Near-infrared spectroscopy (NIRS) and diffuse optical tomography (DOT) detect changes in brain blood volume and oxygenation by measuring light that has passed through the head, including the scalp and the skull. Extracerebral and systemic circulation interfere with optical measurements of cerebral hemodynamics, especially when measuring brain responses to stimuli or tasks that evoke strong systemic circulatory changes. We studied the effect of changes in systemic circulation on NIRS responses and DOT reconstructions in thirteen subjects during a hand motor task that increased the heart rate. Both the NIRS responses and the DOT reconstructions depended on the change in the heart rate. The NIRS response amplitudes during epochs with a large change in heart rate (24.8±0.8 bpm; highest third) were significantly larger (p < 0.05) than during epochs with a smaller change in heart rate (5.8±0.5 bpm; lowest third). Accordingly, we propose that comparing epochs associated with large and small changes in heart rate serves as a method for estimating whether NIRS signals are affected by the systemic circulation, given that there is variability in the systemic circulation between epochs.