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Lecture 2 Primary Visual Cortex

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imissyou419's version from 2017-10-26 19:36

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Visual stimuli activates; left and above visual stimulus activatesprimary visual cortex (V1) centered on the calcarine sulcus; right side of brain and below the calcarine
Parvocellular ganglion cells - size, conducting speed, location, codes what, project whereSmall, slow conducting, primarily from the fovea, code the object's detail and project to LGN (thalamus)
Magnocellular ganglion cells - size, conducting speed, location, codes what, project whereLarge, fast conducting, primarily from the peripheral retina, codes where objects are and project to LGN (thalamus) and SC (brainstem)
Visual grasp reflex SC causes the eye and head to turn to location of a visual object that draws your attention i.e. someone waving their hand in your periphery. This in turn points the fovea at the object and foveal parvocellular ganglions can inform the cortex about the object's details
What happens in the LGN?2 eyes maintain their own separate representation in different layers. LGN sends information to visual cortex: what an object is (P cells), and where it is (M cells)
How does a ganglion cell connect to the correct place in the LGN?Ganglion cell axons grow to specific locations within LGN's 5 layers, neighboring cells grow to neighboring locations
1. eye develops a chemical gradient based on its location in the eye socket (e.g nasal vs. temporal),
2. ganglion cells are given a location identity by the position specific chemical gradient in the eye,
3. a similar gradient is set up in the LGN,
4. axons from the nasal retina are guided by this "scent" to the correct location in the contralateral LGN.
5. some time later, axons from temporal retina are guided to ipsilateral LGN; a similar process maps the LGN onto the visual cortex
What do we not know about the LGN?1. why there is so many layers, 2 P layers redundant
2. 80% of the input is not from retina but from the primary visual cortex and surrounding areas, what does it do?
3. receptive fields of LGN neurons same as those of ganglions so does not process visual info, why synapse?
Primary visual cortex namesV1, area 17, striated cortex (thick layer 4c produced by massive input from LGN)
What is V1 made of?thin sheet of folded grey matter at cortical surface - where neurons and their connections are so where memories are formed, 6 layers of grey matter; 1/2 of V1 represents details from the fovea. White matter below grey matter - nerve fibers interconnecting cells in grey matter
Layer 4c cells receptive fieldhave receptive fields that are the same as that of LGN and ganglion cells
Simple cells receptive fieldhave elongated receptive fields which make them maximally sensitive to a line or edge of a particular orientation at a particular location of the retina; several ganglion cells, whose receptive fields lie along a COMMON LINE, converge by way of LGN onto a simple cell
Complex cells receptive fieldhave receptive fields similar to simple cells except the line can lie over a larger area of the retina (positional invariance). Some are more sensitive to a moving stimulus; several simple cells of the SAME ORIENTATION converge onto a complex cell
End stopped complex cell / hyper-complex cell receptive fieldhave receptive fields similar to complex cells except they are maximally activated by lines of a particular length. The activity is less for both longer and shorter lines. Still other end stopped complex cells fire when a line ends in their receptive field
Cells in the inferior temporal lobe respond toparticular faces generalized over the whole of the retina
What happens to patients with damage to their fovea (either all ganglion cells damaged or cortical area that represent it is damaged)?they do not see the face but see stripes (background) where the face should be; end stopped complex cells outside of blind spot become activated by the end of lines. The firing of these cells elicit the percept of a line even within the blind spot. Your visual system fills the blind spot. This filling in property of the brain also explains why damage to peripheral retina in visual cortex often goes unnoticed
How does the visual cortex improve vision?ganglion cells see dots (acuity of the eye). Simple cells in the cortex sees lines (acuity of eye + visual cortex). The visual cortex improves acuity by analyzing line segments that encompasses many ganglion cells, called hyperacuity
Why are there many more cells in the visual cortex than the LGN?1 cell in the LGN shares its information with many simple cells in the visual cortex by grouping different LGN neurons in simple cells, get a variety of orientations
V1 has what kind of organization?Hypercolumn (analyzes information from 1 small region of the retina, extracts stereopsis, color, orientation of line segments) and retinotropic (forms a map of the eye in our brain, with adjacent areas of the eye mapped to adjacent hypercolumns in the brain; map is distorted with fovea having very large representation - as much columns devoted to fovea as there are to rest of retina)
Input from the left and right eye enter at what layer?via the LGN enter at layer 4c
As you go higher or lower in layers from the 4c, what do you find?At 4c you have monocular cells with circular surround receptive fields. As you go higher or lower you find binocular cells, first simple, then complex with elongated receptive fields
StereopsisIn each half, one or the other eye dominants. One see in stereo by combining info from the 2 eyes in binocular cells located above and below 4c. Binocular cells detect disparity and gives the illusion of depth
ColorIn the centre of each cube you can find a column, called a blob, running through all 6 layers except for layer 4. The blob contain color sensitive double opponent cells with circular surround receptive fields. Thus each hypercolumn contains 2 blobs; 1 right eye dominant, 1 left eye dominant. Make up only 10% of cells in column
Orientation of the line segmentsRadiating from the blobs, simple and complex cells (in layer above and below 4c) ordered into pinwheels of the same orientation. These cells are edge sensitive but not color sensitive. Pinwheel arrangement allows cells with similar orientation sensitivities to be grouped together.
Lesion of the corpus callosumloss of stereo vision
Deprivation amblyopiaIn newborns, each eye competes for representation in V1. If vision is impaired in 1 eye, the impaired eye loses its connections and the good eye collaterals take over cortical representation normally occupied by the impaired eye. Result in permanent cortical blindness in the impaired eye, persists even if the eye regains normal function. A cataract as adult has little effect b/c V1 simple and complex cell above and below layer 4c binocular organization already completed. This early sensitivity in infants to thalamic competition from each eye for cortical representation is called critical period. Kittens raised in dark suggest equal deprivation keeps critical period dormant. The visual cortex has other forms of plasticity, such as learning and recovery from damages
Synaptic plasticity1. Synchronous activation causes a strong depolarization.
2. NMDA receptor is activated allowing Ca2+ to enter the cell.
3. Postsynaptic nerve growth factor is released and taken up only by recently activated presynaptic terminals.
4. These particular terminals enlarge at the expense of others
Synapses are strengthened with synchronous activation and weakened with asynchronous activation
Postsynaptic changes take time
Influx of Ca2+ triggers a cascade of molecular processes some rapid but not long lasting, others take days and are semi-permanent so you can remember what you saw
Strabismusall neurons become monocular because binocular cells are never activated simultaneously by the same stimulus. Child loses stereoposis. Causes double vision, the image from 1 eye may be suppressed -> amblyopia;
Normally, simple cells are simultaneously activated by the same optimal line orientation in both eyes - a corresponding line of receptors on the retina of the 2 eyes must be wired to the SAME SIMPLE CELL in the cortex
At what level of the visual pathway is binocular disparity first analyzed?input from the 2 eyes first converge onto cells in V1 (above and below layer 4c) where about 70% are binocularly driven. Each cell is only activated by a particular retinal disparity. Objects farther away project to the medial side of the retina. Objects that are close project to the temporal side of each retina. When the images of the 2 eyes are combined, the direction of the disparity activates different V1 cells.
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