Visual System

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Visual System

Many Parts are not included in the Slide 2 there is quite a lot more to the visual system than visible on this slide

Nystagmus controls Reflexes turning eyes in Opposite Direction of eye movement

Our Visual Field overlaps, the nose hides some parts of the visual field. Center of the Retina Fovea Fibers from the Eyes Cross in the Chiasmata Opticum and are thansferred to the other hemisphere, each hemisphere seesy info from both eyes, bot visual field representation is represented in different Parts of the brain. contralateral

most of the visual cortex is located in the calcarine sulcus The bottom part gets represented on top, while the top part is represented in bottom (just the position in the calcarine sulcus)

The eye

Retina is part of the central nervous system, ther fove is teh place we use to see stuff in detail an there is also the blind spot, the place where the otic nrve leaves the eye. You don't notice your blind spot, because there are mechanisms that specifically interpolate the blind spot. If you have retinal lesions you will not be aware of it, because the brain interpolates. the people are often not aware of their holes in their visual fields (a so called scotoma) they don't see black where they should see soumething, you just don't see it

Photoreceptor asre made out of rods and cones, the rods are sensitive and don't see colour, the cones are the ounes that see colors. At angle 0° (Fovea) the concentration of cones is very high, but reduces drastically in the periphery and get bigger. In the Fovea there are no rods, but the concentration is also high close to the fovea an decreases to the periphery.

rods are small and densely packed

Each of the photoreceptors ahs a wavelenght where they react the best. Cones

  • 437nm - s -b
  • 533nm - m -g
  • 564nm - l -r
  • Rods: 498nm

these wavelenghts are pretty much the same for everybody, they dont vary from human to human, there is very little variation, the genes that code these pigments arre on the x chromosome, thats why women have much less color vision deficit. there is not much variability in wavelenght, some women are tetrachromats (mutation in pigments) these women see differences in colours we percieve as the same. RG-default either missing M or L cones. Most cases of colourblindeness are because of receptors, only very little peaople have colorvisoon problems due to lesions in their brains.

having three different types is what allows us to see different wavelengths as colours, colour visio is a very complex phenomenon Rods change their response with different wavelenghts The brain receives the signal, since the rods react more or less the brain does not know ehter the light gets brighter or the wavelenght changes, so the brain doesn't bother, it just gives you intensity infomration. We don't know why we see red the way we do, we have no proof that the internal experience of your red is the same as my red.

Humans have trichromatic vision (we have three different conetypes) we can replicate all the colours we can see with just 3 basic colours

Retina

  • Rods & Cones
  • Bipolar & horisuonral cells
  • amacrine cells
  • gangliion cells (output nerves)
  • Optic nerves

Only the ganglion cells produce actin potentioal in the retina and have an axon, electrical synapses (gap junctions) Photoreceptors are at the back of the eye, all the rest is in front of the eye, these are all transparent cells the photoreceptors have a high metabolic demand, they are close to the blood vessels, so they can get enough nutrints

Basic retinal circuitry

Many different cell types which are in a close network and process the information It's a compley neural network with a lot of procesing

Visual Pathways

LGN

LGN Lateral genicualate Nucleus Striata, its layered in 6 Layers Each cell will receive signals from either the left or right eye, each layer will receive signals from one eye the top will always be contralateral. each layer responds to one eye.

Receptive Fields

Receptive fields of Ganglion an LGN cells are very similar to one another, All the neurons only "see" part of the whole images it recieves signals from some photoreceptors. each ganglion cell has a small visual field. The part of the viual field where a changing ligth can change response is called receptive fields. Circular region made up of two subregions, center and surround one region reacts if there is an increase in light and one in decrease, the on and off reasons.

there are some cells which have different responses to different colors there are red/green cells and blue/yellow cells (different color combinations don't exist) many cells sont care about colour (only the P cells, not the m -cells)

Parvo cell layers (P cells small ones) do care about colours top 4 Layers of LGN are mostly P calles bottom layers dont care about colours they are Magno Cells (MCells) big ones The signals for short wavelenght response (blue) are very small cells and located in between the layers Konio cells (very small) The Layers are separated So where does the yello come from? Mixture from M and L cones

Extrastriate cortical areas

If you flatten V1 you get many differnet areas almost half of the cortex is otimized and part of our visual system (about 40 regions that have to do with vidion)

Cortical represenation

Wheel with alternating black an white Dts, to the right you can see which parts of the Cortex are responding to the stimulus Th ewhole thing is very orderly organized on the cortex, the organisation is retinotopic, its faithfully organized V1 is retinotropically organized.

Hubel and Wiesel

SIgnals that arrive arrive at layer 4 If you flaten cortex and cut it at layer 4. When the signals arrive in L4 the signals are separated, and form bands of response the signal is segregated. If you move away from Layer 4 the cells get binocular Hubel and wiesel dicovereted that the regions are binocular but are dominated by one eye. (more driven by one eye than the other) Most cells are binocular but dominated by one eye.

Siehe blatt - Ocular dominance Columns

Simple Cells

Receptive Fields have elongated Subregions - orientation specific - simple cells These cells are orientation selected (Hubel & Wiesel got Nobel Prize for this discovery) You can recognize objects by analyzing the differnet lines and edges and code for orientation.

Additionally the found out that if you take a Micro Electrode in the Brain and move in at a 45fi angle), you can measure the preferred orientation of the Neuron (0 - Surface) each Dot represents a Neuron for which they marked the orientation The cells are again very nicely ordered which orientatio they prefer. If you would move the electode perpendicular, you would find that all the Points prefer the same orientation. The whole column prefers the same orientation, the so called orientation columns.

Ice cube Model of Hubel and Wiesel

It's divided inti Right/Left Eye columns and within each of this columns you have perpendicular to it the orientation columns. This is a Model proposed by Hobel and Wiesel.

I Reality it's not quite as nice but much more messy than they pRoposed, Folie Orientierung und Okular Dominance Säule Gray - orientation colums, black Left/right eye colums. Rougly speaking they where rigth its' just not an ice cube, but its not that bad.

Orientation columsn come togeter in pinwheel regions they are in the center of ocular dominance colums

It's a coincidence that LGN and v1 have 6 Layers each, thea are NOT comparable