The parahippocampal gyrus is a turn of the cerebral cortex. It is part of the limbic system, contributes to memory processes, and plays an important role in visual recognition.
What is the parahippocampal gyrus?
The parahippocampal gyrus is located in close proximity to the hippocampus. This is a part of the archicortex, which in turn is part of the cerebrum. Phylogenetically, the archicortex is younger than the neocortex, but older than the paleocortex. Medicine classifies the hippocampus as part of the limbic system, to which the parahippocampal gyrus also belongs. Within this system, the hippocampus is primarily involved in memory processes. Anatomically, the parahippocampal gyrus is not completely demarcated from the surrounding brain mass. It merges into the uncus on one side and borders the occipitotemporal gyrus medialis (lingual gyrus or infracalcarinus gyrus) on the other. Beneath the parahippocampal gyrus and the occipitotemporalis medial gyrus lies the occipitotemporalis lateralis gyrus (subcuneus).
Anatomy and structure
The anterior region of the parahippocampal gyrus contains part of the entorhinal cortex. This is also known as the association cortex and consists of three sections: the frontal, parietal, and limbic association cortex. The latter is the portion located in the parahippocampal gyrus. It corresponds to Brodmann areas 28 and 34. The limbic association cortex can be further divided into a ventral and a dorsal area. The posterior part of the parahippocampal gyrus belongs to the parahippocampal cortex, to which anatomy also assigns areas of the lateral occipitotemporal gyrus. Also located within the cerebral gyrus is the “parahippocampal place area,” which is relevant to visual recognition. The cortex of the parahippocampal gyrus is composed of six layers of cells. Overall, the tissue counts as gray matter because it is composed primarily of neuronal bodies. The actual processing of information takes place in the neuron networks. In contrast to gray matter, the white matter of the brain consists mainly of myelinated nerve fibers. Nerve fibers are the thread-like projections of neurons and carry the electrical signals from the neurons.
Function and tasks
The parahippocampal gyrus forms a component of the limbic system, which is composed of several anatomical structures. These are interconnected and dedicated to tasks such as emotion, memory, learning, and some autonomic control processes. However, these functions are not exclusive to the limbic system. For example, no central store exists for memory in the brain. Instead, memory processes such as memorization and recall are distributed across different brain areas. The default mode network plays a central role in memory processes. It represents a functional network of different brain structures. According to research findings, the parahippocampal gyrus may play a key role within the default mode network by mediating between the network and the medial temporal lobe (Ward et al., 2014). The parahippocampal gyrus also establishes associations. The so-called association cortex is the entorhinal cortex that occupies a central position in Alzheimer’s dementia, among other diseases. In addition, the parahippocampal gyrus may participate in associations in social situations. Furthermore, the parahippocampal gyrus is involved in visual recognition, with the “parahippocampal place area” playing an important role. The activity of this area is related to the viewing of landscapes and spaces. However, the parahippocampal gyrus is not responsible for primary sensory perception (vision proper), but performs a higher cognitive function. Recognition comes into play only after sensory perception and refers to identifying or classifying what is seen.
Diseases
Decreased activity in the parahippocampal gyrus and hippocampus is associated with schizophrenia. Schizophrenia is a mental disorder characterized by delusions and hallucinations. Other possible symptoms include association breakdown, ego disorders, striking expressions (for example, neologisms), emotional arousal, and thought distraction.These signs of illness represent so-called positive symptoms. The counterpart to them are negative symptoms such as emotional flattening, reduced affect, social withdrawal, cognitive and linguistic reduction, apathy, and reduced activity and initiative. Because schizophrenia is a very complex disorder, it can manifest differently from person to person. For the treatment of schizophrenia, accompanying psychotherapy, psychoeducation or special training can be considered in addition to drug therapy. The “parahippocampal place area”, located within the parahippocampal gyrus, is significant for the visual recognition of landscapes and spaces. Lesions in this area therefore typically lead to problems in recognizing these views. The affected person is still able to see and identify individual objects, but he or she can no longer relate the overall picture. Such lesions can result from a tumor, hemorrhage, inflammation, or stroke, for example. Abnormalities of the parahippocampal gyrus are also possible in association with temporal lobe epilepsy. The disease may be associated with hippocampal sclerosis, also known as mesial temporal sclerosis, which manifests as failure of neurons in the affected area. Medical science distinguishes four different types of hippocampal sclerosis, of which type 1B is the most common and is considered severe hippocampal sclerosis. Doctors often treat temporal lobe epilepsy with medication, but in some cases, other treatments such as brain surgery may be considered.
