Visual System

Overview of Pathways – Functional Anatomy

• Another sensory system, like touch and pain.• Therefore, organized in similar way:

thalamus

sensory system (retina) ------- 1° visual cortex.

As in other sensory systems, projections to thalamus and visual cortex is very topographically organized.

Figure 10.36

Neural pathway

• The right visual field maps on the left visual cortex and vice versa

Neural pathway – Regional Anatomy of the Visual System A. Retina

1. Retina regions2. Retina layers and cell types.

B. Optic nerve and optic chiasm.C. Superior Colliculus.D. Lateral geniculate nucleus (of the

thalamus).E. Optic radiations and projections to

the 1° visual cortex.1. magnocellular/parvocellular

projections.2. Column Organization of the 1°

visual cortex.a. ocular dominance

columns.b. orientation columns.c. color blobs.

F. Inputs to higher-order visual cortical areas.

Photoreceptor cells

Inter-neurons

Ganglion cells

Optic nerve

Optic chiasm

Optic tract

LGN Brachium ofsuperior colliculus

Optic radiations

1° visualCortex

(calcarine fissure)

Superiorcolliculus

2 Primary Visual Paths (See Fig. 7-1)

Pathway for Visual Perception: (Fig. 7-1A)•Inputs to the 1° visual cortexserve basic visual perception.

•After this initial input, pathwaysmay then ascend to 2° and higher-order cortical areas in surrounding parts of the occipitallobe.

•These other parts serve other aspectsof vision, such as colour visionand visual motion.

•A decussating path (through corpuscallosum) terminates on contralateral 1° cortex (unify images from 2 eyes).

•A descending path goes tooculomotor centres in midbrain, in orderto focus visual image on the retina.

Pathway for Control of Eye Movement (Fig. 7-1B):

For control of eye movements: Pathway to midbrain.

Superior colliculus: inputs in voluntary control of saccades (REMs to salient stimuli) and incoordinating movements of headin concert with this.

Pretectal nuclei participate inpupillary reflexes

Regional Anatomy of the Visual System

A. Retina and optic nerve are derived from out-pocketings of the diencephalon and are, therefore, part of the CNS, and not the PNS (in case you were wondering).

B. As seen in Fig. 7-1B, temporal half of each eye projects ipsilaterally, where as the nasal half projects contralaterally.

C. Optic disk – where axon exits and blood vessels pass – a “blind spot”.

D. Macula – area of highest visual acuity.

E. Fovea – centre of the macula.

F. Retinal layers and cell types (see preceding and following slide)

Photoreceptor cells

Inter-neurons

Ganglion cells

Optic nerve

Optic chiasm

Optic tract

LGN Brachium ofsuperior colliculus

Optic radiations

1° visualCortex

(calcarine fissure)

Superiorcolliculus

2 Primary Visual Paths (See Fig. 7-1)

Regional Anatomy of the Visual System: Retinal structure

• 3 cell layers,

2 synaptic layers:

Outer layer: photoreceptors- rods

(night vision) and cones (for daylight and colour, densest just around the fovea)

Middle layer: bipolar neurons: horizontal (more superficial) and amacrine (deeper). Both perform lateral interactions, which enhance visual contrast.

Inner layer: ganglion cells. Note that light goes through to outermost layer hits photoreceptor cells.

Retinal Layers:

Cone bipolar cells:Input from small #Of cones for high visualAcuity.

Rod bipolar cells:Input from several rodsfor convergence andincreased sensitivityat low illumination.

At outermost extreme:Pigmented epithelium – Phagocytic role for removingold photoreceptors.This function is absent inretinosa pigmentosum, which can detached retina

Neural processing

• The bipolar neurons and ganglion cells process the signal

• In the fovea where the acuity is the highest: 1 cone 1 bipolar cell 1 ganglion cell

• At the periphery: many rods 1 bipolar cell … acuity is much decreased

• Other cells in the retina participate in signal processing

Fig. 7-3A

Concentration of rods and cones

Optic Nerve and Optic Chiasm

• Ganglion cell axons (which are clear and unmyelinated, while running along the inner surface of the retina) gather together and exit at the optic disk, where they become myelinated and form the optic nerve (actually, the 2nd cranial nerve, but since the CNS, officially a tract).

• Optic nerves from both eyes converge at optic chiasm: partial cross-over.

• Images in the nasal hemiretina from both sides cross over (temporal stay ipsilateral).

• This allows for complete cross-over of each visual field (see Fig. 7-3C).

Figure 10.36

Neural pathway

• The right visual field maps on the left visual cortex and vice versa

Fig. 7-3C

Superior colliculus

• As noted above, some optic tract axons do not go to the LGN of the thalamus, but instead, travel in the brachium of the superior colliculus.

• Laminated - 1st layer receives direct input from the retina (brachium). Other layers receive input from 1st or from somatic sensory (AL system) and auditory systems.

- sensory info from the different modalities is lined up in the different layers output to eye and neck muscles so that one can properly orient to the stimulus.

Superior Colliculus

Superiorcolliculus

Lateral Geniculate Nucleus

A nucleus in the thalamus, which projects to the 1° visual cortex and serves visual perception.

A recurrent theme: this nucleus is also laminated (6 layers) with alternating input from ipslateral and contralateral retina.

There is also a division between 2 important input systems:

Magnocellular – input from M ganglion cells with wide dendritic arbours (integrates visual input info from wide area for motion vision).

Parvocellular – input from P cells (ganglionic cells with small dendritic arbours) – for discriminative aspects of visual form and colour.

These systems project to distinct parts of the 1° visual cortex.

Lateral geniculate nucleus

Optic Radiations and Projections to the Primary Visual Cortex

• Meyer’s Loop: course anterior for a short distance in order to move over the lateral ventricles.

• 1° cortex, in columnar fissure, is also Brodman’s Area 17.

• Densest projection to the 1° visual cortex (like other sensory) is to Layer 4 (Stripe of Gennari).

Optic radiations

Stripe of Gennari in area 17(Fig. 7-10A)

Inputs to primary visual cortex

Magnocellular

Parvocellular

Note: both have distanceProjections and destinations.These, in turn, project to higher-order areas for visual processing.

Columnar Organization

Neurons lined up with similar properties across the different layers.

e.g., somatic sensory cortex: columns from same location in body and same modality.

In visual system, 3 types of aggregates (2 transverse the whole thickness of cortex and, therefore, are considered columns:

a. Ocular dominance columns.

b.Orientation columns.

c. Aggregates of colour-sensitive neurons (“colour blobs”).

Ocular dominance columns: alternating input from each eye

Orientation columns: inputs with similar spatial orientation

“Colour blobs”: aggregates of colour-sensitive neurons

(layers 2 and 3 only)

Inputs to Higher-Order Visual CorticalAreas

• Higher-order areas surround area 17 (areas 18 and 19), and can be distinguished as they lack a Stripe of Gennari.

1. Motion pathway – Magnocellular system 1° cortex higher-order areas for visual form in motion (including medial temporal projection).

2. Colour pathway – parvocellular system 1° cortical projections in 4Cβ colour blobs higher-order areas.

3. Form visual pathway – parvocellular system 4Cβ region between blobs higher-order cortex and inferior temporal lobe.

Two “streams” of projections outside the visual system:

- ventral: features “what”

- dorsal: spatial information: “where”

Higher order cortical areas

Visual Field Deficits after Lesions at Various Levels of the Visual System

A. Optic n.: transection = monocular blindness.

B. Optic chiasm: (e.g., pituitary tumour). Transection bitemporal visual field deficit.

C. Optic tract or LGN: transection contralateral visual field deficits (homonymous hemianopsis).

D. Optic radiations: transection of Meyer’s Loop only (temporal region) contralateral upper quadrant (‘quadranotopia’). Complete transection homonymous hemianopsia.

E. 1° Visual Cortex: most common = infarction homonymous hemianopsia with macular sparing (2° greater cortical representation).

Visual field lesions and defects

Neural pathway – Regional Anatomy of the Visual System

• What will happen if the left optic nerve is severed?

• What will happen if a person has a tumor in the pituitary gland (just below the optic chiasmata) and the inner fibers are destroyed?

• What will happen if a person suffers a brain tumor on the right side of the brain around the lateral geniculate body?