Visual Cortex😊

Organization of Vision

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Vision is a special sense

It has 3 levels of organization:

Retina → Thalamus → Occipital Visual cortex

Note: General senses have 4 levels of organization, including the spinal cord.

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Order of Neuron	Cell Type	ㅤ
1st order neuron	Photoreceptors (Rod, Cone)	ㅤ
2nd order neuron	Bipolar cell	Has two poles: • One to receive information. • One to conduct information.
3rd order neuron	Ganglion cell	Only retinal cell to: - Have an output. ▪ Axons → Optic nerve → Thalamus. - Generate action potential (For impulses to travel long distances).
4th order neuron	Lateral Geniculate Body (LGB) neuron	Has 6 layers, 3 pathways - Magnocellular pathway - Parvocellular pathway - Koniocellular pathway

Feature	Rods	Cones
Sensitive to	Dim lights (Night vision)	Colour vision (Day vision)
Number	100 million	5 million
Pigment	Rhodopsin / Visual purple	Iodopsin
Abundant in	Peripheral retina	Central retina → Foveola: Only cones (Rod-free zone)

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Thalamus: Lateral Geniculate Body (LGB)

Inputs to LGB:

Each LGB receives input from both ipsilateral (I/L) and contralateral (C/L) eyes.
LGB has 6 layers.

Layers 1, 4, 6 → Receive C/L fibers.
Layers 2, 3, 5 → Receive I/L fibers.

Pathways within LGB

Pathway	Origin (LGB layers)	Function	Cell Type	Termination
Magnocellular pathway	1, 2 layers Parietal lobe	Extraocular movements Detection of flickering	M cells (big)	Layer 4 of visual cortex
Parvocellular pathway	3, 4, 5, 6 layers Temporal lobe	Colour vision Finer details in the visual field	P cells (small)	Layer 4 of visual cortex
Koniocellular pathway	Between P&M cells	Colour vision: Blue	K cells	ㅤ

mnemonic : Magno - Big - Big movements

Parvo - Small - Finer details

Blue Cone

Visual Cortex

1. Calcarine cortex/striate cortex

Primary visual cortex

Area 17
Located in posterior part of calcarine sulcus
Lobe: Occipital
Heavily myelinated
Striate appearance:

Striae of Gennasi (also known as striate cortex)

Mnemonic: Genz vision brod anu → 17, 18, 19 yr olds

Accessory visual areas

Area 18 → Secondary visual cortex
Area 19 → Tertiary visual cortex

2. Extrastriate Cortices

LGB

Area	Projections from	Function
Parietal lobe	Magnocellular pathway	Eyeball movements
Temporal lobe	Parvocellular pathway	Colour vision

3. Inferior Temporal Cortex

Function: Face recognition by sight

Area	Functions
Prefrontal Cortex • Anterior frontal lobe	• Executive functions: • Planning, decision-making • Working Memory (Short term) → Eg: Phone numbers
Anterior nucleus of thalamus	• Recent memories • Relay station in the limbic circuit.
Amygdala	• Associates memory with emotions • Especially fear • Applied: Kluver - Bucy syndrome • though not part of original Papez circuit, it's a part of limbic system
Hippocampal formation	• Memory encoding. • Anterograde amnesia ↳ Lesions in hippocampus or Papez circuit.
Mamillary bodies	• Recollective memory • Degeneration → Wernicke-Korsakoff psychosis • (seen in alcoholics)
Parietal Association Cortex	• Spatial awareness • Damage can cause hemispatial neglect. • Parietal - Periphery - spatial awareness Inferior parietal lobule • Hand eye coordination Superior Parietal lobule • A/w Tactile agnosia
Temporal Association Cortex	• Memory, object recognition, and language comprehension. • Includes Wernicke’s area on the dominant hemisphere.
Inferior Temporal Cortex	• Face recognition by sight ↳ Prosopagnosia
Limbic Association Area	• Part of limbic lobe; • Connects with hippocampus and amygdala • Involved in emotion, motivation, and memory.
Entorhinal cortex (of limbic cortex)	• Associates memory with smell • Also in Spatial memory • Interface between hippocampus and neocortex.

Lesion: Prosopagnosia → Loss of recognition of faces

Fusiform Gyrus → Temporo occipital lobe
Mnemonic: Cant recognise face after using Pro Soap (Prosop - Agnosia) → IT jobs

Cells

Cell Type	Function
Simple cell	Higher order processing - Detect minute changes
Complex cell	ㅤ
Blobs	Colour vision Expresses cytochrome oxidase

Color Vision

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Primary Colors:

Red (Prota), Green (Deutra), Blue (Trita).

Mnemonic: 6 GB

Colour Defect	Name
Red	Protanopia
Green	Deuteranopia
Blue	Tritanopia

Etiology:

Acquired:

Blue-yellow:

Associated with

Retinal diseases (CSR, macular edema)
Primary Open Angle Glaucoma (POAG)

By → Po Retina

Red-green:

Associated with optic nerve lesions (optic neuritis, LHON).
ON RGb
Congenital:

X-linked recessive disorders.
Affects males > females (Carriers)
Most common (m/c): Red green defect

Blue:

Associated with old age/Digoxin
Yellow Tinging → Xanthopsia → Yellow vision
Old age → Blue gone & Die (Diogxin)

Types: AD

A cro

A = Mono (Rod, Cone)

Rod Monochromat → Hemeralopia (day blindness)

D cro

D = 2/3 = Anopia/anomaly

Di → Anopia
Trichromatic → Anomaly

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Dyschromatopsia (Defective Perception):

Dichromatic:

Perception of 2 colors;
blindness for 1 color (anopia).
Conditions: Protanopia, Deuteranopia, Tritanopia.

Trichromatic Anomalous:

Perception of all 3 colors
but defective perception (anomaly)
Conditions: Protanomaly, Deuteranomaly, Tritanomaly.

Achromatopsia (Absent Perception):

Rod Monochromat:

Only rod function present;
Total color blind (black & white vision)
A/w Hemaralopia (day blindness)

Cone Monochromat:

One type of cone function present
1 cone perception

Cones

Types of cones:

Cone Type	Wavelength Detection	Colour
L cone	Long	Red
M cone	Medium	Green
S cone	Small	Blue

Colorful (cons) dress (Large, medium, small)

Long → Medium → Small

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