Tone disruptions in Mandarin post-stroke aphasia: an fNIRS study on Broca's area using the auditory oddball paradigm

Tone processing is essential in tonal languages such as Mandarin. Understanding how post-stroke aphasia (PSA) affects tone perception can guide targeted rehabilitation, especially in linguistically unique populations.

We aimed to explore the neural mechanisms of tone perception in Mandarin-speaking PSA patients and examine how brain network reorganization supports or hinders phonological processing.

Using functional near-infrared spectroscopy (fNIRS) and a tone-based auditory oddball paradigm, we compared cortical activation and functional connectivity (FC) patterns between PSA patients and healthy controls.

Patients with PSA showed reduced/reversed hemodynamic responses in the left Broca's area but increased FC with the right motor cortex. Despite this local hyperconnectivity, overall FC was lower in patients than in controls, especially among highly connected links ( $\left|r\right|>0.7$ ), suggesting either compensatory or maladaptive reorganization. Moreover, deoxygenated hemoglobin changes in the left Broca's area were positively associated with language function, as measured by aphasia quotient scores.

The findings highlight altered auditory-motor network dynamics in PSA, with Broca's area playing a central role in tone-based phonological processing. These results support the potential of fNIRS for clinical assessment and underscore the importance of accounting for network-level changes in aphasia rehabilitation strategies for tonal languages.