EEG Signatures of Successful Memory Encoding

The subsequent memory effect (SME) refers to two characteristics of evoked potentials that correspond to successful memory encoding in the medial temporal lobe. Both negativity at 400 ms post-stimulus (in rhinal cortex) and positivity at 800 ms post-stimulus (in hippocampus) can be used to predict whether that stimulus will later be successfully recalled. But what role might synchronized oscillations have in this encoding process, and how would synchrony influence or be influenced by these SME effects?

Sederberg et al set out to answer this question in a 2003 article in the Journal of Neuroscience. By presenting subjects with a sequential list of randomly selected words, while simultaneously recording from intracranial electrodes, and then subsequently asking them to recall the words in any order, the authors were able to correlate specific EEG activity at the time of encoding with successful recall later.

The dominant frequencies for SME effects were in the theta and gamma frequency bands. Right temporal and frontal areas showed a lateralized and positive SME, such that increased theta band power between 600-1300 msec post-stimulus correlated with better subsequent recall. Similar areas showed a positive SME for gamma-band oscillations, but this effect was not lateralized. Negative SME's were found for alpha and beta band oscillations in the same regions, such that activity in those frequency ranges was negatively correlated with successful recall.

Interestingly, one aspect of these results can be interpreted to cast doubt on the Jensen & Lisman model of working memory capacity limits, in which the 7 +/- 2 capacity limit is thought to arise from the multiplexing of gamma within theta frequencies. The current results suggest that the areas showing positive theta and gamma frequencies are only very slightly correlated, indicating that the regions manifesting each oscillation are likely to be at least somewhat distinct from each other.

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