Memories can be fragile and subject to distortion because we literally cannot record and store all of what we learn and experience. People often mistakenly claim to remember having seen a word or object that is similar to something they saw earlier, according to several studies. Such false memories can have an even greater impact when they manifest in such a way that entirely novel events are implanted into an individual's memory. Such an individual can willingly retrieve these completely false memories, such as being lost in a mall, with surprisingly vivid and specific details.
Neuroimaging techniques can help determine if the neural processes driving this retrieval of inaccurate memories are different from those that drive the retrieval of accurate memories. Several research groups are using functional magnetic resonance imaging (fMRI) to address this question. The hope is that neuroimaging can help determine the various potential sources of false memories.
Daniel Schacter, PhD, and his colleagues at Harvard University have looked at neural activity associated with the creation of false memories. Previous studies had focused on neural activity associated with the retrieval of false memories.
Relying upon earlier work that showed the right fusiform cortex is involved in encoding the exact visual details of objects and the left fusiform cortex is involved in more general processing, Schacter's group designed an experiment to test the role of the right fusiform area in avoiding the formation of false memories for objects similar to those seen previously but not exactly the same.
In the study, led by graduate student Rachel Garoff, participants underwent brain scans using fMRI as they made judgments about the size of various objects. A surprise memory test was then given when the patients were outside the MRI scanner. During the test, patients saw objects identical to those seen earlier, objects similar to those they had seen earlier, and new objects they had not seen at all.
Although the study is still in progress, results to date indicate that the right fusiform area was more active in these individuals during the encoding of objects participants later labeled the "same" as objects they had seen before. The right fusiform area was less active when patients incorrectly labeled objects the same when they were only similar, or when they labeled objects similar when they were actually the same.
"This preliminary finding supports the idea that the right fusiform area is tied to the encoding of specific visual details," Schacter said. "It also suggests that false memories of objects can be reduced through additional activity of the right fusiform area during encoding."
In another study, Schacter's group showed that visual processing regions of the brain were reactivated during true memory but not during false memory.
Scott Slotnick and Schacter constructed "prototype" shapes by adjoining four curves into various shapes, then they distorted these prototypes to form "exemplar" shapes. The twelve individuals who have taken part in the study thus far were instructed to remember each shape and whether it appeared to the left or the right on a screen. "True memory" was defined as recognizing a shape that was seen previously, and "false memory" referred to mistakenly recognizing a shape that resembled a shape seen earlier but that was not actually seen. In the next step, fMRI was used to determine which areas of the brain were associated with true and false memory.
"We found that participants gave the same response regarding whether an object was "new" or "old" during true and false memory, which leads you to expect that the associated brain activity might be indistinguishable," Schacter said. "But fMRI revealed there is a different activation of brain regions involved in visual processing during true versus false memory. What we need to do now is understand the meaning of this difference."
In other work, Craig Stark, PhD, and graduate student Yoko Okado used fMRI to understand how the "misinformation effect" contributes to the development of false memories. This effect occurs when, for example, an individual watches a scene, is later given a description of the scene in which several details are altered, and then recollects having originally seen the altered information.
The researchers hypothesized that false memories are more likely to occur if memory for an original event is weakly established in the brain and memory for misinformation is strongly established.
The eight participants in the study first read vignettes about events such as robberies or college friends waiting for class while they were in an fMRI scanner. Each vignette contained 12 critical items, such as a left-handed robber or a student studying for an exam, that participants would be tested on. Next, the participants studied the same vignettes while still in the scanner, but with changes to all of the 12 critical items. This was called the "misinformation phase."
Two days later, the participants took a recognition memory test for the critical items they saw in their first study of the vignettes. Then they were asked to identify from which source they remembered seeing the critical item. Participants' memories were "true" when the individuals correctly said critical items from the first study of vignettes actually came from that study. Memories were considered "false" when the changes made to critical items in the misinformation phase were said to have come from the original study of vignettes. The neural activity that later led to these true and false memories was examined from the fMRI data.
The investigators found that when neural activity in the hippocampus--an area of the brain known to be important in establishing memories--was greater during the original vignette study than it was during the misinformation phase, the subsequent memory was true. When neural activity was greater during the misinformation phase than during the original vignette study, the memory was false.
"These results are significant because they show that the neural processes that establish, or encode, a memory play a substantial role in determining whether recollected memories are true or false," said Okado.
David Beversdorf, MD, and colleagues at Ohio State University looked at formation of false memories when a "critical lure" similar to a geometric shape previously seen was later viewed.
Twenty-three participants were shown images of geometric figures in 24 sets on 12 slides. Each set of slides showed different geometric shapes that were different in size, color, and position. Participants were asked to remember as many of the slides as they could. After seeing the 12 slides in each set, participants were shown five more slides and asked whether or not they had seen them in a previous set. Two of the five slides had in fact been shown, two had not, and one was designed to be strongly related to all of the other slides shown but had not been shown previously. This slide was called a critical lure.
Participants believed they had seen an image that they had actually not seen 59 percent of the time. "This study shows that people can be easily persuaded that they have seen something they haven't," said Beversdorf.
The significance of this study and others using visual stimuli, according to Beversdorf and his group, is that it shows that false memories can develop during nonverbal tasks. Such findings may have implications for those with autism. High functioning autistic individuals can actually perform better than others on verbal false memory tasks. However, because atypical language processing is a defining feature of autism, it remains to be seen whether this unusually good performance for false memories can extend to nonverbal tasks.