Vision II: visual pathways

RETINA AND VISUAL PATHWAYS

  • Anatomy of the Visual System

    • Retina

    • Optic Nerve

    • Lateral Geniculate Nucleus (LGN)

    • Primary Visual Cortex (VI)

    • The information flow of visual pathways processes images into different levels of complexity.

How the Brain Sees Images

  • Questions Addressed:

    • How do our brains see images?

    • What information is the brain perceiving?

    • How is this information processed in our brain?

Levels of Visual Processing

  • Processing Stages:

    • Low-level processing: Basic features detected.

    • Orientation

    • Color

    • Contrast

    • Movement direction

    • Surface properties

    • Intermediate-level processing: Combinations of features and forms.

    • Contour integration

    • Shape discrimination

    • Surface depth & segmentation

    • High-level processing: Recognition and interpretation of objects and movements.

    • Object motion and identification

    • Shape from kinetic cues

Retinofugal Pathways

  • Definition: Connections of the eyes to subcortical visual centers.

  • Key Components:

    • To Lateral Geniculate Nucleus (LGN): Retinogeniculate pathway for vision.

    • To Superior Colliculus (SC): Retinotectal pathway for visual reflexes (orienting the eye towards a peripheral stimulus).

    • To Pretectum: Midbrain area for pupil reflex control.

Structure of the Visual Pathway

  • Diagram Overview:

    • Optic Nerve → Optic Tract → Optic Chiasm → Lateral Geniculate Nucleus → Optic Radiation → Striate Cortex (Primary Visual Cortex, V1)

    • Brodmann's Area 17 (primary visual area).

  • Hypothalamus: Regulation of circadian rhythms.

  • Superior Colliculus: Orienting head and eye movements.

  • Pretectum: Reflex control of pupil and lens.

Eye and Visual Field

  • Key Structures:

    • Retina: The neural structure for light perception.

    • Visual Field: 2/3 overlap in one eye with the opposite eye (binocular and monocular fields).

    • Visual Axis: Line passing from the lens center to the fovea (fovea is dense with cone cells for sharp vision).

    • Fovea Centralis: Site of most acute vision.

    • Macula: Region of highest visual acuity.

  • Optic Disk: Point where optic axons exit the eye, resulting in the blind spot.

Visual Field Representation

  • Key Concepts:

    • Vertical Meridian: Line passing through fovea, dividing retina into temporal and nasal halves.

    • Representation of visual field is inverted in the retina.

  • Visual Field Mapping:

    • Left visual field is represented in the right visual cortex and vice versa.

    • Upper/lower visual fields are represented as inverted in the visual cortex.

Retinofugal Pathway Components

  • Optic Tract:

    • Located laterally on the diencephalon's surface, containing axons from both eyes representing the same half of the visual field.

  • Medial and Lateral Roots:

    • Lateral Root: Projects to LGN.

    • Medial Root: Projects to SC, pretectum, and hypothalamus.

Lateral Geniculate Nucleus (LGN)

  • Function: Relay center in the thalamus.

  • Inputs:

    • Receives input from retina, primary visual cortex, and thalamic reticular nucleus.

    • Projects to primary visual cortex through optic radiation.

  • Layer Composition:

    • 2 Magnocellular layers (1 & 2): Inputs from Y-type ganglion cells, focused on movement perception.

    • 4 Parvocellular layers (3-6): Inputs from X-cells, focused on visual detail and color.

  • Neuronal Inputs: Neurons in each layer receive inputs from one eye (monocular).

Information Projections from LGN to Visual Cortex

  • Functions:

    • X-cells: Visual detail and color input.

    • Y-type ganglion cells: Movement perception inputs.

  • Spatial Processing Mechanisms:

    • Relays different visual components for processing color, form, and movement.

  • Pupillary Reflex Control: Via Pretectal nucleus to Edinger Westphal nucleus (oculomotor nerve).

  • Efferents: Control functions regarding biorhythms, mainly managed by the suprachiasmatic nucleus in the hypothalamus.

Optic Radiation Details

  • Meyer's Loop: Part of the path leading to the visual cortex.

  • Dissection:

    • Axons from LGN terminate in V1.

Retinotopy in Visual Cortex

  • Mapping:

    • Each visual cortex sees the opposite visual hemifield.

    • Central to peripheral axis is represented from posterior to anterior in Area 17.

    • The central region covers larger space in cortical area as compared to peripheral regions, illustrating a disproportional representation.

Higher Visual Centers

  • Visual Association Cortices: Areas V2-V5, process distinct visual features.

  • Flow of Blood:

    • Increased in V4 for color recognition.

    • Increased in V5 for motion perception.

Higher Visual Processing Paths

  • What Pathway (Ventral Pathway):

    • Involves V2 projecting to V4 for color and form-with-color vision, critical for object recognition.

  • Where Pathway (Dorsal Pathway):

    • Involves V2 projecting to V3 and V5 for movement, depth and dynamic form perceptions, important for spatial vision.

Facial Recognition and Color Vision

  • Area 37 located in the inferotemporal cortex is crucial for face recognition, situated lateral to color vision areas in the ventral pathway.

Visual Defects

  • Types of Defects:

    • Differential loss of certain visual features such as color, form or shape due to damage in higher visual centers.

  • Case Studies:

    • Achromatopsic patients show issues distinguishing colors but may preserve shape recognition.

    • Examples illustrate challenges for patients when attempting to reproduce colors or shapes after visual pathway lesions.

References & Further Readings

  • Hubel, D. H. (1988). Eye, Brain, and Vision. Scientific American Library Series.

  • Kandel, E. R., Schwartz, J. H., & Jessell, T. M. (1995). Essentials of Neural Science and Behavior. Prentice Hall International, Inc.

  • Kandel, E. R., et al. (2021). Principles of Neural Science (6th Ed.).

  • M. C. Mtui, Gruener, & Dockery (2016). Fitzgerald’s Clinical Neuroanatomy and Neuroscience (7th Ed.).