Prefrontal cortex supports speech perception in listeners with cochlear implants

Cochlear implants are neuroprosthetic devices that can restore hearing in people with severe to profound hearing loss by electrically stimulating the auditory nerve. Because of physical limitations on the precision of this stimulation, the acoustic information delivered by a cochlear implant does not convey the same level of acoustic detail as that conveyed by normal hearing. As a result, speech understanding in listeners with cochlear implants is typically poorer and more effortful than in listeners with normal hearing. The brain networks supporting speech understanding in listeners with cochlear implants are not well understood, partly due to difficulties obtaining functional neuroimaging data in this population. In the current study, we assessed the brain regions supporting spoken word understanding in adult listeners with right unilateral cochlear implants (n=20) and matched controls (n=18) using high-density diffuse optical tomography (HD-DOT), a quiet and non-invasive imaging modality with spatial resolution comparable to that of functional MRI. We found that while listening to spoken words in quiet, listeners with cochlear implants showed greater activity in the left prefrontal cortex than listeners with normal hearing, specifically in a region engaged in a separate spatial working memory task. These results suggest that listeners with cochlear implants require greater cognitive processing during speech understanding than listeners with normal hearing, supported by compensatory recruitment of the left prefrontal cortex

A systematic review of neuroimaging approaches to mapping language in individuals

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

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

Task-based functional magnetic resonance imaging (fMRI) has become the method of choice for studying localized function in the human brain. Functional MRI studies often rely on group-level results to derive conclusions about the neurobiology of language. However, doing so without accounting for the complexities of individual brains may reduce the validity of the findings. Furthermore, understanding brain organization in individuals is critically important for both basic science and clinical application. To assess the state of single-subject language localization in the functional neuroimaging literature, we carried out a systematic review of studies published through April 2020. Out of 977 papers identified through our search, 121 met our inclusion criteria for reporting single-subject fMRI results. Of these, 20 papers reported using a single-subject test-retest analysis to assess reliability. Specific metrics included overlap measures (like the Dice coefficient), correlation measures (like Intraclass Correlation Coefficient), Euclidean Distance between peak activation/center of mass, and Sensitivity/Specificity. These papers varied substantially in their experimental paradigms and stimuli, making more detailed comparisons impossible. In the absence of quantified reproducibility, results from paradigms used for single-subject language localization may need to be treated with caution. Incorporating reliability and validity measures in language mapping paradigms increases the likelihood that task-based activations are reproducible. Our search found that a relatively modest number of papers reporting single-subject results quantified single-subject reliability. Future endeavors to optimize the localization of language networks in individuals will benefit from the broader reporting of reliability metrics for different tasks and acquisition parameters