The SEM is in parentheses. The row with heading “4-D In focus” lists values for sparks detected in x-y-z-t series, the 4-D mode, which satisfy the in-focus criterion. x-y-z-t series were obtained from 34 cells; each series consists of 1,000 trios of x–y images successively at three z positions, for a total of 3,000 images per series. The row with heading “3-D” includes all sparks detected in x-y-t series at the z₂ position (z = 0), whether or not they were in focus. “2-D” refers to sparks detected in line-scan images constructed from the x-y-t series at position z₂ as described in Materials and methods. Fig. 4 has an example line-scan image constructed this way. It should be noted that the three modes (4-D, 3-D, and 2-D) are just different ways to analyze the full 4-D database.