-
Vision: CNS
Pathway from the eye to the cortex
-
Themes of this lecture
- Visual information is analyzed in more complicated ways than in the retina
- One major pathway from the eye leads to the striate cortex and from there to many other visual centers
- Cortical lesions can lead to difficulty in determining what an object is or where it is
-
Optic nerve
- one of the major targets of MS
- is an outgrowth of the diencephalon and is surrounded by meninges
- Optic axons become myelinated by oligodendrocytes after they leave the retina
- The myelin of optic nerve is one of the commonest locations of damage in multiple sclerosis
-
Optic nerve and tract
- Inversion of the visual field; the left half of each eye sees the right half of the visual field
- the right half of the visual field is "seen" by the left half of the brain
- Nasal vs. temporal hemiretina: the fovea is the dividing point
- Axons leave the eye at the optic disc (optic papilla)
- Half of the optic axons cross to the opposite side of the brain at the chasm
-
The lateral geniculate nucleus (LGN)
- About 90% of optic axons project to the LGN (but optic axons account for only about 20% of the input to the LGN)
- The receptive field properties of cells in the geniculate are almost the same as the properties of retinal ganglion cells
-
The layers of the LGN
- most textbooks say there are 6 layers, but there are really 12!
- Information from the two eyes is kept separate: 6 layers get input from the left eye; 6 layers get input from the right eye
- Retinotropic maps: each layer has a "copy" of half of the retina, with axons coming from one type of ganglion cell in one eye
- What's different about these layers?: different kinds of ganglion cells project to different layers
- For example, a cell with non-color selective responses will project to a different layer than a cell that is color-selective
-
Lateral geniculate nucleus activity
- Recent imaging studies indicate that the lateral geniculate plays a role in selective attention.
- The activation comes from the cortex
- During sleep and periods of inattentiveness, transmission from the eye through the lateral geniculate is reduced and cells fire spontaneous bursts
-
The optic radiations
- Fibers from cells in the LGN enter the optic radiations, sweep around the lateral side of the lateral ventricle and pass to the occipital lobe
- The visual fibers are also called the geniculocalcarine tract
-
Primary Visual Cortex
- Area 17, Striate cortex, V1
- Located in the pole of the occipital lobe above and below the CALCARINE SULCUS on the medial aspect of the hemisphere
-
Cortical circuitry
- Axons from lateral geniculate terminate in layer 4
- Information is then passed to upper layers and deeper layers
-
Receptive fields in striate cortex
- Most cells in ayer 4 have circular receptive fields with center-surround organization
- The vast majority of cells in other layers respond best to straight bars or edges: and prefer different orientations
- Some respond best to dark bars or lines
- Some respond best to light bars or lines
- Typical cells outside of layer 4
-
Output of striate cortex: more than 40 other regions!
or why you should know that there's more than one way to be blind
-
The "where" stream
- Cells in some of these regions respond almost exclusively to moving objects
- Some respond to circular or spiraling movement
- Some respond to "visual flow"
- Some respond best to approaching or receding objects
-
Lesions of the "where" stream
- loss of speed and motion perception
- loss of ability to use visual information information to grasp objects
- visual neglect in peripersonal space
-
Balint's syndrome
- Due to bilateral parietal sesion
- Simultanagnosia: inability to use visual information to grasp objects
- Optic apraxia: difficulty voluntarily shifting gaze
- Optic ataxia: inability to reach correctly toward an object
-
The "what" stream
- some areas have many cells that are color-sensitive
- some areas have cells that respond to complex shapes
- some areas are particularly important for face perception
-
Lesions of the "what" stream
- achromatopsia: inability to percieve colors
- Prosopagnosia: inability to recognize familiar faces
|
|
