How the brain weaves a memory

A central question has been whether the hippocampus receives an “episodic packet,” or a collection of perceptual strands that it must integrate into a memory.

In an article in the November 9, 2006, issue of the journal Neuron, published by Cell Press, Melina Uncapher and colleagues report experiments with human volunteers that shed important light on this process.

In their experiments, the researchers presented the subjects with series of “study” words on a display screen, as their brains were scanned using functional magnetic resonance imaging (fMRI). This technique involves using harmless radio waves and magnetic fields to measure blood flow to brain regions, which reflects brain activity. As their brains were scanned, the subjects were shown words of different colors and that were located in different quadrants of the display screen.

Later, the subjects were presented a collection of words including both the study words and new words. They were asked to recall whether the words were old or new, and for the old words, their color and location.

The researchers then correlated fMRI data on the brain regions active during the study phase with the data on the subjects' later retrieval of the features of these study words. They found that, indeed, regions of the brain involved in processing color and location were active during formation of memories for those features.

However, importantly, they found that the subjects' successful retrieval of both features–versus only color or location–was uniquely associated with enhanced activity in yet another brain region called the intraparietal sulcus, which has been strongly implicated in other studies as important in “perceptual binding” of multiple features of stimuli.

Thus, the researchers concluded that “The findings suggest that the encoding of disparate features of an episode into a common memory representation requires that the features be conjoined in a common perceptual representation when the episode is initially experienced.”