“…We recently suggested that a distinction be made between delta activity at sleep onset (DASO) and slow wave activity that does not occur in the context of sleep onset (e.g., FIRDA) (Kawai et al, 2016). Although not termed DASO, previous reports consistently show a higher prevalence of this type of activity in older adults (Katz and Horowitz, 1983;Shiohama et al, 1993;Stam, 2011;Zurek et al, 1985), with some suggesting that it may represent normal variation among older adults because it occurs within a normal EEG background (Katz and Horowitz, 1983). DASO is characterized with the sequences of rhythmic, bilateral anterior slow activity in the delta range (mostly 1.5-2.5/s), duration varies between 2 and 10 s, and the voltage output may be considerably high (Katz and Horowitz, 1983;Stam, 2011).…”