Learning Objectives

After studying this topic, students will be able to:

• Trace the main visual pathway from the retina to the visual cortex.
• Explain the older and newer visual pathways.
• Describe the functions of the dorsal lateral geniculate nucleus.
• Compare magnocellular and parvocellular pathways.

Introduction

• Visual information from both retinas travels through a highly organized pathway toward the visual cortex.
• Some optic nerve fibers cross at the optic chiasm, while others remain on the same side.
• The main conscious visual pathway passes through the dorsal lateral geniculate nucleus of the thalamus before reaching the primary visual cortex.
• Other visual fibers travel to older brain areas concerned with:
  • Circadian rhythms
  • Pupillary light reflexes
  • Eye movements
  • Behavioral functions
• The dorsal lateral geniculate nucleus does more than relay signals.
• It also:
  • Preserves accurate spatial information.
  • Keeps signals from the two eyes separate.
  • Controls how much visual information reaches the visual cortex.

PRINCIPAL VISUAL PATHWAY

Visual pathway: The neural route by which visual signals travel from the retina toward the brain and visual cortex.

The principal visual pathways are shown in Fig. 52.1.

Main Sequence

Retina
→ optic nerve
→ optic chiasm
→ optic tract
→ dorsal lateral geniculate nucleus
→ optic radiation
→ primary visual cortex

Optic Nerves

Optic nerve: The pathway through which visual nerve signals leave the retina.

• Visual nerve signals leave both retinas through the optic nerves.
• The optic nerves travel toward the optic chiasm.

Optic Chiasm

Optic chiasm: The point where optic nerve fibers from the nasal halves of the retinas cross to the opposite side.

At the optic chiasm:

• Fibers from the nasal half of each retina cross to the opposite side.
• Fibers from the temporal retina remain on their side.
• After the crossing:
  • Nasal retinal fibers join fibers from the opposite temporal retina.
  • Together, they form the optic tract.

Key Concept

Nasal retinal fibers
→ cross at optic chiasm

Temporal retinal fibers
→ remain uncrossed

Then:

Crossed nasal fibers + opposite temporal fibers
→ optic Tracts

Optic tract: The visual pathway formed after fibers pass through the optic chiasm.

• Each optic tract contains fibers from both eyes.
• Its fibers travel toward the thalamus.
• They synapse mainly in the dorsal lateral geniculate nucleus.

Dorsal Lateral Geniculate Nucleus

Dorsal lateral geniculate nucleus: A thalamic nucleus that receives optic tract signals and relays them toward the visual cortex.

• Optic tract fibers synapse in this nucleus.
• From there, visual signals travel through the geniculocalcarine fibers.

Optic Radiation

Optic radiation: Fibers carrying visual signals from the dorsal lateral geniculate nucleus to the primary visual cortex.

It is also called the:

• Geniculocalcarine tract

Signal Flow

Dorsal lateral geniculate nucleus
→ optic radiation
→ primary visual cortex

Primary Visual Cortex

Primary visual cortex: The cortical area receiving the main visual signals from the lateral geniculate nucleus.

• It lies in the:
  • Calcarine fissure area
  • Medial occipital lobe

Complete Main Pathway

Retina
→ optic nerve
→ optic chiasm
→ optic tract
→ dorsal lateral geniculate nucleus
→ optic radiation
→ primary visual cortex in the medial occipital lobe

VISUAL FIBERS TO OLDER BRAIN AREAS

Not all visual fibers travel directly toward the visual cortex.

Some pass to older areas of the brain.

Suprachiasmatic Nucleus

Suprachiasmatic nucleus: A hypothalamic nucleus involved in controlling circadian rhythms.

Pathway

Optic tract
→ suprachiasmatic nucleus of hypothalamus

Function

• Presumably helps control circadian rhythms.
• These rhythms synchronize physiological changes of the body with:
  • Night
  • Day

Circadian rhythm: A physiological rhythm synchronized with the day-night cycle.

Pretectal Nuclei

Pretectal nuclei: Midbrain nuclei involved in reflex eye responses and the pupillary light reflex.

Pathway

Visual fibers
→ pretectal nuclei in midbrain

Functions

• Produce reflex movements of the eyes.
• Help the eyes focus on important objects.
• Activate the pupillary light reflex.

Pupillary light reflex: The reflex response of the pupil to light.

Superior Colliculus

Superior colliculus: A brain region involved in rapid directional movements of the eyes.

Pathway

Visual fibers
→ superior colliculus

Function

• Controls rapid directional movements of both eyes.

Ventral Lateral Geniculate Nucleus and Basal Brain Regions

Some visual fibers pass to:

• Ventral lateral geniculate nucleus of the thalamus.
• Surrounding basal regions of the brain.

Function

• Presumably help control certain behavioral functions of the body.

OLD AND NEW VISUAL SYSTEMS

The visual pathways can be divided roughly into:

• An old visual system
• A new visual system

Feature	Old Visual System	New Visual System
Main destination	Midbrain and base of forebrain	Visual cortex
Major role	Reflex and older visual functions	Conscious vision
Important areas	Suprachiasmatic nucleus, pretectal nuclei, superior colliculus, basal regions	Occipital visual cortex
In humans	Limited compared with cortical system	Responsible for nearly all conscious visual perception
In primitive animals	Can detect visual form	Less dominant compared with mammals

Old Visual System

Old visual system: Visual pathways projecting mainly to the midbrain and base of the forebrain.

• These pathways participate in:
  • Circadian control
  • Reflex eye movements
  • Pupillary responses
  • Rapid directional eye movements
  • Some behavioral functions
• In many primitive animals:
  • Even visual form is detected by this older system.
  • The superior colliculus performs a role similar to that of the visual cortex in mammals.

New Visual System

New visual system: The pathway that directly transmits visual signals toward the visual cortex.

• In humans, this system is responsible for the perception of virtually all:
  • Visual form
  • Colors
  • Other aspects of conscious vision

Key Concept

Old visual system
→ mainly reflex and older visual functions

New visual system
→ conscious visual perception

FUNCTION OF THE DORSAL LATERAL GENICULATE NUCLEUS OF THE THALAMUS

Dorsal lateral geniculate nucleus: The main thalamic relay nucleus of the newer visual pathway.

• It is located at the dorsal end of the thalamus.
• It is also called the lateral geniculate body.
• Optic nerve fibers of the new visual system terminate here.

The nucleus has two major functions:

• Accurate relay of visual signals.
• Gating of visual transmission toward the cortex.

RELAY OF VISUAL INFORMATION

The first major function is to relay visual information.

Pathway

Optic tract
→ dorsal lateral geniculate nucleus
→ optic radiation
→ visual cortex

• This relay is extremely accurate.
• It provides exact point-to-point transmission.
• Spatial relationships are preserved with a high degree of fidelity from:
  • Retina
  • Through the thalamus
  • To the visual cortex

Spatial fidelity: Accurate preservation of the location and arrangement of visual information during transmission.

Key Concept

A specific retinal point
→ corresponding geniculate neuron
→ corresponding visual cortical point

Therefore:

Retinal spatial organization is preserved.

SEPARATION OF SIGNALS FROM THE TWO EYES

After the optic chiasm:

• About half the fibers in each optic tract come from one eye.
• About half come from the other eye.
• These fibers represent corresponding retinal points.

However:

• Signals from the two eyes remain separated in the dorsal lateral geniculate nucleus.

Six Layers of the Dorsal Lateral Geniculate Nucleus

The nucleus contains six layers.

Layers II, III, and V

• Receive signals from the lateral half of the ipsilateral retina.

Ipsilateral: On the same side.

Layers I, IV, and VI

• Receive signals from the medial half of the retina of the opposite eye.

Key Concept

Some layers
→ receive signals from the same-side retina

Other layers
→ receive signals from the opposite-side retina

Yet:

• Corresponding retinal areas are represented in paired layers.
• Neurons representing corresponding points are positioned over one another.
• Parallel organization continues toward the visual cortex.

GATING OF VISUAL SIGNALS

The second major function of the dorsal lateral geniculate nucleus is gating.

Gating: Controlling how much visual information is allowed to pass to the visual cortex.

• Not every incoming visual signal is passed equally.
• The lateral geniculate nucleus can control transmission through selected regions.

Sources of Gating Control

Two major sources provide gating signals.

Corticofugal Fibers

Corticofugal fibers: Fibers traveling backward from the visual cortex toward the lateral geniculate nucleus.

Pathway

Primary visual cortex
→ backward fibers
→ lateral geniculate nucleus

Reticular Areas of the Mesencephalon

A second source comes from:

• Reticular areas of the mesencephalon.

Mesencephalon: The midbrain.

Effect of Gating Signals

• Both sources are inhibitory.
• When activated, they can reduce or stop transmission through selected parts of the dorsal lateral geniculate nucleus.
• These mechanisms help highlight the visual information that is allowed to pass onward.

Key Concept

Incoming visual signals
→ lateral geniculate nucleus

Inhibitory gating control
→ selected signals reduced

Important allowed information
→ continues toward visual cortex

MAGNOCELLULAR AND PARVOCELLULAR LAYERS

The dorsal lateral geniculate nucleus can also be divided according to cell size and input.

It contains:

• Magnocellular layers
• Parvocellular layers

MAGNOCELLULAR LAYERS

Magnocellular layers: Layers I and II of the dorsal lateral geniculate nucleus containing large neurons.

• Layers I and II are magnocellular.
• Their neurons are large.
• They receive input almost entirely from M retinal ganglion cells.

Functional Features

• Rapidly conducting pathway.
• Carries information quickly toward the visual cortex.
• Color blind.
• Transmits mainly black-and-white information.
• Point-to-point spatial transmission is relatively poor.

Why Spatial Accuracy Is Poorer

• There are relatively few M ganglion cells.
• Their dendrites spread widely across the retina.

Therefore:

Wide retinal sampling
→ less precise point-to-point localization

PARVOCELLULAR LAYERS

Parvocellular layers: Layers III through VI of the dorsal lateral geniculate nucleus containing many small to medium-sized neurons.

• Layers III to VI are parvocellular.
• They contain large numbers of:
  • Small neurons
  • Medium-sized neurons
• Their input comes almost entirely from P retinal ganglion cells.

Functional Features

• Transmit color information.
• Provide accurate point-to-point spatial information.
• Conduct signals at a moderate velocity.
• Their conduction is slower than the magnocellular pathway.

COMPARISON OF MAGNOCELLULAR AND PARVOCELLULAR PATHWAYS

Feature	Magnocellular System	Parvocellular System
LGN layers	I and II	III through VI
Cell size	Large neurons	Small to medium-sized neurons
Main retinal input	M ganglion cells	P ganglion cells
Conduction speed	Rapid	Moderate
Color information	No	Yes
Main visual information	Black and white	Color
Point-to-point accuracy	Poorer	Accurate
Retinal ganglion distribution	Fewer M cells with widely spreading dendrites	P cells provide more precise spatial information

KEY CONCEPT

The main visual pathway can be remembered as:

Retina
→ optic nerve
→ optic chiasm
→ optic tract
→ lateral geniculate nucleus
→ optic radiation
→ visual cortex

At the optic chiasm:

Nasal retinal fibers
→ cross

Temporal retinal fibers
→ remain on the same side

The lateral geniculate nucleus then:

• Relays signals accurately.
• Keeps signals from the two eyes separated.
• Controls how much visual information reaches the cortex.
• Separates information into magnocellular and parvocellular pathways.

CLINICAL / FUNCTIONAL IMPORTANCE

• Crossing of nasal retinal fibers allows visual information from the two eyes to be organized appropriately after the optic chiasm.
• The dorsal lateral geniculate nucleus preserves precise spatial relationships between the retina and visual cortex.
• Its gating function controls which visual signals are emphasized.
• Older visual pathways help control:
  • Pupillary light reflexes
  • Reflex eye movements
  • Directional eye movements
  • Circadian rhythms
• The newer cortical pathway is responsible for almost all conscious visual perception in humans.
• Magnocellular pathways provide rapid black-and-white information.
• Parvocellular pathways provide detailed spatial and color information.

HIGH-YIELD POINTS

• Nasal retinal fibers cross at the optic chiasm.
• Temporal retinal fibers remain on the same side.
• The major conscious pathway is:

Retina
→ optic nerve
→ optic chiasm
→ optic tract
→ dorsal lateral geniculate nucleus
→ optic radiation
→ primary visual cortex

• Primary visual cortex lies in the calcarine fissure region of the medial occipital lobe.
• Suprachiasmatic nucleus:
  • Circadian rhythms.
• Pretectal nuclei:
  • Reflex eye focusing.
  • Pupillary light reflex.
• Superior colliculus:
  • Rapid directional eye movements.
• Older visual system:
  • Midbrain and forebrain-base pathways.
• Newer visual system:
  • Visual cortex.
  • Conscious form and color vision.
• Dorsal lateral geniculate nucleus:
  • Accurate relay.
  • Point-to-point spatial transmission.
  • Keeps signals from the two eyes separate.
  • Gates transmission to the cortex.
• The nucleus contains six layers.
• Layers I and II:
  • Magnocellular.
• Layers III through VI:
  • Parvocellular.
• Magnocellular pathway:
  • Rapid
  • Black and white
  • Poorer spatial accuracy
• Parvocellular pathway:
  • Color
  • Accurate spatial detail
  • Moderate conduction speed

COMMON STUDENT MISTAKES

• Do not think all optic nerve fibers cross at the optic chiasm.
  • Only fibers from the nasal retinal halves cross.
• Do not confuse the optic tract with the optic nerve.
  • Optic nerve is before the chiasm.
  • Optic tract is after the chiasm.
• Do not think both eyes’ signals mix completely inside the lateral geniculate nucleus.
  • They remain separated in different layers.
• Do not confuse old and new visual systems.
  • Old system → midbrain and basal forebrain.
  • New system → visual cortex.
• Do not confuse magnocellular with parvocellular pathways.
  • Magnocellular → fast, black and white.
  • Parvocellular → color and precise detail.
• Do not assume the lateral geniculate nucleus is only a passive relay.
  • It also gates visual transmission.

QUICK REVISION

Main Visual Pathway

Retina
→ optic nerve
→ optic chiasm
→ optic tract
→ lateral geniculate nucleus
→ optic radiation
→ visual cortex

Chiasm

Nasal retina
→ crosses

Temporal retina
→ remains uncrossed

Older Visual Pathways

Suprachiasmatic nucleus
→ circadian rhythms

Pretectal nuclei
→ pupillary light reflex + reflex eye movements

Superior colliculus
→ rapid directional eye movements

Basal brain regions
→ behavioral functions

New Visual Pathway

Lateral geniculate nucleus
→ optic radiation
→ visual cortex

Function:

→ conscious visual form
→ color
→ other conscious vision

Lateral Geniculate Functions

Visual signal relay
→ accurate point-to-point transmission

Eye signals
→ remain separated

Gating
→ controls how much information reaches visual cortex

Magnocellular

Layers I and II
→ M ganglion cells
→ fast
→ black and white
→ poorer spatial precision

Parvocellular

Layers III–VI
→ P ganglion cells
→ color
→ accurate point-to-point information
→ moderate speed

CONCEPTUAL SUMMARY

Visual information begins in the retina and travels toward the brain through an organized pathway.

Retina
→ optic nerve
→ optic chiasm

At the chiasm:

Nasal retinal fibers
→ cross to the opposite side

Temporal retinal fibers
→ remain on the same side

These fibers combine to form:

→ optic tracts

Optic tract fibers then travel mainly to:

→ dorsal lateral geniculate nucleus of the thalamus

From there:

→ optic radiation
→ primary visual cortex in the medial occipital lobe

Some visual fibers also travel to older brain regions.

Suprachiasmatic nucleus
→ circadian rhythm control

Pretectal nuclei
→ pupillary and reflex eye responses

Superior colliculus
→ rapid directional eye movements

Basal brain regions
→ some behavioral functions

Therefore, the visual system can be divided into:

Old system
→ mainly reflex and older visual functions

New system
→ conscious vision through the visual cortex

The dorsal lateral geniculate nucleus has two major roles.

First:

Retinal signal
→ precise point-to-point relay
→ visual cortex

Second:

Incoming signal
→ gating mechanisms
→ selected information allowed to pass

The nucleus also contains separate magnocellular and parvocellular pathways.

Magnocellular system

M ganglion input
→ fast conduction
→ black-and-white information
→ poorer spatial precision

Parvocellular system

P ganglion input
→ moderate conduction
→ color information
→ precise spatial transmission

Final Take-Home Concept

Nasal retinal fibers cross; temporal retinal fibers do not.

The lateral geniculate nucleus is both a relay and a gate.

Magnocellular pathway = fast, black-and-white information.

Parvocellular pathway = color and precise spatial detail.

The visual cortex is the major center for conscious vision in humans.

Reference Guyton and Hall Textbook of Medical Physiology 15th Edition, page. 663 Chapter52