Correlation of visual-evoked hemodynamic responses and potentials in human brain

The interaction of brain hemodynamics and neuronal activity has been intensively studied in recent years to yield better understanding of brain function. We investigated the relationship between visual-evoked hemodynamic responses (HDRs), measured with near-infrared spectroscopy (NIRS), and neuronal activity in humans, approximated with the stimulus train duration or with visual-evoked potentials (VEPs). Concentration changes of oxyhemoglobin (HbO₂) and deoxyhemoglobin (HbR) in tissue and VEPs were recorded simultaneously over the occipital lobe of ten healthy subjects to 3, 6, and 12 s pattern-reversing checkerboard stimulus trains having a reversal frequency of 2 Hz. We found that the area-under-the-curves \((\Upsigma)\) of HbO₂ and HbR were linearly correlated with the stimulus train duration and with the \(\Upsigma\hbox{VEP}\) summed over the 3, 6, and 12 s stimulus train durations. The correlation was stronger between the \(\Upsigma\hbox{HbO}_2\) or the \(\Upsigma\hbox{HbR}\) and the \(\Upsigma\hbox{VEP}\) than between the \(\Upsigma\hbox{HbO}_2\) or the \(\Upsigma\hbox{HbR}\) and the stimulus train duration. The \(\Upsigma\hbox{VEP}\hbox{s}\) explained 55% of the \(\Upsigma\hbox{HbO}_2\) and 74% of the \(\Upsigma\hbox{HbR}\) variance, whereas the stimulus train duration explained only 45% of the \(\Upsigma\hbox{HbO}_2\) and 51% of the \(\Upsigma\hbox{HbR}\) variance. We used \(\Upsigma\) of the NIRS responses and VEPs because we wanted to incorporate all possible processes (e.g., attention, habituation, etc.) affecting the responses. The results indicate that the relationship between brain HDRs and VEPs is approximately linear for 3–12 s long stimulus trains consisting of checkerboard patterns reversing at 2 Hz. To interpret hemodynamic responses, the measurement of evoked potentials is beneficial compared to the use of indirect parameters such as the stimulus duration. In addition, interindividual differences in the HbO₂ and HbR responses may be partly explained with differences in the VEPs.