You can use a math node to subtract a bit from the displacement map.  Some areas will be sub-black (less than zero, or negative);  other areas will still be lighter than black (positive).  With a positive value in the displacement value box, this will make some areas engrave, and other areas pop out.

     If you subtract 0.5 from the displacement map and then multiply by 2, you can use a map in which grey is no displacement, white is full outward displacement, and black is full inward displacement.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5