Abstract
Working memory (WM) and short-term memory (STM) supposedly rely on the phase-amplitude coupling (PAC) of neural oscillations in the theta and gamma frequency ranges. The ratio between the individually dominant gamma and theta frequencies is believed to determine an individual’s memory capacity. The aim of this study was to establish a causal relationship between the gamma/theta ratio and WM/STM capacity by means of transcranial alternating current stimulation (tACS). To achieve this, tACS was delivered at a frequency below the individual theta frequency. Thereby the individual ratio of gamma to theta frequencies was changed, resulting in an increase of STM capacity. Healthy human participants (N = 33) were allocated to two groups, one receiving verum tACS, the other underwent a sham control protocol. The electroencephalogram (EEG) was measured before stimulation and analyzed with regard to the properties of PAC between theta and gamma frequencies to determine individual stimulation frequencies. After stimulation, EEG was recorded again in order to find after-effects of tACS in the oscillatory features of the EEG. Measures of STM and WM were obtained before, during and after stimulation. Frequency spectra and behavioral data were compared between groups and different measurement phases. The tACS- but not the sham stimulated group showed an increase in STM capacity during stimulation. WM was not affected in either groups. An increase in task-related theta amplitude after stimulation was observed only for the tACS group. These augmented theta amplitudes indicated that the manipulation of individual theta frequencies was successful and caused the increase in STM capacity.
Highlights
Brain oscillations, in particular in the theta (3–8 Hz) and gamma (>30 Hz) frequency ranges, have been suggested to be key features in cognitive processes (Buzsáki, 2006; Lisman and Buzsáki, 2008; Lisman and Jensen, 2013), including memory performanceShort-term memory increase by theta-transcranial alternating current stimulation (tACS) (Düzel et al, 2010; Hanslmayr and Staudigl, 2014)
Digit Span Task An ANOVA (3 × 2 × 2) with factors stimulation block, direction and group on mean list lengths from the digit span task recorded during stimulation revealed that performance levels in backward and forward digit span differed significantly (main effect of direction: F(1,31) = 67.111, p < 0.001, η2 = 0.684)
The forward digit span performance was significantly better in the tACS than the sham control group (planned comparison, F(1,31) = 4.962, p = 0.033), whereas no such effect was found for the backward condition (F(1,31) = 0.018, p = 0.894)
Summary
In particular in the theta (3–8 Hz) and gamma (>30 Hz) frequency ranges, have been suggested to be key features in cognitive processes (Buzsáki, 2006; Lisman and Buzsáki, 2008; Lisman and Jensen, 2013), including memory performanceShort-term memory increase by theta-tACS (Düzel et al, 2010; Hanslmayr and Staudigl, 2014). The theta-gamma coding theory of STM and WM states that each gamma cycle represents one memory item while each theta-cycle represents the rehearsal of the list of all items held in storage (Lisman and Idiart, 1995; Jensen and Lisman, 1998; Lisman, 2010) This explains the limited capacity of these systems, which is determined by the number of gamma-cycles that ‘‘fit’’ onto one theta wave (Figure 1A). In support of this theory, it has been shown that the strength of PAC between theta and gamma frequencies as well as the underlying theta frequency depends on WM load (Axmacher et al, 2010). Most direct support for this theory was provided in an experiment showing that the theta to gamma cycle-length-ratio correlates with STM capacity at fronto-central EEG electrodes (Kaminski et al, 2011)
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