Induced theta oscillations mediate large‐scale synchrony with mediotemporal areas during recollection in humans

Abstract

One functional aspect of theta oscillations that recent studies have begun to explore in humans is their role for the recollection of personal events. A specific role of theta for recollection but not for stimulus-familiarity would support links between cortical theta oscillations and hippocampal functioning, given that the hippocampus seems to be more critical for recollection than for stimulus-familiarity. During recollection, theta oscillations might mediate a dynamic link between hippocampal and neocortical areas, thereby allowing to recruit and bind distributed cortical representations. We recorded theta oscillations using whole-head magnetoencephalography while nine healthy subjects made recognition memory judgments on previously studied and unstudied pictures of faces. For each recognized face, subjects indicated whether they also recollected the background image in front of which that face was studied. Theta oscillations were higher in amplitude during recollection than during recognition in the absence of accurate memory for the background. These theta oscillations were induced in nature, meaning that they showed considerable phase variability from trial to trial. To nevertheless extract the field distribution of coherent theta oscillations from single trials, we calculated phase differences between sensor pairs at each time point of each single trial. This field information was used to localize the brain sources of synchronized theta-generators. The results suggest that recollection is associated with induced activity increase in a distributed synchronous theta network, including prefrontal, mediotemporal, and visual areas. These findings are compatible with the notion that theta oscillations are related to the binding of distributed cortical representations during recollection.

Medial temporal theta state before an event predicts episodic encoding success in humans

“…First, a partial least squares (PLS) analysis (22,23) was computed on theta amplitudes from Ϫ400 to 800 ms relative to word onset at study. PLS is a multivariate technique that examines the relationship between a set of design variables (here, frequency bands and experimental contrasts) and corresponding measures of brain activity (here, amplitude for each sensor, time point in the epoch, and participant).…”

“…PLS is a multivariate technique that examines the relationship between a set of design variables (here, frequency bands and experimental contrasts) and corresponding measures of brain activity (here, amplitude for each sensor, time point in the epoch, and participant). In this first analysis, theta amplitudes between 3 and 8 Hz were derived from a convolution of the raw signals with Morlet wavelets (22,23) having Ϸ6 oscillation cycles and a frequency spacing of 0.25 Hz, resulting in 21 transforms. The high frequency resolution of these long wavelets permitted detection of the theta frequency most strongly associated with later recall.…”

“…Theta field distributions were assessed by calculating between-sensor phase differences on a trial-by-trial basis for each participant for laterrecalled and later-forgotten words. This approach was necessary because the prestimulus theta activity did not show above-chance phase locking, and averaging random phase values would lead to distorted field information (23). Inverse wavelet transformation (IWT) was used to bring the resulting phase information, together with the corresponding amplitude information, back to the time domain (see ref.…”

“…Inverse wavelet transformation (IWT) was used to bring the resulting phase information, together with the corresponding amplitude information, back to the time domain (see ref. 23 for more detail). Minimum-norm current-density reconstruction of the resulting theta waveforms revealed strong sources of theta activity bilaterally in the MTL between Ϫ100 and 0 ms, with later-recalled words being associated with stronger sources than later-forgotten words (Fig.…”

Dynamic causal modelling of induced responses

“…In this model, temporal changes of power in a source are explained as a network function of power in all sources. This sort of model can deal with situations in which oscillations in one area become amplitude modulated by oscillations in another band in the same or another areas; e.g., theta-band modulation of gamma activity (e.g., Guderian and Duzel, 2005). Here, high levels of theta activity would engender increases in gamma.…”