Visual Pathways: Visual Pathways: The Road to VisionThe Road to Vision

Anthony DeSimone LDOAnthony DeSimone LDO

Visual PathwayVisual Pathway

““Focus on the Eye”Focus on the Eye”• Concerned aboutConcerned about

CorneaCornea LensLens RetinaRetina

There is more to vision than meets There is more to vision than meets the eyethe eye

Retinal Fields vs. Visual FieldsRetinal Fields vs. Visual Fields

What’s the differenceWhat’s the difference• Retinal Field – describes the area that includes Retinal Field – describes the area that includes

neural fibers of the retina that are receiving neural fibers of the retina that are receiving light from some objectlight from some object

• Visual Field – describes the area in space Visual Field – describes the area in space where the object lieswhere the object lies

They are the reversal of one anotherThey are the reversal of one another• The nasal retinal field receives light from the The nasal retinal field receives light from the

temporal visual fieldtemporal visual field• The temporal retinal field receives light from The temporal retinal field receives light from

the nasal visual fieldthe nasal visual field

Temporal Temporal

Visual field Visual Field

Nasal Visual Nasal Visual

Field Field

Temporal TemporalRetinal field Retinal Field

Nasal Retinal Nasal RetinalField Field

Optic ChiasmOptic Chiasm

Partial decussation (cross-over) of Partial decussation (cross-over) of Optic Nerve fibers occurs at the level Optic Nerve fibers occurs at the level of the Optic Chiasmof the Optic Chiasm• Only nasal retinal fibers (from the nasal Only nasal retinal fibers (from the nasal

retinal field) cross overretinal field) cross over• Temporal nasal fibers (from the Temporal nasal fibers (from the

temporal retinal field) do not.temporal retinal field) do not.

Lateral Aspect

1. Optic Nerve (stump) 2. Optic Chiasm 3. Optic Tract

Inferior Aspect

Optic TractOptic Tract Optic tractOptic tract It is important for the It is important for the sense of sightsense of sight. . By convention, the optic tract is defined as By convention, the optic tract is defined as

that extent of the visual system pathway that extent of the visual system pathway from the optic chiasm to the lateral from the optic chiasm to the lateral geniculate nucleus of the thalamus. geniculate nucleus of the thalamus.

Each optic tract contains axons from Each optic tract contains axons from ganglion cells in the retinas of both the left ganglion cells in the retinas of both the left and right eyes, but information from only and right eyes, but information from only one half (i.e either left or right) of each one half (i.e either left or right) of each eye's visual field eye's visual field

Nerve CellsNerve Cells

LGN

Optic Tracts

Chiasm

Lateral Geniculate Body Lateral Geniculate Body

After the optic tract, the next stop is After the optic tract, the next stop is the Lateral Geniculate Body the Lateral Geniculate Body (or (or Lateral Geniculate Nucleus)Lateral Geniculate Nucleus)

LGNLGN• Optic nerve fibers from the optic tracts terminate at two bodies in the thalamus (a structure in the middle of the brain) known as the Lateral Geniculate Nuclei (or LGN for short). • One LGN lies in the left hemisphere and the other lies in the right hemisphere. • Each has six layers

 The optic tract wraps around the cerebral peduncles of the midbrain to get to the lateral geniculate nucleus (LGN), which is a part of the thalamic sensory relay system.  There are two geniculate nuclei, located on either side of the rear end of the thalamus. They each consist of six cellular layers, forming a threefold representation of the opposite binocular visual hemifield in exact anatomic registration. 

This apparently complicated arrangement is engineered so that the right LGN receives information about the left visual field, and the left LGN receives information about the right visual field.

LGNLGN This layered structure is This layered structure is

exquisitely precise in two exquisitely precise in two ways. ways. • First, cells in different layers First, cells in different layers

that align (like the numbers in that align (like the numbers in the picture) have receptive the picture) have receptive fields in the same area of fields in the same area of retina. retina.

• Second, optic nerve fibers Second, optic nerve fibers from the two eyes are from the two eyes are segregated in different layers. segregated in different layers. If you look carefully at the If you look carefully at the projections to the LGN, you projections to the LGN, you will see that ipsilateral fibers will see that ipsilateral fibers arrive in layers 2, 3, and 5, arrive in layers 2, 3, and 5, while contralateral fibers while contralateral fibers arrive in layers 1, 4, and 6 (no-arrive in layers 1, 4, and 6 (no-one knows why).one knows why).

LGN Cell TypesLGN Cell Types

All cells in the LGN have concentric All cells in the LGN have concentric receptive fields, just like the ganglion cells receptive fields, just like the ganglion cells whose fibers terminate in the LGN. whose fibers terminate in the LGN.

Layers 1 and 2 are made up of cells with Layers 1 and 2 are made up of cells with large bodies ("magnocellular") that have large bodies ("magnocellular") that have monochromatic responses (ie. mediate monochromatic responses (ie. mediate responses to light and dark)responses to light and dark)

Layers 3 to 6 are made up of cells with Layers 3 to 6 are made up of cells with small bodies ("parvocellular") that mediate small bodies ("parvocellular") that mediate color vision. color vision.

Optic RadiationsOptic Radiations

Leaving the LGN are optic radiationsLeaving the LGN are optic radiations Optic radiationsOptic radiations are a collection of axons are a collection of axons

from relay neurons in the lateral from relay neurons in the lateral geniculate nucleus of the thalamus.geniculate nucleus of the thalamus.

They carry visual information to the visual They carry visual information to the visual cortex (also called cortex (also called striate cortexstriate cortex) along the ) along the calcarine fissure.calcarine fissure.

There is one such tract on each side of the There is one such tract on each side of the brain.brain.

Meyer’s LoopMeyer’s Loop The optic radiations follow a very wide three The optic radiations follow a very wide three

dimensional arc. Here is how the radiations are dimensional arc. Here is how the radiations are conventionally drawn, and how they look from the conventionally drawn, and how they look from the side side

The longer loop actually dives into the temporal The longer loop actually dives into the temporal lobe before it heads back to the occipital lobe. lobe before it heads back to the occipital lobe.

This group of fibers is called Meyer's loop. Recall This group of fibers is called Meyer's loop. Recall that, since the lens inverts all images, the lower half that, since the lens inverts all images, the lower half of the retina sees the upper half of the world. This of the retina sees the upper half of the world. This orientation is preserved through the pathway, so orientation is preserved through the pathway, so that the lower optic radiations, or Meyer's loop, are that the lower optic radiations, or Meyer's loop, are carrying information from the upper visual world. carrying information from the upper visual world.

Meyer’s LoopMeyer’s Loop

Striate Cortex (V1) Striate Cortex (V1)

Retinotopic MappingRetinotopic Mapping In lower visual areas (e.g., V1 through V5) the In lower visual areas (e.g., V1 through V5) the

neurons are organized in an orderly fashion called neurons are organized in an orderly fashion called topographic or retinotopic mappingtopographic or retinotopic mapping• they form a 2D representation of the visual image they form a 2D representation of the visual image

formed on the retina in such a way that neighboring formed on the retina in such a way that neighboring regions of the image are represented by neighboring regions of the image are represented by neighboring regions of the visual arearegions of the visual area

But this retinotopic representation in the cortical But this retinotopic representation in the cortical areas is distorted. areas is distorted.

The foveal area is represented by a relatively The foveal area is represented by a relatively larger area in V1 than the peripheral areas. larger area in V1 than the peripheral areas.

Visual CortexVisual Cortex

Much of the primate cortex is Much of the primate cortex is devoted to visual processing. devoted to visual processing. • In the macaque monkey at least 50% of In the macaque monkey at least 50% of

the neocortex appears to be directly the neocortex appears to be directly involved in vision, with over twenty involved in vision, with over twenty distinct areas. distinct areas.

• Some of the areas concerned are quite Some of the areas concerned are quite well understood, others are still a well understood, others are still a complete mystery.complete mystery.

Visual CortexVisual Cortex

Nearly all visual information reaches the Nearly all visual information reaches the cortex via V1, the largest and most cortex via V1, the largest and most important visual cortical area. important visual cortical area.

Because of its stripey appearance this Because of its stripey appearance this area is also known as striate cortex, area is also known as striate cortex, amongst other things. amongst other things.

Other areas of visual cortex are known as Other areas of visual cortex are known as extrastriate visual cortex extrastriate visual cortex • the more important areas are V2, V3, V4 and the more important areas are V2, V3, V4 and

MT (also known as .....V5!).MT (also known as .....V5!).

Areas of Visual CortexAreas of Visual Cortex

V1V1

In primates nearly all visual information In primates nearly all visual information enters the cortex via area V1. This area enters the cortex via area V1. This area is located in the occipital lobe at the is located in the occipital lobe at the back of the brain. It is also known as: back of the brain. It is also known as:

- primary visual cortex - primary visual cortex - area 17 - area 17 - striate cortex.- striate cortex.

V1V1

This region represents about 5% of This region represents about 5% of the neocortex in man. the neocortex in man.

It is the most complex region of the It is the most complex region of the cortex with at least 6 identifiable cortex with at least 6 identifiable layers layers • layer 1 is close to the cortical surface, layer 1 is close to the cortical surface,

layer 6 adjoins the white matter belowlayer 6 adjoins the white matter below

V1V1

Simple CellsSimple Cells Simple cell receptive fields contain sub-regions Simple cell receptive fields contain sub-regions

that exert an excitatory influence on the cell's that exert an excitatory influence on the cell's response (light grey in the picture), and sub-response (light grey in the picture), and sub-regions that exert an inhibitory influence (dark regions that exert an inhibitory influence (dark grey in the picture). The blue lines in the picture grey in the picture). The blue lines in the picture are time traces that plot the onset and offset of are time traces that plot the onset and offset of stimulation. The black vertical lines below them stimulation. The black vertical lines below them indicate individual nerve impulses. The most indicate individual nerve impulses. The most effective stimulus for this particular receptive effective stimulus for this particular receptive field (left) is one that puts a lot of light in the field (left) is one that puts a lot of light in the excitatory region, and only a little in the inhibitory excitatory region, and only a little in the inhibitory region. region.

Response of Simple CellsResponse of Simple Cells

Simple CellsSimple Cells

It must have the right orientation, the right It must have the right orientation, the right position, and the right size. Stimuli that position, and the right size. Stimuli that are non-optimal in terms of position are non-optimal in terms of position (middle left), or orientation (middle right), (middle left), or orientation (middle right), or size (right) are less effective. Simple cell or size (right) are less effective. Simple cell receptive fields could be 'built' in the receptive fields could be 'built' in the cortex by collecting responses from LGN cortex by collecting responses from LGN cells whose receptive fields fall along a cells whose receptive fields fall along a line across the retina, but the exact wiring line across the retina, but the exact wiring is still the subject of debate. is still the subject of debate.

Complex CellsComplex Cells Complex cells are the most numerous in Complex cells are the most numerous in

V1 (perhaps making up three-quarters of V1 (perhaps making up three-quarters of the population). Like Simple cells, they the population). Like Simple cells, they respond only to appropriately oriented respond only to appropriately oriented stimuli, but unlike Simple cells, they are stimuli, but unlike Simple cells, they are not fussy about the position of the not fussy about the position of the stimulus, as along as it falls somewhere stimulus, as along as it falls somewhere inside the receptive field (left and middle-inside the receptive field (left and middle-left examples above). Many complex cells left examples above). Many complex cells are also direction-selective, in the sense are also direction-selective, in the sense that they respond only when the stimulus that they respond only when the stimulus moves in one direction and not when it moves in one direction and not when it moves in the opposite direction moves in the opposite direction

Response of Complex CellsResponse of Complex Cells

Orientation CellsOrientation Cells Hubel and Wiesel were the first to discover that cells in V1 Hubel and Wiesel were the first to discover that cells in V1

are arranged in a beautifully precise and orderly fashion. are arranged in a beautifully precise and orderly fashion. Hubel and Wiesel found that as one advances deeper into Hubel and Wiesel found that as one advances deeper into

the cortex through successive layers perpendicular to the the cortex through successive layers perpendicular to the surface, all cells that have orientation tuning prefer the surface, all cells that have orientation tuning prefer the same orientation. same orientation.

On the other hand, moving across the surface of the cortex, On the other hand, moving across the surface of the cortex, orientation tuning mostly changes in an orderly fashion (as orientation tuning mostly changes in an orderly fashion (as shown by the small lines in the picture).shown by the small lines in the picture).

Hubel and Wiesel used the term "orientation columns" to Hubel and Wiesel used the term "orientation columns" to describe this arrangement, but they are really slabs rather describe this arrangement, but they are really slabs rather than columns.than columns.