According to the National Center for PTSD, 7-8 out of every 100 people will experience PTSD at some time in their lives. Additionally, it’s estimated that 21.4% of US adults will experience some kind of mood disorder (including depression), while 31.1% will experience any anxiety disorder at some point across their lifespan. We’re talking on the order of up to 65 million people; clearly, these are issues that touch the lives of many.

Post-Traumatic Stress Disorder, or PTSD, is characterized by the continued experience of fear-induced symptoms such as anxiety and flashbacks after a scary or shocking event. Anxiety, depression, and other mood disorders are similarly characterized by symptoms related to fear and emotion regulation. A common experience across these disorders is the formation of inappropriate negative responses to neutral circumstances leading to the development of negative associations linked to harmless stimuli (Ježek et al., 2010). For example, such an experience in the case of PTSD might look something like this:

Following her return home after combat, a veteran hearing her ceiling fan is reminded of the churning blades of a helicopter; she feels intense fear and may experience a flashback to a negative experience in a war zone. For this individual, the blades of the fan thus trigger a negative fearful association each time she hears them.

In 2010, a study by Ježek et al. sought to better understand the potential link between stress and these inappropriate memory-event associations. Knowing that both stress and memory played critical roles in the phenomenon, they reasoned that perhaps stress could be what was promoting these unwanted connections.

To sort this out, they used rats as an animal model in which to induce stress through a “forced-swim task.” Essentially, this is as unpleasant as it sounds; rats were trained to perform a task, and then one group was put in a deep bucket of water, forcing them to swim, while the other group was put in a container of shallow water they could stand in.

Ultimately, after a multitude of follow-up experiments, the researchers came to the conclusion that the stressed out rats had better retention for the task they had learned than their non-stressed counterparts. But is this all to suggest that we should be religiously stressing ourselves to the breaking point in the name of learning? Well… no.

Interestingly, the researchers also showed that a stressful experience can re-activate consolidated memories (those that are stabilized/stored after being acquired) unrelated to the experience itself. Accordingly, the researchers dubbed this phenomenon “out-of-context activation of memory,” or OCAM. To summarize, OCAM is looking awfully similar to the hallmark “inappropriate memory-event association” so often seen in PTSD, anxiety, and mood disorders.

Lastly, these researchers also showed that the ability of stress to induce OCAM was reliant on the hippocampus (the region of our brain so often implicated in memory formation and retrieval), although the memories alone were not. To put it concisely, the hippocampus was necessary to control memories stored elsewhere in the brain.

Following this research, Ježek et al. ultimately proposed that stressful events trigger OCAM, creating the potential for formation of inappropriate associations that in turn promote and perpetuate anxiety and mood disorders. Essentially, stressful events reactivate and link pre-trauma memories with an individual’s memory for the traumatic event, blending those seemingly innocuous memories with the threatening ones.

So while stress, at face value, may promote memory retention, there is a staggering potential for this equation to go wrong. Like so much else about the brain, the process of memory storage and retrieval is convoluted and messy. When a jarring dose of stress is thrown in the mix, we face the potential that things may go awry as trauma sews together incompatible memories, leading our brains to signal the wrong response for the situation.

If you or a loved one are struggling with PTSD, anxiety, or a mood disorder, please refer to this resource provided by the NIH for more information and support.

References:

Ježek K., Lee B.B., Kelemen E., McCarthy K.M., McEwen B.S., & Fenton, A.A. (2010). Stress-induced out-of-context activation of memory. PLOS Biology, 8(12). doi: 10.1371/1000570

NIMH. (2017). Any anxiety disorder. Retrieved from https://www.nimh.nih.gov/health/statistics/any-anxiety-disorder.shtml

NIMH. (2017). Any mood disorder. Retrieved from https://www.nimh.nih.gov/health/statistics/any-mood-disorder.shtml

NIMH. (2019). Post-Traumatic Stress Disorder. Retrieved from https://www.nimh.nih.gov/health/topics/post-traumatic-stress-disorder-ptsd/index.shtml

Neuroscientifically Challenged. (2014). Know your brain: Hippocampus. Retrieved from https://www.neuroscientificallychallenged.com/blog/2014/5/23/know-your-brain-hippocampus

VA.gov: Veterans Affairs. (2019). How common is PTSD in adults? Retrieved from https://www.ptsd.va.gov/understand/common/common_adults.asp