Sight is one of the most evolved and important senses in the human being. Thanks to it we can see the existence of advantageous or threatening stimuli or 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 any 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 sense, highlights the role of the visual cortex of the 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 from the retinal photoreceptors . This is one of the most represented senses at the cortex level, with most of the occipital lobe and a small part of the parietal lobes occupying its processing.

The visual information passes from the eyes to the lateral geniculated nucleus of the thalamus and the upper colliculus, in an ipsilateral way, to finally reach the cerebral cortex for processing. Once there, the different information captured by the receptors is worked on and integrated to give them a sense and allow us to really perceive fundamental aspects such as distance, colour, shape, depth or movement , and finally to give them a joint sense.

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 extra-striated 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 initial processing of this information. It is made up of both simple cells (which respond only to stimulations with a specific position in the visual field and analyse very specific fields) and complex cells (which capture larger visual campuses), and is organised into a total of six layers. The most relevant of these is layer 4, as it is in this layer that the information is received from the genomic nucleus.

In addition to the above, it should be noted that this crust is organized into hypercolumns, composed of functional columns of cells that capture similar elements of visual information . These columns capture a first impression of the orientation and predominance of the eye, depth and movement (which happens in the columns called interblob) or a first impression of the colour (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 the Penfield homunculus as far as the somatosensory and motor systems are concerned.

2. Extruded or associative bark

In addition to the primary visual cortex, we may encounter various associative brain areas of great importance in the processing of different characteristics 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 be later analyzed again in the primary area to be reanalyzed.

Their functions are diverse and they handle different information. For example, the V2 area receives information about colour from the regions and information about spatial orientation and movement from the inter-blocks. The information passes through this area before going to any other area, forming part of all visual pathways. The V3 area contains a representation of the lower visual field and has directional selectivity, while the posterior ventral area has it from the upper visual field determined by colour and orientation selectivity.

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

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

Main visual processing routes

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

1. Ventral line

The ventral pathway, also known as the “what” pathway, is one of the main visual pathways of the brain, which would run from V1 towards the temporal lobe . It includes areas such as V2 and V4, which are mainly responsible for observing the shape and colour of objects, as well as depth perception. In short, it allows us to observe what we are observing.

Also, it is in this path where the stimuli can be compared with the memories when passing through the lower part of the temporal lobe, as for example in areas such as the fusiform in the case of face recognition.

2. Dorsal route

The dorsal route runs through the upper part of the skull towards the parietal. It is called the “where” pathway , as it works especially with aspects such as movement and spatial location. The participation of the visual cortex V5 stands out, with a great role in this type of processing. It allows us to visualize where and how far away the stimulus is, whether it moves or not and its speed.

Alterations caused by injury to the different visual pathways

The visual cortex is a very important element for us, but sometimes different injuries can 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 despite the fact that the subject’s eyes function correctly and receive the information, this cannot be processed by the brain, so it cannot be perceived. Also hemianopsia can appear if only one hemisphere is damaged , with blindness appearing in only one visual field

Injuries to other brain regions can cause different visual disturbances. A lesion of the ventral duct will probably generate some kind of visual agnosia (either aperceptive in which it is not perceived or associative in which although it is perceived it is not related to emotions, concepts or memories), as we are not able to recognize the objects and stimuli that are presented to us. For example, it could generate prosopagnosia or lack of identification of faces at a conscious level (although not necessarily at an emotional level).

Damage to the dorsal duct may cause acinetopsy , inability to detect movement at a 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 happens in the aforementioned hemianopsia or quadrantal scan (in this case we would be facing a problem in one of the quadrants).

Also, vision problems such as depth perception difficulties or blurred vision may occur (similar to what happens with eye problems such as myopia and hyperopia). Problems similar to colour blindness (we are talking about monochromatism or dichromatism) or lack of recognition for colour may also appear.

Bibliographic references:

  • 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.
  • Peña-Casanova, J. (2007). Behavioral neurology and neuropsychology. Editorial Médica Panamerica.
  • Possin, K.L. (2010). Visual spatial cognition in neurodegenerative disease. Neurocase 16 (6).