The temporoparietal junction (TPJ) is an area of the brain where the temporal and parietal lobes meet, at the posterior end of the lateral sulcus (Sylvian fissure). The TPJ incorporates information from the thalamus and the limbic system as well as from the visual, auditory, and somatosensory systems. The TPJ also integrates information from both the external environment as well as from within the body. The TPJ is responsible for collecting all of this information and then processing it. This area is also known to play a crucial role in social cognition, including perspective-taking, empathy, self–other distinctions processes and theory of mind (ToM). The TPJ is also involved in attentional reorientation, enabling individuals to shift attention toward relevant environmental stimuli. Furthermore, damage to the TPJ has been implicated in having adverse effects on an individual's ability to make moral decisions and has been known to produce out-of-body experiences (OBEs). Electromagnetic stimulation of the TPJ can also cause these effects. Apart from these diverse roles that the TPJ plays, it is also known for its involvement in a variety of widespread disorders including anxiety disorders, amnesia, Alzheimer's disease, autism spectrum disorder, and schizophrenia.
Anatomy and function
The brain contains four main lobes: temporal lobe, parietal lobe, frontal lobe, and the occipital lobe. The temporoparietal junction lies in the region between the temporal and parietal lobes, near the lateral sulcus (Sylvian fissure). Specifically, it is composed of the inferior parietal lobule and the caudal parts of the superior temporal sulcus. There are two halves to the temporoparietal junction, with each component in their respective hemispheres of the brain. Each half of the TPJ pertains to various aspects of cognitive function. Often, however, the separate halves of the TPJ will work in coordination. The TPJ is mainly involved in information processing and perception.
Right temporoparietal junction The right temporoparietal junction (rTPJ) is involved in the processing of information in terms of the ability of an individual to orient attention to new stimuli. Evidence from neuroimaging studies as well as lesion studies revealed that the rTPJ plays a pivotal role in analyzing signals from self-produced actions as well as with signals from the external environment. For example, an individual with lesions in their rTPJ would more than likely exhibit a sense of hemi-neglect, wherein they would no longer be able to pay attention to anything they observe on the left. So, if someone were to have a lesion in their rTPJ, then over time the awareness of the left limbs may fade without treatment. Visual signals provide the sensory information necessary for the brain to process spatial recognition of the world. When vision is limited, knowledge of existence begins to fade away since as far as the brain is concerned the object does not exist. Furthermore, the rTPJ plays a role in the way individuals observe and process information, thus impacting social interaction. Empathy and sympathy require an individual to simultaneously distinguish between different possible perspectives on the same situation. Imaging studies show that this ability depends upon the coordinated interaction of the rTPJ to identify and process the social cues presented to it. This rapid process allows for an individual to quickly react to situations.
Left temporoparietal junction The left temporoparietal junction (lTPJ) contains both Wernicke's area and the angular gyrus, both prominent anatomical structures of the brain that are involved in language cognition, processing, and comprehension of both written and spoken language. Steven Pinker discusses this brain region, theorising that it underlies an amodal 'language of thought' or Mentalese. The lTPJ, in this account, takes in observations from external environments, such as conversations, makes connections in the brain regarding memories or incidents and then converts those thoughts and connections to written and spoken language. Pinker's full account of this is explained in The Language Instinct: How the Mind Creates Language. The lTPJ also plays an important role in reasoning of other's beliefs, intentions, and desires. Activation of the lTPJ was observed in patients processing mental states such as beliefs when an fMRI was used on patients as they were asked to make inferences regarding the mental states of others such as lying. This study was further supplemented by a study which identified that lesions to the left TPJ can impair cognitive processes specifically involved in the inference of someone else's belief, intention, or desire. Individuals with lesions in the lTPJ were no longer able to correctly identify when someone was lying or insinuating a false sense of belief or desire. The lTPJ is also involved in the processing of associating and remembering the names of individuals and objects.
Disorders The dopaminergic-serotonergic system mediates our ability to distinguish and understand others' beliefs as well as predict their behavior in light of that understanding. In certain disorders involving the dopaminergic-serotonergic system, this mentalizing process is disrupted and part or all of the process is impaired; this includes amnesia, Alzheimer's disease, and schizophrenia.
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