Recent studies at New York University and the New York–Presbyterian Hospital/Weill Cornell Medical Center show enduring psychological and neurological repercussions in adult witnesses near the World Trade Center that day, and in children who lost a parent in the tragedy.

According to the researchers, adults who appeared hardy and functional—and who weren't diagnosed with post-traumatic stress disorder after 9/11—nevertheless do show some symptoms of PTSD and may be vulnerable to mental disorders in the future. People afflicted with full-blown PTSD relive their terrifying ordeal through nightmares, flashbacks, and upsetting thoughts, and lose interest in activities that were once important to them. They also feel alone, are unable to relax, and remain on guard.

The researchers compared the brain scans of adults who were within two miles of the World Trade Center during the attacks with those who were further away. Four years later, relatively healthy and seemingly resilient 9/11 witnesses near the twin towers—people who witnessed the events with their own eyes—were more sensitive to certain emotional stimuli than people several miles away who learned of the events secondhand. Both studies found that these relatively asymptomatic witnesses had an overactive amygdala—the part of the brain that judges emotional intensity and creates emotional memories.

To compare the participants' amygdalae, the researchers did brain scans that illuminate parts of the brain when they are active. The researchers showed that the experiences of these immediate witnesses created a type of memory similar to "flashbulb memory," the exceptionally vivid and confident recollection of a shocking public event. Such recollections cause people to replay in their minds the traumatic event—as when John F. Kennedy was shot—with unusual detail. "We clarified the concept of flashbulb memory, when and how it occurs, and showed that only those close to Ground Zero had memories with these qualities," says Dr. Elizabeth A. Phelps, who led the NYU team.

In the Cornell study, researchers showed photographs of fearful and calm faces to adults who were within one and a half miles of the site that day, and to adults who lived 200 miles away. Even though the measurements were taken as much as four years later, those people who were near Ground Zero had hyperactive amygdalae compared to those who were out of town. Dr. Barbara Ganzel, one of the Cornell researchers, says that even though these effects were elicited using mildly upsetting images and not with specific reminders of the trauma, "people who appear to have recovered quickly from traumatic experiences may have differences in their brains long-term, even if they are not sick." It may take years for these people to recover from such changes in their brains, and they may be vulnerable to developing mental disorders in the future.

Another study, conducted by Dr. Cynthia Pfeffer, a psychiatrist at Cornell, followed 45 children who had lost a parent on 9/11. Despite receiving psychotherapy during the two years following the catastrophe, the children's rate of psychiatric illnesses doubled, from 32 percent before 9/11 to nearly 73 percent afterwards. PTSD, general anxiety, and separation anxiety were the most common.

The grieving children also had high levels of cortisol, a stress hormone, in their saliva. Pumping out an above-normal level of cortisol means that their bodies were still in emergency mode. "Their fight-or-flight mechanisms were switched on," says Pfeffer. The high cortisol levels could make them overly sensitive to stress later in life.

Cortisol is produced by the adrenal glands, which interact with two other glands near the brain, the pituitary and the hypothalamus. Problems arise in other parts of the body when their secretions are out of balance. Cognitive abilities like reasoning, intuition, and perception in the bereaved children could be affected; they may also develop diabetes, because the way their bodies handle insulin may be impaired. Another concern is weak bones.

"Grief is a normal process. However, forms of grieving may be different, depending on the nature of loss," says Pfeffer. "Variations in grieving should be identified and addressed so that these children don't develop other problems." Understanding the underlying biology of post-traumatic stress will yield clues to treating those suffering after 9/11 and other traumas, the researchers say.