Visual cortex of the brain: structure, parts and pathways

Sight is one of the most evolved and important senses in the human being. 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 we have some type of food available).

But seeing is not as simple a process as it may seem: it is not only required to capture the image but also to interpret 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 sense, highlights the role of the visual cortex of the brain.

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.

• Related article: "Parts of the human brain (and functions)"

Visual cortex: what is it and where is it?

The part of the cortex mainly dedicated to the visual cortex is known as the visual cortex.

processing visual stimulation from retinal photoreceptors. It is one of the most represented senses at the level of the cortex, its processing occupying most of the occipital lobe and a small part of the parietals.

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 receivers are worked on and integrated to give them a meaning and allow us the real perception of fundamental aspects such as distance, color, shape, depth or movement, and finally to give them a joint meaning.

However, the full integration of visual information (that is, the last step of its processing) does not it takes 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 is not made up of a single uniform structure, but rather 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 the striated cortex, is the first cortical area that receives visual information and performs a first processing of it. 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) as complex (which capture broader visual campuses), and is organized into a total of six layers. The most relevant of all of them is number 4, since it 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 similar elements of visual information. These columns capture a first impression of the orientation and ocular dominance, depth and movement (what happens in the columns called interblob) or a first impression of the 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 by itself, 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 in terms of the somatosensory and motor system.

• You may be interested: "Penfield's sensory and motor homunculi: what are they?"

2. Extra-striated or associative cortex

In addition to the primary visual cortex, we can find various associative brain areas of great importance in the processing of different characteristics and elements of visual information. Technically there are around 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 re-analyzed in the primary area to be re-analyzed.

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

The V4 participates in the processing of information in the form of stimuli and in their recognition. Area V5 (also called medial temporal area) is primarily involved in detection and processing of the movement of stimuli and depth, being the main region in charge of the perception of these aspects. The V8 has color perception functions.

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

Main visual processing pathways

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

1. Ventral route

The ventral pathway, also known as the “what” pathway, is one of the main visual pathways of the brain, which 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 the perception of depth. Ultimately it allows us to observe what we are observing.

Likewise, it is in this way that stimuli can be compared with memories as they pass through the lower part of the temporal lobe, for example in areas such as fusiform in the case of face recognition.

2. Dorsal route

Regarding the dorsal pathway, it runs through the upper part of the skull, going towards the parietal. It is called the "where" route, since it works especially with aspects such as movement and spatial location. The participation in it of the visual cortex V5 stands out, with a great 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 damage to the different visual pathways

The visual cortex is an element of great importance to us, but different injuries can sometimes occur that can alter and endanger its functionality.

The damage or disconnection of the primary visual cortex generates what is known as cortical blindness, in which although the eyes of the subject work correctly and receive the information, this cannot be processed by the brain, so it is not possible to perceive. Also hemianopia may occur if damage occurs in only one hemisphere, appearing blindness only in a visual hemifield

Injuries to other brain regions can cause different visual disturbances. A lesion of the ventral pathway will probably generate some type of visual agnosia (be it apperceptive in which it is not perceived or associative in the that although it is perceived, it is not related to emotions, concepts or memories), not being able to recognize the objects and stimuli that are present. For example, you could generate prosopagnosia or lack of identification of faces on a conscious level (although not necessarily on an emotional level).

Damage to the dorsal tract could cause acinetopsia, inability to detect movement visually.

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

Also, vision problems such as difficulties in depth perception or blurred vision (similar to what happens with eye problems such as nearsightedness and farsightedness). Problems similar to color blindness may also appear (let's be talking about monochromaticism or dichromatism) or lack of recognition for color.

Bibliographic references:

• Horton, J.C.; Adams, D.L. (2005). The cortical column: a structure without a function. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 360 (1456): pp. 837 - 862.
• Kandel, E.R.; Schwartz, J.H.; Jessell, T.M. (2001). Principles of Neuroscience. Madrird: MacGrawHill.
• Kolb, B. & Wishaw, I. (2006). Human neuropsychology. Madrid: Editorial Médica Panamericana.
• Lui, J.H.; Hansen, D.V.; Kriegstein, A.R. (2011). Development and evolution of the human neocortex. Cell. 146 (1): pp. 18 - 36.
• Peña-Casanova, J. (2007). Behavioral neurology and neuropsychology. Panamerica Medical Publishing House.
• Possin, K.L. (2010). Visual spatial cognition in neurodegenerative disease. Neurocase 16 (6).
• Richman, D.P.; Stewart, R.M.; Hutchinson, J.W.; Caviness, V.S. (1975). Mechanical model of brain convolutional development. Science. 189(4196): 18 - 21.