic gradients in retinal cells.

Rods vs. Cones

• Rods are cylindrical, more numerous (120 million), and concentrated in the peripheral retina.
• Cones are cone-shaped, fewer (6 million), and concentrated in the fovea.
• Rods are specialized for low-light (scotopic) and night vision but do not detect color.
• Cones are responsible for color (photopic) and high-resolution vision in bright light.
• Rods have high light sensitivity but low spatial resolution, and are effective at detecting motion.
• Cones have lower light sensitivity but high spatial resolution for sharp vision.
• Rods do not detect color, while cones detect red, green, and blue.
• Rods are crucial for dark adaptation, whereas cones adapt quickly to bright light.
• Rods have a higher degree of convergence, leading to poorer spatial resolution.
• Cones have a lower degree of convergence, which leads to higher visual acuity.
• Disorders involving rods result in night blindness, as seen in retinitis pigmentosa.
• Disorders involving cones result in color blindness or central vision loss, as seen in macular degeneration.

Acuity, Color Perception, and Night Vision Trade-offs

• Visual acuity is sharpness and detail of vision.
• Night vision is the ability to see in low light, determined by the number of rods for lower resolution.
• Predatory animals sacrifice night vision for excellent visual acuity, which allows them to spot small prey at a distance.
• Nocturnal animals sacrifice acuity for enhanced night vision with a higher proportion of rods.
• Color vision depends on cones sensitive to different wavelengths.
• Night vision relies heavily on rods, but they do not provide color vision.
• Diurnal primates have trichromatic vision for foraging and distinguishing objects, but less effective low-light vision.
• Nocturnal mammals have fewer cones used for movement and detection in low light.
• Adaptations depend on ecological niche and survival needs.
• Deep-sea animals have high sensitivity to light or night vision, and sacrifice color vision.

Inhibitory Interneurons in Signal Sharpening

• Center-surround organization shapes ganglion cell receptive fields, which creates center areas and surrounds to detect stimuli.
• Lateral inhibition sharpens contrast via horizontal and amacrine cells.
• Activated horizontal cells inhibit neighboring photoreceptors in the surround region to reduce influence from stimuli from areas around the center.
• Improved spatial resolution results from emphasized signal over surrounding areas
• Dynamic range compression uses smaller differences in brightness to make it easier to discern.
• Horizontal cells mediate lateral inhibition between photoreceptors.
• Amacrine cells provide inhibitory signals between bipolar and ganglion cells.

Pathway from Retina to Occipital Cortex

• Photoreceptors process light into electrical signals in the retinas.
• Bipolar cells transmit signals to ganglion cells in the retina.
• Ganglion cells generate action potentials, forming the optic nerve.
• Nasal retina fibers cross at the optic chiasm, while temporal retina fibers stay on the same side.
• Fibers from the left visual field is processed in the right hemisphere, and vice versa.
• The optic tract carries visual information from the contralateral visual field.
• The lateral geniculate nucleus (LGN) of the thalamus is the major relay station that is organized in layers and preserves spatial structure.
• Optic radiations split into Meyer's loop (inferior retinal fibers - temporal lobe), and Baum's loop (superior retinal fibers - parietal lobe).
• The primary visual cortex (V1) is within the occipital lobe along the calcarine fissure, which has retinotopic organization.
• The visual cortex begins processing features like edges, orientation, motion, and contrast.

Steps in Image Processing

• Phototransduction begins in retina, where photoreceptors hyperpolarize, and reduced glutamate release.
• Horizontal & amacrine cells provide lateral inhibition (sharpening contrast & edges).
• Partial decussation at optic chiasm, directs optic input to hemispheres.
• The LGN thalamus organizes input by eye, location, and information type.
• Meyer’s loop (upper visual field) through temporal lobe, and Baum’s loop (lower visual field) through parietal lobe occur.
• V1 detects edges, orientation, direction of motion, basic contrast & brightness, and color blobs.
• Parallel processing pathways include The dorsal stream (“Where/How” Pathway) which projects to the parietal lobe.
• The Ventral Stream (“What” Pathway) projects to inferotemporal cortex

Role of Rostral Colliculi

• The rostral colliculi, or superior colliculi, are in the midbrain and helps initiate rapid, involuntary movements toward visual stimuli
• Sensorimotor integration receives sensory input and generates motor commands to cranial nerve nuclei.
• Saccadic eye movements are helped with quick, simultaneous movements for visual tracking.

Decussation and Eye Position Effects on Visual Fields

• Decussation and eye position impacts visual fields across species.
• Binocular vision uses overlapping field of view from both eyes
• In relation to species, predators exhibit ~50% decussation resulting in visual information from the contralateral visual field.
• Prey animals are 80-100% in decussation with each eye sending information to the opposite hemisphere
• Front-facing eyes have larger binocular fields than small monocular fields, a trait seen in predators.
• Side-facing eye have minimized binocular overlap while maximizing the total field of view, which is a trait seen in prey animals.
• Panoramic vision can see around wide fields.

Pupil Shape Effects on Perception

• Vertical slit pupils enhance depth perception and light control.
• This improves prey detection and precision for ambush predators.
• Horizontal slit pupils provides a wide panoramic view.
• Round Pupils are a versatile shape allowing for dynamic vision tasks in primates.
• It is a balanced sensitivity between light and movement and allows for high visual acuity.
• W-shaped Pupils have a high-contrast in vision in deep, varying depths.

Pathway Mediating the Pupillary Light Response (PLR)

• The Pupillary Light Reflex (PLR) pathway is follows this circuit in order: Retina, Optic Nerve, Optic Chiasm, Pretectal Nucleus, Edinger-Westphal Nucleus, Oculomotor Nerve, Ciliary Ganglion, Iris Constriction.
• Each species has a variable percentage of decussation, such as the Rabbit having up to 100%
• Humans and Primates tested PLR used to asses integrity
• In prey species, strong direct PLR with weak response is common
• Some bird and Reptiles lack consensual response

Sympathetic Innervation of the Eye

• Sympathetic innervation facilitates pupil dilation, eyelid elevation, and vasoconstriction.
• The first order neuron starts, descends the brainstem and terminates in the ciliospinal center of Budge.
• The second order neuron ascends the chain the the to superior cervical ganglion.
• The third order neuron is located above the Superior cervical ganglion, travel internal to target tissues like pupillae and muscles.
• Horner’s syndrome is a loss of pupillae and other impairments that results in the disruption the normal process.

Anisocoria, Mydriasis, and Miosis Definitions

• Anisocoria is unequal pupil sizes between the two eyes
• Mydriasis is dilation of of the pupil, which is larger than normal
• Miosis is smaller construction of the pupils

Signs Seen with Loss of Oculomotor Nerve

• Eyelid droops (ptosis)
• The eye becomes more and and out from position (Down-and-out eye)
• Loss of focus(accommodation)
• Double vision(diplopia)

Loss of Sympathetic Innervation of the Eye

• constricted pupils.(miosis)
• dropping eyelid(Ptosis)
• Sunken eyeball(Enophthalmos)