Vision lectures 1-3

Introduction

• Course: BIOL2052 (Vision Studies)

• Instructor: Herman Wijnen

• Contact: hw3a11@soton.ac.uk

Objectives

• Aim: Understand molecular, cellular, and higher-order mechanisms underlying vision.

• Key Texts:

  • Purves, Neuroscience

  • Pocock, Human Physiology

  • David Hubel’s "Eye, Brain and Vision"

Challenges in Vision

• Key observations from a quick look:

  • Shape

  • Size

  • Colour

  • Location

  • Speed

  • Direction

Learning Objectives for Lectures

By the end of these lectures:

• Describe the anatomy of the eye, focusing on the retina.

• Explain the function of rods and cones.

• Discuss the roles of on-center and off-center ganglion cells.

• Describe the central visual pathways.

• Understand the concept of retinotopic organization.

• Describe the columnar organization in the visual cortex.

Resources for Revision

• Textbook: Purves: Neuroscience, chapters focusing on vision and central visual pathways.

Visual Scene Components

• Encoded attributes of visual scenes include:

  • Location: Where is it?

  • Identity: What is it?

  • Distance: How far away is it?

  • Colour: What colour is it?

  • Movement: Is it moving?

The Visual System

• Optic Nerve: Contains axons from retinal ganglion cells.

• Partial decussation at the optic chiasm (temporal visual fields cross).

• Terminations:

  • Lateral Geniculate Nucleus (LGN): Major relay station to the visual cortex (~40% cortical area).

  • Pretectal Nuclei: Reflexive eye movements and pupillary constriction.

  • Suprachiasmatic Nucleus: Regulation of sleep-wake cycle.

Anatomy of the Eye

• Length: Approximately 25 mm.

• Layers:

  • Sclera: Outer protective layer.

  • Choroid: Middle layer with blood vessels.

  • Retina: Inner layer containing photoreceptors.

• Components:

  • Cornea and lens: Essential for light refraction.

  • Ciliary muscles: Adjust lens shape for accommodation.

Accommodation

• Refers to the process of changing the eye's optical power to focus on objects at varying distances.

• Factors involved:

  • Cornea: Provides primary refraction.

  • Lens: Adjustable to allow accommodation.

Refractive Errors

• Emmetropia: Normal vision.

• Myopia: Nearsightedness; difficulty seeing far distances.

• Hyperopia: Farsightedness; difficulty seeing close objects.

The Retina

• Part of the CNS evolving from the diencephalon (optic vesicle).

• Contains five neuronal types:

  • Photoreceptors

  • Bipolar cells

  • Ganglion cells

  • Amacrine cells

  • Horizontal cells

Structure of the Retina

• Key layers include:

  • Photoreceptor Layer: Contains rods and cones.

  • Inner and Outer Nuclear Layers: Contain different neuronal cell types.

  • Ganglion Cell Layer: Connects to the optic nerve.

Function of Photoreceptors

• Rods: Sensitive to low light; responsible for night vision.

• Cones: Responsible for colour vision and high acuity under bright light conditions.

Photoreceptor Responses

• Detection of light involves hyperpolarization of photoreceptors leading to graded responses.

Vision Across Light Intensities

• The human visual system can adapt to a wide range of light intensities (approximately 12 logs of intensities).

Colour Vision

• Based on the presence of three types of cones:

  • Short (S): Blue light sensitivity.

  • Medium (M): Green light sensitivity.

  • Long (L): Red light sensitivity.

• Trichromatic theory explains how different cones contribute to colour perception.

Central Visual Pathways

• Projection Mechanisms:

  • Optic tract, optic nerve, optic chiasm, lateral geniculate nucleus, optic radiation, and finally to the striate cortex.

• Functions include pupillary reflexes and circadian rhythms.

Orientation Detection in Visual Processing

• Neurons in the visual cortex respond to specific orientations due to the arrangement in ocular dominance columns.

Higher Order Visual Processing

• Dorsal Pathway: Involves spatial awareness and coordination.

• Ventral Pathway: Involves object recognition and memory.

Eye Movement Mechanisms

• Types include:

  • Saccades: Rapid movements to shift focus.

  • Smooth Pursuit: Follow moving objects.

  • Vergence: Adjust the eyes for distance.

  • Vestibulo-Ocular: Stabilizes gaze during head movements.