Hippocampal Activity Patterns Carry Information about Objects in Temporal Context. Hsieh, Gruber, Jenkins, Rangarath. Neuron 2014.

  1. Hippocampus is a key part of episodic memory, but a key questions is “… whether the hippocampus represents specific objects or assigns context-dependent representations to objects.”
  2. fMRI results “… are consistent with models proposing that the hippocampus represents objects within specific temporal contexts, a capability that might explain its critical role in episodic memory.”
  3. Episodic memories are memories based around a specific context
  4. “A strong version of the view that the hippocampus represents stimulus attributes in memory would suggest that the hippocampus should assign similar representations to events that include the same objects.  In contrast, a strong version of the context-based view would suggest that the hippocampus assigns distinct representations to multiple encounters with the same object in different temporal contexts. Thus, a fundamental, and currently unresolved, question is whether the hippocampus supports memory for temporal context, over and above memory for specific objects.”
  5. The presented sequences of 5 images to the subjects.  In ‘X’ trials, the same image appears at the 2nd and 3rd positions, whereas in the ‘Y’ trials, the same image appears in the first three positions.
    1. Because the ‘X’ sequences start differently, the hypothesis was that it would initiate a different context, whereas ‘Y’ would be identified more as the same episode because they initialize the same way
  6. Use multivoxel pattern similarity analysis on fMRI data
  7. They measured RTs but I’m not clear on what the response was actually based on – it says just “… semantic decisions about each object in the sequence.” <could be it was explained properly but missed it due to mental state>
  8. “… results demonstrate that learning of the object sequences facilitated participants’ semantic decisions during the scan session. Because participants performed different semantic tasks in each scanning run, the results suggest that the learning was not at the level of motor responses or of object-response associations, but rather driven by learning about the temporal relationships among the objects.”
  9. <I see one potential confound here, which is that there are two differences between X and Y trials.  One is the point in which the matched subsequence starts (Ys at beginning, and Xs at the middle), but also there is a difference in how long those matched subsequences are (Ys with length 3 and Xs with length 2), I don’t think they’ve addressed this so far in the paper.  They have it set up this way (I think) so that the 4th object in the X and Y sequences are not part of the subsequence…>
  10. Activity in hippocampus was greatest among trials that had objects match at the same position
    1. A modification of the setup shows hippcocampal activity was not “… solely driven by serial position information.  Furthermore, there was no evidence of purely temporal coding in the hippocampus…”
  11. The next modification tested for activity of objects when they occurred at different positions in the sequence.
  12. “The results therefore clearly support the hypothesis that the hippocampus is specifically involved in the binding of object and position information during temporal sequence retrieval.”
    1. <Although this finding was not significant for the left hippocampus?>
  13. “…participants who showed more behavioral enhancement for the learned sequences (i.e. faster RTs for the learned sequences than the ‘Random’ sequence) also showed more of a hippocampal pattern similarity effect for learned, relative to random, sequences.”
  14. “… when individual differences in sequence learning are taken into consideration, the left hippocampus also carries information about the serial positions associated with objects in temporal sequences.”
  15. X and Y trials are different because the X trials can be disambiguated based on the first item (which is unique across them).  Analysis of right and left hippocampus didn’t show distinction between them, but difference did show up in right posterior hippocampus
  16. “Previous studies have indicated hippocampal involvement in processing boundaries in spatial contexts… Consistent with our predictions, pattern similarity was higher for ‘Within” than for ‘Between’ trial pairs in the left hippocampus …”
  17. Discussion
  18. “We found that hippocampal activation patterns specifically carried information about objects in particular temporal positions (i.e. ‘object-position binding), and this could not be explained by the processing of object or temporal position information alone.  Moreover, individual differences in hippocampal voxel pattern information explained over one-third of the interindividual variance in reaction time indices of sequence learning.  Individuals who exhibited more robust hippocampal object-position binding showed more behavioral facilitation during sequence retrieval.  Using overlapping sequences, we also found that hippocampal voxel pattern similarity was higher for pairs of adjacent trials that belonged to the same sequence context… Together, these results are consistent with the idea that the hippocampus represents information about the temporal context associated with specific items.”
  19. Results here are consistent with view that hippocampus codes not only for items but also for the context it exists in
  20. “We found no evidence to support the idea that hippocampal activity patterns carry information about objects when the temporal order was random.”  Other brain areas did activate in this situation, however
  21. “We speculate that the capability of the hippocampus to encode objects in relation to a temporal context might relate to the ability to distinguish between temporally distinct events that share common elements.  For instance, parking a car in the same parking structure on different days requires the formation of distinct memory representations in order to efficiently retrieve the car at a later time.”
  22. “…the hippocampus may assign distinct representations to overlapping but psychologically distinct events… Specifically, we found that, even when comparing pairs of trials corresponding to the same object in the same temporal position, hippocampal pattern similarity was higher for pairs of trials in the same learned sequence (‘X1-X1’ or ‘X2-X2’ paris) than across pairs of trials in different sequences (‘X1-X2’ pairs).”
  23. This is also true between the Y pairs which can only be distinguished first by the 4th item in the sequence.    “These results suggest that the hippocampus only differentiates between overlapping sequences that are psychologically distinct.”
  24. “Taken together, the results are consistent with the idea that temporal context coding in the hippocampus may help disambiguate overlapping events in episodic memory, thereby contributing to ‘pattern separation’ (…).”
  25. <Lots of related work>
  26. “The context-sensitive hippocampal activation patterns observed here might support a wide tange of memory capacities, including the ability to learn spatial maps (…), differentiate highly similar, yet distinct memories (…), and the ability to segment continuous incoming information into distinct episodic memories (…).  More generally, the results underscore the importance of temporal information in understanding hippocampal function, potentially explaining how the hippocampus supports the ability to remember what happened when.”

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