Subsequent memory effect in intracranial and scalp EEG

Abstract

Successful memory encoding is marked by increases in 30-100 Hz gamma-band activity in a broad network of brain regions. Activity in the 3-8 Hz theta band has also been shown to modulate memory encoding, but this effect has been found to vary in direction across studies. Because of the diversity in memory tasks, and in recording and data-analytic methods, our knowledge of the theta frequency modulations remains limited. The difference in the directionality of these theta effects could arise from a distinction between global cortical and deeper subcortical effects. To address this issue, we examined the spectral correlates of successful memory encoding using intracranial EEG recordings in neurosurgical patients and scalp EEG recordings in healthy controls. We found significant theta (3-8 Hz) power modulations (both increases and decreases) and high gamma (44 - 100 Hz) power increases in both samples of participants. These results suggest that (1) there are two separate theta mechanisms supporting memory success, a broad theta decrease present across both the cortex and hippocampus as well as a theta power increase in the frontal cortex, (2) scalp EEG is capable of resolving high frequency gamma activity, and (3) iEEG theta effects are likely not the result of epileptic pathology.

Slow-theta power decreases during item-place encoding predict spatial accuracy of subsequent context recall

“…The study demonstrates that theta power decreases co occurred and were co localized with BOLD signal increases in MTL, a region clearly driven by task's pro cesses. The same reasoning could be applied to decreases in slow theta (~2 5 Hz) activity found in our study, taking also into account the high overlap in frequency with broadband negative SMEs (Burke et al, 2013;Greenberg et al, 2015;Long et al, 2014;Sederberg et al, 2007). In concordance with these two perspectives, decreased slow theta activity could mean that higher degree of visual and spatial in formation was processed and encoded into memory, enabling a more accurate retrieval of the spatial location during the test phase.…”

“…However, other studies have reported negative SMEs in the broad theta band (Burke et al, 2013;Guderian et al, 2009;Long et al, 2014;Sederberg et al, 2007). These theta power decreases are seen during the successful encoding of single items and inter item associations (Greenberg et al, 2015) and are widely detected with surface sensors or iEEG contacts targeting medial temporal lobe (MTL) and fronto temporal structures (Greenberg et al, 2015;Long et al, 2014).…”

“…These theta power decreases are seen during the successful encoding of single items and inter item associations (Greenberg et al, 2015) and are widely detected with surface sensors or iEEG contacts targeting medial temporal lobe (MTL) and fronto temporal structures (Greenberg et al, 2015;Long et al, 2014). Since negative SMEs manifest in key regions for successful encoding (e.g., hippocampus and left prefrontal cortex), the traditional vision that increased theta activity is necessary for memory formation faces a challenge.…”

“…Positive or negative SMEs manifest when study and test contexts match or mismatch, re spectively, which constitutes evidence that theta power plays a crucial role in context item binding (Staudigl and Hanslmayr, 2013). SMEs also vary across brain regions, time windows and frequency ranges (e.g., early narrow band frontal power increases vs. late broad band hippocampal power decreases; Long et al, 2014) and seem to reflect different phase synchronization patterns (Burke et al, 2013). Besides these parameters, opposite SMEs can coexist in hippocampus at distinct frequency components (positive SMEs at~3 Hz vs. negative SMEs at 8 Hz; Lega et al, 2012), suggesting the existence of functionally disso ciable theta bands (for more details, see discussion in Lega et al, 2012).…”

Alpha oscillations during incidental encoding predict subsequent memory for new “foil” information

“…Our oscillatory findings are consistent with prior literature highlighting a role for alpha frequencies in successful semantic encoding (Hanslmayr et al, 2009;Hanslmayr & Staudigl, 2014;Zion-Golumbic et al, 2009) and semantic processing (Klimesch et al, 2006;Long et al, 2014). In the oscillations literature, alpha frequencies have been linked with a wide variety of cognitive functions ranging from inhibitory processes during memory suppression (Park et al, 2014), to fine-grained resolution of visual processing (Samaha & Postle, 2015), working memory (Sauseng et al, 2009, Myers et al, 2014, and active inhibition of a not-tobe applied rule (Buschman et al, 2012).…”