Wednesday, July 13, 2011

Watching contextual memories

Our memories from a particular time and place are linked together (recall Proust's Madeleine cookie) and referred to as contextual memory. Benedict Carey points to an elegant study by Manning et al. that has studied 69 neurosurgical patients who were implanted with subdural electrode arrays and depth electrodes during treatment for drug-resistant epilepsy. As electrocorticographic (ECoG) signals were recorded, the patients volunteered to participate in a free recall memory experiment, in which they studied lists of common nouns and then attempted to recall them verbally in any order following a brief delay. Their results suggest that memory is like a streaming video that is bookmarked, both consciously and subconsciously. New memories of even abstract facts are encoded in brain-cell firing sequences that also contains information about what else was happening during and just before the memory was formed. Here is their abstract, followed by a striking figure from the paper:
Psychological theories of memory posit that when people recall a past event, they not only recover the features of the event itself, but also recover information associated with other events that occurred nearby in time. The events surrounding a target event, and the thoughts they evoke, may be considered to represent a context for the target event, helping to distinguish that event from similar events experienced at different times. The ability to reinstate this contextual information during memory search has been considered a hallmark of episodic, or event-based, memory. We sought to determine whether context reinstatement may be observed in electrical signals recorded from the human brain during episodic recall. Analyzing electrocorticographic recordings taken as 69 neurosurgical patients studied and recalled lists of words, we uncovered a neural signature of context reinstatement. Upon recalling a studied item, we found that the recorded patterns of brain activity were not only similar to the patterns observed when the item was studied, but were also similar to the patterns observed during study of neighboring list items, with similarity decreasing reliably with positional distance. The degree to which individual patients displayed this neural signature of context reinstatement was correlated with their tendency to recall neighboring list items successively. These effects were particularly strong in temporal lobe recordings. Our findings show that recalling a past event evokes a neural signature of the temporal context in which the event occurred, thus pointing to a neural basis for episodic memory.

Evidence for context reinstatement in the temporal lobe. (A) Each dot marks the location of a single electrode from our dataset in Montreal Neurological Institute space. We divided our dataset into four regions of interest: temporal lobe (blue, 1,815 electrodes), frontal lobe (red, 1,737 electrodes), parietal lobe (yellow, 512 electrodes), and occipital lobe (green, 138 electrodes).

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