Vision (Central processing)

Question 1

What is the structure and function of the optic chiasm?

Answer 1

Structure:

• Partial decussation of the optic nerves occur at the optic chiasm.

Fibres on the nasal side carrying information regarding the temporal retinae cross over.

Function: Partial decussation at the optic chiasm ensures that information regarding the same side of the visual field is carried by the same (contralateral) optic tract.

Question 2

What are the types of visual defects associated with lesions in the pre-LGN visual pathway?

Answer 2

• Lesion in optic nerve: Unilateral anopia (blindness in one eye)
• Lesion in optic chiasm: Heteronymous (bi-temporal) hemianopia (loss of temporal aspects of visual field - i.e. tunnel vision)
• Lesion in optic tract: Homonymous hemianopia (loss of one half of visual field)

Question 3

What are the parts of the brain into which ganglion cell axons project?

Answer 3

• Lateral geniculate nucleus (90%) - Visual processing
• Others (10%):
  1. Pretectum - Pupillary reflex
  2. Suprachiasmatic nucleus - Circadian rhythms
  3. Superior colliculus - Eye motor control

Question 4

Why is the organisation of information throughout the whole of the visual pathway considered retinotopic?

Answer 4

Spatial information from the retina is preserved, so that fibres carrying information from adjacent parts of the retina remain adjacent to each other.

Question 5

What is the structure of the lateral geniculate nucleus (LGN)?

Answer 5

• The LGN contains 6 layers, each receiving inputs from distinct (ipsi-[I]/contralateral[C]) retinae.
• The arrangement of inputs from different sides is:

I. C

II. I

III. I

IV. C

V. I

VI. C

• Layers I, II receive inputs from magnocellular ganglion cells.
• Layers III-VI receive inputs from parvocellular ganglion cells.
• Koniocellular cells (specialised ganglion cells carrying information from blue cones) project into thin layer between the main layers.

Question 6

What is the function of the LGN?

Answer 6

• Initially, it was believed that the LGN was mainly relay station (since 1:1 association between ganglion cells and geniculate cells).
• The LGN may be involved in feedback mechanisms from the cortex and act as a ‘gating’ mechanism for inputs into the primary visual cortex.
• E.g. Responsible for reduced input into V1 from flashes during sleep.

Question 7

What evidence is there against idea that LGN is simply relay station?

Answer 7

• The idea of ‘relay’ stations is physiologically wasteful.
• Only 10% of LGN input is from the retina, with 90% from elsewhere, suggesting its involvement in many functions.
• 30% of input from cortical cells. This input may be responsible for feedback gating of visual information to V1.

Question 8

What are the differences between the image received by primary visual cortex (V1) compared to real image?

Answer 8

1. Image is inverted, indicating that spatial configuration of retina is conserved, not image itself.
2. Cortical amplification, meaning that the fovea is disproportionally represented compared to the rest of the retina (with 50% of V1 dedicated to processing information from central 10% of retina in foveal region).

Question 9

What is the location of the primary visual cortex?

Answer 9

• Located in the occipital lobe, around the calcarine sulcus (striate cortex).
• Brodmann’s area 17

Question 10

What are the types of cells in V1?

Answer 10

• Pyramidal cells: Main cell responsible for outputs from V1, either to other parts of cortex or back to thalamus and LGN.
• Stellate cells: Interneurones that form part of connection between geniculate cells and pyramidal cells.

Question 11

What is the structure of V1?

Answer 11

• V1 is ~2mm thick and like other parts of the neocortex, contains 6 layers.
• Magnocellular inputs are received in layer 4Cα.
• Parvocellular inputs are received in layer 4Cβ.
• Pyramidal cells projecting from V1 into other parts of the cortex are located in layers 2 & 3.
• Pyramidal cells projecting into deep structures (e.g. superior colliculus) are located in layer 5.
• Pyramidal cells projecting back into thalamus are located in layer 6.

Question 12

What are the classes of cells in V1?

Answer 12

• Simple cells
• Complex cells

Question 13

What are the properties of simple cells?

Answer 13

• Responds to bars of light and edges of a specific orientation and within a defined position.
• Mainly found in input areas (e.g. layer 4, even 6).

Question 14

What are the properties of complex cells?

Answer 14

• Respond to bars and edges of specific orientation irresective of position.
• Maximum response produced for moving stimuli.
• Found mainly in layers 2,3 and 5.
• Makes up ~75% of cells in V1.

Question 15

What are end-stopping cells?

Answer 15

Simple or complex cells that show additional property of only responding to stimuli up to certain length, and producing sub-maximal response when length is exceeded (even if orientation is correct).

Question 16

What is the overall structure of V1?

Answer 16

• Cells in V1 can be divided into perpendicular columns, with each column containing cells that have preference for same orientation. These are called orientation columns.
• Adjacent columns have slightly different orientation preferences.
• Over distance of ~1 mm along V1, all orientations are represented.
• Strips of cells also respond differently to inputs from different eyes, with one eye usually being dominant (ocular dominance), although this only occurs in 4c, while cells in other layers respond more evenly to both eyes.
• There are “blobs” scattered throughout V1 containing cells responsible for colour analysis. Surrounding cells called interblob.
• Within area of 1 mm by 2 mm of V1, cells sensitive to all possible orientations and from both eyes can be found, along with blob for colour analysis. This is called a hypercolumn.
• Within a hypercolumn, every aspect of a small part of the retinal image can be analysed.

Question 17

What is the mechanism of disparity detection?

Answer 17

• When eyes are focused onto objects in one plane of fixation, all objects in that plane map onto areas of retina at same distance and direction from fovea. These are called corresponding points and a lot of cortical neurones receive inputs from these.
• Objects not in plane of focus maps onto parts of the retina not at the same distance/direction from fovea. These are called disparate images. Some neurones are specialised at detecting disparate images (disparity detectors) in order to distinguish between objects in and out of plane of fixation.
• Important for depth perception and stereopsis (3D vision).

Question 18

What enzyme is stained within blobs?

Answer 18

Cytochrome oxidase

Question 19

What clinical problems may be associated with defects in ocular dominance/orientation tuning?

Answer 19

Amblyopia (lazy eye): One eye fails to achieve full acuity despite optical corrections, indicating neural problem. Can be caused by 2 childhood defects:

1. Monocular deprivation during childhood
2. Strabismus (misalignment of 2 eyes)

Question 20

What are the higher visual pathways beyond V1?

Answer 20

1. Ventral (what) pathway
2. Dorsal (where) pathway

Question 21

What is the structure of the ventral pathways?

Answer 21

1. Parvocellular neurone → Blob (V1) → Thin stripe (V2) → V4: Colour
2. Parvo-/magnocellular neurone → Interblob (V1) → Interstripe (V2) → V4: Edge/fine form analysis
3. V4 → Infratemporal cortex: Faces/hands

Question 22

What are disorders associated with lesions in ventral pathway?

Answer 22

• Achromatopsia (V4): Cortical colour blindness
• Prosopagnosia (Infratemporal cortex): Inability to recognise faces

Question 23

What is the structure of the dorsal (where) pathway?

Answer 23

Magnocellular neurone → 4B-4Cα (V1) → Thick stripe (V2) → V5 + V5a → Posterior parietal cortex: Movement and binocular depth cues.

Question 24

What is the function of end-stopping cells?

Answer 24

Possibly to detect curved edges

Question 25

What may be the functions of all types of cells in the hypercolumn?

Answer 25

1. Simple cell: Stationary straight edges & orientation 2. Simple cell (end-stopping): Stationary curved edges & orientation 3. Complex cell: Moving straight edges, orientation & direction 4. Complex cell (end-stopping): Moving curved edges, orientation & direction 5. Blob cells: Colour 6. Disparity detecting cells: Stereopsis

Question 26

What disorders are associated with lesion in the dorsal "where" pathway?

Answer 26

Akinetopsia (V5): Inability to see movement