Causality in Covariant String Field Theory
  Causality is studied in the covariant formulation of free string field theory (SFT). We find that, though the string field in the covariant formulation is a functional of the ghost coordinates as well as the space-time coordinate and the latter contains the time-like oscillators with negative norm, the condition for the commutator of two open string fields to vanish is simply given by $\int_0^\pi d\sigma\left(\Delta X^\mu(\sigma)\right)^2 >0$, which is the same condition as in the light-cone gauge SFT. For closed SFT, the corresponding condition is given in a form which is manifestly invariant under the rigid shifts of the $\sigma$ parameters of the two string fields. 
Hiroyuki Hata and Hajime Oda