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C82COG - Perception > Geniculate and cortical processing > Flashcards

Geniculate and cortical processing Flashcards

1
Q

What do axons of ganglion cells gain when leaving the eye?

A

An insulating myelin coat.

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2
Q

Where do the optic nerves from the two eyes converge?

A

The optic chiasm.

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3
Q

What are ipsilateral fibres?

A

Optic nerve fibres that remain on the same side of the brain.

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4
Q

What are contralateral fibres?

A

Optic nerve fibres that cross to the opposite side of the brain.

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5
Q

Define optic tract.

A

The pathways between the optic chiasm and the brain.

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6
Q

What does each optic tract contain?

A

Fibres from both eyes - the temporal retina from one eye and the nasal retina from the other.

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7
Q

In humans, what percentage of fibres cross at the chiasm?

A

About 50%.

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8
Q

What is the degree of nerve fibre decussation (crossing over) related to?

A

The position of the eyes in the head (and consequent overlap of visual fields) - e.g. fish and birds’ fibres are completely rather than partially crossed.

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9
Q

Where do images presented to the left of fixation fall?

A

On the temporal retina of the right eye and nasal retina of the left eye, both of which converge in the right hemisphere.

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10
Q

Why does nerve fibre decussation occur?

A

So that the right hemisphere processes visual information from the left visual field (LVF), whilst the left hemisphere processes information from the right visual field (RVF).

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11
Q

What is the old name for M cells?

A

Alpha (α) cells.

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12
Q

What is the old name for P cells?

A

Beta (ß) cells.

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13
Q

What is the old name for K cells?

A

γ cells.

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14
Q

What proportion of axons from ganglion cells project to the LGN?

A

About 80%.

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15
Q

Other than the LGN, where do axons from ganglion cells project to?

A

Midbrain structures, primarily the superior colliculus.

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16
Q

How many LGNs and SCs are there?

A

Two of each - one in each hemisphere.

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17
Q

Describe cells in layers 1 and 2 of the LGN.

A

They’re larger than other layers’ cells and receive input from M ganglion cells.

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18
Q

What are the Magnocellular layers of the LGN?

A

Layers 1 and 2.

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19
Q

Describe cells in layers 3-6 of the LGN.

A

They’re smaller and receive input from P ganglion cells.

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20
Q

What are the Parvocellular layers of the LGN?

A

Layers 3-6.

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21
Q

Where are K cells located in the LGN?

A

Sandwiched between the M and P layers.

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22
Q

Which layers of the LGN do ipsilateral fibres input to?

A

2, 3 and 5.

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23
Q

Which layers of the LGN do contralateral fibres input to?

A

1, 4, and 6.

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24
Q

Which eye are the contralateral and ipsilateral fibres from for each hemisphere’s LGN?

A

Contralateral from one eye, ipsilateral from the other, so the right LGN receives left eye contralateral and right eye ipsilateral fibres.

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25
Q

What is the retinotopic map of the LGN?

A

The way in which each layer of the LGN contains an orderly map of the retina (adjacent areas represented in adjacent regions - same region for each layer), preserving retinal topography.

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26
Q

Describe the receptive properties of (M and P) LGN cells.

A

Centre-surround antagonistic arrangement, circular in shape, orientation independent.

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27
Q

What are the two configurations of centre-surround antagonistic arrangement, and what LGN layers are they found in?

A

On centre/off surround, and off centre/on surround. Both found in both M and P layers.

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28
Q

What influence is stronger in LGN than in RGC receptive fields?

A

The inhibitory influence of surround - amplifies differences between neighbouring regions of the RGC receptive field.

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29
Q

What does the P vs. M RGC input to M and P layers of the LGN suggest?

A

A strong sub-division in visual function, for example in colour - P cells are colour sensitive, whereas M cells respond to all colours.

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30
Q

What forms the basis of colour opponency in P cells?

A

They respond best when a particular wavelength falls on their receptive field centre, e.g. red=strong excitatory response when green=response inhibition. This pairing of opposite colours forms the basis of colour opponency.

31
Q

How does acuity vary in (M and P) LGN cells?

A

Receptive field sizes vary in each layer, with the smallest devoted to the fovea.

32
Q

Where are the largest LGN cell receptive fields found?

A

The magnocellular layers - 2-3x larger.

33
Q

Where is the best spatial resolution in LGN cells?

A

Parvocellular layers.

34
Q

How does temporal sensitivity vary in M and P LGN cells?

A

Cells in the M layer respond vigorously to rapid light intensity fluctuation, P cells respond slowly - M cells are sensitive to motion and P cells are not.

35
Q

What are the functions of the LGN?

A
  1. Relays information between the retina and visual cortex.
  2. Receives input from the Reticular Activating System (brainstem) - modulation of response intensity depending on arousal.
  3. Receives feedback from visual cortex (feedback loop modulates signal quality).
  4. Segregates information in M and P systems for cortical processing.
36
Q

Where does the LGN project to?

A

The occipital cortex - V1, primary visual cortex, area 17, or striate cortex.

37
Q

How many cells are contained in each hemisphere’s V1?

A

About 100 million.

38
Q

How is V1 structured?

A

In layers - 1 at the top to 6 at the bottom.

39
Q

What layer of V1 do LGN cells connect with?

A

Layer 4 - M cells in the upper part (4Cα) and P cells in the lower (4Cß).

40
Q

Where are connections made from level 4 of V1?

A

Upper and lower layers.

41
Q

Where do K cells from the LGN connect to in V1?

A

Directly to layers 1-3.

42
Q

If a particular cell in layer 4 of V1 receives input from the right eye, where will cells above, below, and adjacant to it receive input from?

A

Above and below from the same eye, adjacent from the other eye.

43
Q

What are ocular dominance columns?

A

The pattern of alternate eyes driving all the cells within a penetration perpendicular to the surface of the brain.

44
Q

How is the retinotopic map maintained in the visual cortex?

A

Similarly to in the LGN (adjacent regions etc.)

45
Q

What is different about the retinotopic map in the visual cortex compared to the map in the LGN?

A

The distribution - 80% of all cortical cells are devoted to the central 10* of the visual field.

46
Q

What is cortical magnification?

A

The fact that different numbers of cells are devoted to different regions of visual space (i.e. 80% of all cortical cells are devoted to the central 10* of the visual field.)

47
Q

What does cortical magnification mirror?

A

The devotion of the vast majority of RGCs to the fovea - the bias is also present but smaller in the LGN.

48
Q

Why are more foveal receptive fields required to cover the same region of visual space?

A

Because foveal receptive fields are small relative to their peripheral counterparts.

49
Q

What similarities are there between the functional properties of cortical neurons and RGCs and geniculate cells from which they receive their input?

A

The presence of a retinotopic map, relative insensitivity to illumination levels, and responding well to abrupt luminance gradients (lines and bars).

50
Q

What differences are there between the functional properties of cortical neurons and RGCs and geniculate cells from which they receive their input?

A

Their selectivity to various dimensions: orientation, binocularity, size selectivity, colour, and direction of motion.

51
Q

How is the visual cortex more orientation selective than the retina and LGN?

A

The receptive fields are elongated rather than circular, and the ‘on’ and ‘off’ regions are differentially arranged so that a maximum response is obtained for a line of a particular orientation.

52
Q

How can orientation selectivity in the visual cortex be demonstrated?

A

Gratings of diagonal lines looked at above and below fixation line, when switch to straight lines they appear to be tilted in the opposite direction; following adaptation, sensitivity of active cells is reduced, changing the shape of population response to new stimuli. The shift in peak of distribution makes perceived orientation shift away from the adapting orientation.

53
Q

How are orientation columns organised?

A

Cells in the visual cortex with similar orientation preference are grouped together. Orientation preference changes gradually across the cortical surface. Iso-orientation patches are organised around orientation ‘pinwheels’.

54
Q

What are the two types of cortical cells?

A

Simple and complex.

55
Q

What are simple cells?

A

Phase sensitive cells which have their optimum response to an appropriately oriented stimulus placed in a certain position within the receptive field.

56
Q

What are complex cells?

A

Phase insensitive cells which have their optimum response to an appropriately oriented stimulus placed anywhere within the receptive field.

57
Q

What are hypercomplex cells?

A

A property that both simple and complex cells can have - optimum response also depends on contour length (max. when bar matches width of receptive field) = ‘end stopping’ or ‘length width’ inhibition.

58
Q

Are cells in layer 4 of V1 monocular or binocular?

A

Monocular - input from one eye only.

59
Q

Are the layers that layer 4 of V1 signals to monocular or binocular?

A

Binocular - can be driven by either eye.

60
Q

How is a visual stimulus response in binocular cortical cells different when the stimulus is delivered to both eyes?

A

The response is more vigorous.

61
Q

What are the receptive fields of binocular cortical cells, and how are they matched?

A

LE and RE, matched in type (simple or complex), preferred orientation, direction of motion, and region of visual space.

62
Q

Under what conditions do binocular cells respond maximally?

A

When corresponding regions in each eye are stimulated by stimuli of similar size and orientation.

63
Q

Where are colour sensitive cortical cells concentrated?

A

The cortical blobs, each of which is centred on an ocular dominance column.

64
Q

What is not mixed within a blob?

A

Cell opponency - either red/green or blue/yellow.

65
Q

Where do blobs receive their input from?

A

The lower subdivision of layer 4 - P layers.

66
Q

How are cells in blobs different from virtually all other cells in V1?

A

They show no preference for a particular orientation.

67
Q

What is direction selectivity?

A

The property of motion sensitive cells whereby they only respond to movement in one direction.

68
Q

What kinds of motion are detected by simple and complex cells?

A

Simple cells - slow motion

Complex cells - faster motion

69
Q

The visual cortex is composed of columns of cells. What do cells in a stack have in common?

A

Orientation and ocular dominance preference.

70
Q

What is a hypercolumn?

A

The collection of 18-20 adjacent columns which traverse a complete range of orientations and ocular dominance.

71
Q

What is the structural arrangement of the visual cortex into columns and hypercolumns referred to as?

A

The ‘ice cube’ model.

72
Q

What does each hypercolumn contain?

A

The requisite neural machinery to simultaneously analyse multiple attributes of an image (orientation, size, direction of motion, colour) falling on a localised region of the retina.

73
Q

To summarise, what does the striate cortex do?

A

Breaks down information from the retina into many different measurements. For each location there are 500x as many striate cells as RGCs, organised as hypercolumns. Neighbouring hypercolumns “look at” neighbouring regions of the retina.

C82COG - Perception (9 decks)

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