Rate of decay of the calcium signal. (A) Fluorescence was measured in response to pairs of brief flashes (90 ms) spaced 3 s apart. Light was only collected from the photosensitive microvillar lobe. The first light evoked a large increase in Ca fluorescence above the basal level. At the time the second flash was presented, fluorescence remained significantly elevated, and no further increase was elicited. (B) Average amplitude of the fluorescence signal during the second stimulus, normalized with respect of the initial peak amplitude, as the interval separating the two flashes was increased from 3 to 5 s. Error bars indicate standard deviation.