Here is my hopefully successful attempt at creating a presentation exploring Yoko Okado and Craig Stark's findings regarding the encoding of false memories.
What is false memory?
False memory refers to altered or distorted memories which are experienced by the subject as accurate. Okado and Stark describe a number of forms false memories take: "changes in the context of a memory," for example (in which the source of a memory is inaccurately attributed, so that a subject may report that he or she personally experienced an event rather than having seen it on television, heard about it from a friend, or dreamed it), or "changes in the content of a memory," in which details in the memory (such as the season or year in which an event took place, what someone was wearing, et cetera) are confused.
How were false memories created in this study?
Okado and Stark describe an experiment in which subjects were twice shown eight vignettes: first in an Original Event phase, and secondly in a Misinformation phase. In the Misinformation phase, the vignettes contained changes made to 12 "critical items." In order to further clarify subjects' experiences of memory, a "source memory test" was administered which asked the subjects to attribute their memory to a specific source.
What did the fMRIs show?
I quote from Okado & Stark to ensure that I don't misrepresent their findings:
"During encoding of the Original Event, there was significantly more activity when participants subsequently remembered the item shown in this phase (a true memory), compared with when participants subsequently did not remember the item. That is, there was more encoding activity when an item was later remembered compared with when it was forgotten (or another item was remembered instead). During the Misinformation phase, this same Dm effect was observed. There was more activity for subsequent false memories than for subsequent true memories. This difference in activity was significant in the left hippocampus tail, but not in the left perirhinal cortex. Again, there was more encoding activity for items subsequently remembered (Misinformation items) compared with items subsequently not remembered (Original Event items).
"In the left hippocampus tail, activity for subsequently true memories was greater during the Original Event phase than during the Misinformation phase, and activity for subsequently false memories was significantly greater during the Misinformation phase than during the Original Event phase. In the left perirhinal cortex, the same pattern of results was observed. Thus, the relative activity during the two encoding phases was predictive of which version of the item would later be remembered. " (italics mine)
"The traditional Dm effect (greater activity for subsequently remembered items than for forgotten items) was observed in the left hippocampus tail and left perirhinal cortex. When encoding activity was greater during the Original Event phase, the Original Event items (true memories) were subsequently recollected. When encoding activity was greater during the Misinformation phase, the Misinformation items (false memories) were subsequently recollected. Thus, in these two regions, activity was correlated with successful encoding of an item later remembered, whether it was from the Original Event phase or from the Misinformation phase."
What is the significance of these results?
What is significant about this study is that it allows us to look at the formation of false memories, and shows that activity in specific regions of the brain can be correlated to memory formation regardless of whether the memory is "true" or "false."
It can be misleading to talk about "true" or "false" memories in this context, as what is really being addressed in this study is misattribution and confusion (in the literal sense of two experiences being mixed together and thus remembered as one). The "false" memories (as I understand it) were memories which either attributed the Misinformation phase changes in vignettes to the original vignette or registered the Misinformation phase changes in the vignettes as the only version seen. In this sense these are not "false" memories; the subject did, in fact, see these versions of the vignettes-- but the activity levels shown in the fMRIs suggest that they encoded the Original Event vignettes weakly, and the Misinformation vignettes strongly, leading to the errors in memory described.
This suggests that memories are in some sense "rewritable." A weakly encoded memory, when confronted by a more strongly encoded memory with an overlapping context, can be altered to reflect the more strongly encoded memory-- without the subject becoming aware of the alteration. It is truly a sub-conscious change.
My remaining questions include: what prompts this "rewriting"? How weak or strong must an encoded memory be for it to be susceptible or resistant to this? Is there any distinction between the process of "original" memory formation and memory alteration, or are all our memories essentially shifting, constantly edited narratives?
Okado and Stark's paper, "Neural activity during encoding predicts false memories created by misinformation," can be found here.