  The Apache Point Observatory Galactic Evolution Experiment (APOGEE) provides
spectroscopic information of regions of the inner Milky Way inaccessible to
optical surveys. We present the first large study of the metallicity
distribution of the innermost Galactic regions based on homogeneous
measurements from the SDSS Data Release 12 for 7545 red giant stars within 4.5
kpc of the Galactic center, with the goal to shed light on the structure and
origin of the Galactic bulge.
  Stellar metallicities are found, through multiple-Gaussian decompositions, to
be distributed in several components indicative of the presence of various
stellar populations such as the bar, or the thin and the thick disk. A
super-solar ([Fe/H]=+0.32) and a solar ([Fe/H]=+0.00) metallicity components,
tentatively associated with the thin disk and the Galactic bar, respectively,
seem to be the major contributors near the midplane. The solar-metallicity
component extends outwards in the midplane but is not observed in the innermost
regions. The central regions (within 3 kpc of the Galactic center) reveal, on
the other hand, the presence of a significant metal-poor population ([Fe/H]
=-0.46), tentatively associated with the thick disk, and which becomes the
dominant component far from the midplane ($|Z| > +0.75 kpc). Varying
contributions from these different components produce a transition region at
+0.5 kpc < |Z| < +1.0 kpc characterized by a significant vertical metallicity
gradient.
