Visual+System

=**Visual System**=


 * 1. Recognize the retinal layers and name the main classes of neurons in the retina that are in each layer.**

The retina has 10 layers:

(1) Retinal Pigment Epithelium (2) Photoreceptor Layer (3) External Limiting Membrane (4) Outer Nuclear Layera (5) Outer Plexiform Layer (6) Inner Nuclear Layer (7) Inner Plexiform Layer (8) Ganglion Cell Layer (9) Retinal Axon Layer (10) Internal Limiting Membrane

The photoreceptors reside from the photoreceptor layer all the way to the outer plexiform layer. There, they synapse with second order neural cells such as bipolar, horizontal, and amacrine cells that extend to the inner plexiform layer. At that point, the second order cells synapse with third order ganglion cells that extend to the retinal axon layer. Glia-like support cells called Muller cells extend from the external limiting membrane to the internal limiting membrane.


 * 2. Explain the basics of how visual transduction occurs and how signal amplification results from the biochemical cascade following absorption of photons by visual pigments.**

Photopigment for rods includes a Retinal plus an opsin protein to make rhodopsin. Cones use photopigments formed from retinal plus one of 3 variant opsin proteins.

A photon absorbed by rhodopsin converts it from a cis form to a trans form. This activated trans-rhodopsin stimulates a G protein called transducin. Transducin activates a phosphodiesterase which breaks down cGMP.

Na+ channels on the cell membrane of rods are usually bound to cGMP which they require to remain opened and depolarize the rod. However, when the phosphodiesterase breaks down the cGMP, it causes the Na+ to close, leading to hyperpolarization of the rod.

This amplification cascade leads to a -1 mV hyperpolarization. The higher the intensity the light, the longer the G-protein coupled cascade keeps the Na+ channels closed and the longer the rods stay hyperpolarized. Sensitivity of the cones shift and adapt appropriately to the level of background light.


 * 3. Define the differences in the anatomical organization of peripheral and central retina and explain how these relate to the receptive field properties of the neurons in these regions and correspond to the "alerting" and "analysis" functions of the cells.**

Cones have a more tapered structure, allowing light to converge and funnel into a smaller area than rods, optimizing detection of fine detail. Rods are untapered for greater sensitivity.

Anatomically, cones are found at peak density at the fovea and fall off rapidly by about 5 degrees away. Rods are absent from the fovea and have a peak density outside of the fovea that gradually declines in the periphery. The fovea is specialized not only because of its high number of cones, but because the ten retinal layers that light usually must pass through before reaching the photoreceptors is reduced in the fovea/ this allows for the highest acuity and best color vision in the fovea. In contrast, highest sensitivity in dim light is about 20 degrees from where rod density is maximal.


 * 4. Describe the differences between the properties and central projections of the P (beta) and M (alpha) systems.**

M cells and P cells are ganglion cells in the retina. In general, M cells have greater dendritic spread than P cells. M cells are also broadly distributed but dense centrally while P cells are densest in the fovea. P cells have well developed chromatic coding receptive fields while M cells do not and instead have high contrast receptiveness.

//LGN – lateral geniculate nucleus//
 * **Attribute** || **M Cells** || **P Cells** ||
 * Percentage of all ganglion cells || <4% || ~60-80% ||
 * Distribution on retina || Dense centrally but broadly distributed || Densest in fovea ||
 * Conduction velocity || ~15 m/s || ~6 m/s ||
 * Central projections of ganglion cells || LGN: Magnocellular || LGN: Parvocellular ||
 * Central projections of LGN cells || V-I: Layer 4Calpha || VI: layer 4Cbeta ||
 * Chromatic oppenency (i.e. color coding receptive fields) || Almost none || Well developed with L vs. M and S vs. L/M types ||
 * Rod input || Yes || Little, if any ||
 * Contrast receptiveness || High || Low ||
 * Spatial resolution || Low || High, especially for fovea ||
 * Temporal resolution || High (>60 Hz) || Moderate (>30 Hz) ||

The main pathways from the retina to the Primary Visual Cortex are: M&P Cells> LGN-->PVC(V-I) for Color, form, motion, position and depth M& Other Cells> Superior Colliculus>Motor Pathways (Pulvinar, Pons, Spinal Cord) for Eye and head movements related to visual attention Each eye leads to one group of layers in the LGN on the ipsilateral side of the brain. Layers 1-2 of the LGN correspond to M-cell input, whereas layers 3-6 of the LGN correspond to P-cell input. The visuotopy is maintained into the primary visual cortex in area V-I 17, where the M cells correspond to 4c alpha layer, and the P cells correspond to 4c beta. In addition, the cortex is arranged into "Ocular Dominance Columns", which are really more like stripes, alternating with coded information from the left and right eyes. Also of note is the fact that the fibers from the nasal portion of the retina (peripheral field of vision) decussate at the optic chiasm to join the temporal fibers of the contralateral optic nerve. This portion of the field of vision that is transmitted in the decussating fibers is also known as the binocular field of vision.
 * 5. Characterize how the visual field is represented (i.e., the visuotopic map) in visual pathways and structures from the optic nerve to primary visual cortex.**

Scotomas are areas of visual field deficit due to lesions in the eye, retina or brain.
 * 6. Relate visual field deficits ("scotomas") to sites of damage of visual pathways or nuclei.**
 * Lesion || Deficit || Name ||
 * Transection of R. Optic Nerve || Right visual field scotoma || Right anopsia ||
 * Transection of Optic Chiasm || Loss of lateral visual field bilaterally (nasal retinal areas) || Hemianopsia ||
 * Lesion of R. Optic Tract || Loss of left field of vision bilaterally || Left homonymous hemianopsia ||
 * Lesion of Loop of Meyer || Upper part of visual field for both eyes lost || Homonymous upper quadrianopsia ||

Dorsal stream pathways lead into parietal cortex and carry information on image movement and spatial relationships. Ventral stream pathways lead into temporal cortical regions and carry information for color and form perception.
 * 7. Give examples of how cortical processing "streams" distill information regarding perceptions of color, shape, and motion of visual images.**

Binocularity applies to the decussation of nasal-side retinal sensory information onto the contralateral optic nerve and to the contralateral portion of the brain, to combine with informatrion from the contralateral eye. Ocular dominance indicates the visuotopic organization of cortical neurons where information from R and L eyes is kept separate in alternating stripes, or ocular dominance columns. Information from each side is kept separate through the LGN, until it reaches the PVC, where it is combined in cortical layers 4c alpha and beta.
 * 8. Describe the concepts of binocularity and ocular dominance as they apply to neurons in visual cortex. Identify the cortical areas and layers where information from both eyes is kept separate and where it is combined.**

Both eyes will image the same point within the binocular field, and the information will then be carried to the LGN, where it is maintained separately. After leaving the LGN, they will converge onto points in the PVC (Ocular Dominance Columns) that are organized into sublayers that collect information from M or P cells, organized by eye.
 * 9. Describe how binocular receptive fields are formed in primary visual cortex.**

--Not covered in lecture, will have to reference Purves p271.--
 * 10. Explain how binocular vision gives rise to distance perception via disparity coding by binocular cortical neurons.**

Ocular dominance columns have pinwheel-type arrays of orientation selective columns within them that contain groups of cells selective to stimuli in a specific orientation. The sensitivity of these orientation columns can be influenced postnatally by one's environment (see lecture notes regarding kittens raised in cages with vertical vs horizontal stripes). I can only assume from this evidence that the best treatment for infants wtih crossed eyes is to put them in striped cages for a few months at a time, then put electrodes in their brains.
 * 11. Describe the development of ocular dominance columns and relate consequences for brain development of ocular misalignment and treatment options for strabismus in infants.**

See table listed in question 6.
 * 12. Predict sites of visual pathway damage from patterns of vision loss (scotomas).**


 * 13. For each of the following eye movements, describe the sensory input required, movement characteristics, and contributions of cortical and subcortical control centers:**

__Vestibulo-ocular Response (VOR)__- This refers to the involuntary ocular muscle action that "rotates" the eye in response to balance changes. It involves the oculomotor and abducens and trochlear cranial nerves. Sensory input would come from information obtained via the CN VIII.

__Optokinetic Nystagmus (OKN)__- This term is used clinically to refer to movements of the eye slowly in one direction (slow phase) usually towards the lesioned side, followed by a quick correctional movement in the other direction. OKN specifically refers to the movements when tracking a stimulus across the visual field-- a tracking phase followed by brief "snapping back" movements, that persist for a short time even after the stimulus is removed. This is a typically normal response, unlike the general usage of the term 'nystagmus'. The direction of the nystagmus and the circumstances under which it occurs tends to indicate the lesion causing it.

__Saccades__- Saccades are similar to nystagmus, but instead of being a slow phase-fast phase, they are composed entirely of fast-phase movements, intended to aim images optimally at the fovea.

__Visual Pursuit-__A tracking movement involved with attending to visual stimuli, i.e. watching a ping pong game.