Biology subjects

Johari, K.

Publications and source records attributed to Johari, K..

2 recordsLinked to original sources

bioRxiv2026

Age-Related Changes in the Neural Dynamics of Speech Production Following Noninvasive Brain Stimulation

IntroductionNon-invasive brain stimulation is a promising technique to restore neural function in older adults by enhancing cortical excitability. In the present study, we examined the behavioral and neural effects of three stimulation protocols over the supplementary motor area on speech production in older adults. These protocols included fixed-frequency and personalized high-definition transcranial alternating current stimulation (HD-tACS) and high-definition transcranial random noise stimulation (HD-tRNS). We also compared the effects of personalized HD-tACS and tRNS between younger and older adults. Two groups, older adults (mean age: 69 y {+/-} 8.18) and younger adults (mean age: 19.87 y {+/-} 1.54), participated in this study. Older adults attended a four-session experiment including sham, fixed-frequency beta-HD-tACS (17 Hz), personalized beta-HD-tACS, and HD-tRNS. Younger participants also followed a similar study design but without fixed-frequency beta-HD-tACS. Following 25 minutes of stimulation, participants were asked to complete a speech production task while their EEG activity was recorded. Both personalized tACS and tRNS significantly reduced reaction times compared to sham in younger and older adults. In older adults, personalized tACS and tRNS both effectively modulated speech motor network but exhibited distinct spatial profile. While personalized tACS produced more focal modulation, tRNS elicited broader modulation across distributed neural networks and engaged multiple oscillatory frequencies. Broadband and personalized stimulation approaches (personalized tACS and tRNS) were more effective than fixed-frequency beta-tACS in modulating behavioral and neural responses measured throughout a distributed speech-motor network in older adults.

neuroscience↗

bioRxiv2025

High-Frequency Spinal Cord Stimulation Reorganizes Cortical Cross-Frequency Coupling in a Region- and Time-Dependent Manner

Pain management strategies have progressed beyond traditional pharmacologic and physical interventions, integrating advanced neuromodulation techniques such as deep brain stimulation, peripheral nerve stimulation, and high-frequency spinal cord stimulation (hSCS). Despite its clinical efficacy, the supraspinal mechanisms underlying hSCS remain poorly understood. Prior work in sheep demonstrated that hSCS modulates gamma ({gamma}) band (70-150 Hz) activity in the primary somatosensory and association cortices, implicating cortical involvement in pain modulation. Given, the interaction between low and high oscillations, we hypothesized that hSCS modulates {gamma} activity in a region- and time-dependent manner through specific coupling with theta ({square}) rhythms (4-8 Hz). Using 96-channel subdural electrocorticography (ECoG), we computed {square}-{gamma} phase-amplitude coupling (PAC) and the corresponding modulation index (MI) to quantify the effects of hSCS. While the preferred {square}phase of {gamma} activity remained consistent across conditions and regions, MI increased significantly post-stimulation--most prominently in the association cortex, where robust -{gamma} phase locking was observed. In contrast, the somatosensory cortex exhibited weaker and more variable locking. Temporally, both cortices demonstrated an early, rapid increase in MI post-hSCS, accompanied by a shift (association) and attenuation (somatosensory) of the secondary peak. These findings reveal distinct regional and temporal dynamics in PAC following hSCS and suggest complementary roles of somatosensory and association cortices in processing neuromodulatory input. hSCS appears to reorganize cortical cross-frequency interactions, supporting its role in reorganizing functional network dynamics relevant to sensory processing and the subjective pain experience.

neuroscience↗