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PHYSIOLOGY OF VISION II - 49

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Terms

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What is the visual system?
It is the part of the nervous system which allows organisms to see. It interprets the information from visible light to build a representation of the world surrounding the body. Note that different species are be able to see different part of the light spectrum; for example, some can see into the ultraviolet, while others can see into the infrared.
Give a synopsis of the eye in terms of the visual system?

The eye is a complex biological device. The functioning of a CCD camera makes an apt metaphor for the workings of the eye, which takes visible light and converts it into a stream of information that can be transmitted via nerves.
Light entering the eye is refracted as it passes through the cornea. It then passes through the pupil (controlled by the iris) and is further refracted by the lens. The lens inverts the light and projects an image onto the retina.

The retina consists of a large number of photoreceptor cells which contain a particular protein molecule: the photopigment called rhodopsin. When rhodopsin is struck by a photon (a particle of light) it transmits a signal to the cell; the more photons strike the cell, the stronger the signal will be. In some animals, like humans, cone cells contain cone opsin molecules attuned to specific wavelengths of light; i.e., a blue cone cell contains opsin most attuned to blue-wavelength light and will most strongly be stimulated by blue-wavelength light, while a yellow-red cone cell will only be weakly stimulated by blue-wavelength light. This gives the ability to distinguish color.
What are 3 Parallel Visual Paths that originate in the retina?
1. Day vs. Night vision
2. Form vs. Spatial
3. Stereopsis by binocular
disparity
What are the 2 types of vision under the Day vs. Night vision retina parallel pathway?

1. Scotopic vision:
a. Retinal rods
b. Dim light
c. “Color blind”
d. Low acuity
e. Non-foveal, peripheral vision

2. Photopic vision:
a. Retinal cones
b. Bright light
c. Trichromatic
d. High acuity
e. Foveal, central vision

NOTES:

1. Scotopic vision is the monochromatic vision of the eye in dim light. Since cone cells are nonfunctional in low light, scotopic vision is produced exclusively through rod cells.

2. Photopic vision is the vision of the light-adapted eye; in many animals, color vision, mediated by cone cells.
What are the 2 categories in the Form vs. Spatial retina parallel pathway?

1. “What is it?”
a. Parvo-cellular path
b. High acuity
c. Color rich
d. Inferior temporal cortex

2. “Where is it?”
a. Magno-cellular path
b. High sensitivity to motion
c. “Color blind”
d. Posterior parietal cortex




NOTES:

1. Parvocellular parts (also called P-cells), are slow-conducting neurons; transmitting information about colour vision, texture, pattern, and visual acuity. The cells transmit the information to the lateral geniculate nucleus, the area of the brain responsible for analyzing and interpreting the information. Parvocellular cells have small cell bodies, use a relatively long time to process information, and are part of a visual processing system that tells the brain what something is. This system operates more slowly and with lots of information about details. For example, these cells carry color information while magnocellular cells do not. Parvocellular cells are found in layers 3, 4, 5 and 6.

P Cells are the retinal ganglion cells that project their axons to the parvocellular layers of the LGN.

2. Magnocellular parts, also called M cells, are cells in the brain concerned primarily with visual perception. In particular these cells are responsible for resolving motion and coarse outlines. The cells have large, fast-conducting neurons and transmit information to the lateral geniculate nucleus, the area of the brain responsible for analyzing and interpreting the information. This system of cells operates with great speed at the expense of detail. Magnocellular cells have large cell bodies, use a relatively short time to process information, and are part of a visual processing system that tells the brain where something is. This system operates quickly but without much detail. They are found in layers 1 and 2 of the LGN, those layers more ventrally located which are next to the incoming optic tract fibers.

M Cells are the retinal ganglion cells that project their axons to the magnocellular layers of the LGN.
Describe the Stereopsis by binocular disparity retina parallel pathway?
1. Cortical specialization where signals from both eyes are finally combined

NOTES:

Stereopsis is the process in visual perception leading to perception of the depth or distance of objects. It comes from two Greek roots, stereo meaning solidity, and opsis meaning vision or sight. That means it could refer to any sort of visual depth perception, but since about the 1960s it has come to refer to depth perception from binocular vision, requiring two eyes. Prior to then, it was often referred to as "binocular stereopsis". Because each eye views the visual world from slightly different horizontal positions, each eye's image differs from the other. Objects at different distances from the eyes project images in the two eyes that differ in their horizontal positions, giving the depth cue of horizontal disparity, also known as retinal disparity and as binocular disparity.
What are the 6 structures involved in the central visual pathway?

1. Optic nerve
2. Optic chiasm
3. Optic tract
4. Accesory optic system
5. Lateral Geniculate Nucleus
(LGN)
6. Optic radiations
Describe the role of the OPTIC NERVE in the central visual pathway?

OPTIC NERVE:
1. Axons of retinal ganglion cells
2. Both visual hemifields; ipsilateral eye

NOTE:
The optic nerve is the nerve that transmits visual information from the retina to the brain. Following some rudimentary processing (mostly involving color boundaries), the information about the image received by the eye is transmitted to the brain via the optic nerve. In humans, the optic nerve is the only sensory system that is connected directly to the brain and does not connect through the medulla, due to the necessity of processing the complex visual information quickly. The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during embryonic development. Optic nerve damage produces irreversible blindness. The fibers from the retina run along the optic nerve to nine primary visual nuclei in the brain, from whence a major relay inputs into the primary visual cortex. The optic nerve is composed of retinal ganglion cell axons and support cells. It leaves the orbit (eye) via the optic canal, running postero-medially towards the optic chiasm where there is a partial decussation (crossing) of fibers from the temporal visual fields of both eyes. Most of the axons of the optic nerve terminate in the lateral geniculate nucleus from where information is relayed to the visual cortex. Damage to the optic nerve typically causes permanent and potentially severe loss of vision, as well as an abnormal pupillary reflex, which is diagnostically important. The type of visual field loss will depend on which portions of the optic nerve were damaged. Generally speaking, damage before the optic chiasm causes loss of vision in the visual field of the same side only.
Describe the role of the OPTIC CHIASM in the central visual pathway?

OPTIC CHIASM:
1. Temporal axons do not cross
2. Nasal axons do cross

NOTES:
The optic chiasm (from the Greek χλαζειν 'to mark with an X', after the letter 'Χ' chi) is the part of the brain where the optic nerves partially cross, those parts of the right eye which see things on the right side being connected to the left side of the brain, and vice versa. The optic nerves from both eyes meet and cross at the optic chiasm, at the base of the frontal lobe of the brain. At this point the information from both eyes is combined and split according to the field of view. The corresponding halves of the field of view (right and left) are sent to the left and right halves of the brain, respectively (the brain is cross-wired), to be processed. That is, though we might expect the right brain to be responsible for the image from the left eye, and the left brain for the image from the right eye, in fact, the right brain deals with the left half of the field of view, and similarly for the left brain. (Note that the right eye actually perceives part of the left field of view, and vice versa).
Describe role of the OPTIC TRACT in the central visual pathway?

OPTIC TRACT:
1. Contralateral visual
hemifield
2. Both eyes

NOTES:

The optic tract is a part of the visual system in the brain.

It is a continuation of the optic nerve and runs from the optic chiasm (where half of the information from each eye crosses sides, and half stays on the same side) to the lateral geniculate nucleus. The right optic tract consists of temporal retinal fibers from the right eye and nasal retinal fibers from the left eye; conversely, the left optic tract consists of temporal retinal fibers from the left eye and nasal retinal fibers from the right eye. Information from the right visual field (now on the left side of the brain) travels in the left optic tract. Information from the left visual field travels in the right optic tract. Each optic tract terminates in the lateral geniculate nucleus (LGN) in the thalamus.
What are the 3 structures located in the accessory optic system of the central visual pathway?

1. Superior Colliculus
(Optic tectum)
2. Pretectum
3. Suprachiasmatic nucleus
What is the role of the superior colliculus (optic tectum)?

Superior Colliculus (Optic Tectum) is involved in multi-modal orientation.

NOTE:
The superior colliculus (Latin: hill) is part of the brain that sits below the thalamus and surrounds the pineal gland in the mesencephalon of vertebrate brains. This structure comprises the rostral aspect of the midbrain, anterior to the periaqueductal gray and adjacent to the inferior colliculus. The inferior and superior colliculi are known collectively as the corpora quadrigemina, or four twins.

In humans the superior colliculus (SC) is involved in the generation of saccadic eye movements and hand-eye coordination. Afferents to the SC originate in the cerebral cortex, inferior colliculus, retina, basal ganglia, and spinal cord. In humans, as in most larger vertebrates, sensory information that goes to the mesencephalon will be relayed via the thalamus to the cerebral cortex for interpretation. However, the SC can also mediate some oculomotor movements without cortical involvement.

The SC receives visual, as well as auditory, inputs in its superficial layers, and the deeper layers of the colliculus are connected to many sensorimotor areas of the brain. The colliculus as a whole is thought to help orient the head and eyes toward something seen or heard.

In echolocating bats the SC is also implicated in orienting movements. In this animal the SC has been shown to influence vocalization parameters and ear movements, both orienting components of the bat's biosonar system.

The comparable area of the mesencephalon of non-mammalian vertebrates is called the optic tectum. In amphibians, reptiles and fish, the optic tectum is the main visual processing area. In contrast, the role of the SC for visual discrimination is less prominent in more complex vertebrates.
What are 3 roles of the Pretectum in the central visual pathway?

PRETECTUM:
1. Pupillary light reflex
2. Optokinetic reflex
3. Accommodation of lens

NOTES:
Pretectum is a structure located in the midbrain. It receives binocular input from the eyes and is involved with the pupillary light reflex.
Therefore, it is the transitional zone of the brainstem between the midbrain and the diencephalon that is associated with the analysis and distribution of light impulses.

The pretectum, after receiving binocular input, outputs to the Edinger-Westphal nucleus, which is a pre-ganglionic nucleus also located in the midbrain. The Edinger-Westphal nucleus projects onto the ciliary ganglion, whose output controls pupillary diameter (mydriasis or myosis).
What is the role of the Suprachiasmatic nucleus in the central visual pathway?
Suprachiasmatic nucleus is involved in circadian rhythms .

NOTES:

The suprachiasmatic nucleus (SCN) is a nucleus in the hypothalamus and is so named because it resides immediately above the optic chiasm (OX). It consists of two nuclei each of which lies on either side of the hemisphere separated by the third ventricle (3V). Its principal function is to create the circadian rhythm, which regulates the body functions over the 24-hour period. The suprachiasmatic nucleus is one of four nuclei that receive nerve signals from the retina, the other three being the lateral geniculate nucleus (aka LGN), superior colliculus, and the pretectum. The LGN is responsible for passing information about color, contrast, shape, and movement on to the visual cortices. The superior colliculus is responsible for controlling the movement and orientation of the eyeball itself. The pretectum is responsible for controlling the size of the pupil.
What are the 6 major roles of Lateral Geniculate Nucleus (LGN) in the central visual pathway?

1. The visual “relay” nucleus
2. Segregation of parallel streams is conserved at the thalamus
3. Parvocellular layers (form and color)
4. Magnocellular layers (motion and broadband luminance)
5. Konio cellular layers
6. Gating by relay cell response mode
Describe 4 characteristics of the visual “relay” nucleus in LGN?
1. receptive field (RF) properties unchanged: antagonistic center/surround

2. surprisingly, only about 10% of the input synapses are from retina

3. feedback from cortex via layer 6 pyramidal cells and the thalamic reticular nucleus

4. modulatory synapses from aminergic and cholinergic non-visual systems


NOTES:

The lateral geniculate nucleus (LGN) of the thalamus is a part of the brain, which is the primary processor of visual information, received from the retina, in the central nervous system.
Schematic diagram of the primate lateral geniculate nucleus. The LGN receives information directly from the retina, and sends projections directly to the primary visual cortex. In addition, it receives many strong feedback connections from the primary visual cortex. Ganglion cells of the retina send axons to the LGN through the optic nerve. Although it is generally considered to be a cranial nerve, and is always listed as cranial nerve II, in reality the retina and optic nerve arise as an outpocketing of the developing diencephalon. Rather than a proper nerve, then, the optic nerve is really a tract of the brain.
Describe how Segregation of parallel streams is conserved at the thalamus in the LGN?

(1) alternation of eye inputs to separate layers

(2) each layer has retinotopic map (contralateral visual hemifield) in register with layers above/below

NOTES:

The LGN is a distinctively layered structure ("geniculate" means "bent like a knee"). In most primates, including humans, it has six layers of cell bodies with layers of neuropil in between, in an arrangement something like a club sandwich or layer cake, with cell bodies of LGN neurons as the "cake" and neuropil as the "icing".

These six layers contain two types of cells. The cells in layers 1 and 2 are large, or magnocellular ; others in layers 3, 4, 5, and 6 are smaller, or parvocellular. (The Latin prefix "parvo-" means "small"; some authors prefer the term parvicellular. If you're searching for more information, try both spellings.)

Between each of the M and P layers lies a zone of very small cells: the interlaminar, or koniocellular (K), layers. K cells are functionally and neurochemically distinct from M and P cells and provide a third channel to the visual cortex.

The magnocellular, parvocellular, and koniocellular layers of the LGN correspond with the similarly-named types of ganglion cells.
Describe 7 characteristics of the Parvocellular layers (form and color) of LGN?

1. dorsal principle layers 3-6

2. input from P retinal ganglion cells concentrated in central vision

3. sustained activity to sustained stimulus

4. color opponent receptive fields

5. small RFs and high acuity

6. sensitive to high spatial frequencies (fine details)

7. relatively insensitive to flicker/motion (temporal frequencies)

NOTES:

Parvocellular cells have small cell bodies, use a relatively long time to process information, and are part of a visual processing system that tells the brain what something is. This system operates more slowly and with lots of information about details. For example, these cells carry color information while magnocellular cells do not. Parvocellular cells are found in layers 3, 4, 5 and 6.

P Cells are the retinal ganglion cells that project their axons to the parvocellular layers of the LGN. Additionally, the layers are divided up so that the eye on the same side (the ipsilateral eye) sends information to layers 2, 3 and 5 while the eye on the opposite side (the contralateral eye) sends information to layers 1, 4 and 6. (A simple mnemonic for this is that 2 + 3 = 5 while 1 + 4 does not equal 6, so it is "contra"ry to your knowledge of math.)

Remember that in visual perception, the right eye gets information from the right side of the world (the right visual field) as well as the left side of the world (the left visual field). You can confirm this by covering your left eye: the right eye still sees to your left and right, but on the left side, your vision is partially blocked by your nose.

In the LGN, the corresponding information from the right and left eyes is "stacked" so that a toothpick driven through the club sandwich of layers 1 through 6 would hit the same point in visual space six different times. Therefore, Parvocellular parts (also called P-cells), are slow-conducting neurons; transmitting information about colour vision, texture, pattern, and visual acuity. The cells transmit the information to the lateral geniculate nucleus, the area of the brain responsible for analyzing and interpreting the information.
Describe 7 characteristics of the Magnocellular layers (motion and broadband luminance) of LGN?

1. ventral principle layers 1-2
2. input from M ganglion cells, esp. peripheral retina
3. transient activity to sustained stimulus
4. sensitive to luminance, not color
5. relatively large RFs and coarse acuity
6. relatively insensitive to high spatial frequencies
7. sensitive to high temporal frequencies

NOTES:

Magnocellular cells have large cell bodies, use a relatively short time to process information, and are part of a visual processing system that tells the brain where something is. This system operates quickly but without much detail. They are found in layers 1 and 2 of the LGN, those layers more ventrally located which are next to the incoming optic tract fibers.

M Cells are the retinal ganglion cells that project their axons to the magnocellular layers of the LGN. Therefore, Magnocellular parts, also called M cells, are cells in the brain concerned primarily with visual perception. In particular these cells are responsible for resolving motion and coarse outlines. The cells have large, fast-conducting neurons and transmit information to the lateral geniculate nucleus, the area of the brain responsible for analyzing and interpreting the information. This system of cells operates with great speed at the expense of detail. Additionally, the layers are divided up so that the eye on the same side (the ipsilateral eye) sends information to layers 2, 3 and 5 while the eye on the opposite side (the contralateral eye) sends information to layers 1, 4 and 6. (A simple mnemonic for this is that 2 + 3 = 5 while 1 + 4 does not equal 6, so it is "contra"ry to your knowledge of math.)

Remember that in visual perception, the right eye gets information from the right side of the world (the right visual field) as well as the left side of the world (the left visual field). You can confirm this by covering your left eye: the right eye still sees to your left and right, but on the left side, your vision is partially blocked by your nose.

In the LGN, the corresponding information from the right and left eyes is "stacked" so that a toothpick driven through the club sandwich of layers 1 through 6 would hit the same point in visual space six different times.
Compare the Receptive Field properties of the Parvo and Magno cell layer of LGN, based on these 3 criterias:

1. Size of Receptive Fields (RF)

2. Concentration at fovea

3. Color blind component luminance only

SEE PICTURE
Describe the koniocellular layer of LGN and its role in the central visual pathway?

1. They are tiny cells (“dust”) between principle layers

2. They are poorly understood; ?specialized for color?

NOTES:

Between each of the M and P layers lies a zone of very small cells: the interlaminar, or koniocellular (K), layers. K cells are functionally and neurochemically distinct from M and P cells and provide a third channel to the visual cortex. The magnocellular, parvocellular, and koniocellular layers of the LGN correspond with the similarly-named types of ganglion cells. Therefore, a neurochemically distinct population of koniocellular (K) neurons makes up a third functional channel in primate lateral geniculate nucleus. As part of a general pattern, K neurons form robust layers through the full representation of the visual hemifield. Similar in physiology and connectivity to W cells in cat lateral geniculate nucleus, K cells form three pairs of layers in macaques. The middle pair relays input from short-wavelength cones to the cytochrome-oxidase blobs of primay visual cortex (V1), the dorsal-most pair relays low-acuity visual information to layer I of V1, and the ventral-most pair appears closely tied to the function of the superior colliculus. Throughout each K layer are neurons that innervate extrastriate cortex and that are likely to sustain some visual behaviors in the absence of V1. These data show that several pathways exist from retina to V1 that are likely to process different aspects of the visual scene along lines that may remain parallel well into V1.
What is the snellen chart?

A Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Hermann Snellen who developed the chart in 1862. The traditional Snellen chart is printed with eleven lines of block letters. The first line consists of one very large letter, an "E." Subsequent rows have increasing numbers of letters that decrease in size. Patients taking the test cover one eye, and read aloud the letters on each row, beginning at the top. The smallest row that can be read accurately indicates the patient's visual acuity in that eye. In the most familiar acuity test, a Snellen chart is placed at a standard distance, twenty feet in countries where that is the customary unit of measure. At this distance, the symbols on the line representing "normal" acuity subtend an angle of five minutes of arc, and the thickness of the lines and of the spaces between the lines subtends one minute of arc. This line, designated 20/20, is the smallest line that a person with normal acuity can read at a distance of twenty feet. Three lines above, the letters have twice the dimensions of those on the 20/20 line. The chart is at a distance of twenty feet, but a person with normal acuity could be expected to read these letters at a distance of forty feet. This line is designated by the ratio 20/40. If this is the smallest line a person can read, the person's acuity is "20/40," meaning, in a very rough kind of way, that this person needs to approach to a distance of twenty feet to read letters that a person with normal acuity could read at forty feet. In an even rougher way, this person could be said to have "half" the normal acuity. Acuity charts are used during many kinds of vision examinations, such as "refracting" the eye to determine the best eyeglass prescription. During such examinations, acuity ratios are never mentioned.

The biggest letter on an eye chart often represents an acuity of 20/200, the value that is considered "legally blind." Many people with refractive errors have the misconception that they have "bad vision" because they "can't even read the E at the top of the chart without my glasses." But in most situations where acuity ratios are mentioned, they refer to best corrected acuity. Many people with moderate myopia "cannot read the E" without glasses, but have no problem reading the 20/20 line or 20/15 line with glasses. A legally blind person is one who cannot read the E even with the best possible glasses. Visual acuity = Distance at which test is made / distance at which the smallest letter read subtends an angle of 5 arcminutes.

The symbols on an acuity chart are formally known as "optotypes." In the case of the traditional Snellen chart, the optotypes have the appearance of block letters, and are intended to be seen and read as letters. They are not, however, letters from any ordinary typographer's font. They have a particular, simple geometry in which:

* the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter "C"
* the height and width of the optotype (letter) is five times the thickness of the line.

Only the nine letters C, D, E, F, L, O, P, T, Z are used in the traditional Snellen chart.
Compare Magno, Parvo, and Kornio LGN layers based on these 7 characteristics:

1. Color contrast

2. Lum. contrast

3. Spatial freq.

4. Temporal freq

5. Ascending input

6. Ascending output

SEE DIAGRAM
What is the gating by relay cell response mode determined by?
T-type Ca++ channels
Describe how the gating by relay response mode is determined by T-type Ca++ channels?

1. There is a transient inward Ca++ current; “low threshold” Ca++ spike and this triangular voltage “spike” has bursts of Action Potentials (APs) riding on it and ,
A. The size of the spike depends on resting potential
B. The number of spikes depends on size
(b) only APs propagate to cortex

2. Also, it is only the Action Potentials of the T-type Ca++ channels that propagate to cortex.
Differentiate between the 2 types of gating by relay cell response mode?

1. TONIC MODE:

(a) linear input/ouput
(b) relatively depolarized “resting” membrane voltage
(i) inactivates T-type Ca++ channels
(c) predominates during wakefulness

2. BURST MODE
(a) non-linear “bursty” input/output
(b) high detectability (?“wake up call” during wakefulness?)
(c) relatively hyperpolarized “resting” membrane voltage
(d) de-inactivates T-type Ca++ channels
(e) most abundant during slow wave sleep (esp. rhythmic bursting)
Elaborate on the burst mode cell response?

Burst mode is mediated by a voltage-gated “T-type” Ca++ channel that gives rise to a transient inward Ca++ current, IT , called a “low threshold Ca++ spike” or LTS. This LTS events usually triggers a high-frequency burst of Na+ action potentials. Moreover, the T channels are inactivated at relatively depolarized potentials and de-inactivated by hyperpolarization.
Describe OPTIC RADIATIONS and their involvement in the central visual pathway?

OPTIC RADIATIONS:

1. They are fan-like projections to the visual cortex

2. They are a direct path for superior retinal quadrents (lower visual field)

3. They loop through temporal lobe (Meyer’s loop) for inferior retinal quadrents (upper visual field)

NOTES:

The geniculo-calcerine tract (known as the optic radiation) is a collection of axons carrying visual information from the lateral geniculate nucleus of the thalamus to the primary visual cortex (also called striate cortex).

Much of this tract can go straight to the occipital lobe; however, the fibres carrying information from the lower retina must loop around the lateral ventricle via Meyer's loop (passing into the temporal lobe) to get to the visual cortex.

For this reason, a lesion in the temporal lobe can cause a loss of vision to the superior quadrant, (the top half of what we see, for instance the sky). Therefore, Information leaving the LGN travels out on the optic radiations, which form part of the retrolenticular limb of the internal capsule. The axons which leave the LGN go to V1 visual cortex and generally end in layer IV. Axons from layer VI of visual cortex send information back to the LGN.
Describe Primary Visual cortex?

The primary visual cortex is the most well-studied visual area in the brain. It is the part of the cerebral cortex that is responsible for processing visual stimuli. It is the simplest, earliest cortical visual area. It is highly specialized for processing information about static and moving objects and is excellent in pattern recognition. The functionally defined primary visual cortex is approximately equivalent to the anatomically defined striate cortex. The name "striate cortex" is derived from the stria of Gennari, a distinctive stripe visible to the naked eye that represents myelinated axons from the lateral geniculate body terminating in layer 4 of the gray matter. The primary visual cortex is divided into six functionally distinct layers, labelled 1 through 6. Layer 4, which receives most visual input from the lateral geniculate nucleus (LGN), is further divided into 4 layers, labelled 4A, 4B, 4Cα, and 4Cβ. Sublamina 4Cα receives most magnocellular input from the LGN, while layer 4Cβ receives input from parvocellular pathways.
What are the 6 characteristics of the primary visual cortex?

1. It is located at the posterior pole of occipital cortex and calcarine fissure.

2. It receives direct input from the thalamic relay (LGN)

3. It serves as a functional magnification for central retina

4. It has a distinct nomenclature

5. It has modular organization

6. It receives input layer 4C from LGN
List the 3 nomenclature of the primary visual cortex?

The 3 nomenclature are:

1. area 17
2. striate cortex
3. area V1

NOTES:

Visual cortex is the term applied to both the primary visual cortex (also known as striate cortex or "V1") and upstream visual cortical areas also known as extrastriate cortical areas (V2, V3, V4, V5). The primary visual cortex is anatomically equivalent to Brodmann area 17, or BA17. Brodmann areas are based on a histological map of the human brain created by Korbinian Brodmann. The visual cortex occupies about one third of the surface of the cerebral cortex in humans. It is thought to be divided into as many as thirty interconnected visual areas, but at the present time there is good evidence for only 4 of these areas, V1, V2, V3 and MT (aka V5). The first cortical visual area, the one that receives information directly from the lateral geniculate nucleus, is the Primary Visual Cortex, or V1. V1 transmits information to two primary pathways, called the ventral stream and the dorsal stream
What are the 2 primary pathways that the Primary Visual Cortex, or V1 transmits information to?

The two primary pathways, called the ventral stream and the dorsal stream.

1. The ventral stream begins with V1, goes through Visual area V2, then through Visual area V4, and to the inferior temporal lobe. The ventral stream, sometimes called the "What Pathway", is associated with form recognition and object representation. It is also associated with storage of long-term memory.

2. The dorsal stream begins with V1, goes through Visual area V2, then to Visual area V3, Visual area MT (also known as V5) and to the inferior parietal lobule. The dorsal stream, sometimes called the "Where Pathway" or the "How Pathway", is associated with motion, representation of object locations, and control of the eyes and arms, especially when visual information is used to guide saccades or reaching.
What are the 3 types of modular organization present in the primary visual cortex?

1. Ocular dominance columns
2. Orientation columns
3. Cytochrome oxidase 'blobs'
Describe ocular dominance columns?

Scientists studying the visual cortex discovered columns of neurons that selectively respond to visual information from one eye or the other. They learned that normal visual experience during a critical period in early childhood is crucial for these columns to form properly. These discoveries shed light on normal brain development and revolutionized treatment of a childhood eye disease called strabismus. Neuroscientists found that specific types of nerve cells, or neurons, of the mature visual cortex respond to specific shapes or orientations of light. They also discovered that isolated within the visual cortex are sets of alternating columns of cells that process information sent from either the left or right eye.
These columns called ocular dominance columns, are a major feature of the organization of the visual cortex. They are part of the neural circuitry that gives us one unified view of the visual world, even though the brain gets information from both eyes. Using radioactive molecules to make neurons that respond to each eye, researchers found that ocular dominance columns run across the cortex as a series of alternating stripes, like a zebra's black and white stripes. With research on cats and monkeys, scientists discovered that ocular-dominance columns are not fully wired at birth, but take shape during the first several months of life. If one eye is not used during this critical period of visual cortex development, neurons in the ocular dominance column that should receive visual information from the unused eye do not develop normally, and instead become wired to the normal eye. The ocular dominance columns representing the eye that is not used waste away. Once the critical period ends, sight is permanently impaired.

What does A anb B illustrate?
A = Cortical Occularity during development

B = Ocular Dominance Stripes
Describe orientation columns?

In the earliest recordings from the striate cortex, it was noticed that
whenever two cells were recorded together, they agreed not only in their eye
preference, but also in their preferred orientation. You might reasonably ask at
this point whether next-door neighboring cells agree in all their properties: the
answer is clearly no. As I have mentioned, receptive-field positions are usually not quite the same, although they usually overlap; directional preferences are
often opposite, or one cell may show a marked directional preference and the other show none. In layers 2 and 3, where end-stopping is found, one cell may show no stopping when its neighbor is completely stopped. In contrast, it is
very rare for two cells recorded together to have opposite eye preference or
any obvious difference in orientation. Orientation, like eye preference, remains constant in vertical penetrations
through the full cortical thickness. In layer 4C Bata, as described earlier, cells
show no orientation preference at all, but as soon as we reach layer 5, the cells
show strong orientation preference and the preferred orientation is the same as
it was above layer 4C. If we pull out the electrode and reinsert it somewhere
else, the whole sequence of events is seen again, but a different orientation very
likely will prevail. The cortex is thus subdivided into slender regions of con-
stant orientation, extending from surface to white matter but interrupted by
layer 4, where cells have no orientation preference.
Describe cytochrome oxidase blobs?

The mitochondria in all living cells except bone and hair contain an enzyme called cytochrome oxidase. It is possible to stain brain cells so that various concentrations of cytochrome oxidase can be identified. Cortical areas with high concentrations of cytochrome oxidase were named "blobs" by Livingstone and Hubel. In macaque monkeys it would appear that a great deal of color information is processed in the blob areas and in areas of low concentration little color information is processed. The other thing of note is that little or no orientation specificity is exhibited in the blobs but orientation specificity is evident in areas of low concentration.

Unfortunately, the scientific waters are a bit muddied with the following observation. It would appear that retinas of nocturnal monkeys with very little color vision exhibit blobs indicating a richness of cytochrome oxidase. We will have to wait for future research to obtain a more complete understanding of the role played by cortical areas exhibiting blobs.
Show the model of the modular organization of striate cortex?

see diagram
What happens to the Input layer 4C from LGN?

It relays the input to other layers in the 'interblob' regions

NOTE:
The primary visual cortex is divided into six functionally distinct layers, labelled 1 through 6. Layer 4, which receives most visual input from the lateral geniculate nucleus (LGN), is further divided into 4 layers.
What are 6 layers in the interblob region that layer 4C from LGN relays input to?

1. 4Calpha
2. 4Cbeta
3. cytochrome oxidase blobs
4. output layer 4B and 6
5. output layer 2 and 3 interblob
6. output layer 2 and three blob
Describe layer 4Calpha in the interblob region?

1. It receives input from LGN magno cells and then projects to layers 4B and 6 interblob regions.

2. It responds transiently to visual stimuli and it is sensitive to stimulus motion

NOTES:
The primary visual cortex is divided into six functionally distinct layers, labelled 1 through 6. Layer 4, which receives most visual input from the lateral geniculate nucleus (LGN), is further divided into 4 layers, labelled 4A, 4B, 4Cα, and 4Cβ. Sublamina 4Cα receives most magnocellular input from the LGN.
Describe layer 4Cbeta in the interblob region?

1. It receives input from LGN parvo cells and then projects to layers 2 and 3, interblob regions.

2. It responds tonically to visual stimuli and it is sensitive to fine detail and color

NOTES:

The primary visual cortex is divided into six functionally distinct layers, labelled 1 through 6. Layer 4, which receives most visual input from the lateral geniculate nucleus (LGN), is further divided into 4 layers, labelled 4A, 4B, 4Cα, and 4Cβ. Layer 4Cβ receives input from parvocellular pathways.
Describe the cytochrome oxidase blobs?

1. It inputs directly to layers and 3 from LGN and it is selective for color
Describe output layer 4B and 6?

It gets input from 4Calpha and it outputs to V2 'thick stripes'.
Describe output layer 2 and 3 interblob?

Receives input from 4Cbeta and output to V2 'pale interstipes'
Describe output layer 2 and three blob?

It receives input from LGN parvo neurons and outputs to thin stripes
Describe the Extrastriate visual cortex?

It receives inputs from V1 and it consists of V2, V3, V4, V5 (MT), MST, etc. Moreover, it has two general streams of processing

NOTES:

Visual cortex is the term applied to both the primary visual cortex (also known as striate cortex or "V1") and upstream visual cortical areas also known as extrastriate cortical areas (V2, V3, V4, V5). The visual cortex occupies about one third of the surface of the cerebral cortex in humans. It is thought to be divided into as many as thirty interconnected visual areas, but at the present time there is good evidence for only 4 of these areas, V1, V2, V3 and MT (aka V5). Neurons in the visual cortex fire action potentials when visual stimuli appear within their receptive field. A receptive field is a small region within the entire visual field. Any given neuron only responds to a subset of stimuli within its receptive field. This property is called tuning. In the earlier visual areas, neurons have simpler tuning. For example, a neuron in V1 may fire to any vertical stimulus in its receptive field. In the higher visual areas, neurons have complex tuning. For example, in the inferior temporal cortex (IT), a neuron may only fire when a certain face appears in its receptive field.
List the two general streams of processing in the Extrastriate visual cortex?

1. The dorsal 'what' pathway (form).

2. The ventral 'where' pathway (spatial relations)
Describe the dorsal 'what' pathway (form)?

It is involved in object and form recognition and it is the highest order function in the inferotemporal cortex.

NOTE:

The dorsal stream begins with V1, goes through Visual area V2, then to Visual area V3, Visual area MT (also known as V5) and to the inferior parietal lobule. The dorsal stream, sometimes called the "Where Pathway" or the "How Pathway", is associated with motion, representation of object locations, and control of the eyes and arms, especially when visual information is used to guide saccades or reaching.
What would happen if there is a lesion in the inferotemporal cortex?

There will be an inability to recognize objects that the patient otherwise would be completely familiar with (form agnosias), for example, a patient might be unable to recognize a pencil held before him. Allow the patient to reach out to grasp the object (he would be accurate in locating it), then he would recognize the object as a pencil by its 'feel'
Describe the ventral 'where' pathway (spatial relations)?

It is involved in motion and spatial perception and its highest order function is in the posterior parietal cortex.

NOTES:

The ventral stream begins with V1, goes through Visual area V2, then to Visual area V3, Visual area MT (also known as V5) and to the inferior parietal lobule. The ventral stream, sometimes called the "Where Pathway" or the "How Pathway", is associated with motion, representation of object locations, and control of the eyes and arms, especially when visual information is used to guide saccades or reaching.
What would happen if there is a lesion in the posterior parietal cortex?

It can produce an inability to appreciate the spatial relations of objects that he can see and recognize such as hemineglect syndromes, and movement apraxias
Describe hemineglect syndrome?

Patients with hemineglect syndrome can accurately perceive sensory information but behave as if it does not exist. Since a lesion usually affects only one hemisphere most neglect syndromes only involve sensory information on one side (i.e.., hemi). Their primary sensory systems (e.g., vision ) are intact, but they do not attend to or behave consistent with perception. They behave as if they only perceive information from the unaffected side. Patients with severe forms of hemi-neglect may even refuse to accept that the affected limbs even belong to them. They may even complain that someone else's leg is in bed with them.

Patients with hemineglect who are touched on the affected side may report being touched on the intact side. This error is called allesthesia. Allesthesia may be present in any sensory modality.
What are movement apraxias?

Apraxia is a neurological disorder characterized by loss of the ability to execute or carry out learned (familiar) movements, despite having the desire and the physical ability to perform the movements.

The root word of Apraxia is praxis which is Greek for an act, work, or deed.

Types

There are several types of apraxia including:

* limb-kinetic (inability to make fine, precise movements with a limb),
* ideomotor (inability to carry out a motor command),
* ideational (inability to create a plan for or idea of a specific movement),
* buccofacial or facial-oral (inability to carry out facial movements on command, i.e., lick lips, whistle, cough, or wink) - which is perhaps the most common form,
* verbal (difficulty coordinating mouth and speech movements),
* constructional (inability to draw or construct simple configurations),
* and oculomotor (difficulty moving the eyes).

Apraxia may be accompanied by a language disorder called aphasia.

Developmental Apraxia of Speech (DAS) presents in children who have no evidence of difficulty with strength or range of motion of the articulators, but are unable to execute speech movements because of motor planning and coordination problems. This is not to be confused with phonological impairments in children wtih normal coordination of the articulators during speech.

Symptoms of Acquired Apraxia of Speech (AOS) and Developmental Apraxia of Speech (DAS) include inconsistent articulatory errors, groping oral movements to locate the correct articulatory position, and increasing errors with increasing word and phrase length. AOS often co-occurs with Oral Apraxia (during both speech and non-speech movements) and Limb Apraxia.
What is the Retino-tecto-thalamo-cortical path?
It is indirect' inputs to extrastriate cortex via superior colliculus and pulvinar. It also consists of “blind sight”: vague, residual orienting to visual stimuli in blind hemifield through this phylogenetically old pathway
Describe the Progression of receptive field properties from V1 to extrastriate?
1) rf size gets larger
(2) retinotopy becomes less well organized
(3) rf properties become more complex and selective for certain properties and non-selective for others
(a) e.g., face selective neurons in monkeys
(b) prosoagnosia, inability to recognize familiar faces from temporal lobe lesion
What are the 2 categories under depth perception?
1. Monocular cues
2. Binocular disparity
What are 3 involvements of Binocular disparity?
1. Relative positions on retinae due to image depth with respect to binocular fixation point

2. Tuning of cortical visual neurons for disparity

3. Stereopsis
What are the 4 types of tuning of cortical visual neurons for disparity?
(1) tuned excitatory
(2) tuned inhibitory
(3) far tuned
(4) near tuned
What is stereopsis?
It characterized by seeing objectd in three dimensions and it is strictly a cortical function since eye information is segregated until convergence after layer 4 inputs at V1.
What is Amblyopia?
Amblyopia, or lazy eye, is a disorder of the eyes. It is characterised by poor or blurry vision that is not correctable with glasses in an eye that is otherwise physically healthy and normal. The problem is due to either no transmission or poor transmission of the visual image to the brain for a sustained period of dysfunction or disuse during early childhood. The condition will only arise at this young age because most of the visual system's development in humans is complete and "locked in" by a few years of age. Amblyopia normally only affects one eye, but it is possible to be amblyopic in both eyes if both are similarly deprived of a good, clear visual image.

Amblyopia affects 2-5% of the population. Amblyopia is a developmental problem in the brain, not an organic problem in the eye. The part of the brain corresponding to the visual system from the affected eye is not stimulated properly and develops abnormally. This has been confirmed in brain specimens.

Many children who have amblyopia, especially those who are only mildly amblyopic, are not even aware they have the condition until tested at older ages, since the vision in their stronger eye is normal. However, people who have severe amblyopia may experience associated vision disorders, most notably poor depth perception. Therefore, it is : Decreased, uncorrectable visual abilities without detectable anatomic damage.
Describe Strabismic amblyopia?
Strabismus, sometimes known as lazy eye, will result in normal vision in the fixating eye, but abnormal vision in the strabismic eye due to the brain discarding its information. Strabismus in an adult usually causes double vision (diplopia), since the two eyes are not fixated on the same object. Children's brains, however, are more plastic, and therefore can more easily adapt by ignoring images from one of the eyes, getting rid of the double vision. This plastic response of the brain, however, interrupts the brain's normal development, resulting in the amblyopia.

Strabismic amblyopia is best treated by treating the strabismus through the use of prescription glasses, vision therapy, surgery or patching.
Therefore, Strabismus (“Lazy eye”) and astereopsis can occur to toddlers with uncorrected visual misalignment during critical period for cortical disparity development.
Illustate the entire M, P, and K pathways in diagram?

SEE PIC
Illustate the entire M, P, and K pathways in a color coded diagram?

SEE PIC
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