The visual cortex (visual cortex) is the part of the cerebral cortex that enables vision. It is located in the occipital lobe of the brain. Failures in the visual cortex lead to disturbances in image processing, resulting in visual field defects.
What is the visual cortex?
The visual cortex (visual cortex) represents the area of the cerebral cortex where image processing takes place from visual stimuli received in the eye to the complex representation of what is seen. It occupies the largest part of the occipital lobe of the brain. In Korbinian Brodmann’s brain map, it corresponds to brain areas 17, 18, and 19. The visual cortex is further divided into the primary visual cortex (V1) and the secondary and tertiary visual cortex. In primates including humans, the cell density of the visual cortex is very large. However, their thickness is very small, only 1.5 to 2 millimeters in humans. Area 17 represents the primary visual cortex and directly represents the opposite half of the visual field. It also has a retinotopic structure. This means that the dots mapped on the retina are also arranged in the same way in the visual cortex. Because area 17 (primary visual cortex) has a striate structure, it is also called area striata.
Anatomy and structure
The visual cortex is divided into the primary, secondary, and tertiary visual cortex, as mentioned earlier. The primary visual cortex initially receives visual stimuli relayed from the retina via the thalamus. The primary visual cortex consists of six cell layers. The first two layers contain so-called Magno cells. These are large cells responsible for motion perception. The next four layers are represented by Parvo cells. The Parvo cells are small and control the perception of objects through color and structure representation. The ganglion cells in the primary cortex are arranged like the receptors in the retina. Thus, the cells in the primary cortex that are supposed to represent the fovea are the most numerous. The fovea is the area of sharpest vision in the retina and therefore contains the most optical receptors. In addition to the division into layers, there is also a division into columns. There are orientation columns, dominance columns and hypercolumns. The downstream cells in each column are arranged in the same way as the dots mapped in the retina. Thus, each orientation column responds only to a line of a specific point in the retina. The system of lines is recorded as an image of the environment in contours. A dominance column is composed of several orientation columns of differently oriented lines from the same point in the retina. Moreover, in addition to orientation columns, dominance columns also consist of so-called blobs. Blobs represent columns that respond to colors. Hypercolumns, in turn, consist of dominance columns from both eyes of the same visual field. Thus, they are each composed of two dominance columns (one per eye). From the primary visual cortex, the image information is transmitted to the secondary and primary visual cortex via two different pathways for further processing.
Function and tasks
The visual cortex has the task of receiving optical stimuli and processing them step by step in such a way that the environment is imaged. In this process, after the stimulus is received, the information is decomposed, analyzed, abstracted, and passed on in an orderly form to the next processing stage. While the processes in the primary visual cortex are largely known, the further processing of information is not so easily understood. From the primary visual cortex, the stimulus is relayed via a dorsal parietal and a ventral temporal pathway. The parietal processing stream is used to perceive motion as well as position and is also referred to as the Wo stream. The temporal stream is used to recognize objects through color, pattern, and shape perception. Accordingly, it is also referred to as the what stream. In the further course of image processing, the links between image representation, reaction and behavior become more and more complex. Not only the current image serves as the basis for action, but also the images stored in memory. Thus, similar processes occur in visual representations as in image processing.
Diseases
Lesions in the visual cortex lead to visual perception dysfunction.The failure symptoms depend on which areas of the visual cortex fail. If the primary visual cortex is damaged, visual field deficits occur. In the worst case, complete blindness can occur. This form of blindness is also called cortical blindness. The function of the visual pathway is still completely intact, but the image information is no longer transmitted. Unconsciously, the patient still reacts to visual stimuli, although he no longer sees anything. However, he is still able to grasp and name objects when prompted to do so. This condition is colloquially called blindsight. When the secondary or tertiary visual cortex fails, blindness does not occur. The image is still perceived completely. However, the reference to the persons or objects is partially lost here. Since in this phase of image processing the complex relationships between visual perception and recognition of the objects are controlled, the persons or objects can partly no longer be recognized. This is a case of agnosia. Hallucinations may also occur. Often, secondary or tertiary visual cortex dysfunction also results in synesthesia, in which different sensory perceptions are combined to form a subjective sensation.
