Vision and Light Processing

Review of Eyes

  • Accessory Eye Structures: Structures that support vision and protect the eye.

  • Eye Layers:

    • Sclera: The white, outer layer of the eye providing structure and protection.

    • Choroid: A layer containing blood vessels that nourish the eye.

    • Retina: The innermost layer containing photoreceptors.

  • Lens: A transparent structure that focuses light onto the retina.

  • Humours of the Eye:

    • Aqueous Humor: The watery fluid in the front part of the eye.

    • Vitreous Humor: The gel-like substance in the main part of the eye.

Vision

  • Rods vs. Cones:

    • Rods:

    • Function: Responsible for vision in low light conditions.

    • Sensitivity: About 100 times more sensitive to light than cones.

    • Color: Perceive images in black and white (one type of pigment).

    • Cones:

    • Function: Responsible for color vision and function best in bright light.

    • Types: Three types (red, green, blue) corresponding to different pigments.

    • Sharpness: Each cone connects directly to the optic nerve, yielding a sharper visual image.

Eye Disorders

  • Cataracts: Clouding of the lens leading to decreased vision.

    • Causes: Aging, diabetes, exposure to UV light.

    • Consequences: Blurred vision, difficulty with glare, and eventual blindness if untreated.

  • Glaucoma: Increased pressure in the eye damaging the optic nerve.

    • Causes: Blockage of aqueous humor drainage, leading to pressure build-up.

    • Consequences: Loss of peripheral vision, leading to blindness if untreated.

  • Astigmatism: Irregular curvature of the cornea or lens causing distorted vision.

    • Causes: Genetic factors affecting the shape of the eye.

    • Consequences: Blurred or distorted vision at all distances.

  • Myopia (Nearsightedness): Difficulties in seeing distant objects clearly.

    • Causes: Elongated eyeball or overly curved cornea.

    • Consequences: Clear near vision, blurry distance vision.

  • Hyperopia (Farsightedness): Difficulties in seeing nearby objects clearly.

    • Causes: Shortened eyeball or insufficiently curved cornea.

    • Consequences: Clear distant vision, blurry near vision.

Light and Optics

  • Electromagnetic Radiation: Energy existing as waves, including:

    • Wavelengths: Refers to the distance between wave peaks, measured in nanometers.

    • Visible Light: Electromagnetic radiation with wavelengths between 400 and 700 nm detectable by humans.

    • Detection: The eye is a special organ used to perceive this radiation.

    • Colors: Different wavelengths correspond to different colors perceived.

Wavelengths of Light

  • Prism Refraction:

    • A prism can split white light into its component wavelengths due to refraction.

  • Color Perception:

    • The colors we perceive are reflections of specific wavelengths off objects.

    • Example: Grass appears green because it reflects green wavelengths and absorbs others.

Color Vision

  • Tetrachromats:

    • Ancestors of birds, reptiles, mammals, and some fish with four types of cone cells (red, green, blue, ultraviolet).

  • Dichromats:

    • Most mammals that lost two cone types, exhibiting reduced color differentiation (effects on red/green differentiation).

  • Trichromats:

    • Humans and some mammals regain the ability to distinguish red and orange using three types of cone cells.

Phototransduction

  • Definition: The process by which light energy generates receptor potentials in photoreceptors.

  • Photoreceptors:

    • Modified neurons with photoreceptive ends embedded in the pigmented layer of the retina.

    • Photopigments: Molecules in photoreceptors that change shape upon absorbing photons.

  • Photoreceptor Damage: Vulnerable to intense light; outer segments are renewed every 24 hours.

Rods and Cones

  • Rods:

    • Approximately 92 million in the retina, containing one type of pigment for monochromatic vision.

    • Highly sensitive but contribute to fuzzy images due to convergence of 50 rods to a single bipolar neuron.

  • Cones:

    • Approximately 6 million in the retina, each with one of three color-detecting pigments (red, green, blue).

    • Provide sharp images due to direct connections to the optic nerve.

Visual Pigments

  • Structure: Made by combining light-absorbing molecules (retinal) with opsin proteins (determine color absorption).

    • Rhodopsin: The pigment found in rods.

    • Cone Opsins: Similar but optimized for specific wavelengths corresponding to color vision.

  • Pigment Breakdown: Exposure to light causes breakdown of pigments (bleaching) as retinal and opsin separate.

Changing Resting Membrane Potential

  • Communication Signals in Neurons:

    • Changes in membrane potential due to sensory stimuli create graded potentials that influence neuron communication.

  • Graded Potentials:

    • Temporary changes in polarization, either depolarizing (more positive) or hyperpolarizing (more negative).

  • Action Potentials:

    • All-or-none events generated when graded potentials exceed a threshold, leading to a full depolarization.

    • Depolarization facilitates action potentials (more positive), while hyperpolarization inhibits them (more negative).

Membrane Potentials and Special Senses

  • Graded Potentials: Normal signaling mechanisms, but photoreceptors in the eye behave differently by depolarizing in the dark and hyperpolarizing in the light.

Photoreceptors in the Dark

  • Dark Conditions:

    • Photoreceptors are slightly depolarized, releasing inhibitory neurotransmitters that inhibit bipolar cells, preventing them from stimulating ganglion cells.

Photoreceptors in the Light

  • Light Conditions:

    • Light exposure hyperpolarizes photoreceptors, stopping inhibitory neurotransmitter release.

    • Bipolar cells depolarize and release excitatory neurotransmitters to ganglion cells, generating action potentials along the optic nerve.

Light and Dark Adaptation

  • Light Adaptation:

    • Transition from darkness to bright light, bleaching photopigments and reducing sensitivity.

    • Initial glare followed by pupil constriction and gradual improvement in visual acuity.

  • Dark Adaptation:

    • Transition from bright to dark environments, requiring time for rod pigments to reactivate.

    • Initial difficulties in seeing due to dilated pupils and rhodopsin accumulation to enhance retinal sensitivity over time.

Visual Pathway

  • Optic Nerve: Formed by ganglion cell axons, crossing at the optic chiasma to connect to the thalamus.

  • Thalamic Processing: Information is integrated for depth perception, color, and motion before reaching the primary visual cortex in the occipital lobes.

Visual Processing

  • Ganglion Cells: Simplify and condense information from the retina, emphasizing edges in visual scenes.

  • Lateral Geniculate Nucleus: Integrates visual information and aids in depth perception.

  • Primary Visual Cortex: Further processes information for contrast, color, and movement.

  • Two-Streams Hypothesis:

    • Ventral Stream: For memory and emotional processing via the temporal lobes.

    • Dorsal Stream: For spatial recognition and interaction, projecting to parietal and occipital lobes.

  • Depth Perception: Generated by overlapping visual fields from both eyes, allowing the brain to fuse these images into a three-dimensional view.

Learning Objectives

  • Define the events converting light into neural signals.

  • Compare and contrast the roles of rods and cones in vision.

  • Differentiate light and dark adaptations.

  • Trace the visual pathway to the visual cortex and describe visual processing steps.