A comprehensive protocol to study the effects of multilingualism on cognition and the brain in patients with progressive neurological diseases

Multilingualism, the cognitively demanding experience of speaking multiple languages, has been linked to measurable effects on cognition and brain structure and function. These effects, attributed to the constant activation of multiple languages, may contribute to cognitive reserve, expressed as delayed cognitive decline in multilinguals diagnosed with progressive neurological diseases such as Alzheimer's, Huntington's and Parkinson's diseases, and Multiple Sclerosis. However, the evidence remains inconsistent due to methodological shortcomings, including variations in defining multilingualism and inadequate accounting for other lifestyle factors that may also promote cognitive reserve. To address these limitations, we developed a comprehensive protocol which incorporates detailed measurement of factors that are related to cognitive reserve, including multilingual experiences, standardized clinical and experimental cognitive assessments suitable for multilingual patients diagnosed with the above progressive neurological diseases, as well as thorough neuroimaging testing batteries. This protocol enables systematic investigation across linguistic and cultural contexts, facilitating cross-laboratory data pooling to advance understanding of multilingualism's neuroprotective potential in aging and disease. Its purpose is to spearhead an open-ended project which will keep adding to an increasing dataset of multilingual patients internationally; crucially, it sets a new standard in how to gather evidence investigating multilingualism as a factor contributing to cognitive reserve.