A moment lasts all of a second, but the memory lives on forever?
A woman looks familiar, but you can’t remember her name or where you met her. New research by UC Irvine neuroscientists suggests the memory exists – you simply can’t retrieve it.
Using advanced brain imaging techniques, the scientists discovered that a person’s brain activity while remembering an event is very similar to when it was first experienced, even if specifics can’t be recalled.
“If the details are still there, hopefully we can find a way to access them,” said Jeff Johnson, postdoctoral researcher at UCI’s Center for the Neurobiology of Learning & Memory and lead author of the study, appearing Sept. 10 in the journal Neuron.
“By understanding how this works in young, healthy adults, we can potentially gain insight into situations where our memories fail more noticeably, such as when we get older,” he said. “It also might shed light on the fate of vivid memories of traumatic events that we may want to forget.”
In collaboration with scientists at Princeton University, Johnson and colleague Michael Rugg, CNLM director, used functional magnetic resonance imaging to study the brain activity of students.
Inside an fMRI scanner, the students were shown words and asked to perform various tasks: imagine how an artist would draw the object named by the word, think about how the object is used, or pronounce the word backward in their minds. The scanner captured images of their brain activity during these exercises.
About 20 minutes later, the students viewed the words a second time and were asked to remember any details linked to them. Again, brain activity was recorded.
Utilizing a mathematical method called pattern analysis, the scientists associated the different tasks with distinct patterns of brain activity. When a student had a strong recollection of a word from a particular task, the pattern was very similar to the one generated during the task. When recollection was weak or nonexistent, the pattern was not as prominent but still recognizable as belonging to that particular task.
“The pattern analyzer could accurately identify tasks based on the patterns generated, regardless of whether the subject remembered specific details,” Johnson said. “This tells us the brain knew something about what had occurred, even though the subject was not aware of the information.”
However valuable these results are, they seem to be missing half of the picture. At least in the amygdala, a brain structure involved in emotional control, scanning for neuronal activity would not be enough in order to trap the building of memory. Next to neurons themselves, their embedding matrix too is involved in memory protection and recollection.
Neuron, Volume 63, Issue 5, 697-708, 10 September 2009
Recollection, Familiarity, and Cortical Reinstatement: A Multivoxel Pattern Analysis
Jeffrey D. Johnson1, 4, Go To Corresponding Author, , Susan G.R. McDuff2, 4, Michael D. Rugg1 and Kenneth A. Norman2, 3
1 Center for the Neurobiology of Learning and Memory and Department of Neurobiology and Behavior, University of California, Irvine, Irvine, CA 92697, USA
2 Department of Psychology, Princeton University, Princeton, NJ 08544, USA
3 Princeton Neuroscience Institute, Princeton University, Princeton, NJ 08544, USA
4 These authors contributed equally to this work.
SummaryEpisodic memory retrieval is thought to involve reinstatement of the neurocognitive processes engaged when an episode was encoded. Prior fMRI studies and computational models have suggested that reinstatement is limited to instances in which specific episodic details are recollected. We used multivoxel pattern-classification analyses of fMRI data to investigate how reinstatement is associated with different memory judgments, particularly those accompanied by recollection versus a feeling of familiarity (when recollection is absent). Classifiers were trained to distinguish between brain activity patterns associated with different encoding tasks and were subsequently applied to recognition-related fMRI data to determine the degree to which patterns were reinstated. Reinstatement was evident during both recollection- and familiarity-based judgments, providing clear evidence that reinstatement is not sufficient for eliciting a recollective experience. The findings are interpreted as support for a continuous, recollection-related neural signal that has been central to recent debate over the nature of recognition memory processes.
Science 4 September 2009: Vol. 325. no. 5945, pp. 1258 – 1261
Perineuronal Nets Protect Fear Memories from Erasure
Nadine Gogolla, Pico Caroni, Andreas Lüthi, Cyril Herry
In adult animals, fear conditioning induces a permanent memory that is resilient to erasure by extinction. In contrast, during early postnatal development, extinction of conditioned fear leads to memory erasure, suggesting that fear memories are actively protected in adults. We show here that this protection is conferred by extracellular matrix chondroitin sulfate proteoglycans (CSPGs) in the amygdala. The organization of CSPGs into perineuronal nets (PNNs) coincided with the developmental switch in fear memory resilience. In adults, degradation of PNNs by chondroitinase ABC specifically rendered subsequently acquired fear memories susceptible to erasure. This result indicates that intact PNNs mediate the formation of erasure-resistant fear memories and identifies a molecular mechanism closing a postnatal critical period during which traumatic memories can be erased by extinction.