Examining the optimal timing for closed loop auditory stimulation of slow wave sleep in young and older adults

Study Objectives
Closed loop auditory stimulation (CLAS) is a method for enhancing slow oscillations (SOs) through the presentation of auditory clicks during sleep. CLAS boosts SOs amplitude and sleep spindle power, but the optimal timing for click delivery remains unclear. Here, we determine the optimal time to present auditory clicks to maximise the enhancement of SO amplitude and spindle likelihood.

Methods
We examined the main factors predicting SO amplitude and sleep spindles in a dataset of twenty-one young and seventeen older subjects. The participants received CLAS during slow-wave-sleep in two experimental conditions: sham and auditory stimulation. Post-stimulus SOs and spindles were evaluated according to the click-phase on the SOs and compared between and within conditions.

Results
We revealed that auditory clicks applied anywhere on the positive portion of the SO increased SO amplitudes and spindle likelihood, although the interval of opportunity was shorter in the older group. For both groups, analyses showed that the optimal timing for click delivery is close to the SO peak phase. Click-phase on the SO wave was the main factor determining the impact of auditory stimulation on spindle likelihood for young subjects, whereas for older participants the temporal lag since the last spindle was a better predictor of spindle likelihood.

Conclusions
Our data suggest that closed-loop auditory stimulation can more effectively boost SOs during specific phase windows, and these differ between young and older participants. It is possible that this is due to the fluctuation of sensory inputs modulated by the thalamocortical networks during the SO.