Understanding how the brain allocates resources to match the demands of active neurons
under physiological conditions is critically important. Increased metabolic demands
of active brain regions are matched with hemodynamic responses known as neurovascular
coupling (NVC). Several methods that allow noninvasive assessment of brain activity
in humans detect NVC and early detection of NVC impairment may serve as an early marker
of cognitive impairment. Therefore, non-invasive NVC assessments may serve as a valuable
tool to detect early signs of cognitive impairment and dementia. Working memory tasks
are routinely employed in the evaluation of cognitive task-evoked NVC responses. However,
recent attempts that utilized functional near-infrared spectroscopy (fNIRS) or transcranial
Doppler sonography (TCD) while using a similar working memory paradigm did not provide
convincing evidence for the correlation of the hemodynamic variables measured by these
two methods. In the current study, we aimed to compare fNIRS and TCD in their performance
of differentiating NVC responses evoked by different levels of working memory workload
during the same working memory task used as cognitive stimulation. Fourteen healthy
young individuals were recruited for this study and performed an n-back cognitive
test during TCD and fNIRS monitoring. During TCD monitoring, the middle cerebral artery
(MCA) flow was bilaterally increased during the task associated with greater cognitive
effort. fNIRS also detected significantly increased activation during a more challenging
task in the left dorsolateral prefrontal cortex (DLPFC), and in addition, widespread
activation of the medial prefrontal cortex (mPFC) was also revealed. Robust changes
in prefrontal cortex hemodynamics may explain the profound change in MCA blood flow
during the same cognitive task. Overall, our data support our hypothesis that both
TCD and fNIRS methods can discriminate NVC evoked by higher demand tasks compared
to baseline or lower demand tasks.
