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 Cortex
The visual cortex is sectioned into five distinct areas- V1 to V5. These distinctions are determined by structure and function. Visual sensory information proceeds to the visual cortex via the thalamus’s lateral geniculate nucleus (LGN). The sensory input path from the retina concludes in the primary visual cortex.
The sensory neurons in 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
Neuronal tuning is a brain cell characteristic explored by psychologists and neurologists. Accordingly, brain cells are selective when responding to stimuli, and this is based on particular patterns recognized through experience. Typically, higher arousal rates of neurons generate significant responses.
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 winners Hubel 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.
Neurons of the visual cortex discharge action potentials as their receptive field is visually stimulated. Therefore, a visual receptive field is an entire area delineated by specific sensory neurons that elicits a charge or action potential when activated. The receptive field also involves the neurons belonging to the visual pathways.
The Primary Visual Cortex (V1) Or Striate Cortex
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 of cortical processing of 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 is a network of multiple neuronal tributaries. The network structure is significant as the visual processing system operates by transferring information to distinct specialized areas. The visual tributaries connect with acute precision to other optical and non-visual cranial sections. The precise interconnections include feedback projections to the LGN.
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%. This complete sulcus is 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 the parieto-occipital sulcus. The central visual field is 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 sulcus anteriorly.
The Line Of Gennari And The Striate Cortex
The striate cortex is 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 the fourth layer of 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
Like other cranial cortexes, the striate cortex is structured according to layers. There are six layers in the V1; their variation is determined by the neuron density, the number of axons, dendrites, and synapses. They were proposed by Brodmann, the first layer is dorsal, and layer six is ventral.
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 are inhibitory 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 layers innervate the LGN and provide modulatory feedback to this relay region. This feedback mechanism creates a neural loop.
The Extrastriate Cortex
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 Of The Visual System
The ventral-dorsal model is also referred to as the two-stream hypothesis. This psychological theory endeavors 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 the temporal lobe, and the dorsal stream in the parietal 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.
These connections assist in determining human behaviors. The ventral stream is frequently referred to as the “what” stream. In comparison, the dorsal stream is called the “how” or “where” pathway. It’s important to remember that both the how and what functions of these visual pathways are impacted by optical illusions.
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 dorsal stream or “how” network assists the brain in spatial awareness. Whereas the ventral stream is perceptual, the ventral stream is defined by action. It’s hypothesized to facilitate or direct the visual coordination of various bodily actions. It also assists with spatial awareness-particularly regarding seen objects. It’s also referred to as the parietal stream.
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 the dorsomedial area (DM/V6) and the medial temporal area (MT/V5). The dorsal stream terminates in the posterior parietal cortex (PPC).
The Occipital Lobe And The Rest Of The Brain
To better appreciate the make-up of the primary visual cortex, a rudimentary grasp of the brain’s broader structure is required. The cerebral cortex, the brain’s peripheral layer, is characterized by a wrinkly appearance. This region is associated with higher cognitive capacity, including language, memories, and consciousness.
The brain has five distinct areas or lobes: the frontal, parietal, temporal, insular, and occipital lobes (smallest). Some medical sources reference parts of the limbic system as a sixth lobe. In medical terms, the occipital lobe is positioned posterior to the parietal and temporal lobes. Therefore, it belongs to the caudal part 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. The corpus callosum joins 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 the brain’s neocortex and 18% of the neocortical 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 the tentorium 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.
Medical Conditions And Cortical Mapping Of The Visual Cortex
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 stokes can cause cortical blindness. If a patient suffers an ischemic stroke, the resulting optical deterioration could be reversed with reperfusion. Although, frequently, patients don’t recover their sight fully.
Prosopagnosia is 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 the inferior 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 as Anton-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.
Cortical mapping is done by stimulating the brain parenchyma with an electrode. Neurosurgeons then ask the patient to report what they’re seeing. The feedback response from the patient is vital when diagnosing malignant cells from normal cortical tissue. Doctors can execute procedures with considerably more safety with cortical mapping.
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.