This is an invited research digest contributed by Dr. Adriana del Palacio-Gonzalez at the Center on Autobiographical Memory Research, Aarhus University, Denmark.
Have you ever caught yourself smiling when you were by yourself walking on the street or driving? It happens to me and many times it is because a funny memory comes to mind. From a subjective perspective, personal memories may seem like a simple mental act. In the end, they are in our minds virtually every day for one reason or another. However, for personal memories to come to mind and us being aware of them, there is an array of cognitive and affective processes that are put in motion. They are a complex psychological process and so psychologists and neuroscientists study them from many different angles, ranging from how the brain lights up when memories come to mind, to how people “see” memories in their mind’s eye, to how they make us feel, to what they mean in the bigger picture of our lives and our identity.
I study the emotional reactions triggered by personal memories, with a special focus on memories that come to mind unexpectedly or out of the blue. Researchers call them involuntary memories (1). In some cases, they may also be referred to as “intrusive” memories, especially in the context of psychological distress and (or) psychiatric disorders (2). Thus, studying personal memories has also implications for understanding how we think about our past when we experience various types of symptoms or mental illness, such as depression, social anxiety, and posttraumatic stress disorder. But to get there, to the side of distressing, intrusive, or traumatic memories, it is also important to understand the general operation of ‘involuntary memories.
Here are some highlights of the characteristics of involuntary memories. First off, they are very short-lived, they pop-up and then they go away quickly (1) (although in some cases they might trigger other processes that may make you feel stuck, like rumination) (3).
Second, they are frequent and (may be) universal. Most of us experience them. The frequency varies a lot from one person to the next, and even from one day to the next for the same person. There are many reasons for that variation including, how busy (or distracted) we are, whether there are things that have you worried or having recent exciting events happening, etc (4). It also has to do with a personal difference, some people are just simply more likely to have many involuntary memories. But if you would like some reference point, a good estimate of the average for the daily frequency of involuntary memories is 20 per day (5).
Third, involuntary memories are mostly positive! (1) This might come as a surprise to some. Maybe because people notice more easily and (or) are more affected by the involuntary memories that are distressing. However, many studies have been conducted to show that the proportion of positive involuntary memories is higher than the proportion of negative memories. However, the proportion of positive vs. negative memories may vary when we are in distress. The proportion is closer to a 50 – 50 during clinical depression (6) and with very elevated PTSD symptoms (1).
Fourth, although involuntary memories seem to come out of the blue, most of them are triggered by something in our environment (a smell, a song in the radio), or something internal (your own mood, another idea that you just had) (7). Generally, environmental cues are more frequent than internal (1).
Lastly, involuntary memories have a greater emotional impact than other memories we experience. In addition to experiencing spontaneous memories, we also remember more purposefully. For example, when someone asks you about your childhood you may start searching for past situations to explain how it was. Similarly, when you tell your friend about your last vacation trip you search for certain memories to share with her. Compared with these more purposeful memories, the emotional reaction coming with involuntary memories is greater (8). Part of this reaction also entails making a greater effort to regulate or control the emotions triggered by involuntary memories. A possible explanation for that is that due to the unexpected and automatic nature of involuntary memories, we do not have the opportunity to emotionally prepare for them (whether they are positive or negative).
What emotions are common for involuntary memories and how do we regulate those emotions?
A few studies have addressed this question. One study found that the most frequent emotions associated with involuntary memories were happiness, followed by sadness (9). Along the same lines, in one my studies we found that happiness was the most intense emotion experienced with involuntary memories, followed, by sadness (10). However, a single memory may trigger more than one emotion. We found that about 65% of involuntary memories triggered at least two emotions simultaneously (11). Virtually any emotion may be experienced with involuntary memories.
When we experience both positive and negative emotions, there may be conscious or automatic efforts to control such emotions. Some of these efforts may include analyzing the remembered situation, trying to push away the memory, hiding your emotions, or simply letting it be (10). In other words, when involuntary memories show up, we notice them more emotionally and so we co-respond with more emotion regulation efforts.
How does this fit with intrusive memories during psychological distress?
There is an ongoing scientific debate trying to clarify the nature of intrusive memories. Some researchers think that these memories are different from other spontaneous memories (or any other memory to be precise) and that they are saved in our brains in a different way from non-traumatic or non-stressful memories (12). Other studies, like those many of those cited above, suggest that much of the intrusive nature of those stressful and/or traumatic memories follow the same rules or patterns as involuntary memories: They come to mind unexpectedly, so they have a strong emotional impact. But that is not the only factor that explains the high distress caused by some memories.
We know from other studies that when people are experiencing high distress, whether in the form of depression, anxiety, or trauma symptoms, all memories (even if they are not traumatic) take a higher emotional toll on them (6, 10, 13). Thus, when you add the normal high emotional impact of involuntary memories to the heightened emotionality presented during distress, people may experience involuntary memories as intrusive and or highly stressful.
Other factors may contribute to the strong emotional reaction provoked by some memories, including the degree to which you think that an event or situation was central to who you are or for how your life unfolds (14). Psychological science is still in the process of understanding more deeply the nature of highly emotional memories, including intrusive memories.
 Berntsen, D. (2009). Involuntary autobiographical memories: An introduction to the unbidden past. Cambridge University Press, New York, NY.
 Berntsen, D. (2015). From everyday life to trauma: Research on everyday involuntary memories advances our understanding of intrusive memories of trauma. Clinical perspectives on autobiographical memory (pp. 172–196). New York: Cambridge University.
 Smets, J., Wessel, I., Schreurs, E., & Raes, F. (2012). The interplay between rumination and intrusions in the prediction of concurrent and prospective depressive symptoms in two nonclinical samples.The Psychological Record, 62(4), 777-788.
 Johannessen, K. B., & Berntsen, D. (2010). Current concerns in involuntary and voluntary autobiographical memories. Consciousness and Cognition, 19, 847-860.
 Rasmussen, A. S., & Berntsen, D. (2011). The unpredictable past: Spontaneous autobiographical memories outnumber autobiographical memories retrieved strategically. Consciousness and Cognition: An International Journal, 20(4), 1842-1846.
 Watson, L. A., Berntsen, D., Kuyken, W., & Watkins, E. R. (2012). The characteristics of involuntary and voluntary autobiographical memories in depressed and never depressed individuals. Consciousness and Cognition: An International Journal, 21, 1382-1392.
 Mace, J. H., Clevinger, A. M., & Martin, C. (2010). Involuntary memory chaining versus event cueing: which is a better indicator of autobiographical memory organisation? Memory, 18(8), 845-854.
 Berntsen, D., & Hall, N. M. (2004). The episodic nature of involuntary autobiographical memories.Memory & Cognition, 32, 789-803.
 Krans, J., de Bree, J., & Moulds, M. L. (2015). Involuntary cognitions in everyday life: Exploration of type, quality, content, and function.Frontiers in Psychiatry, 6.
 Del Palacio-Gonzalez, A., Berntsen, D., & Watson, L. A. (2017). Emotional intensity and emotion regulation in response to autobiographical memories during dysphoria. Cognitive Therapy and Research,4, 530-542.
 del Palacio Gonzalez, A., Berntsen, D., & OToole, M. S. (2017). Emotion regulation flexibility in dysphoria upon retrieval of everyday memories. Abstract from International Convention of Psychological Science (ICPS), Vienna, Austria.
 Brewin, C. R., Dalgleish, T., & Joseph, S. (1996). A dual representation theory of posttraumatic stress disorder. Psychological Review, 103, 670-686.
 Rubin, D. C., Bernsten, D., & Bohni, M. K. (2008). A memory-based model of posttraumatic stress disorder: evaluating basic assumptions underlying the PTSD diagnosis. Psychological Review, 115, 985-1011.
 del Palacio-Gonzalez, A., & Berntsen, D. (2017). Emotion regulation of events central to identity and their relationship with concurrent and prospective depressive symptoms.Behavior Therapy, doi 1016/j.beth.2017.11.002.