Abstract
The speed of theta brain oscillatory activity is thought to play a key role in determining working memory (WM) capacity. Individual differences in the length of a theta cycle (ranging between 4 and 7 Hz) might determine how many gamma cycles (>30 Hz) can be nested into a theta wave. Gamma cycles are thought to represent single memory items; therefore, this interplay could determine individual memory capacity. We directly tested this hypothesis by means of parietal transcranial alternating current stimulation (tACS) set at slower (4 Hz) and faster (7 Hz) theta frequencies during a visuospatial WM paradigm. Accordingly, we found that 4-Hz tACS enhanced WM capacity, while 7-Hz tACS reduced WM capacity. Notably, these effects were found only for items presented to the hemifield contralateral to the stimulation site. This provides causal evidence for a frequency-dependent and spatially specific organization of WM storage, supporting the theta–gamma phase coupling theory of WM capacity.
Highlights
The theta–gamma cross-frequency coupling theory [1] (Fig 1A) proposes that individual fast brain waves represent individual memory items that are bound together to a multi-item memory by slow brain waves
Previous studies have left unanswered a long-lasting question regarding the exact mechanism by which theta oscillations orchestrate working memory (WM) capacity: does the cycle length of theta oscillations play a mechanistic role in determining interindividual variability of WM capacity? Here, we test the prediction based on the theta–gamma phase coupling theory [1] that inducing slower theta cycles will enhance WM capacity, while inducing faster theta cycles will reduce WM capacity
We tested this prediction in a visuospatial WM paradigm based on seminal work by Vogel and Machizawa [7,8,9], who showed interindividual differences in visuospatial WM capacity to positively correlate with the amplitude of evoked responses localised over parietal areas contralateral to the hemifield where the stimulus to be kept in memory was presented
Summary
The theta–gamma cross-frequency coupling theory [1] (Fig 1A) proposes that individual fast brain waves (gamma cycles) represent individual memory items that are bound together to a multi-item memory by slow brain waves (theta oscillations). Recent neurostimulation work has shown that entraining parietal theta oscillations via transcranial alternating current stimulation (tACS) [4] or rhythmic Transcranial Magnetic Stimulation (TMS) [5] has proven effective in enhancing WM performance, providing causal evidence for the role of theta oscillations in WM performance. We test the prediction based on the theta–gamma phase coupling theory [1] that inducing slower theta cycles will enhance WM capacity, while inducing faster theta cycles will reduce WM capacity We tested this prediction in a visuospatial WM paradigm based on seminal work by Vogel and Machizawa [7,8,9], who showed interindividual differences in visuospatial WM capacity to positively correlate with the amplitude of evoked responses localised over parietal areas contralateral to the hemifield where the stimulus to be kept in memory was presented. These effects were obtained for the visual hemifield contralateral to the stimulation site
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