Vision L4

Question 1

is the optic nerve a bottle neck?

Answer 1

yep

Question 2

The primary projection from the retina is to the ….

Answer 2

The primary projection from the retina is to the lateral geniculate nucleus (LGN) in the thalamus

Question 3

A minority (~10%) of ganglion cells project to other areas including …

Answer 3

A minority (~10%) of ganglion cells project to other areas including the pretectum (for pupillary responses), the suprachiasmatic nucleus (circadian rhythms) and the superior colliculus (involved in eye movements).

Question 4

V1 is largely buried in the medial aspect of the hemispheres, in the ____ sulcus.

Answer 4

V1 is largely buried in the medial aspect of the hemispheres, in the calcarine sulcus.

Question 5

describe how the retina is represented in V1?

Answer 5

Throughout this projection an orderly point-for-point (retinotopic) representation of the retina is maintained.

Question 6

which part of the retina is over represented in the V1

Answer 6

fovea

Question 7

dewscribe LGN strucuter

Answer 7

The LGN consists of 4 parvocellular layers (receiving input from P ganglion cells) and 2 magnocellular layers (receiving input from M ganglion cells).

Question 8

T ro F

Each layer receives input from only one eye and from only one of the two classes of ganglion cells.

Answer 8

T

Question 9

The koniocellular layers containing. what ? what do they do?

Answer 9

The koniocellular layers containing very small cell bodies are located between layers 1-6 and receive specific input from ganglion cells serving blue cones.

Question 10

The LGN is often considered as a relay station

how so?

Answer 10

The LGN is often considered as a relay station: ganglion cell axons make direct excitatory connections with LGN cells which then project directly to the cortex.

However, there are also local interneurons and about 30% of the synaptic input to the LGN comes from feedback pathways from the cortex.

Question 11

LGN diagram

Answer 11

Question 12

V1 is ____ matter?

Answer 12

grey

Question 13

how thick is grey matter of V1?

Answer 13

2mm

Question 14

how many layers is V1?

Answer 14

6

Question 15

label LGN

Answer 15

Question 16

Fibres from the LGN end primarily on s……

Answer 16

Fibres from the LGN end primarily on spiny stellate neurons in layer 4.

Question 17

Outputs to higher visual areas from v1 come from…. which layes?

Answer 17

Outputs to higher visual areas come primarily from pyramidal neurons in layers 2 & 3.

Question 18

V1 outputs:

Projections back to the thalamus exit from layer _, whilst cells in layer - project to deep brain structures such as the superior colliculus.

Answer 18

Projections back to the thalamus exit from layer 6, whilst cells in layer 5 project to deep brain structures such as the superior colliculus.

Question 19

LGN neurones recepetive fields?

Answer 19

The receptive fields (RFs) of LGN neurons are similar to those of the ganglion cells with a centre-surround organization

Question 20

L1 receptive fields?

Answer 20

majority of cells in V1 respond optimally to bars or edges with a particular orientation. They are therefore said to be orientation tuned.

Question 21

2 main lcasses of V1 cells?

Answer 21

Two broad classes are recognised: simple cells and complex cells.

Question 22

describe simple cells:

Answer 22

Simple cells respond only to an edge of a particular orientation in a very well-defined position. The receptive field has an inhibitory flank on one or both sides, and with diffuse illumination excitation and inhibition cancel. Simple cells are commonest in input areas (layers 4, also 6).

Question 23

describe complex cells?

Answer 23

Complex cells (75% of cells) also respond best to oriented bars or edges, but position is not so critical. They respond hardly at all to spots or stationary patterns, and best to moving edges over a somewhat larger visual field.

They are often directionally selective and are found mainly in layers 2,3 & 5.

Question 24

complex neurone sV1 image

Answer 24

Question 25

what are orientation columns

Answer 25

Question 26

whata re disparity detectors?

Answer 26

But some respond instead to stimuli located at slightly different positions on the two retinae, to function as disparity detectors for detecting objects nearer or further than the plane of fixation. They are used in stereopsis, the binocular judgement of relative depth. This is especially important when judging the shape of complex objects.

Question 27

what do the blobs in V1 contain?

Answer 27

cytochrome oxidase

respond to colour

Question 28

whats a hyper column

Answer 28

We can thus imagine a 1-2 mm2 region of cortex, known as a hypercolumn, representing a small region of visual space, in which all orientations are represented, and which contains both left and right eye ocular dominance columns each with a blob for colour analysis. It therefore contains all the cortical machinery necessary to analyse this small region of the image. Within each hypercolumn, receptive fields also vary in size, typically with the finest detail in 4C and layer 3, with larger fields in layers 5 & 6. In other words, each area of visual space is simultaneously analysed in V1 by ~105-106 neurons with respect to contrast, orientation, length of line elements, direction and movement, spacial ‘grain’ and colour

Question 29

describe the development of occular dominance columns, and how this can be important clinically?

Answer 29

Ocular dominance columns and orientation tuning are established during a critical period of neonatal development (3-5 weeks in kittens, 1-2 years in humans). In monocularly deprived kittens, cells in the primary visual cortex come to be dominated by input from the unoccluded eye. Alternatively, if raised in an environment containing only vertical stripes, most cells are tuned to vertical bars. These cortical “wiring defects” cannot be corrected later in life.

There are some obvious clinical implications of these results: a visual problem in early infant life such as an uncorrected squint, astigmatism or wearing an eye-patch can lead to a permanent defect in cortical function known as amblyopia. Such problems affect up to 5% of the population.

Question 30

Higher-order visual processing is often simplified to what?

Answer 30

These are often simplified into two major streams: the dorsal “where” pathway which is dominated by cells of the magnocellular pathway, and the ventral “what” pathway which has mainly parvocellular input.

Question 31

Answer 31

Question 32

The first visual assocation area, V2, directly adjoins V1. With cytochrome oxidase staining V2 is subdivided into ….

Answer 32

The first visual assocation area, V2, directly adjoins V1. With cytochrome oxidase staining V2 is subdivided into thick and thin dark stripes and pale interstripes.

Question 33

describe teh ventral what pathway

Answer 33

Area V4 receives its input from the thin stripes and interstripes within V2. Within this stream there seem to be two parallel systems: a parvo - blob - thin stripe - V4 stream which is particularly concerned with colour and a parvo/magno - interblob - interstripe - V4 stream concerned with edges and analysis of fine form.

Question 34

lesions in V4 lead to?

Answer 34

Lesions in V4 lead to cortical colour blindness (achromatopsia).

The final destinations of this parvo-magno stream include the inferotemporal (IT) cortex where some of the most highly sophisticated cells have been found, responding for example to faces and hands. Lesions in this area can result in the inability to recognise faces (prosopagnosia).

Question 35

Answer 35

Question 36

describe the dorsal where pathwy

Answer 36

The magnocellular input into V1 projects primarily only from 4Cα to layer 4B from where it projects to the thick stripes of V2.

Cells here project on to V5 & V5a (also known as MT & MST) and thence to posterior parietal cortex.

All levels within this pathway are dominated by rapidly responding, movement detecting cells with little colour preference.

The cells are also responsive to binocular depth cues.

Cells in MST show sophisticated response selectivity, responding to motion illusions and flow- fields (whole field patterns of movement normally generated by the motion of the eyes, head or body through space).

Some of the output destinations of MT include brain stem nuclei controlling eye movements. A lesion in MT may lead to the inability to see movement.

Question 37

Lesion in Mt leads to what?

Answer 37

inability to see movement.

Question 38

fat

Answer 38

mamba