Ever noticed that it is difficult to think clearly when you are acutely stressed? A recent study from the Douglas Mental Health University Institute could help explain why stress makes it harder to concentrate on what you are doing. This work, published in Biological Psychiatry, is the first to show that when you are being stressed by a demanding situation, specific parts of your brain turn off. It seems that this in turn allows others areas to become active and react to the stress.

“When you are being stressed, large areas of the brain including the hippocampus seem to turn off,” says Jens Pruessner, PhD, Director of the Douglas’ Aging and Alzheimer’s disease Research Theme and lead researcher of the study. “It was surprising to see that the activity in the hippocampus region was reduced in response to stress. The hippocampus is an important structure in the brain involved in spatial orientation, understanding new information, and learning and memory. It appears that during non-stressful periods, the hippocampus is continually active and acts like a brake, inhibiting the stress response. When hippocampal activity decreases in response to stress, the brake is released, and hormones are released to help the brain and the body cope with the stressor.”

Pruessner, an Assistant Professor in McGill’s Department of Psychiatry, Neurology and Neurosurgery, and his colleagues from the Douglas and McGill University, used the advanced technologies of Positron Emission Tomography (PET) and Functional Magnetic Resonance Imaging (fMRI) for the study.

“Thanks to the neuroimaging methodologies, we were able to see what happens in the brain while the subject perceives the stress,” says Pruessner. “This is the first time that this type of finding has been observed.”

Fifty healthy volunteers participated in the study. They were asked to perform difficult mental arithmetic tasks while their brains were scanned, either by PET or fMRI. Stress hormone levels were measured during the scans as well. The findings revealed that when subjects reacted to the task, as measured by an increase in stress hormone levels, the activity in their hippocampus declined. This was observed in both imaging modalities, cross-validating the findings. In addition, the degree of deactivation of the hippocampus was linked to the amount of cortisol released, suggesting a linear relationship between the two.

“As the hippocampus is involved with a number of important reasoning abilities, our next step is to determine if this hippocampal shut-down effects these functions as well, as would be predicted,” says Pruessner.

This research was supported by grants from the Canadian Institutes of Health Research and the Natural Sciences and Engineering Research Council of Canada.