Visual Special Sensory

Visual Perception

• Light energy converts to electrical energy via photoreceptors in the retina, then is transmitted along CN II.
• Light energy can be described in wavelengths and as particles (photons)
• Brightness or intensity corresponds to the number of photons hitting photoreceptors.
• Neuronal signals that carry visual information relay into the thalamus, and then to the primary visual cortex, which plays a role in visual perception.

Anatomy of the Eye

• The eye is a fluid-filled sphere within three tissue layers: the sclera, choroid, and retina.
• Light goes through the cornea, anterior chamber, pupil, lens, vitreous humor, to finally arrive at the retina.
• An optical image of the surroundings focuses on approximately 130 million photoreceptors in the human retina.

Modification of Light

• Light rays bend through the cornea and lens to focus images onto the retina.
• Lens curvature is changeable using parasympathetic innervation (Edinger-Westphal nucleus of CN III) on the ciliary muscle
• Ciliary muscle contraction rounds the lens for near vision, while its relaxation flattens the lens for far vision.
• Pupillary dilator and sphincter muscles control the amount of light entering the retina.
• The dilator muscle widens the pupil to let in more light and is sympathetically controlled, innervated by the sympathetic nerves of the ciliary ganglion.
• The sphincter muscle, which has parasympathetic control via the Edinger-Westphal nucleus of CN III, narrows the pupil to let in less light.

Inner Surface of the Eye

• The retina has a macula, which contains a center called the fovea, which has a high density of rods and cones, but no blood cells.
• A capillary bed supplies blood in the area behind the pigmented epithelium.
• The optic disk ("blind spot") lacks photoreceptors where the axons forming CN II exit; so no visual data comes from this location.
• Fovea contain:
  • High density of photoreceptor cells (mostly cones)
  • Low sensitivity photoreceptors (cones)
  • Sensitivity to color
  • Lateral displacement of retinal layers
• The eye acts like a "window to the brain" because the conditions within the body are reflective in the eyes
• An ophthalmoscopic view of a normal right eye is visible by observing the optic nerve
• Elevated intracranial pressure can result in swelling of the optic nerve head via an ophthalmoscope

Photoreceptors of the Retina

• Photoreceptors lie at the back of the retina, so light needs to pass through the other retinal parts to get there.

• Parts of the Retina:

  • Pigmented epithelium - located posteriorly to absorb light
  • Photoreceptors - Rods and Cones - both are hyperpolarized by light energy
  • Bipolar cells
  • Horizontal and amacrine cells (not shown)
  • Ganglion cells (axons of ganglion cells form cranial nerve II)

• There are 130 million rods containing rhodopsin that are sensitive to low light, but saturate in bright light, giving achromatic vision with decreased resolution because multiple rods converge on one ganglion cell.

• There are 7 million cones with less photopigment than rods that function in bright light because they are less sensitive to bright light, and have high densities in the central retina; they come in red, green and blue

Rods and Cones

• In the dark, photoreceptors are depolarized and liberate glutamate.
• Light hyperpolarizes photoreceptors, thus stopping the release of glutamate.
• Rods and cones contain opsins, which bind retinal (a Vitamin A derivative).
• When exposed to light retinal transforms, triggering a second messenger cascade that leads to hyperpolarization of these cells.

Cone Opsins

• There are 3 types of opsins in cones.
• Each cone has one type of opsin, called conopsins, with a unique absorption spectrum.
• Detection of color depends on the expression of the 3 conopsins.
• Mutations can produce color blindness, leading to a lack of perception for a specific color
• The most common form of color blindness is red/green, and is seen at 2% in men (dichromats).

Projection of Visual Field on Retina

• The receptive field is a region of the visual world influencing neuronal activity.
• The location of the star in is the receptive field of that photoreceptor
• Light refraction through a convex lens turns the visual field upside down and reverses it laterally on the retina.
• Retinotopic map orderliness is preserved throughout the visual pathway for perception.

Visual Pathways

• Ganglion cells send their axons from the eye, forming the optic nerve.

• The area where the optic nerve departs the eye lacks rods and cones, hence is called the blind spot.

• The thalamus sends LGN axons through Meyer's loop in the temporal lobe synapsing in the occipital cortex's lingual gyrus, responsible for the upper visual fields.

• Axons from lateral geniculate nuclei project via parietal loop to the cuneate gyrus responsible for lower visual fields

• The optic or visual radiations combine Meyer's and Parietal loops together.

• V1 - Cortical area is the Area 17 of the occipital cortex lower lingual gyrus and upper cuneus gyrus.

• Other visual pathways:

  • The suprachiasmatic nucleus of the hypothalamus get axons through the retino-hypothalamic tract and regulates circadian rhythms.
  • Axons bypass the LGN and go to the superior colliculus for input from the contralateral visual field in order to orient eyes, head and neck, or the pretectal area for both the pupillary light reflex and accomodation reflex for near vision.

Gross Anatomy of the Visual Pathway

• Visual areas visible in a gross brain include Area 17 of the occipital cortex: the lingual gyrus and upper cuneus gyrus.

Visual Pathways Beyond Primary Visual Cortex

• At each stage of the visual pathway, more complex features of the visual world are extracted.
• The primary visual cortex's projections to other visual cortical areas are organized in the following ways:
  • Dorsal pathway (goes to the parietal lobe): important for spatial vision, including motion
  • Ventral pathway (goes to the temporal lobe): important for object recognition

Additional Info

• Ganglion cells project axons outside the eye.
• Ganglion cell axons from the nasal hemiretina (temporal visual fields) intersect at the optic chiasm to synapse in the lateral geniculate nucleus of the thalamus.
• The optic radiations and visual cortex transmit visual information from the contralateral visual hemifield
• Lesions in the optic radiations and visual cortex cause deficits in both eyes, not just one eye.
• The optic radiations consist of:
  • Parietal loop → cuneus brings visual data from the contralateral hemifields’ inferior quadrant.
  • Meyer loop → lingual is from the contralateral hemifields’ superior quadrant.
• Calcarine cortex lower bank gets input from superior visual fields, whereas the upper bank receives input from the inferior visual field.
• The macula also has a disproportionately large representation, the central 10 degrees occupies about half of the visual cortex.
• Each retina sees light separately from different visual fields (left and right).
• The visual field projects onto the retina and is flipped upside down and reversed from left to right.
• Visual cortex mirrors contralateral half of visual world.

Lesions in Vision

• Damage to the optic nerve results in ipsilateral blindness.
• Damage to the optic chiasm leads to bitemporal hemianopia. Pituitary tumors may cause this damage.
• Damage to the optic tract leads to contralateral homonymous hemianopia.
• The Parietal loop brings visual information from the contralateral hemifields’ inferior quadrant, while damage causes contralateral homonymous inferior quadrantanopia.
• The Meyer loop transmits optic information from the contralateral hemifields’ superior quadrant; damage causes contralateral homonymous superior quadrantanopia
• Damage to the cuneus gyrus leads to contralateral homonymous inferior quadrantanopia with macular sparing.
• Damage to the lingual gyrus leads to contralateral homonymous superior quadrantanopia with macular sparing
• Damage to the occipital cortex (cuneus + lingual) results in contralateral homonymous hemianopia with macular sparing.
• Macular sparing preserves foveal vision, and happens due to damage in the cortex as well as along the visual pathway with some degree
• IPLateral means on the same side as the lesion, with CONTRAlateral meaning the opposite side as the lesion.