The Visual System: Eye Anatomy and Retina

• The visual system is the part of the central nervous system which gives organisms the sense of sight, as well as enabling the formation of several non-image photo response functions.
• It detects and processes visible light to allow for sight.
• The major components of the visual system are the eyes, the optic nerve, the optic tract, the lateral geniculate nucleus (LGN), and the visual cortex.

Eye Anatomy

• The eye captures light and converts it into electrical signals.
• Light enters through the pupil, whose size the iris controls.
• The lens focuses the light onto the retina, a layer of light-sensitive cells at the back of the eye.
• The retina contains photoreceptor cells called rods and cones.
• Rods are sensitive to low light levels and facilitate night vision.
• Cones facilitate color vision and function best in bright light.
• The optic nerve transmits signals from the retina to the brain.

Retina

• The retina is a layered structure in the back of the eye containing photoreceptors and neurons.
• Photoreceptors (rods and cones) convert light into electrical signals.
• Horizontal and amacrine cells modulate signals between photoreceptors and ganglion cells.
• Retinal ganglion cells (RGCs) are the output neurons of the retina, and their axons form the optic nerve.
• The fovea is a central retinal region with a high cone density and high visual acuity.
• The optic disc is the "blind spot" where the optic nerve exits the eye and lacks photoreceptors.

Phototransduction

• Phototransduction converts light into electrical signals in photoreceptors.
• In rods, light activates rhodopsin, a light-sensitive pigment.
• Activated rhodopsin triggers a cascade that reduces cGMP levels.
• The reduced cGMP closes ion channels, hyperpolarizing the cell.
• In cones, a similar process occurs with different photopigments sensitive to different light wavelengths.

Visual Pathways

• The optic nerves from each eye meet at the optic chiasm.
• At the optic chiasm, fibers from the nasal side of each retina cross to the opposite side of the brain.
• After the optic chiasm, the optic tracts carry visual information to the lateral geniculate nucleus (LGN) of the thalamus.
• The LGN relays visual information to the visual cortex.
• Visual information travels from the LGN to the visual cortex in the occipital lobe.
• Some fibers also project to the superior colliculus, which contributes to eye movements.

Lateral Geniculate Nucleus (LGN)

• The LGN is a nucleus in the thalamus that relays visual information to the visual cortex.
• It receives input from the retinal ganglion cells.
• The LGN is organized into layers, with different layers receiving input from different types of retinal ganglion cells and eyes.
• Magnocellular layers receive input from M-type ganglion cells, important for motion perception.
• Parvocellular layers receive input from P-type ganglion cells, important for color and detail perception.
• Koniocellular layers receive input from non-M-non-P ganglion cells.

Visual Cortex

• The primary visual cortex (V1) resides in the occipital lobe and is the first cortical area to receive visual information.
• V1 neurons respond to specific orientations, spatial frequencies, and motion directions.
• V1 is organized retinotopically, so adjacent retina areas are represented in adjacent V1 areas.
• Visual information flows from V1 to other visual areas, including V2, V3, V4, and V5 (MT).
• These areas process more complex visual information, such as shape, color, and motion.

Visual Processing Streams

• The dorsal stream processes spatial information and motion, answering the "where" question.
• It projects from the visual cortex to the parietal lobe.
• The ventral stream processes object recognition, answering the "what" question.
• It projects from the visual cortex to the temporal lobe.

Color Vision

• Color vision is mediated by three cone types, each sensitive to different wavelengths of light (red, green, and blue).
• The relative activity of these cones determines the perceived color.
• Color information is processed by specific neurons in the retina, LGN, and visual cortex.
• Color constancy is the ability to perceive colors as relatively constant despite illumination changes.

Depth Perception

• Depth perception is the ability to perceive the distance of objects.
• Binocular cues rely on information from both eyes, like stereopsis (the difference between the images each eye sees).
• Monocular cues rely on information from one eye, such as relative size, linear perspective, and texture gradient.

Eye Movements

• Eye movements are controlled by muscles that rotate the eye in different directions.
• Saccades are rapid, ballistic eye movements that shift gaze from one location to another.
• Smooth pursuit movements allow the eyes to follow moving objects.
• Vergence movements align the eyes to focus on objects at different distances.

Visual Disorders

• Myopia (nearsightedness) occurs when the eye is too long, causing light to focus in front of the retina.
• Hyperopia (farsightedness) occurs when the eye is too short, causing light to focus behind the retina.
• Astigmatism occurs when the cornea is irregularly shaped, causing blurred vision.
• Cataracts are clouding of the lens, which can cause blurred vision.
• Glaucoma is damage to the optic nerve, often caused by increased pressure inside the eye.
• Macular degeneration is damage to the macula, the central part of the retina, leading to loss of central vision.
• Color blindness is a deficiency in distinguishing certain colors, typically due to a lack of one or more cone types.
• Visual agnosia impairs the ability to recognize objects, despite intact basic visual processing.