The ventral stream is an important part of the brain. Although the brain is divided into various regions, the different parts work together to achieve complicated processes. Vision is one example of a highly complex series of processes involving information passing through different brain regions. But what links these brain regions, how is the information divided, and where is it processed?
The ventral stream is an essential neurological pathway that carries information from the primary visual cortex to the inferior temporal lobe and other important structures. Information processing takes place pertaining to what the item is, its shape, and assisting in memory forming.
The ventral stream is essential for interpreting optical stimuli. Without it, the organism would be unable to see effectively. However, the ventral stream forms part of a greater system that processes visual information. Below we’ll investigate how it works, what it connects to, complications to the system, and other aspects.
The Physical Characteristics Of The Ventral Stream
The ventral stream is also known as the “vision-for-perception” pathway or the ventral visual pathway. It is one of two pathways leading from the brain region known as the striate cortex.
The ventral stream/pathway, along with the dorsal visual pathway, is a “large” fiber bundle of multisynaptic connections.
The ventral stream's main " highway " is the inferior longitudinal fasciculus. This elongated fiber bundle is composed of white matter (axons).
The Function And Location Of The Ventral Stream
The ventral stream is one of many information highways in the brain. It is one of two described pathways that takes sensory information from the primary visual cortex to other brain regions.
The ventral stream carries information to the temporal lobe and areas of the occipital lobe surrounding the primary visual cortex.
The ventral stream's information allows the brain to identify and recognize what the eye looks at (one reason the ventral stream is often called the “what” pathway).
The dorsal stream is the “where” pathway. This pathway runs across the top of the brain to the parietal lobe and allows the brain to process information relating to spatial relationships between objects and their movement.
These systems function together to help the brain process what the organism looks at and where it fits in time and space according to the surrounding environment.
There are still many uncertainties concerning the exact function of each step in the network. One of the areas of uncertainty is whether the ventral stream connects to the parietal lobe through other connecting pathways.
Researchers believe the ventral stream is necessary for memory, recognition, and conscious perception.
Where Is The Ventral Stream Located?
The ventral stream is a cortical pathway (in the brain) connecting different lobes. "Ventral" refers to the bottom area, and in this case, of the brain.
The ventral stream stretches from the primary visual cortex on the occipital lobe to the extrastriate areas surrounding the primary visual cortex (the areas of the occipital cortex that assist with processing vision) and to the inferior part of the temporal lobe.
The primary visual cortex is referred to as the "striate" because Gennari's stria characterize area. These striae are prominent bands of myelinated axons that appear as “striped or banded.” Cells in this area have a laminar cell structure.
Both occipital lobes contain a calcarine sulcus along its medial surface. Within this calcarine sulcus, we find the striate cortex, which makes up around 10% of the brain.
The ventral pathway passes through the occipitotemporal cortex. Researchers believe that it might extend into the ventrolateral prefrontal cortex as well.
Aside from the primary visual cortex (V1) and the temporal lobe, the ventral stream passes optical information to the secondary visual cortex, or prestriate cortex (V2), and visual area 4 (V4).
How The Ventral Stream Works
The ventral stream works as part of a system in the brain to process visual stimuli (i.e., what the eye sees) and gains meaning and understanding. This processing of visual information occurs across several areas of the brain.
Where The Sensory Information Comes From
Visual stimuli enter through the retina and travel along the optic nerves until it reaches the lateral geniculate nucleus inside the thalamus (think of catching different trains at various stations which make up the various legs of a journey).
The Division Of Optical Information In The Brain
Once at the thalamus, the optical information takes one of three “train lines” (pathways).
- Magnocellular. This pathway features large neurons (nerve cells).
- Parvocellular. Cells along this pathway have smaller neurons.
- Koniocellular. This pathway also has small neurons.
These three distinct pathways are specialized and serve different functions by transporting different types of visual information/stimuli.
- The magnocellular pathway transports movement-related stimuli, including speed, location, and the direction of movement.
- The parvocellular pathway relates to objects' shape, size, and color, making it important for the spatial aspects of whatever the organism views. The ability to see in color also stems from this pathway.
- The koniocellular pathway is mostly unknown but appears to aid in color vision.
What Happens At The Primary Visual Cortex
The information then “transfers” to the optical radiation, a tract (pathway) that loops around the lateral ventricle in both cerebral hemispheres. This leg of the journey terminates in the occipital lobe, specifically at the primary visual cortex (V1).
The optic radiation's axons (and axon terminals) transport the specific information from the visual field to the related parts of the primary visual cortex (V1). I.e., The parts of the V1 that deal with color receive color information, those dealing with shape receive shape, etc.
This process of sorting and delivering specific information to a specific region within the brain is known as retinotopy.
Information pertinent to the inferior visual field passes through axons that terminate above the calcarine sulcus of the primary visual cortex. Conversely, information concerning the superior portion of the visual field travels through axons that terminate below the calcarine sulcus.
The various neurons making up the V1 area of the occipital lobe are arranged into columns of similar neurons according to their function. I.e., neurons responsible for processing orientational information from the contralateral eye are grouped.
Neurons receiving similar information from the ipsilateral eye are grouped in separate columns.
These various columns group together to form “modules.” Modules function to analyze information from a particular visual field area. This analysis is made possible by the various types of neurons functioning together as a unit.
Each module builds a small “pieceof the visual puzzle,” which works together to generate the “whole” picture.
The processing of visual stimuli is not limited to the primary visual cortex. The areas surrounding this region are called the extrastriate cortex (still within the occipital lobe), and they play a fundamental role in analyzing optical information.
How The Ventral Stream Fits In
The ventral stream becomes relevant at this junction. As the sensory information enters the V1, the ventral stream is responsible for relaying it to the extrastriate regions and eventually to the temporal lobe.
Its functionality is directly related to the number of connections the ventral stream makes within the brain (like a highway, with many roads branching off to the various regions).
The ventral stream is a network of connections within the occipitotemporal regions that links the primary visual cortex to specialized cortical and subcortical structures. These structures are critical for emotions, habit-forming, learning, and long and short memory forming and recalling.
Unlike the dorsal stream (which deals with dynamic relationships between items), the facilities connected to the ventral stream are geared toward stable visual stimuli and creating associations with them. These associations are known as "the processing of object quality.”
Criteria like shape, size, brightness, and color relating to the object seen by the eye are conveyed and processed.
However, there is a degree of overlap. I.e., spatial information from the dorsal stream flows to the same "stations" connected to the ventral stream. This overlap in information processing allows the brain to more fully comprehend what the organism is looking at.
The ventral stream connects to at least six cortical and subcortical regions along its length. All of these pathways contribute to processing object quality.
Researchers believe that the information is not limited to movement in one direction but rather a complex network of feedback and forward loops that allow the various connections to communicate.
They also believe that some parts of the pathway bypass the additional connections and move straight to the temporal cortex from V1.
Visual Processing Regions Connect To The Ventral Stream
Once in the ventral stream, the information moves to the V2 and the V4. The V2 area is fundamentally a more refined version of V1. If V1 is where the information is sorted, V2 is where the information goes to be further processed, particularly patterns.
The V4 area of the brain is important for processing color and other aspects. There are still many knowledge gaps regarding the V4’s function.
The Anterior Inferior Temporal Cortex
After passing through the various connections along the pathway, the ventral stream links with the temporal lobe, specifically at the anterior inferior temporal cortex.
Scientists believe that this region of the brain further processes complex shapes. The temporal lobe is also important regarding memories.
Why The Ventral Stream Is Important
Without the ventral stream, the brain could not process optic information, particularly when recognizing and identifying objects.
The brain processes edges, simple shapes, contours, and colors in the primary visual cortex and surrounding areas. The inferior temporal cortex is important for processing complex shapes.
Without the connection via the ventral stream, the organism would not effectively process whatever they were looking at.
The number of connections linking to the ventral stream allows the brain to quickly process optical information sent by the retina.
Without the compartmentalization of information, the brain would be unable to quickly form an understanding, a delay that could have life or death consequences for the organism.
Complications Of The Ventral Stream
The ventral stream is implicitly linked to the organism's visual capabilities. If the pathway, or regions of the brain that it connects to, sustain damage, it may impair or completely cut off the organism's vision.
Some of the effects of damaging these regions of the brain lead to:
- Difficulty discerning and discriminating different hues, 2D patterns, 3D shapes, and brightness.
- Damage to the anterior part of the temporal cortex results in loss of visual memory, while damage to the posterior part leads to loss of discriminative capabilities.
- Alexia without agraphia. The patient cannot read what they’ve written.
- Prototypic syndromes of achromatopsia. Loss of color vision, either completely or partially.
- General visual agnosia. The inability to recognize objects by eyesight (although the eyesight works).
- Prosopagnosia. The patient cannot recognize people's faces.
- Topographagnosia. The patient cannot recognize landmarks in the environment.
Damage to these regions of the brain is either through trauma or disease.
Lesions On The Occipital/Temporal Lobe, Or Along The Pathway
Lesions are any damage the brain sustains through disease, genetics, or trauma. Lesions may start as asymptomatic and then later develop symptoms.
Lesions to the ventral stream result in the loss of discriminatory identification and recognition functions.
Lesions are often caused by:
- A stroke
- Vascular injury
- Impaired blood flow to the brain
- Infection (bacterial or viral)
- Certain diseases like muscular sclerosis, lupus, and tumors cause lesions in the brain.
- Genetic predispositions
- Toxins, radiation, and other chemicals
- Plaque build-up
- Trauma
- A natural process of aging
Some typical symptoms of brain lesions include:
- Blurry vision
- Convulsions (in server stages) and uncontrolled muscle movements
- Delayed speech
- Impaired movement
- Impaired hearing
- Headaches
- Loss of concentration/short concentration span
- Nausea
Where the lesion is in the brain will determine its treatment plan. Most critical is that you should seek medical advice if you suspect lesions.
The Effects Of Alzheimer’s Disease On The Ventral Stream
Alzheimer's is a degenerative disease that attacks parts of the brain, causing loss of function, cognitive ability, and motor skills, among others.
The patient loses sight when Alzheimer's disease attacks visually orientated brain cells (atrophy). The cortex of the brain dealing with memory is also affected.
Although the mechanics of what causes Alzheimer's are not fully understood, scientists believe that the disease is due to proteins not functioning properly, which has a knock-on effect on the synapses in the brain.
There is usually a plaque build-up in the brain. These aggregations create a toxic effect on the brain where they are found.
The symptoms of Alzheimer’s disease include:
- Dementia
- Memory loss (short and long term)
- Loss of cognitive abilities (logic, planning, performing, etc.)
- A change in behavior and social skills (they become depressed, angry, irritable, etc.)
Although there is no cure for Alzheimer's, many people manage their symptoms with medication.
The Effects Of Trauma On The Ventral Stream
Trauma is a severe cause of many issues relating to the brain and vision. Trauma occurs in many shapes and forms and has varying effects under different circumstances.
PTSD (Post-traumatic stress disorder) is a lasting effect trauma has on an individual. PTSD often causes the sufferer to become overwhelmed by sensory stimuli, such as light or sounds.
When subjected to visual stimuli, scientists discovered that those suffering from PTSD had lessened activity in the ventral stream (and associated areas).
Physical damage (trauma) can also inhibit the ventral stream and surrounding areas from functioning properly.
Conclusion
The brain is an important network of neurons and their various parts, working in unison to produce a functional organism. The ventral stream is part of this network, linking the visual processing areas of the occipital lobe to the memory-orientated temporal lobe. The ventral stream is critical for effective analysis of the visual field, and damages to this pathway (via disease and trauma) result in a loss of vision in the organism.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3532569/
https://pubmed.ncbi.nlm.nih.gov/22810517/
https://pubmed.ncbi.nlm.nih.gov/15265653/
https://www.ncbi.nlm.nih.gov/books/NBK482504/
https://www.sciencedirect.com/topics/neuroscience/retinotopy
