Most modern neurological studies of bilingualism employ functional neuroimaging techniques to uncover the neurological underpinnings of multilingualism and how multilingualism is beneficial to the brain. Neuroimaging and other neurological studies have demonstrated in recent years that multilingualism has a significant impact on the human brain. The mechanisms required by bilinguals to
code-switch (that is, alternate rapidly between multiple languages within a conversation), not only demonstrate increased connectivity and density of the neural network in multilinguals, but also appear to provide protection against damage due to age and age-related pathologies, such as Alzheimer's. Multilingualism, especially bimodal multilingualism, can help slow to process of cognitive decline in aging. It is thought that this is a result of the increased work load that the executive system, housed mostly in the frontal cortex, must assume in order to successfully control the use of multiple languages at once. This means that the cortex must be more finely tuned, which results in a "neural reserve" that then has neuroprotective benefits. Gray matter volume (GMV) has been shown to be significantly preserved in bimodal bilinguals as compared to monolinguals in multiple brain areas, including the hippocampus, amygdala, anterior temporal lobes, and left insula. Similarly, neuroimaging studies that have compared monolinguals, unimodal bilinguals, and bimodal bilinguals provide evidence that deaf signers exhibit brain activation in patterns different from those of hearing signers, especially in regards to the left superior temporal sulcus. In deaf signers, activation of the
superior temporal sulcus is highly lateralized to the left side during facial recognition tasks, while this lateralization was not present in hearing, bimodal signers. Bilinguals also require an effective and fast neural control system to allow them to select and control their languages even while code-switching rapidly. Evidence indicates that the left
caudate nucleus—a centrally located brain feature that is near the thalamus and the basal ganglia—is an important part of this mechanism, as bilinguals tend to have significantly increased GMV and activation in this region as compared to monolinguals, especially during active code-switching tasks. As implied by the significant preservation of gray matter in the
hippocampi (an area of the brain largely associated with memory consolidation and higher cognitive function, such as decision-making) of bimodal bilinguals, areas of the brain that help control phonological working memory tend to also have higher activation in those individuals who are proficient in two or more languages. There is also evidence that suggests that the age at which an individual acquires a second language may play a significant role in the varying brain functions associated with bilingualism. For example, individuals who acquired their second language early (before the age of 10) tend to have drastically different activation patterns than do late learners. However, late learners who achieve full proficiency in their second language tend to show similar patterns of activation during auditory tasks regardless of which language is being used, whereas early learners tend to activate different brain areas depending upon which language is being used. Along with the neuroprotective benefits that help to prevent onset of age-related cognitive issues such as dementia, bimodal bilinguals also experience a slightly different pattern of organization of language in the brain. While hearing bimodal bilinguals showed less parietal activation than deaf signers when asked to use only sign language, those same bimodal bilinguals demonstrated greater left parietal activation than did monolinguals. Parietal activation is not typically associated with language production but rather with motor activity. Therefore, it is logical that bimodal bilinguals, when switching between speech- and sign-based language, stimulate their left parietal areas as a result of their increased need to combine both motor action and language production. Moreover, it has been proven that there is a parallel or simultaneous language activation during language use. This activation occurs when the bilingual uses language, regardless of the L1 or L2, being used. The dominance or lack of dominance of a language does not impact the stimulation of the brain when a language is being used. This same activation happens with any language modality, meaning the brain is activated whether the language is written, signed, or spoken. == Sociolinguistic and identify considerations ==