• Maps• Plasticity of maps• Retinotopic map

Last Lecture

• More: Retinotopic Map• Parallel Visual Pathways• Blindsight

Today’s Outline

Visual world onto the retina

LVFLeft Nasal retina Right Temporal retina

RVFLeft Temporal retinaRight Nasal retina

Retinotopic Map

Left Eye’s View

Retinotopic Map

Right Eye’s view

Retinotopic Map

Perimetric testing to Diagnose field defects

View from above

Retinotopic Map

Each eye’s field of View (visual field) is mapped out

Can light be detected throughout the visual field?

Retinotopic Map

Left

Right

VF Left Eye VF Right Eye

Combined Visual Field

Retinotopic Map

Left

Right

VF Left Eye VF Right Eye

Combined Visual Field

Retinotopic Map

Function: Perimetric Map shows blindness in lower right quadrant of…

Left Eye Right Eye

Where is the damage?Retinotopic Map

On the retinae?

Retinotopic Map

From Retina to Brain

Geniculo-Striate Pathway Optic nerve carries

signals from retina.• Decussation at optic

chiasm (optic tract)• Synapse at Thalamus:

Lateral Geniculate Nucleus (LGN)

• Optic radiations to• AREA 17; Striate

Cortex, Primary visual cortex

Retinotopic Map

Visual World Mapped onto Cortex (via the retina) >> retinotopic map

Retinotopic Map

Deficit in 1/2 VF (homonymous: both eyes)

Retinotopic Map

CALCARINE FISSURE

Upper Bank

Lower Bank

Function: Perimetric Map shows blindness in lower right quadrant of…

Left Eye Right Eye

Where is the damage?Retinotopic Map

defect affects 1/4 VF

Retinotopic Map

An area of visual loss surrounded by relatively well-preserved vision. Size and shape vary.

Retinotopic Map

Gordon Holmes (1919)Correlated Visual Field Defects with lesion locus to identify theRetinotopic map.

Retinotopic Map

Sensitive areas are “magnified”

Field of View

Cortical Map of Visual Field

Retinotopic Map

Equi-visibility chart

Objects in the periphery must be

Physically larger in order to beas visible as objects falling on fovea

Fovea is more sensitive

Cortical Magnification

AnstisRetinotopic Map

Fovea is a small portion of retina

Retinotopic Map

Cortical Magnification

• Area 17 neurons have receptive fields in the retina

• More neurons have foveal receptive fields

Retinotopic Map

Retinotopic map: Summary

Crossed organization Left 17 --> RVF Right 17 --> LVF

Inverted organization Lower calcarine > Upper VF Upper calcarine > Lower VF

Fovea: Disproportional representation cortical magnification

NOTE: Each visual cortex represents a visual field NOT an eye.

Retinotopic Map

VF Left Eye VF Right Eye

Combined Visual Field

Left

Right

Nasal

Retinotopic Map

Left

Right

Nasal

VF Left Eye VF Right Eye

Combined Visual Field

Retinotopic Map

Left

Right

Nasal

VF Left Eye VF Right Eye

Combined Visual Field

Retinotopic Map

For your review: the connections

Left EyeNasal hemiretina- LVF- projects

to right hemisphereTemporal hemiretina- RVF-

projects to left hemisphere

To cross at optic chiasm

uncrossed uncrossed

Right EyeNasal hemiretina- RVF- projects to left hemisphereTemporal hemiretina- LVF- projects to right hemisphere

nasal

LVF input to Rhem. RVF input to LHem

LEFT EYE’s RETINA

RIGHT EYE’s RETINA

• The Retinotopic Map• Parallel Visual Pathways• Blindsight

Outline

Parallel Pathways

From Retina

To primary visual cortex

To superior colliculus

Parallel Pathways

Path to SC

LGN StriateextraStriate

LORE of neurology until the early 70's...

Vision requires Area 17…or maybe not?

Reports of residual vision in Animals with striate lesions (hamsters; monkeys): Recovery after experimental field defects (cortical ablations)

spared light/dark discrimination spared localization abilities

Implication: Other pathways can compensate for some geniculo-striate function.

Can this also be true in humans??

Parallel Pathways