Visual Fiber Pathways

• The visual nerve signals exit the retinas via the optic nerves.
• Optic nerve fibers from the nasal halves of the retinas cross to the opposite sides at the optic chiasm, joining fibers from the opposite temporal retinas to form the optic tracts.
• Optic tract fibers synapse in the thalamus' dorsal lateral geniculate nucleus, then geniculocalcarine fibers travel through the optic radiation to the primary visual cortex in the medial occipital lobe's calcarine fissure area.

Visual Fiber Pathways

• From optic tracts to the hypothalamus' suprachiasmatic nucleus, regulating circadian rhythms aligning body changes with day and night.
• To midbrain pretectal nuclei, triggering reflex eye movements for focusing on important objects and activating the pupillary light reflex.
• Into the superior colliculus for controlling rapid directional eye movements.
• To the thalamus' ventral lateral geniculate nucleus and surrounding basal brain regions, potentially aiding in regulating behavioral functions.
• Visual pathways are separated into old and new systems
• The old system goes to the midbrain and base of the forebrain
• The new system directly transmits visual signals into the visual cortex in the occipital lobes
• The "new system" is responsible for conscious vision and form, color etc while, a primitive relies on the superior colliculus

Function of the Dorsal Lateral Geniculate Nucleus of the Thalamus

• The dorsal lateral geniculate nucleus is located at dorsal end of thalamus
• Optic nerve fibers of the new visual system end in the dorsal lateral geniculate nucleus, or the lateral geniculate body
• First, it transmits visual information from the optic tract to the visual cortex using optic radiation and does so with great accuracy
• Fibers from each eye are kept separate in the dorsal lateral geniculate nucleus after passing the optic chiasm; two eyes signals are kept separate.
• The nucleus has six layers; layers II, III, and V get information from the ipsilateral retina's lateral half, and layers I, IV, and VI receive information from the contralateral retina's medial half.
• Retinal areas connect with neurons that are superimposed in paired layers, maintaining parallel transmission to the visual cortex.
• Second major function, regulate signal transmission to visual cortex using sources that are inhibitory
• These sources include corticofugal fibers from cortex, and reticular areas of the mesencephalon
• The dorsal lateral geniculate is divided into magnocellular and parvocellular layers
• Layers I and II, magnocellular, have large neurons getting input from type M retinal ganglion cells for a fast pathway transmitting black-and-white information, but poor point-to-point.
• Layers III-VI, parvocellular, contain small to medium neurons getting input from type P retinal ganglion cells; transmits color and accurate spatial info, but slower conduction.

Organization and Function of the Visual Cortex

• The visual cortex is located primarily on the medial aspect of the occipital lobes.
• Similarly to other systems, it's divided into a primary visual cortex and secondary visual sections.

Primary Visual Cortex

• Situated in the calcarine fissure region, it receives direct visual cues from the eyes-
• Macular area signals end close to the occipital pole, while peripheral retina signals arrive in concentric semicircles ahead of it
• Superior and inferior retinal portions are represented superiorly and inferiorly, respectively.
• The fovea, responsible for top acuity, has a high degree of representation in the primary visual cortex, relative to its retinal area
• The area is also known as visual area I/striate cortex for its striped appearance

Secondary Visual Areas of the Cortex

• Visual association areas lie to the sides, the front, the top, and the bottom of the primary visual cortex and fold outward over the sides of the occipital and parietal cortex
• They analyze visual meanings
• Brodmann's area 18 is visual area II/V-2 and transfers visual signals to primary cortex
• Secondary visual areas are designated V-3, V-4 and analyze elements of the image

Six Layers of the Primary Visual Cortex

• As is true for all of the other areas of the brain; the primary visual cortex has six known layers
• Geniculocalcarine fibers terminate in layer IV, which is subdivided
• Rapid signals from M retinal ganglion cells are relayed vertically from layer IVca
• Vertical signaling goes outward towards the cortical layer, and inward to deeper layers
• Signals from P ganglion cells go to IVa and IVcβ and transmit vision and color

Vertical Neuronal Columns in the Visual Cortex

• The visual cortex is organized into lots of vertical columns
• Each has a diameter of 30-50 micrometers
• Each represents a functional unit with possibly 1000 neurons
• Similarly as in other senses; these also have vertical columnar organizations.
• Optic signals are processed in layer IV, and processed as they travel outward and inward
• Processing breaks down elements of data
• Signals moving up eventually transmit, and go shorter distances in the cortex
• Downward signals activate other neurons, and give feedback over greater distances

"Color Blobs" in the Visual Cortex

• Along with secondary areas: have unique columns getting signals from adjacent visual columns
• Specifically activated by color as primary locations for deciphering color

Interaction of Visual Signals From the Two Separate Eyes

• Signals from different eyes are relayed via different layers of the lateral geniculate nucleus
• The signals are separated in layer IV of the primary visual cortex
• Layer IV interlaced w/ columns containing stripes .5mm wide
• Signals from each eye enter alternating columns that are spaced apart in a pattern
• The cortical area gauges if certain visual areas are in register with each other from the two eyes
• The data adjusts eye gaze to fuse, to be brought together
• The degree of register is crucial for distance judgment

Two Major Pathways for Analysis of Visual Information

• Fast “Position” and “Motion” Pathway
• Accurate Color Pathway
• After leaving the primary visual cortex, visual information is analyzed in varying pathways
• One pathway sees position in 3-D, form, and motion, depicted with black arrows.
• It shows where everything is and if its moving
• The system relies on large fibers

Detail and Color Analysis

• Uses red arrows that passes through the cortex from the primary visual cortex
• This passes into secondary areas
• Analyzes texture and color and recognizes letters; also the meaning of objects

Analysis of Contrasts in Visual Images

• The outer borders are more excited than the inner parts because it has to do with how stimulated adjacent retinal receptors limit one another
• It can also decipher contrast and sharpness

Visual Cortex and Orientation

• Identifies lines or edges and the direction
• Neurons in layer IV called "simple cells" are stimulated for each direction
• With complex cells, signal progresses further away for lines that are oriented in same direction
• The lines aren't position specific

Detection

• Neurons in the outer layers of primary visual columns and in some secondary areas are excited by lines of specific lengths, angulated shapes, or images
• It gets very detailed with analysis

Detection of Color

• It is detected with color contrast
• Colors excite unique cells
• Details of the contrast are noticed by simple cells
• complex detection is done by more complex cells

Effect of Removing the Primary Visual Cortex

• A person won't be able to see, but they are still able to respond subconsciously to movement in reactions like turning head

Fields of Vision; Perimetry

• Field of vision = area that an eye can see from a given point
• Diagnosing where portions of the retina are blind is performed by having patients focus on a spot while closing one eye
• A small object is moved around to test

Abnormalities in the Fields of Vision

• Blind spots can reveal results of glaucoma

Eye Movements and their Control

• Important to have cerebral control to direct the eye
• Muscular control of the eye is the 3 pairs of muscles
• The medial and lateral recti contracts to go from side to side
• Superior and inferior recti contracts to go up or down
• Oblique keeps the fields in the upright position

Conjugate Movement

• The three muscles are reciprocally innervated such that one of the pair relaxes while one contracts

Neural Pathways for Control of Eye Movements

• The brainstem and nerves are shown, with interconnections via the medial longitudinal fasciculus

Fixation Movements of the Eye

• Eyes have to fix on parts of vision and controlled by mechanisms
• The systems allow voluntarily moving of eyeballs to focus on vision
• The other fixes the point to be found

Voluntary Fixation

• Regulated by cortical field located in premotor cortex
• Bilateral dysfunction causes eyes to be unlocked
• The involuntary function causes the “locking” is regulated by areas in the occipital cortex
• When this area is damaged; animals have problems focusing

Saccadic Movement of the Eyes

• With scene moving by, the highlights are seen
• The movements happen at an exceedingly fast pace

Saccadic Movements During Reading

• Eyes move with saccades
• Paintings are similar and the eyes make saccades except it is in different directions

Fixation on Moving Objects

• Eyes remain fixed on a moving object

Superior Colliculi

• Makes the eyes and body turns toward something disruptive
• The optic and sensory signals go to the colliculi, or global system

Fusion of the Visual Images From The Two Eyes

• the system is to make perceptions meaningful by corresponding, in the cortex.

Neural Mechanism of Stereopsis

• There will be too much disparity, which helps us tell distances

Strabismus

• Strabismus is lack of fusion and happens in the coordinates
• Is abnormalities in the system

How is A Repressed Eyed Suppressed?

• the good eye will take on all the workload

How Will the System Accomodate?

• there need to be proper development in synaptic connections

Where and What is Autonomic Control?

• Pupilarry Aperture and accommodation are autonomic
• The fibers arise in westphal nucleus goes to the eyeball, and ciliary muscle excites, causing construction

Where Does Sympathetic Arise

• thoracic segment/ chain and ganglia, and innervates the radial fibers of the eyes, causing pupil movement

How Does lens focus?

• It results from the muscle movement

Lens Strength With Eyes

• Red and Blue light and focus

Eyes Converge

• Stronger lens

Degree of Accomadation

• Brain areas are close

Pupillary Diameter

• Construction is where parasympathetic nerve cells are excited and decreases the aperature and called miosis
• Dialation; opens the pupillary aperature. Radial muslces are excited and dialated

Pupillary Light Reflex

• If light is shone; it will constrict
• Fibers are in pretectal nucluei and if there is a problem in reflex then, there may be a disorder suchas an syphillis-related problem

Argyll Robertson Pupil

• It is a sign of syphillis
• if it doesn't respond as a result of damaged visual signals
• they can construct more with the process happening above, but not with light stimulation

Horner's Syndrome

• Eyes end up interrupted, causing problems
• there aren't sympathetic nerve cells working there