Optics and Vision Mechanisms

Learning Objectives

• Explain optical mechanism and its aberrations and corrections.
• Discuss pupillary reflex and accommodation and their clinical significance.

Refraction

• When light encounters a different medium at an oblique angle, it is refracted.
• Refraction depends on:
  • Angle: The greater the angle, the greater the bending.
  • Comparative densities of media (refractive index).

Refraction of Light

• Light passing through a denser medium (like glass) bends towards the normal.
• Light exiting a denser medium bends away from the normal.
• Apparent depth of an object in water viewed from above is less than the real depth due to refraction effects.

Vision

• The process of light reflected through objects being translated into a mental image.
• The parts of the eye are depicted.

Process Involves in Vision

• Light enters the eye and is refracted three times (cornea, lens).
• Total refractive power is 59 diopters.
• The lens adjusts to focus the image on the retina.
• The lens is elastic and its curvature and light-bending power can be adjusted.

Focusing for Distant Vision

• The far point of vision is the distance at which the lens does not need to change its shape for focusing.
• For a normal eye, the far point is 6 meters (20 feet).
• The ciliary muscles relax; the lens flattens, reducing its refractive power.

Focusing for Close Vision

• For close vision, the eyes need to make active adjustments (accommodation):
  • Accommodation of the lenses.
  • Constriction of pupils.
  • Convergence of the eyeballs.
• The near point of vision is the closest point the eyes can focus clearly.
• For healthy young adults, the near point is 10 cm (4 inches) from the eye.

Common Visual Defects

• Myopia (near-sightedness):
  • Difficulty seeing distant objects clearly.
  • The eyeball is too long, or the cornea and lens system is too strong, causing light to focus in front of the retina.
  • Corrected with diverging (concave) lenses.
• Hyperopia (far-sightedness):
  • Difficulty seeing close objects clearly.
  • The eyeball is too short, or the lens does not bend light enough.
  • Corrected with converging (convex) lenses.

Presbyopia

• An increasing inability to focus on near objects with age.
• Caused by the thickening and loss of elasticity in the lens.
• The lens cannot adjust for close-up vision.

Astigmatism

• Unequal curvature in different parts of the cornea or lens.
• Vertical and horizontal planes cannot be focused simultaneously.
• Corrected using cylindrical lenses to focus both planes.

Retina

• Phototransduction: Conversion of light energy into a graded receptor potential.
• Rods:
  • Very sensitive to light, ideal for dim light and peripheral vision.
  • Contain a single photopigment (rhodopsin).
• Cones:
  • Low sensitivity to light, ideal for color vision and high acuity.
  • Contain three types of photopigments (red, green, blue).

Rods and Cones Distribution

• Distribution of rods and cones across the retina.
• Rods are distributed throughout the retina, with lower concentration in the fovea.
• Cones are concentrated in the fovea.

Rods and Cones Arrangement and Connections

• Low convergence in cone-fed circuits (each cone connects to a single bipolar cell).
• High convergence in rod-fed circuits (many rods connect to a single bipolar cell).

Photoreceptors

• Two types: Rods and Cones.
• Outer segments contain photopigments.
• Inner segments connect to the cell body.

Photoreceptor Activity in the Dark

• 11-cis-retinal is present.
• cGMP levels are high, keeping Na+ channels open.
• Na+ influx depolarizes the membrane.
• Ca2+ channels open, increasing inhibitory neurotransmitter release.
• Bipolar cells are inhibited.
• No action potential in the ganglion cell axon.

Photoreceptor Activity in the Light

• All-trans-retinal is present.
• cGMP levels are low, closing Na+ channels.
• Photoreceptors hyperpolarize.
• Ca2+ channels close, reducing inhibitory neurotransmitter release.
• Bipolar cells are depolarized, releasing a neurotransmitter.
• Action potential is formed in the ganglion cell axon.

On-center/Off-center Bipolar Cells

• Light causes photoreceptors to hyperpolarize, reducing glutamate release.
• Dark causes photoreceptors to depolarize, increasing glutamate release.

Dark Adaptation

• Transition from cone-dominant to rod-dominant vision.
• Cones adapt quickly; rods adapt slowly to low light.

Light Adaptation

• Transition to bright light.
• Initial glare due to high levels of photopigment breakdown.
• Recovery of cones and increased retinal sensitivity over time (5-10 minutes)

Night Blindness (Nyctalopia)

• Impaired rod function.
• Prolonged vitamin A deficiency can lead to rod degeneration
• Retinitis pigmentosa.

Color Vision

• Absorption spectra overlap
• Different types of cones respond to maximum wavelengths of red, green, blue.
• Intermediate colors result from combining the signals of different types of cones.

Color Blindness

• Hereditary condition affecting the ability to distinguish between colors, usually red and green.
• Sex-linked, affecting more men than women.
• Different types of color blindness exist; difficulties in identifying colors.
• Ishihara charts are used for identifying color blindness.

Visual Field

• The perimeter is used to map the visual field, in humans the image of the object differs because they have different positions.
• With one eye closed, the sense of depth and perspective are lost.
• Temporal and nasal field

Visual Pathway

• Pathway from the photoreceptors to the visual cortex.
• Cones, Bipolar neurons, Ganglion cells, Optic Nerve, Optic Chiasm, Optic Tract, Lateral Geniculate Nuclei, Optic Radiations, Visual Cortex

Pupillary Light Reflex

• The ability of the pupils to constrict in response to light stimulation.
• Direct and consensual light reflexes.
• Involvement of the optic nerve, pretectal nucleus, Edinger-Westphal nuclei etc.

Pupil Fail to Response to Light

• Blockage of visual signals to Edinger-Westphal nucleus.
• CNS syphilis, alcoholism, encephalitis.
• Argyll Robertson pupils.

Visual Acuity

• Determining the ability to see detail.
• Measured using visual acuity charts, such as determining distances needed for recognizing specific letters.
• Expressed as a fraction (e.g., 6/6).

Brain Interpretation

• Processing sensory data by the brain to create a subjective perception of images, objects, or events (involves illusions, etc).

Eye Examination Chart

• Used to assess visual acuity.
• Visual acuity is determined by noting the smallest characters that can be identified from a certain distance.

Visual Illusions

• Examples of how the brain interprets data in confusing or ambiguous ways.

This quiz explores the fundamentals of optical mechanisms, including refraction and its effects on vision. Understand the principles of light behavior as it passes through different media, along with the significance of pupillary reflex and accommodation. Test your knowledge on these essential aspects of vision science.