Advantages of double density alignment of fNIRS optodes to evaluate cortical activities related to phonological short-term memory using NIRS-SPM.

Phonological short-term memory (STM) and working memory (WM) capacity, which is temporal storage of phonological information, facilitates language development. This capacity is often impaired in cochlear implant (CI) users who have congenital hearing loss. Functional near-infrared spectroscopy (fNIRS) is potentially useful to reveal underlying mechanisms of this impairment due to its tolerance to magnetic and electrical artifacts generated by CIs. The spatial resolution of the standard fNIRS, however, seems inadequate to evaluate cortical activity associated with the maintenance of phonological information in the STM/WM tasks. In the present study recruiting 14 normal hearing adults, we applied a double density alignment of fNIRS optodes to improve spatial resolution in the generalized linear model (GLM)-based statistical analysis using NIRS-SPM, in which cortical activities were estimated during each of three stages (encoding, maintenance, and retrieval) in pseudoword STM tasks with auditory or visual presentation. Since the double density alignment of fNIRS optodes contains two sets of standard density arrays, in the off-line analysis the measured cortical hemodynamic responses can be analyzed as data from two independent standard density arrays as well as those from one double density array (DD-array) which has two times higher density of channels. The two standard arrays demonstrated a similar pattern of cortical activation at each stage of the auditory and visual tasks, which proved the reliability of our fNIRS analysis, but failed to detect significant cortical activation in 2 of 12 conditions including the maintenance stage in the visual task. On the other hand, DD-array revealed significant cortical activation in all conditions. These differences were observed when estimated cortical activation was localized in small regions, which suggests higher spatial resolution in DD-array than the standard arrays. In our knowledge, this is the first clinical study supporting the previous experimental phantom study which demonstrated improvement of spatial resolution in the double density arrangement of fNIRS optodes. These findings imply that the double density alignment of fNIRS optodes improves reliability and spatial resolution in fNIRS-based estimation of cortical activity in the STM/WM studies, although further studies are required to determine usefulness of this method in the CI population.