The persistence of these mental pictures is what experts at the University of Essex in Colchester, England, call "attention bias" -- the tendency of threatening images to rivet human attention long after danger has passed.
"The reason people can't easily shift their attention back to their normal routines is that threatening images hold our attention much longer than non-threatening ones," says Elaine Fox, senior lecturer in the university's Department of Psychology and lead author of a study of attention bias that will appear in the December issue of the Journal of Experimental Psychology.
"Humans have an adaptive element that keeps us focused on threatening images," Fox says. "The delay in disengaging our attention allows us to conduct a better analysis of potential threats, a practice that probably evolved centuries ago as a protection against attack by other animals."
The researchers' study of anxiety and visual attention was completed before the September 11 terrorist attacks, but Fox and her colleagues believe its findings may help explain why the images of planes crashing, buildings collapsing and the Pentagon on fire have been so slow to fade.
According to Nancy Smyth, a trauma expert and an associate professor at the School of Social Work at the University of Buffalo in New York, attention bias isn't the only reason it's hard for people to forget what they saw happening in New York, Washington, D.C., and Pennsylvania on the day of the attacks.
"There's a mounting body of evidence that our brains don't process traumatic images into long-term memories in the same way that ordinary events are processed," Smyth says. "Different parts of the brain seem to be involved in processing and storing threatening sensory data and more conventional experiences. In fact, it may be that some traumatic images are never completely processed in the usual fashion."
Smyth notes that the persistence of sensory images from traumatic events -- whether actually experienced or viewed on television -- is also affected by the amount of exposure to the images, how horrific the images are, and how much prior trauma an individual has experienced.
"Those who viewed the planes crashing and the buildings collapsing over and over may experience greater difficulty assimilating the images," she says, "as will people who perceive a personal connection between their lives and the traumatic event they have seen images of."
For example, Smyth says, if a person recently flew the same route as one of the hijacked airplanes, or works in a high-rise building, or is a fire fighter, he or she may well experience the events of September 11 as if they had actually happened to them.
"When a person makes an emotional connection to a traumatic event, saying, in effect, 'It could have been me,' the event has a more lasting impression on them," she says. "The images may persist much longer than from a traumatic event where there's no connection perceived."
Mental health professionals report such reactions have been common across the country in recent weeks, as Americans struggle with stress-related sleep and eating disorders, difficulties in concentration, and feelings of numbness, powerlessness or lack of control over their lives.
According to Smyth, it is now appropriate for professional counselors and therapists to diagnose these symptoms as post-traumatic stress disorder (PTSD), rather than as an acute stress reaction.
"PTSD is the appropriate term when stress-related symptoms last longer than a month," she says. "The good news is that PTSD is often responsive to relatively brief, very focused therapeutic interventions."
Cognitive-behavioral therapies, group therapies and exposure therapies -- in which a person gradually and repeatedly re-lives a frightening experience under controlled conditions -- are all proven approaches to resolving PTSD, say experts.
Smyth has another, simpler approach that helps some people who continue to feel overwhelmed by the indelible images: Turn off your television.
"I encourage people having trouble coping to limit their TV time to 20 minutes -- enough to get informed -- then turn it off," she says. "Images are very powerful traumatic stress triggers for most people, especially when they are already stressed."