Not remembering what you shouldn't

Are there brain regions that help us to fill in gaps in memory? What would happen when those regions are damaged? My co-authors and I recently published a short report in the Journal of Neuroscience describing findings that may begin to address this question. A very nice follow-up piece discussed our work last week, and we wrote a brief response.

We know that memory is at least partially reconstructed when recalled. That is, a remembered experience isn't a perfect reproduction of that experience; instead, a remembered experience is the gist of a particular experience with some of the detail filled in from other, similar experiences. If you've ever remembered something typically true but wrong about a specific episode (e.g., Aunt Alice being at Thanksgiving last year when she actually didn't make it, or Derek Jeter starting for the Yankees in a game when he was actually on the DL), you've experienced this kind of memory error.

Recent functional neuroimaging data have pointed toward medial prefrontal cortex (mPFC) as a brain region that could potentially support this kind of generalized, or schematic, memory. Here at UIowa, we're very fortunate to have access to an extraordinary resource in the form of a registry of neurological patients with focal brain injuries, including many with damage limited to the ventral mPFC (vmPFC).

How it worked

We tested these individuals with a memory task designed to produce benign false memories for words that were not studied. The task is simple: participants listened to lists of words such as bed, pillow, blanket, etc., and then recalled as many words as possible. Our healthy normal comparison participants showed the expected effect by often recalling a critical word that was missing from the list, such as sleep. People with vmPFC damage recalled these non-studied words less often than the participants without brain injuries¹. This suggests that the vmPFC may normally play a role in filling gaps in memory with plausible information, and that damage to vmPFC reduces this effect.

Can brain damage improve memory?

We observed that damage to the vmPFC made people less susceptible to false memories while leaving their true memory unaffected. Does this mean that people with specific patterns of brain damage have better memory? Not necessarily. Two key points:

1. Memory is a cognitive process that we define by studying healthy individuals. That is, if healthy normal people typically remember things in a certain way, that's what we should call normal memory. From this perspective, false memories for related words would be normal and vmPFC damage made memory abnormal.
2. The same characteristics of memory that make healthy normal people susceptible to false memories in the DRM task probably help memory performance more often than not. For example, outside of a laboratory setting, how often would you expect to encounter a dozen words related to sleep and later be penalized for thinking about and recalling sleep? Most of the time, the influence of generalized world knowledge such as schemas and semantic relations is beneficial to memory performance, and may have evolved to relieve pressure on other memory systems.

Take-home message

Our memory often takes shortcuts, using general knowledge derived from experience to fill in gaps. Although often harmless or even useful, this can lead to false memories. These memory effects may be supported in part by a specific brain region, the vmPFC. We're continuing to investigate other ways in which vmPFC may influence memory.

¹ Important note: people with vmPFC damage still had some false recall, but it was substantially reduced. Critically, their memory was otherwise identical to that of healthy individuals.