Visual ability is a series of intricate networks of different cortexes within the occipital lobe. The primary visual cortex (V1) is the most researched and understood feature of the immense and complex visual process system. The V1 is considered a neuronal gateway as neuronal projections travel from the V1 to the extrastriate regions within the occipital lobe.
The primary visual cortex (V1) is in the occipital lobe. This is the site of the brain’s optical processing system. It’s tasked with converting external stimuli into meaningful responses and facilitating visual input memory. The V1 responds to basic aspects of vision- orientation and direction.
The occipital lobe consists of the visual cortex. The specialized area of the visual cortex facilitates normal functioning as it discerns patterns and objects immediately and without conscious effort. Therefore, the visual system is an unconscious process, which can become problematic as it can result in misconceptions and optical illusions.
The Visual CortexThe Primary Visual Cortex (V1) Or Striate CortexThe Extrastriate CortexThe Ventral-Dorsal Model Of The Visual SystemThe Occipital Lobe And The Rest Of The BrainMedical Conditions And Cortical Mapping Of The Visual CortexConclusion
The Visual Cortex
The Primary Visual Cortex (V1) Or Striate Cortex
The Extrastriate Cortex
The Ventral-Dorsal Model Of The Visual System
The Occipital Lobe And The Rest Of The Brain
Medical Conditions And Cortical Mapping Of The Visual Cortex
Conclusion
The sensoryneuronsin the individual visual areas respond to specialized visual stimuli. This specialization enables the different cortical regions of the brain to freely and independently execute other computations- those tasked with decision-making and executive operations.
For example, the middle temporal area (MT) consists of neurons that react selectively to the direction of moving edges, independent of the object’s color. Whereas neurons in the cortical area V4 are activated according to the item’s color, independent of the direction of movement.
Neuronal activation is relevant in understanding behavioral capacity. If an aspect of the visual cortex is damaged, the ability of an individual to discern and function is severely impacted. Evidence of selective neuron specialization of the visual cortex is observed in sectional cortex impairment.
Neurologists and psychologists’ surmise that there is a hierarchy to the distinction of the neuronal make-up and activity of each cortical region. As visual information travels, each succeeding cortical area is more specialized than the previous. However, as mentioned above, the automatic processes could lead to misinterpretation and optical illusions.
Neuronal Tuning Of The Visual Cortex
The tuning curve peak is the neuronal state in which a stimulus is encoded. In the primary visual cortex area, neuronal tuning is sharp and strong. Nobel prize winnersHubel and Wiesel(1981) mapped out the neuronal activity of the visual cortex. They concluded that the optimal visual stimulus becomes more intricate as neuronal electro-activity travels toward the anterior brain.
The primary visual cortex is also called the visual area 1 (V1),Brodmann area 17, or the striate cortex. As it receives visual information from the LGN, the primary visual cortex relays information output to the succeeding cortical visual areas. The V1 consists of simple cells which react to distinct visual fields like edges and lines. There are approximately 140 million neurons in each hemisphere.
The area designated as V1, or the primary visual area, is the initial phase ofcortical processingof visual information. Area V1 consists of the entire visual field representation informed through the eyes. The primary visual cortex is the most understood visual cortex area. It’s understood as the cortical gateway for the optical sensory network.
The V1 has substantial neuronal density, which extends over a sizeable area. Therefore, the V1 possesses an extensive array of neurons, approximately 140 million neurons per hemisphere. Essentially, 40 primary visual cortex neurons for one LGN neuron. This significant variation enables effective image processing directed from the LGN.
The Calcarine Sulcus Of The Striate Cortex
The V1 sits in the calcarine sulcus (calcarine fissure) within the occipital lobe, comprising 67%. Thiscomplete sulcusis very deep and a prominent feature of the occipital lobe. It divides the occipital lobe into the lingual gyrus (inferior section) and the cuneus (superior area).
This major sulcus curves along a linear path. It aligns from the apex of the occipital pole to midway along theparieto-occipital sulcus. The centralvisual fieldis situated in the posterior part of the calcarine fissure, and the peripheral visual field is in the anterior region. Therefore, the occipital lobe posteriorly and the parieto-occipital sulcusanteriorly.
The Line Of Gennari And The Striate Cortex
Thestriate cortexis responsible for transforming visual sensory stimuli into meaningful information. Striate gets its name from its striped features observable through a microscope. This feature is the line of Gennari, a discernable single, solid, white belt of myelinated sheaths, visual to the naked eye.
The band is formed from a dense axon pathway from the thalamus to thefourth layerof the primary visual cortex. The line of Gennari travels parallel to the periphery of the cerebral cortex and aligns along the calcarine fissure boundary.
The Layered Structure Of The Striate Cortex
The first layer is almost entirely free of neurons. It’s a superficial layer that consists chiefly of axons and dendrites. Layers one and two receive feedback responses from the extrastriate area. Even though layer one contains few neurons, it is a networking layer that directly affects the firing features of pyramidal cells in deeper layers.
About 20% of the neurons in layers two to six areinhibitory interneurons. Layers two and three are super excitatory neurons that transmit axons to the extricate cortex areas. These layers are described as supragranular.
The fourth layer gathers the largest amount of visual stimuli information from the LGN. It’s separated into four individual layers- 4A, 4B, 4Cα, and 4Cβ. While layer four contains a high density of simple cells, layers two, three, and six have complex cells.
The fifth and sixth layers are the infragranular layers and consist of multiple excitatory projection neurons. The neurons of these higher-order layersinnervate the LGNand provide modulatory feedback to this relay region. This feedback mechanism creates aneural loop.
There are approximately 20 additional visual cortical fields that neurologists have uncovered. That is, these regions are the recipients of observable optical inputs. These are significant regarding their anatomical, electrophysical, and psychological contribution to further study.
The extrastriate cortex consists of numerous operational regions. These areas are the visual regions V2, V3, V4, and V5. Situated alongside the striate cortex and within the occipital lobe, these areas are specialized in their function and visual contribution.
The ventral-dorsal model is also referred to as thetwo-stream hypothesis. Thispsychological theoryendeavors to examine the way information is transmitted along the neural connections of the brain. This theory hypothesizes that humans have two separate visual systems- the ventral and the dorsal.
The primary visual cortex transmits visual input to the two principal neural connections- the ventral and dorsal streams. These streams synapse in different areas of the occipital lobe and continue to relay information to the relevant areas of the brain. The ventral stream concludes in thetemporal lobe, and the dorsal stream in theparietal lobe.
The ventral stream also relays perceptual information- object recognition and shape identification. The ventral stream has significant network connections to the medial temporal lobe and the limbic system.
The ventral stream initiates its course in the striate cortex (V1). It then travels through the visual region V2 and, following that, the V4. The data is then transmitted to the inferior temporal cortex (IT cortex). The IT cortex is an integral feature in object recognition and long-term memory.
The parietal or dorsal stream travels from the striate area in the occipital lobe and terminates in the parietal lobe. The stream travels through the extrastriate area V2 to thedorsomedial area(DM/V6) and themedial temporal area(MT/V5). The dorsal stream terminates in theposterior parietal cortex(PPC).
The brain has five distinct areas orlobes: the frontal, parietal, temporal, insular, and occipital lobes (smallest). Some medical sources reference parts of the limbic system as a sixth lobe. Inmedical terms, the occipital lobe is positioned posterior to the parietal and temporal lobes. Therefore, it belongs to thecaudalpart of the brain.
The brain (including all four lobes) is divided into the left and right cerebral hemispheres. Therefore, each lobe can be divided into the left and right hemispheres. Thecorpus callosumjoins the two hemispheres.
Each brain hemisphere possesses an individual visual cortex. The primary visual cortex gathers information from the contralateral eye. Simply put, the left cortical hemisphere interprets input from the right eye, and the right cortical hemisphere translates external stimuli from the left eye.
The occipital lobe contributes to approximately 12% of the surface area of thebrain’s neocortexand 18% of theneocortical volume. The occipital lobe gets its name from its position. It sits below the skull’s occipital bone, under the parietal lobe, above the temporal lobe, and overlays thetentorium cerebelli. It’s the predominant part of the brain’s visual system.
The occipital lobe’s functions are an indispensable part of the visual processing system. The visual processing area of the brain performs within the occipital lobe. This area is tasked with visuospatial awareness, depth and distance perception, color discernment, face and object recognition, and memory development.
If the striate region of the visual cortex is damaged, it results in cortical blindness. This area is prone to mechanical damage. Cortical blindness is partial or absolute impairment if the visual cortex is injured. This condition is distinct from others that result from damage to different visual anatomical processes.
Posterior cerebral artery stokescan cause cortical blindness. If a patient suffers an ischemic stroke, the resulting optical deterioration could be reversed withreperfusion. Although, frequently, patients don’t recover their sight fully.
Prosopagnosiais the inability to recognize facial features and discern individual facial characteristics. This condition could arrive from a stroke that prevents visual input from adequately being transmitted from the visual cortex to theinferior temporal cortex.
Other medical conditions cause medical blindness, like eclampsia, severe brain trauma, infection, encephalitis, some medications, meningitis, and hyperammonemia. These ailments are not related to strokes.
Bilateral lesions can result in absolute cortical blindness and can be associated with a medical condition referred to asAnton-Babinski syndrome. This syndrome details that a patient is blind. However, they contradict possessing any visual impairment.
The Surgical Significance Of The Primary Visual Cortex
In multiple past cases, surgery to this area resulted in cortical blindness. If there’s a tumor or other masses in the occipital region, surgeons construct a cortical map to avoid damage. This highly skilled technique localizes the specific area.
The primary visual cortex is situated in the occipital lobe. This area is the most studied region of the visual cortex. It’s a gateway as multiple neuronal branches transmit external visual stimuli from the LGN to other cortical areas. The ventral-dorsal model is a theory that endeavors to explain the transmission of neurons in the visual cortex.
There are six operationally distinct layers of the striate cortex. The line of Gennari is a band of white myelinated sheaths of a dense axons pathway that travel from the thalamus to the fourth layer of the primary visual cortex. Striate gets its name from the striped appearance of the line of Gennari visible under a microscope. Cortical blindness could result from a stroke or other ailments.
References
https://foundationsofvision.stanford.edu/chapter-6-the-cortical-representation/
https://en.m.wikipedia.org/wiki/Extrastriate_cortex
https://www.ncbi.nlm.nih.gov/books/NBK482504/
https://en.wikipedia.org/wiki/Visual_cortex
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Practical Psychology. (2022, August).Where is the Primary Visual Cortex Located?.Retrieved from https://practicalpie.com/where-is-the-primary-visual-cortex-located/.Practical Psychology. (2022, August). Where is the Primary Visual Cortex Located?. Retrieved from https://practicalpie.com/where-is-the-primary-visual-cortex-located/.Copy
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