Investigating the post-stimulus undershoot of the BOLD signal--a simultaneous fMRI and fNIRS study

Neuroimage. 2006 Apr 1;30(2):349-58. doi: 10.1016/j.neuroimage.2005.09.048. Epub 2005 Oct 27.


Measuring the hemodynamic response with functional magnetic resonance imaging (fMRI) together with functional near-infrared spectroscopy (fNIRS) may overcome limitations of single-method approaches. Accordingly, we measured the event-related hemodynamic response with both imaging methods simultaneously in young subjects during visual stimulation. An intertrial interval of 60 s was chosen to include the prolonged post-stimulus undershoot of the blood oxygenation level dependent (BOLD) signal. During visual stimulation, the BOLD signal, oxy-, and total hemoglobin (Hb) increased, whereas deoxy-Hb decreased. The post-stimulus period was characterized by an undershoot of the BOLD signal, oxy-Hb, and an overshoot of deoxy-Hb. Total Hb as measured by fNIRS returned to baseline immediately after the end of stimulation. Results suggest that the post-stimulus events as measured by fNIRS are dominated by a prolonged high-level oxygen consumption in the microvasculature. The contribution of a delayed return of blood volume to the BOLD post-stimulus undershoot in post-capillary veins as suggested by the Balloon and Windkessel models remains ambiguous. Temporal changes in the BOLD signal were highly correlated with deoxy-Hb, with lower correlation values for oxy- and total Hb. Furthermore, data show that fNIRS covers the outer 1 cm of the brain cortex. These results were confirmed by simultaneous fMRI/fNIRS measurements during rest. In conclusion, multimodal imaging approaches may contribute to the understanding of neurovascular coupling.

MeSH terms

  • Adult
  • Brain Chemistry / physiology
  • Cerebrovascular Circulation / physiology
  • Electron Transport Complex IV / metabolism
  • Female
  • Hemodynamics / physiology
  • Hemoglobins / chemistry
  • Humans
  • Image Processing, Computer-Assisted
  • Magnetic Resonance Imaging / statistics & numerical data*
  • Male
  • Oxygen / blood*
  • Photic Stimulation
  • Spectroscopy, Near-Infrared / statistics & numerical data*
  • Visual Cortex / anatomy & histology
  • Visual Cortex / blood supply


  • Hemoglobins
  • Electron Transport Complex IV
  • Oxygen