Assuming that the dip is due to a local increase of deoxyhemoglobin, one would obviously expect every single time series exhibiting this early negativity to exhibit the subsequent overcompensation as well. In fact, optical imaging experiments have never detected an initial negativity that was not followed by the typical overcompensation (A. Grinvald, personal communication). Furthermore, if the dip reflects—as it should—a signal that is spatially restricted within the parenchyma, one would also expect it to occur almost exclusively outside the areas transpierced by major drainage vessels, such as the sagittal sinuses; this is particularly true of the very early (two-second) phase of the negativity that was analyzed by Kim et al. 1 Yet an exact superposition of Figs. 1c and 1d of ref. 1 shows that the above assumptions do not hold for their data (Fig. 1 below). The areas showing a dip largely overlap with the sinuses, and a dip signal can be also detected in areas showing no statistically significant conventional activation. The former is puzzling because studies in humans by other investigators from the same laboratory5, as well as studies in monkeys4, have shown that large vessels like the sinus show only positive signal changes. The latter is letters to the editor