Is Order and Content Information Processed Separately in Phonological Working Memory? An fNIRS study.

Phonological working memory (pWM) is crucial for language processing as it enables the maintenance and manipulation of representations of speech sounds. This process entails retaining the identity of the perceived stimuli (i.e., phonemes, syllables) as well as their sequential order. While order and content processing are crucial for complex tasks like language comprehension, their neural correlates are debated. This study further investigates the neural mechanisms underlying order and content processing in pWM and explores their modulation by task difficulty and WM load. Thirty young Italian adults participated in a complex nonword recognition task, that included an unrelated processing episode, while their brain activity over frontal, temporal, and parietal regions was recorded using functional near-infrared spectroscopy (fNIRS). The task involved three levels of WM load (five-, seven-, and nine-syllable nonwords) in three conditions: no change, order change, and content change. Behaviorally, increasing WM load led to lower accuracy and slower reaction times, with the order change condition being more affected by load than the no change and content change conditions. The analysis of the fNIRS data revealed several findings: 1) with no memory processes involved, the dorsolateral prefrontal cortex (DLPFC) showed a scaling load effect, consistent with the literature; 2) when WM was involved, a reversed load effect was found in the DLPFC, inferior frontal gyrus (IFG), superior temporal gyrus (STG), and inferior and superior parietal lobes (IPL, SPL); 3) the order condition elicited lower activation compared to both identical and content change conditions in the DLPFC, STG, IPL, SPL and motor areas. These results demonstrate distinct patterns of neural activity for order and content processing in the brain areas involved in pWM. The findings also highlight the usability of the paradigm and fNIRS in assessing order and content processing in pWM, offering promising directions for future research in other populations.