Vision

Advanced Neurosciences Overview

  • Visual Pathways Discussed by Dr. Mary McGahon

  • Contact: m.mcgahon@qub.ac.uk

  • Location: Room 01/024b Whitla Medical Building

History of Vision

  • Richard Feynman Quote:"We are immersed in a sea of electromagnetic waves... the eye selects a small fraction of these wavelengths..."

    • Insight: The complexity of light's interaction with the environment and the eye's role in processing this information.

Learning Objectives

  • Understand visual information processing from photoreceptors to ganglion cells.

  • Perform tests on the visual system (e.g., depth of focus, visual acuity).

  • Explain central visual pathways, including the role of ganglion cells and the optic chiasm.

  • Discuss cortical visual pathways and how the brain interprets visual input.

  • Demonstrate visual compensation techniques for partial visual field loss.

Basic Anatomy of the Eye

  • Key Structures:

    • Pupil, Iris, Sclera, Cornea, Anterior Chamber, Posterior Chamber

    • Lens, Ciliary body, Retina (Macula, Fovea, Blind Spot)

    • Diagram Overview: Eye anatomy related to vision.

Accommodation (Focusing)

  • Components: Lens, pupils, visual orientation

    • Lens Changes:

      • Distant vision requires less refraction; light rays are nearly parallel.

      • Close vision requires more refraction due to diverging light rays.

  • Mechanics:

    • Circular muscle relaxation → lens flattens.

    • Circular muscle contraction → lens rounds.

Visual Acuity Testing

  • Snellen’s Charts:

    • Normalized to ideal vision (6/6).

    • Exercises: Testing for visual acuity (6/9 approx. needed for driving).

  • Pinhole Effect: Helps improve focus by reducing scatter.

Visual Pathway Components

  • Key Elements:

    • Retina → Optic Nerve → Optic Chiasm → Lateral Geniculate Nucleus (LGN) → Visual Cortex

Phototransduction Process

  • Mechanism Overview:

    • Light triggers phototransduction in photoreceptors, leading to neurotransmitter release and subsequent neural activity.

  • Photoreceptors Structure:

    • Rods and Cones: Cones provide high visual acuity and color vision; rods for peripheral and low-light vision.

Visual Field and Blind Spots

  • Field of Vision Limitations:

    • Tests like the wagging finger test to assess peripheral vision.

  • Mapping the Blind Spot: Clinical relevance for conditions like diabetic retinopathy.

Color Vision and Perception

  • Color Blindness:

    • Most common type: Red-green color blindness, which is linked to genetics (X chromosome).

  • Photoreceptor Functionality:

    • Rods and cones detect various color wavelengths, allowing for millions of colors through overlapping sensitivities.

Neuronal Activity and Visual Processing

  • Bipolar Cell Activity:

    • ON and OFF bipolar cells process light changes and contribute to contrast sensitivity.

  • Role of RGCs:

    • RGC axons project to thalamus, with various types tuned for motion, color, and contrast detection.

Visual Cortex Interaction

  • LGN Processing:

    • Different layers of LGN correspond to different types of photoreceptors, processing movement and color distinctly.

  • Cortical Areas (V1-V5):

    • Involves feed-forward processing of visual information, enabling complex interpretations of motion and form.

Disorders of Vision

  • Higher Order Visual Processing Disorders:

    • Examples include agnosia, prosopagnosia, and achromatopsia, each affecting different aspects of visual processing.

Summary Points

  • Overall Pathway: Visual information travels from rods and cones through multiple cell types to reach processing areas in the brain.

  • Exercise: Understanding and mapping the various aspects of visual perception and the implications of visual testing.

  • Next Steps: Continue learning objectives and exercises to solidify understanding.