Vision and Visual Pathways

Vision is a highly adapted sense allowing humans to perceive brightness, color, shape, and movement.
It integrates this information to create a cohesive three-dimensional model of the world.
Visual pathways are crucial for everyday life.
The occipital lobe is entirely devoted to vision.

The eye detects light in the form of photons.
The sclera is a thick, structural layer covering most of the eye's exposed portion.
The cornea is a clear window at the front of the eye not covered by the sclera.
The eye receives nutrients from:
- Choroidal vessels: Intermingling blood vessels between the sclera and retina.
- Retinal vessels.
The retina is the innermost layer containing photoreceptors that transduce light into electrical information.

Light passes through the cornea, which focuses incoming light.
The front of the eye is divided into:
- Anterior chamber: In front of the iris.
- Posterior chamber: Between the iris and the lens.
The iris controls pupil size using two muscles:
- Dilator pupillae: Opens the pupil under sympathetic stimulation.
- Constrictor pupillae: Constricts the pupil under parasympathetic stimulation.
The iris is continuous with:
- The choroid: A vascular layer nourishing the retina.
- The ciliary body: Produces aqueous humor.
Aqueous humor drains into the canal of Schlemm.
The lens controls light refraction.
Accommodation: Contraction of the ciliary muscle (under parasympathetic control) changes the shape of the lens to focus on images.
The vitreous humor is a transparent gel supporting the retina.
The retina converts photons into electrical signals and is considered part of the central nervous system.

The retina contains two types of photoreceptors:
- Rods: For light and dark detection.
- Cones: For color detection.
The retina contains approximately 6,000,000 cones and 120,000,000 rods.
Cones are used for color vision and fine details, most effective in bright light.
Three forms of cones exist, named for wavelengths of light they best absorb.
Rods are more functional in reduced illumination due to their high sensitivity to photons.
Rods contain a single pigment type called rhodopsin.
Color vision requires more light because each cone responds only to certain wavelengths.
Rods allow sensation of light and dark, but not color.
Rods are less useful for detecting fine details because they are spread over a larger area.

The macula, the central section of the retina, has a high concentration of cones.
The fovea, the center-most region of the macula, contains only cones.
Acuity is best at the fovea.
The concentration of rods increases, and cones decrease as one moves away from the fovea.
The optic disc (blind spot) is where the optic nerve leaves the eye and is devoid of photoreceptors.

Rods and cones connect with bipolar cells, which highlight gradients between adjacent photoreceptors.
Bipolar cells synapse with ganglion cells, whose axons form the optic nerve.
Bipolar and ganglion cells are located in front of the rods and cones.
Photons must pass through several layers of cells to reach the photoreceptors.
Information is transmitted forward via action potentials.
There are significantly more photoreceptor cells than ganglion cells, leading to a pruning of details.
Increased convergence of receptors onto one ganglion cell decreases resolution.
On average, the number of cones converging onto an individual ganglion cell is similar than for rods.
Color vision has greater sensitivity to fine detail than black and white vision.
Amacrine and horizontal cells receive input from multiple retinal cells in the same area and accentuate slight differences.
These cells are important for edge detection, increasing contrast perception.

Visual pathways include anatomical connections between eyes and brain and the flow of visual information.
If an object is to your left, photons stimulate the right side of the retina in each eye, and vice versa.
Visual information from objects on the left is processed by the right side of the brain, and vice versa.
Temporal retinal fibers are on the lateral side of the retina, closer to the temple.
Nasal retinal fibers are on the medial side of the retina, closer to the nose.

The nasal visual field refers to objects whose photons must cross in front of the nose to reach the eye.
The temporal visual field refers to objects whose photons can directly enter the eye.
An object in the nasal field of the right eye stimulates the temporal fibers of that eye.
An object in the temporal field of the left eye stimulates the nasal fibers of that eye.

At the optic chiasm, nasal fibers from the left and right eyes cross paths.
Only nasal fibers cross at the optic chiasm.
Visual information from an object on the left is processed by the right side of the brain.
Nasal fibers from the left eye are routed through the optic chiasm to the right side of the brain.
Temporal fibers from the right eye are already on the right side of the body and do not need to cross.
The reorganized pathways after the optic chiasm are called optic tracts.
The temporal field of each eye stimulates the nasal fields of each eye and vice versa.
The nasal fibers cross at the optic chiasm.

Nerve fibers pass to the lateral geniculate nucleus (LGN) of the thalamus.
From the LGN, nerves pass through radiations in the temporal and parietal lobes to the visual cortex in the occipital lobe.
Other nerve fibers branch off from the optic tracks and head directly to the superior colliculi in the midbrain.
The superior colliculi control some reflexive responses to visual stimuli and reflexive eye movements.

Parallel processing is the brain's ability to analyze information regarding color, form, motion, and depth simultaneously using independent pathways.
Cones are responsible for color perception.
Form refers to the shape of an object and the ability to discriminate it from the background.
Neurons carrying information from the fovea synapse with parvocellular cells in the LGN.
Parvocellular cells have high color spatial resolution but low temporal resolution and can only work with stationary or slow-moving objects.
Magnocellular cells are well-suited for detecting motion due to their high temporal resolution.
Magnocellular cells and parvocellular cells are located in distinct layers of the LGN.
Magnocellular cells predominantly receive inputs from the periphery of vision, allowing rapid detection of objects approaching from the sides.
Magnocellular cells have low spatial resolution, providing a blurry but moving image.
Depth perception is based on discrepancies between inputs from two eyes.
Binocular neurons in the visual cortex compare inputs from each hemisphere and detect differences.
Feature detectors in the visual cortex detect particular features of an object in the visual field.
The overall combination of feature detectors is activated in parallel.
Responses to stimuli are stored for feature retrieval.