Visual cortex of the brain: structure, parts and pathways.

This part of the brain processes data from one of the most important senses: sight.

Sight is one of the most evolved and important senses in humans. in human beings. Thanks to it we can see the existence of stimuli or advantageous or threatening situations around us with a high level of precision, especially in daylight (for example, it allows us to observe if there are predators in the environment or if we have some kind of food available).

But seeing is not as simple a process as it may seem: it requires not only capturing the image but also interpreting its parameters, distance, shape, color, and even movement. At the brain level, these processes require processing that takes place in different brain regions. In this regard, the role of the visual cortex the role of the visual cortex of the brain stands out..

Throughout this article we will see what are the characteristics and parts of the visual cortex, through a summary about this part of the human brain.

Visual cortex: what is it and where is it?

The visual cortex is the part of the cortex mainly dedicated to the processing of visual stimulation coming from the human brain. processing visual stimulation from the photoreceptors of the retina.. It is one of the most represented senses at the level of the cortex, occupying most of the occipital lobe and a small part of the parietal cortex.

Visual information passes from the eyes to the lateral geniculate nucleus of the thalamus and to the superior colliculus, ipsilaterally, to finally reach the cerebral cortex for processing. Once there, the different information captured by the receptors are processed and integrated to give them a meaning and allow us the real perception of fundamental aspects such as distance, color, shape, depth or movement. fundamental aspects such as distance, color, shape, depth or movement, and finally to give them an overall meaning.and finally to give them an overall meaning.

However, the total integration of visual information (i.e., the last step of its processing) does not take place in the visual cortex, but in networks of neurons distributed throughout the rest of the cerebral cortex.

Main areas or parts of the visual cortex

The visual cortex does not consist of a single uniform structure, but includes different brain areas and pathways. includes different brain areas and pathways. In this sense, we can find the primary visual cortex (or V1) and the extrastriate cortex, which in turn is subdivided into different areas (V2, V3, V4, V5, V6).

1. Primary visual cortex

The primary visual cortex, also called striate cortex, is the first cortical area that receives visual information and performs its initial processing. It is made up of both simple cells (which respond only to stimulations with a specific position in the visual field and analyze very specific fields) and complex cells (which capture wider visual campuses), and is organized in a total of six layers. The most relevant of all of them is layer 4, which receives the information from the geniculate nucleus.

In addition to the above, it must be taken into account that this cortex is organized in hypercolumns, composed of functional columns of cells that capture elements from the geniculate nucleus. functional columns of cells that capture similar elements of visual information.. These columns capture a first impression of orientation and ocular dominance, depth and movement (which occurs in the columns called interblob) or a first impression of color (in the columns or blob regions also known as spots or drops).

In addition to the above, which the primary visual cortex begins to process on its own, it is worth noting that in this brain region a retinotopic representation of the eye, a topographic map of the eye, a topographic map of the eye's vision.It should be noted that in this brain region there is a retinotopic representation of the eye, a topographic map of vision similar to that of Penfield's homunculus as far as the somatosensory and motor systems are concerned.

2. Extra-striate or associative cortex

In addition to the primary visual cortex, we can find several associative brain areas of great importance in the processing of different features and elements of visual information. Technically there are about thirty areas, but the most relevant are those coded from V2 (remember that the primary visual cortex would correspond to V1) to V8. Part of the information obtained in the processing of the secondary areas will later be reanalyzed in the primary to be reanalyzed.

Their functions are diverse and they handle different information. For example, area V2 receives color information from the color regions and information regarding spatial orientation and movement from the interblob regions. The information passes through this area before going to any other area, being part of all visual pathways. Area V3 contains a representation of the lower visual field and has directional selectivity, while area V3 contains a representation of the lower visual field and has directional selectivity, while the posterior ventral area has a representation of the superior visual field determined with selectivity for color and orientation.

V4 is involved in the processing of stimulus shape information and recognition. Area V5 (also called medial temporal area) is mainly involved in the detection and processing of stimuli motion and depth, being the main region in charge of the perception of these aspects. V8 has color perception functions.

To better understand how visual perception works, however, it is advisable to analyze the passage of information through different pathways.

Main visual processing pathways

The processing of visual information is not static, but occurs along different visual pathways. occurs along different visual pathways of the encephalon, in which visual information is transmitted to the brain.in which the information is transmitted. In this sense, the ventral and dorsal pathways stand out.

1. Ventral pathway

The ventral pathway, also known as the "what" pathway, is one of the main visual pathways of the encephalon. would go from V1 in the direction of the temporal lobe.. Areas such as V2 and V4 are part of it, and are mainly responsible for observing the shape and color of objects, as well as depth perception. In short, it allows us to observe what we are observing.

Also, it is in this pathway where stimuli can be compared with memories as they pass through the inferior part of the temporal lobe, for example in areas such as the fusiform in the case of face recognition.

2. Dorsal pathway

The dorsal pathway runs along the top of the skull, going towards the parietal. It is called the "where" pathwaysince it works especially with aspects such as movement and spatial localization. The V5 visual cortex plays an important role in this type of processing. It allows to visualize where and at what distance the stimulus is, if it is moving or not and its speed.

Alterations caused by the lesion of the different visual pathways

The visual cortex is an element of great importance for us, but sometimes different lesions can occur that can alter and jeopardize its functionality.

The damage or disconnection of the primary visual cortex generates what is known as cortical blindness, in which, although the subject's eyes work correctly and receive the information, it cannot be processed by the brain, so it is not perceived. Also hemianopsia may also appear if damage occurs only in one hemisphere, resulting in blindness only in one hemisphere.blindness appearing only in one visual hemifield.

Lesions in other brain regions can cause different visual alterations. A lesion of the ventral pathway will probably generate some type of visual agnosia (either apperceptive in which it is not perceived or associative in which, although it is perceived, it is not related to emotions, concepts or memories), as the objects and stimuli presented to us cannot be recognized. For example, it could generate prosopagnosia or absence of identification of faces at a conscious level (although not necessarily at an emotional level).

Damage to the dorsal pathway could lead to acinetopsiainability to detect movement at the visual level.

Another probable alteration is the presence of problems in having a congruent perception of space, not being able to consciously perceive a part of the visual field. This is what occurs in the aforementioned hemianopsia or quadrantopsia (in this case we would be facing a problem in one of the quadrants).

Also, vision problems may appear such as difficulties in depth perception or blurred vision (similar to problems such as nearsightedness and farsightedness). (similar to what happens with ocular problems such as myopia and hyperopia). Problems similar to color blindness (monochromatism or dichromatism) or lack of color recognition may also occur.

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