Exploring age and hemispheric differences in cortical plasticity after iTBS using fNIRS

Non-invasive brain stimulation applied to the prefrontal cortex (PFC) has been shown to improve cognitive outcomes in older adults with cognitive impairments (Miller et al., 2023). However, the differential impact of left versus right dorsolateral prefrontal cortex (DLPFC) stimulation on prefrontal oxygenation levels, as well as its modulation across age groups, remains insufficiently understood. 45 adults completed a within-subjects design completing 4 cognitive tasks before and after intermittent TBS (iTBS) stimulation applied to the left and right DLPFC (F3 and F4). FNIRS was recorded concurrently, with 12 optode channels spanning across the left, medial and right prefrontal cortex measuring oxygenation levels in response to cognitive task performance. Age-related effects were observed with younger adults showing significant increased HbO in response to left iTBS during inhibition tasks. Learning tasks revealed bilateral increases in middle-aged participants and decreases in older adults after right DLPFC stimulation. Working memory showed bilateral increased HbO after right iTBS stimulation in both younger and middle-aged adults. Older adults revealed a decrease in HbO post iTBS, that was centred in the midline after right iTBS, but bilateral after left iTBS. Our novel findings show the dispersion of oxygenation level changes in the ipsilateral and contralateral hemispheres during excitatory TBS on the DLPFC was age dependent. In younger adults' subsequent task related brain activity was 'up-regulated', whereas older adults revealed 'down-regulation' reflecting increased efficiency. These findings cast new light on the dynamic properties of brain stimulation that is highly influenced by age-related factors and cognitive task.