Vision

Vision is the special sense associated with interpreting light stimuli through the eyes.

Each eye sits in an orbit of the skull, where bony structures shield it and provide attachment points for supportive soft tissues.

Eyelids, with lashes at the edges, guard against foreign particles.

The inner surface of each lid, known as the palpebral conjunctiva, extends over the sclera (the “white” of the eye), linking eyelids to the eyeball.

Tear production occurs in the lacrimal gland, situated superior and lateral to each eye. Tears flow via the lacrimal duct toward the medial corner, washing debris from the conjunctiva.

Extraocular muscles

Movement of the eye within the orbit depends on six extraocular muscles anchored to the orbit’s bones and inserting on the eye’s surface:

• Superior rectus, inferior rectus, medial rectus, lateral rectus: Each pulls the eye in the direction of the muscle’s name (e.g., the superior rectus rotates the eye upward).
• Superior oblique: Originates near the posterior orbit, threading through the trochlea (a cartilaginous pulley) before attaching obliquely. Contraction rotates the eye laterally.
• Inferior oblique: Originates from the orbit floor and attaches inferolaterally, also rotating the eye laterally but opposing the superior oblique.

Because the eye is not perfectly aligned with the sagittal plane, the oblique muscles provide slight rotation when the eye looks up or down.

A seventh muscle, the levator palpebrae superioris, raises the upper eyelid, often in tandem with the superior rectus elevating the eye.

Layers of the eye: The eye is a hollow sphere composed of three tunic layers:

1. Fibrous tunic: Outermost layer, including the white sclera and transparent cornea. The cornea covers the anterior tip of the eye, permitting light entry.
2. Vascular tunic: Middle layer, featuring the choroid, ciliary body, and iris.
   • Choroid: Highly vascular tissue supplying blood to the eye.
   • Ciliary body: Contains muscles attached to the lens via suspensory ligaments (also called zonule fibers). Contraction or relaxation shifts the lens shape to focus light.
   • Iris: The colored portion of the eye, with smooth muscle that adjusts the pupil (the central opening) in response to light intensity.
3. Neural tunic (retina): The innermost layer containing photoreceptive nervous tissue. Responsible for converting light into neural signals.

Retina and photoreceptors

• Within the retina, specialized cells (rods and cones) detect light energy:
  • Rods: Contain the pigment rhodopsin in stacked discs; specialized for low-light and peripheral vision.
  • Cones: Contain infolded membranes with opsins, sensitive to distinct wavelengths (red, green, blue).

Retinal layers

• Photoreceptors (rods and cones) alter their membrane potential in response to light, modulating neurotransmitter release onto bipolar cells.
• Bipolar Cells connect to retinal ganglion cells (RGCs), where amacrine cells further refine signals.
• RGCs form the optic nerve at the optic disc, creating a blind spot lacking photoreceptors.

At the center of the retina is the fovea, lacking extra layers and blood vessels, allowing maximum visual acuity. Each photoreceptor in the fovea links to a single RGC, whereas peripheral regions converge multiple photoreceptors onto fewer RGCs, reducing detail.

Light transduction

• Light induces chemical changes in rod and cone pigments, initiating a cascade that modifies RGC output. Differences in rod vs. cone structure and pigment composition underlie the eye’s range of color and intensity detection.

Visual pathway to the brain

1. Retina: Detects light through photoreceptors.
2. Optic nerve: Conveys neural signals from the retina.
3. Optic chiasm: Partial crossing of optic nerve fibers for binocular vision.
4. Optic tracts: Transmit visual information to the lateral geniculate nucleus (LGN) of the thalamus.
5. Optic radiations: Carry signals to the primary visual cortex (V1) in the occipital lobe.

Visual field processing

• Ensures both brain hemispheres process input from each eye, crucial for depth perception and binocularity.

Parallel processing

• Magnocellular pathway (magno): Movement and spatial recognition (the “where” pathway).
• Parvocellular pathway (parvo): Color and detail (the “what” pathway).

Dorsal stream (“where”): Identifies location and motion in the parietal lobe.

Ventral stream (“what”): Recognizes shapes and objects in the temporal lobe.

Feature detection and parallel processing

• Neurons in the visual cortex specialize in aspects of a stimulus:
  • Simple cells: Edge orientation.
  • Complex cells: Motion direction.
  • Hypercomplex cells: Complex shapes or specific lengths.

Color detection relies on S-cones (blue), M-cones (green), and L-cones (red). Motion detection involves areas like the middle temporal region, aided by the magnocellular pathway.

Clinical correlates

• Damage along specific points in the visual pathway (e.g., the optic nerve, optic tract, or occipital lobe) results in distinct visual field defects (e.g., homonymous hemianopia). Conditions like visual agnosia (inability to recognize objects) or motion blindness highlight the necessity of these intricate pathways for normal sight.