Towards optimization of oscillatory stimulation during sleep
Background Oscillatory rhythms during sleep such as slow oscillations (SO) and spindles, and most importantly their coupling, are thought to underlie processes of memory consolidation. External slow oscillatory transcranial direct current stimulation (so-tDCS) with a frequency of 0.75 Hz has been shown to improve this coupling and memory consolidation, however, effects varied quite markedly between individuals, studies and species.
Objective Here, we aimed to determine how precisely the frequency of stimulation has to match the naturally occurring SO frequency in individuals to optimally improve SO-spindle coupling. Moreover, we systematically tested stimulation durations necessary to induce changes.
Methods We addressed these questions by comparing so-tDCS with individually adapted SO frequency to standardized frequency of 0.75Hz in a cross-over design with 28 healthy older participants during napping while systematically varying stimulation train durations between 30s, 2min and 5min.
Results Stimulation trains as short as 30s were sufficient to modulate the coupling between SOs and spindle activity. Contrary to our expectations, so-tDCS with standardized frequency indicated stronger aftereffects with regard to SO-spindle coupling in comparison to individualized frequency. Angle and variance of spindle maxima occurrence during the SO cycle were similarly modulated.
Conclusion Short stimulation trains were sufficient to induce significant changes in sleep physiology allowing for more trains of stimulation, which provides methodological advantages and possibly even larger effects in future studies. With regard to individualized stimulation frequency, further options of optimization need to be investigated, such as closed-loop stimulation to calibrate stimulation frequency to the SO frequency at time of stimulation onset.