A systematic review of neuroimaging approaches to single-subject studies of language processing

Abstract

Background. Task-based functional MRI has become the method of choice for researchers studying functional localization in the human brain. However, for a deeper understanding of brain function beyond group level generalizations, it is crucial to account for the reliability of mapping in a single subject. Individual differences can influence group results in a multitude of ways and consequently lead to the mischaracterization of functional areas. Such errors can be detrimental to the accuracy of both basic research and clinical prognosis. Methodology. We performed a systematic review with the goal of understanding the state of the literature pertaining to mapping language using fMRI in individual participants. A thorough database search was carried out on published literature through September 2018. Results. Out of 473 papers identified through our literature search, 54 met our inclusion criteria for reporting single-subject fMRI results. Of these, only 11 papers reported using single-subject level reliability metrics (such as Dice coefficient, Intraclass Correlation Coefficient, Euclidean distance, or Receiver Operating Characteristic) in their results. Among other categories, papers focused on multimodal validation of fMRI results, comparing performance between language tasks, technical development of protocols and clinical case studies on specific disease conditions. Conclusion. Incorporating reliability and validity measures in language mapping paradigms increases the likelihood that task-based activations in the brain are reproducible. However, very few papers reported this approach. In the absence of quantified reproducibility, results from paradigms used for single-subject language mapping may need to be treated with caution.