Are voluntary and involuntary memories encoded by different brain systems?

Some memories we aim to remember, others just show up. One proposal is that uninvited memories, such as those that intrude in Post-Traumatic Stress Disorder (PTSD), are encoded and stored in a distinct memory system. But a new neuroimaging study led by Shana Hall suggests that similar brain areas are involved whether our memories come spontaneously or by intent.

During a functional imaging brain scan, 26 participants made perceptual judgments about a sequence of 100 sounds piped into headphones. All the sounds had been repeatedly presented earlier, half of them paired with images the participants were instructed to associate with the sound. At scan-time, 14 participants in a voluntary recall condition attempted to call to mind each relevant, related image. In contrast, the involuntary recall group did not perform this deliberate recall task. Afterwards, all participants worked through the sounds once more and indicated which ones had triggered an image in the scanner, either spontaneously or by intent.

Regardless of whether a participant was trying to recall images or not, the trials where image recall occurred led to activation in a range of areas including medial temporal lobe, posterial midline cortex, and angular gyrus. These areas aren't particularly surprising, with links to various memory and visual processes already established in previous studies. But what the results imply is that there is considerable overlap between what happens in the brain when we're remembering on purpose compared with by accident.

Surely some neural activity differentiates the two experiences? Yes, but it's fairly specific: participants in the voluntary recall condition showed more activation in their left dorsolateral prefrontal cortex (DLPFC). This was true across all trials, including those that involved sounds previously unassociated with any image. It seems the brain region wasn't involved in reproducing a memory, but in the strategic search for memories. This fits with what we already know about the function of DLPFC. Beyond this activity (which we would be surprised not to find, given that these participants were specifically asked to search for memories), there was no significant difference in brain activation between the voluntary and involuntary recall groups.

So, all in all, the study suggests that the systems that support everyday voluntary and involuntary memories may have more in common than differences. However, researchers of clinical disorders, such as PTSD, that involve involuntary memories, focus proposals for a distinct system around the especially traumatic nature of such memories. As such, it would be interesting to bring this methodology to explore memories for negative emotional content, and indeed to clinical groups who experience traumatic involuntary memories, to better understand what makes such memories like and unalike.

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Hall, S., Rubin, D., Miles, A., Davis, S., Wing, E., Cabeza, R., & Berntsen, D. (2014). The Neural Basis of Involuntary Episodic Memories Journal of Cognitive Neuroscience, 1-15 DOI: 10.1162/jocn_a_00633