The Pentagon reports that the incidence of traumatic brain injury among U.S. troops has dropped from 644.6 per month in 2011 to 372.8 for 2012, through October. TBIs – the so-called "signature wound" of the post 9/11 wars – happen when a soldier's brain is violently shaken, and bruised, inside the skull, usually due to improvised explosive devices. It can cause all sorts of mental problems later on, ranging from anxiety, to depression, to suicide.