3. The tendency of the hippocampus to malfunction under extreme stress can lead to disorganized memory formation (encoding) without the inclusion of crucial information about the time and location of the event.

Disruptions in learning also are correlated with effects of trauma on the hippocampus, cortical tissue deep within the temporal lobe and adjacent to limbic (emotion-related) brain structures. Early research involving veterans with combat experiences consistently suggested that veterans with chronic or lifetime PTSD had reduced hippocampus volume compared with veterans without PTSD (McEwen & Magarinos, 1997; Pitman, 1997; Stein, Hanna, Koverola, Torchia, & McClarty, 1997). Researchers are not quite sure of the significance of the size reduction—the atrophy seems correlated with but not necessarily caused by PTSD (Stein et al.); cognitive abilities do not seem irreversibly impaired by PTSD (McEwen & Magarinos); and the atrophy itself seems reversible (McEwen & Magarinos). They do agree, however, that the hippocampus under the influence of stress tends to malfunction (Armony & LeDoux, 1997), contributing particularly to disruptions in encoding the context of traumatic memory in time and space (Wolfe & Schlesinger, 1997). In addition, hippocampal failure to contribute vital contextual information in the formation of conscious memories of events results in the amygdala, which modulates the encoding of emotional valence with memories, being unable to evaluate whether the stressor stimuli is current or from the past (Armony & LeDoux; Grolier, Harvey, Steiner, & Yehuda, 1997; van der Kolk et al., 1997). Thus, if new sensory stimuli sufficiently resembling former traumatic stimuli^{Endnote 20}, the new sensory stimuli are liable to be evaluated as a current threat and provoke inappropriate SNS arousal.