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Pathways from retina to brain

Retinal ganglion cells project to 4 major subcorticaltargets:

1. The suprachiasmatic nucleus of hypothalamus - entrainment of circadian rhythms to the light dark cycleto the light-dark cycle.

2. The pretectal area of midbrain - control of pupillary response to light.

3. The superior colliculus – multisensory mapping of extrapersonal space, mainly for reflex control.

4. The lateral geniculate nucleus of the thalamus (LGN) – project to layer 4 of the primary visual cortex (V1).p y ( )

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THE RETINOTHALAMIC‐OCCIPITAL PATHWAY FOR CONSCIOUS VISION. 

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(LGN)

Lateral geniculate nucleus of thalamus(LGN)

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Striate cortex, located in the occipital lobe, is the primary visual projection area.

Each visual cortex receives input from both eyes: the right cortex sees the left visual field and the left cortex sees the right visual field.

Visual Field

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This pattern is achieved by the partial crossing of the optic nerve fibers at the optic chiasm: axons from the medial retina cross, those ,from peripheral retina do not.

To understand how this works, examine the projections of the medial and lateral parts of the receptive field onto the medial and lateral parts of the retina.

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The visual pathway is retinotopically organized.

Retinotopy is the topographic organization of visual pathways in which neighboring ll h ( ) hb ll hcells in the retina project to (i.e., innervate) neighboring cells in the target structure.  

Retinotopy maintains the precise features of the retinal map and therefore also the visual field.

nonM‐nonP

M MP

PP

P

LR

Projections of retinal ganglion cell types onto separate layers of the lateral geniculate nucleus(LGN) of the thalamus.  Each layer is retinotopically organized and in register with the other layers.

L LR RR

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Retinotopic Organization in the Right Occipital Lobe

The receptive field of a visual neuron is the f th ti th t h ti l t darea of the retina that, when stimulated 

with light, changes the cell’s membrane potential.

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Ganglion cells also have receptive fields witih concentric, mutually antagonistic center-surround properties.

Neurons in the lateral geniculate nucleus (LGN) of the thalamus have receptive field properties similar to ganglion cells. p p g g

Neurons in the primary visual cortex have elongated receptive fields.

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• PN12091.JPG

3 thalamic neurons converging on a cortical neuron.The alignment of their receptive fields determine the features of the “best stimulus” of the cortical cell.

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Thalamic neurons

Cortical neuron

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The best stimulus will be a horizontal bar of light.

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Thalamic neurons

A “simple cell” in primary visual cortex

A “simple cell” in primary visual cortex responds to a bar and is orientation-specific –

A simple cell is defined as having a receptive field with mutally antagonistic inhibitory andfield with mutally antagonistic inhibitory and excitatory portions

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All neurons in a given “cortical column” have the same orientation preference.

Columns representing all orientation preferences form a cortical module.

Columnar Organization of Orientation Selectivity in the Striate Cortex

All neurons in a given “cortical column” have the same orientation preference.  Different preference columns change in an orderly way so that preferences in adjacent columns differ only slightly.

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In addition to differences in orientation specificity, cortical neurons differ in their response to pbinocular inputs and wavelength.

Binocular inputs are sorted into “ocular dominance columns”.

• PN12100.JPG

Ocular dominance columns in layer IV

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Ocular dominance columns labeled by anterogradetransport of radiolabeled proline from the LGN

Columnar Organization ofColumnar Organization of Ocular Dominance

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Columnar Organization of Ocular Dominance

Types of columns

The preference for left vs right eye changes gradually across adjacent columns.  A particular preference is found in all cells within a column.  All preferences are represented across p pcolumns.

Vision with two eyes (i.e., “binocular vision” is the the

Why two eyes?

basis of depth perception. 

Binocular disparities are generally thoughtare generally thought to be the basis of stereopsis or depth perception.

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Cortical neurons also differ in their response to wavelength.

l h f dWavelength specificity is sorted into cytochrome oxidase “blobs” (parvo, magno, nonM‐nonP).

Interblob areas have the smallest receptive fields (parvo). They have orientation selective receptive fields.

Cytochrome oxidase blobs are superimposed on the ocular dominance columns.

Wavelength specificity isWavelength specificity is sorted into cytochromeoxidase “blobs”. Blobs receive input from parvo, magno, nonM‐nonPganglion cells.

Interblob areas receive input from parvocellularganglion cells.

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Cortical modules contain all elements for image processing for a particular point in visual field.

P = parvocellularM = magnocellular

GANGLION CELLS

Three classes of ganglion cells have been identified:P‐type and M‐type, and nonM‐nonP type.

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Note: Both rods and cones contribute to these receptive fields, but cones of all types contribute equally to center and surround, so they lack wavelength specificity.

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Dorsal Pathway for Spatial Vision and Motion Detection arises from M-type ganglion cells. All cells in this pathway are direction selective.

Ventral Pathway for object and shape recognition and color perception arises from P type and non-M non-P type ganglion cells