Remembering returns brain states to when the actual experience happened

William James, the influential American philosopher and psychologist of the late 1800's argued that remembering events reactivated motor and sensory brain regions involved during the original event. How right he was! Danker and Anderson have written an extensive review of the research literature looking at how this all happens, cleverly titled "The Ghosts of Brain States Past". Here is there abstract from the latest issue of Psychological Bulletin.

There is growing evidence that the brain regions involved in encoding an episode are partially reactivated when that episode is later remembered. That is, the process of remembering an episode involves literally returning to the brain state that was present during that episode. This article reviews studies of episodic and associative memory that provide support for the assertion that encoding regions are reactivated during subsequent retrieval. In the first section, studies are reviewed in which neutral stimuli were associated with different modalities of sensory stimuli or different valences of emotional stimuli. When the neutral stimuli were later used as retrieval cues, relevant sensory and emotion processing regions were reactivated.

In the second section, studies are reviewed in which participants used different strategies for encoding stimuli. When the stimuli were later retrieved, regions associated with the different encoding strategies were reactivated. Together, these studies demonstrate not only that the encoding experience determines which regions are activated during subsequent retrieval but also that the same regions are activated during encoding and retrieval. In the final section, relevant questions are posed and discussed regarding the reactivation of encoding regions during retrieval.

Some interesting points about brain region reactivation:

1. reactivation of the visual system occurs at the multi-granular level
2. reactivation is stronger during retrieval of more info
3. there's a reduction in reactivation when info is falsely remembered
4. reactivation is correlated with subjective reports of remembering (a bit surprising given how fallible our memories often are)

The authors conclude by asking whether reactivation of brain states are more easily activated when info is more accessible. What about the role of speed and other varying factors? All of these questions, I am sure, will be answered before we forget.

Danker, J., & Anderson, J. (2010). The ghosts of brain states past: Remembering reactivates the brain regions engaged during encoding. Psychological Bulletin, 136 (1), 87-102 DOI: 10.1037/a0017937