Although post-traumatic stress disorder (PTSD) and traumatic brain injury (TBI) are categorized as separate and discrete disorders, the boundary between them is sometimes indistinct. Their separation is based on the assumption that PTSD results primarily from psychological stress, while TBI is the consequence of an identifiable injury to the brain. This distinction is based on an antiquated polarity between mind and brain, and the separation of the two disorders often becomes arbitrary in day-to-day psychiatric practice and research.This issue of Dialogues in Clinical Neuroscience is titled "Trauma, Brain Injury, and Post-traumatic Stress Disorder." The articles between its covers address both post-traumatic stress disorder (PTSD) and traumatic brain injury (TBI). As the combination of these articles indicates, the various recent wars in the Middle East have awakened an old controversy about the relative impact of physical and psychological stress in causing neuropsychiatric disorders. Although the term TBI suggests the occurrence of some type of physical lesion, while PTSD suggests a disorder occurring as a consequence of psychological stress, the boundary between the two is often unclear and sometimes permeable or overlapping. The first systematic discussion of the relationship between physical and psychological stress dates back to World War I. The history of that discussion provides an informative context for current controversies concerning PTSD and TBI. 1 Combat techniques in World War I introduced new types of combat stress that had not existed during previous wars. Soldiers engaged in trench warfare were relatively immobile and therefore more vulnerable. They were also chronically exposed to new and perversely lethal threats, such as poison gas, machine gun fire, mortar attacks, land mines, and tanks. Casualties were devastating, and fatality rates were frightening. Men watched their friends die beside them, and they confronted the possibility of their own demise on a daily basis. Alternatively they might be maimed and consigned to a life of chronic disability. As the war progressed, the high casualty rate made it clear that Britain and continental European countries were losing many of an entire generation of young men-a social loss from which they would be slow to recover. In this context of brutal bloodshed and omnipresent fear, a new and somewhat unfamiliar type of disability emerged that had not been described in previous wars: a syndrome characterized by confusion, memory impairment, headache, difficulty concen-
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