Visual System 2: Central Visual System Flashcards
what is seeing?
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.
Where does the optic nerve project? (5 places)
_ superior colliculus _ pretectal complex _ accessory optic nuclei _ suprachiasmatic nucleus _ lateral geniculate nucleus
what does superior colliculus do?
generates orienting head and eye movements. Also gets extensive inputs from the visual and other cortices. Mostly M-cell input from the retina.
what do pretectal nuclei do?
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
how are R & L visual fields represented in the brain?
R half of the body is represented in the L cerebral hemisphere, the R visual field is represented in the L hemisphere
How are visual representations achieved in the brain?
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
how much of visual field is binocular?
120 degrees - each eye sees 60 degrees into opposite field
how is the lateral geniculate nucleus organized?
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).
what are the two majorlayer types in LGN?
magnocellular layer - parisol cells
parvocellular layer - midget cells
describe the layers in LGN
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.
which layers in LGN are from ipsilateral eye?
2,3,5
how is the LGN organized relative to the retina?
LGN exhibits a retinotopic organization.
adjacent points on the retina are represented by adjacent points in the LGN
what is an important feature of the retinotopic map in LGN?
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.
describe the LGN RF
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.
describe the mechanism (2 parts) supporting the thalamic gate function?
(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.
what is the ratio of non-retinal to retinal inputs in the LGN?
nonretinal inputs outnumber the retinal inputs (in terms of synapse counts) by about 10 to 1.
Striate cortex
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.
what is represented on the lower bank of the calcarine fissure?
upper visual fields
where is the fova represented in the occipital lobe?
occipital pole
describe the M pathway in the cortex
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).
where is the first binocular cells in the visual pathway
IVB
where in cortext do M and P cells send collaterals?
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.
describe the P pathway in the cortex
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.
describe cortical receptive fields
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.
what are the features of the modular organization of the cortex?
orientation selectivity
direction selectivity
ocular dominance
color
describe ocular dominance in V1
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.
describe the color modular organization in cortex
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.
in what layer are the CO blobs?
II & III
in what layer are the OD columns?
IVC
what is the relationship between the CO blobs and OD columns?
The CO blobs are centered on OD bands.
Color information and eye dominance are represented in an overlapping, coordinated set of cortical modules.
describe orientation modules in the cortex
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
how are orientation modules demonstrated in brain?
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.
describe (summary) overall modular organizatin of V1
_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
what is the function of lateral connectivity in V1
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.
features of lateral connectivity in V1
(1) excitatory
(2) like orientations
(3) Between columns whose receptive Fields are non-overlapping in space
describe how kaniza triangle works
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
what is strange about schizoprenia and visual discrimination
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.
main points of LGN organization in Retino-geniculo-striate Pathway
Retinotopocally organized
Simple map but with central magnification
M & P cells in separate layers
Ipsi- & contralateral eyes in separate layers
Gating by extraretinal inputs
main points of cortex organization in Retino-geniculo-striate Pathway
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
