Ivy Tso of the University of Michigan says the study was conducted within one week of the attacks. The researchers assessed 31 university students in Boston who were not directly connected to the attacks and therefore represented the wider American public.
The participants were shown a series of 90 pictures -- 30 containing images of the attacks while the others were defined as either "negative" but not related to the attacks, or "neutral."
The research team measured the brain activity of the participants to detect signs of anxiety and stress.
"The results of our study indicate that participants' brainwave responses during processing of the images deviated from normal in proportion to their self-report distress level directly related to the Sept. 11 attacks," Tso says in a statement.
The study, published in the Journal of Traumatic Stress, finds the stress-related neural deviations are similar to the abnormal cognition observed in people with post-traumatic stress disorder -- including diminished attention, hypervigilance and suppression of unwanted thoughts.
"This finding is significant as our participants were young, unmediated, highly functional individuals and while their distress was clearly below clinical threshold, their brain responses to emotional information were affected the same way, though not to the same degree, as in PTSD," Tso says in a statement. "This makes us rethink whether distress reactions should be considered a spectrum of severity, rather than simply divided into normal vs. clinical categories."