Using near–infrared spectroscopy, we investigated the time–course of the concentrations of oxygenated haemoglobin, [oxy–Hb], and deoxygenated haemoglobin, [deoxy–Hb], in the occipital cortex of healthy human adults during sustained visual stimulation. Within a few seconds after stimulation (coloured dodecahedrons) we observed a decrease in [deoxy–Hb], peaking after 13 s (‘initial undershoot’). In the subsequent 1–2 min, in seven out of ten subjects, [deoxy–Hb] gradually returned to a plateau closer to the baseline level. After cessation of stimulation, there was a ‘post–stimulus overshoot’ in the concentration of deoxyhaemoglobin. There was a statistically significant correlation between the size of the ‘initial undershoot’ and the ‘post–stimulus overshoot’.
The concentration of oxyhaemoglobin increased upon functional activation. However, in the mean across all subjects there was no initial overshoot. After approximately 19 s it reached a plateau and remained constantly elevated throughout the activation period. After cessation of activation there was a ‘post–stimulus undershoot’ of oxyhaemoglobin.
It is important to consider the time–course of haemoglobin oxygenation when interpreting functional activation data, especially those data obtained with oxygenation–sensitive methods, such as BOLD contrast fMRI.