Figure 1.

Coherence is invariant to timing and shape differences in the hemodynamic response function (HRF). Time series generated by convolving the same impulse function with two different HRFs have a high average coherence across the hemodynamic frequency range (0–0.15 Hz), reflecting a highly linear relation between the time series. The correlation coefficient of these time series reflects a difference in the shape of the HRF, and is therefore considerably lower (adapted from Sun et al. [2004], with permission). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]