Closed-loop auditory stimulation of sleep slow oscillations: basic principles and best practices

Abstract

Sleep is an indispensable part of our life and plays a critical role in our physical and mental well-being. During sleep, despite the paucity of behavior, our brain stays active and exhibits a wide range of coupled brain oscillations. This activity in sleep-characteristic brain oscillations has been linked to various functions of sleep. However, whether these sleep oscillations mediate these functions or reflect mere epiphenomena is not yet fully understood. To disentangle the causality of these relationships, experiments utilizing non-invasive stimulation techniques have been essential. In particular, auditory stimulation aligned with neural activity in a closed-loop controlled fashion has drawn increasing attention during the last years due to its specificity and practical advantages. In this review, we summarize closed-loop auditory stimulation experiments that generated evidence to support a causal role of slow oscillations in information reprocessing functions of sleep. Furthermore, we provide technical details and guidelines regarding best practices in closed-loop auditory stimulation, from how to perform the stimulation to analysis strategies. Besides discussing important caveats and open questions, we also touch on various areas in which closed-loop auditory stimulation is applicable, from fundamental investigations on memory processing and endocrine function, to its potential for clinical applications. Eventually, we propose potential topics for future research.