Visual System 2: Central Visual System

Question 1

what is seeing?

Answer 1

Seeing can be defined as using optical input to the two eyes to know what things are, where they are, and to guide action based on this knowledge.

Question 2

Where does the optic nerve project? (5 places)

Answer 2

_ superior colliculus
_ pretectal complex
_ accessory optic nuclei
_ suprachiasmatic nucleus
_ lateral geniculate nucleus

Question 3

what does superior colliculus do?

Answer 3

generates orienting head and eye movements. Also gets extensive inputs from the visual and other cortices. Mostly M-cell input from the retina.

Question 4

what do pretectal nuclei do?

Answer 4

primarily involved in mediating behavioral responses to acute changes in ambient light such as the pupillary light reflex, the optokinetic reflex, the accommodation reflex, and temporary changes to the circadian rhythm

Question 5

how are R & L visual fields represented in the brain?

Answer 5

R half of the body is represented in the L cerebral hemisphere, the R visual field is represented in the L hemisphere

Question 6

How are visual representations achieved in the brain?

Answer 6

axons from RGCs whose cell bodies lie in the nasal hemiretina cross at the chiasm whereas the axons from RGCs whose cell bodies lie in the temporal hemiretina do not cross in the chiasm

Question 7

how much of visual field is binocular?

Answer 7

120 degrees - each eye sees 60 degrees into opposite field

Question 8

how is the lateral geniculate nucleus organized?

Answer 8

The LGN has 6 obvious cellular layers, each many cells thick (the layers are just visible to the naked eye). The layer are numbered from 1 to 6 from inside to outside.
You can see that layers 1 and 2 have larger cells (magnocellular layers) than layers 3-6 (parvocellular layers).

Question 9

what are the two majorlayer types in LGN?

Answer 9

magnocellular layer - parisol cells
parvocellular layer - midget cells

Question 10

describe the layers in LGN

Answer 10

First, the eyes are kept separate: The contralateral eye terminates in layers 1, 4, and 6. The ipsilateral eye terminates in layers 2, 3, and 5. There are no binocular cells in the LGN no cells receiving input from both eyes.

Second, midget and parasol RGC axons are kept separate: midget cells terminate in the Parvocellular layers only (3,4,5,6, and hence midget RGCs are called P-cells) and the parasol RGCs terminate in the Magnocellular layers only (1,2, hence parasol cells are called M-cells).

Third, within the parvocellular layers only, ON-center and OFF-center cells are segregated.

Question 11

which layers in LGN are from ipsilateral eye?

Answer 11

2,3,5

Question 12

how is the LGN organized relative to the retina?

Answer 12

LGN exhibits a retinotopic organization.

adjacent points on the retina are represented by adjacent points in the LGN

Question 13

what is an important feature of the retinotopic map in LGN?

Answer 13

it is distorted - the representation of the central 5 degrees of the visual field (0.3% of the visual field) occupies about 25% of the space in the LGN.

Question 14

describe the LGN RF

Answer 14

LGN receptive fields: same as in retina
M cells (10%)_ large, motion sensitive, no color selectivity
P cells (80%) _ small, center-surround, color selective

In the foveal part of the map, the connectivity is one-to- one in primates.

Question 15

describe the mechanism (2 parts) supporting the thalamic gate function?

Answer 15

(1) The presence in the membrane of thalamic cells of special voltage-sensitive Ca++ channels.
(2) The presence of numerous nonretinal inputs whose main function appears to be the control of the resting membrane potential of the thalamic cells.

Question 16

what is the ratio of non-retinal to retinal inputs in the LGN?

Answer 16

nonretinal inputs outnumber the retinal inputs (in terms of synapse counts) by about 10 to 1.

Question 17

Striate cortex

Answer 17

one of the largest functional areas in the cerebral cortex, on the upper and lower banks of the calcarine fissure.

AKA V1, area 17 (Brodmann), primary visual cortex, striate cortex.

Question 18

what is represented on the lower bank of the calcarine fissure?

Answer 18

upper visual fields

Question 19

where is the fova represented in the occipital lobe?

Answer 19

occipital pole

Question 20

describe the M pathway in the cortex

Answer 20

The M pathway in cortex: neurons in IVC alpha send their axons up a very short distance where they spread out and terminate on neurons in layer IVB. Cells in IVB send their axons out of the cortex to two extrastriate areas of cortex, areas V2 (Brodmann_s area 18) and MT (not an area recognized by Brodmann).

Question 21

where is the first binocular cells in the visual pathway

Answer 21

IVB

Question 22

where in cortext do M and P cells send collaterals?

Answer 22

layers V and VI which are the origins of (a) major pathways to the pontine nuclei and the superior colliculus from layer V and (b) major feedback pathway to the LGN from layer 6.

Question 23

describe the P pathway in the cortex

Answer 23

neurons in IVC_ send their axons up into layers II and III. These cells in layer II-III in turn send their axons out to two extrastriate cortical areas _ area V4 (not a Brodmann area) and area V2.

Question 24

describe cortical receptive fields

Answer 24

Single M-cells in layer IVC_ send their axons up into layer IVB where they innervate many neurons. Similarly, single cells in layer IVB receive input from many M-cells in layer IVC_. The projection from IVC_ to IVB is one-to-many and many-to-one. Inputs to single cells in IVB are a mixture of ON- and OFF-center cells. Also, the RFs of cells in IVB are much larger than those in IVC.

Question 26

what are the features of the modular organization of the cortex?

Answer 26

orientation selectivity
direction selectivity
ocular dominance
color

Question 27

describe ocular dominance in V1

Answer 27

In layer IVC there are non-overlapping alternating bands of cells completely dominated by one eye (still monocular input)

IN the layers above and below, cells receive stronger input from one eye or the other but are dominated by the eye providing input to layer IVC in that band.

OD columns (or bands) have approximately constant dimensions over all of V1.

Question 28

describe the color modular organization in cortex

Answer 28

In sections cut parallel to the cortical surface, cytochrome oxidase stains reveal regularly spaced blobs (groups of cells that massively concentrate the CO stain) throughout layers II and III.

Cells in the CO blobs have center-surround RFs, are color coded, and are part of the P-pathway.

Question 29

in what layer are the CO blobs?

Answer 29

II & III

Question 30

in what layer are the OD columns?

Answer 30

IVC

Question 31

what is the relationship between the CO blobs and OD columns?

Answer 31

The CO blobs are centered on OD bands.
Color information and eye dominance are represented in an overlapping, coordinated set of cortical modules.

Question 32

describe orientation modules in the cortex

Answer 32

pinwheels

The structure is more complex than that for ocular dominance, but far from random as well. It is characterized by _pinwheels _ wherein the optimal orientation of the underlying cells changes systematically around the clock, each pinwheel surrounding a tiny singularity where all orientations are effective. This module is repeated over and over again across the retinotopically organized extent of V1

Question 33

how are orientation modules demonstrated in brain?

Answer 33

imaging of the cortex using voltage sensitive dyes (VSDs).
VSDs incorporate themselves into the membranes of cells in cortex. They are fluorescent dyes but the amount of fluorescence depends on the membrane potential.

Question 34

describe (summary) overall modular organizatin of V1

Answer 34

_alternate band of eye dominance _ this is absolute in layer IVC, cells are driven by the R or L eyes but never both
_CO blobs above (also below but function less clear) which are always exactly centered on OD bands _ color coded centersurround RFs
_Orientation pinwheels usually (but not always???) centered on the CO blobs

Question 35

what is the function of lateral connectivity in V1

Answer 35

in addition to extracting features of the scene at each locus in space (orientation etc_), is also in some way comparing the features extracted over adjacent and even distant parts of the scene.

Question 36

features of lateral connectivity in V1

Answer 36

(1) excitatory
(2) like orientations
(3) Between columns whose receptive Fields are non-overlapping in space

Question 37

describe how kaniza triangle works

Answer 37

there are cortical cells of the same orientation with RFs that will be driven to high firing rates by the features (edges) of the cutouts in the pac men.
The cortical cells in between the pac men will be activated by lateral pathways from the cells above as if there really is a triangle

Question 38

what is strange about schizoprenia and visual discrimination

Answer 38

Normal subjects exhibit powerful theta activity during presentation of the illusory square but not during the identical but rotated pac men. Schizophrenic subjects lack the theta response to the illusory square.

Question 39

main points of LGN organization in Retino-geniculo-striate Pathway

Answer 39

Retinotopocally organized
Simple map but with central magnification
M & P cells in separate layers
Ipsi- & contralateral eyes in separate layers
Gating by extraretinal inputs

Question 40

main points of cortex organization in Retino-geniculo-striate Pathway

Answer 40

Retinotopically organized
LGN axons terminate in layer IVC with M and P inputs and LE/RE inputs separate
Modular: orientation, ocular dominance, color
Binocular cells (stereopsis) _ layer IVb
Diverse outputs: superior colliculus, pontine nuclei, LGN, other cortical areas
M and P largely separate all the way through
Long range connections modify responses elicited from the receptive field