[Cortical distribution of 2 delta frequencies in slow wave sleep]

Many persons show two different types of delta-activity during synchronized sleep; these can already be differentiated visually as follows: one is located over the anterior brain areas, has a high frequency and is monomorph, the other one is located over the posterior brain areas, has a low frequency and is polymorph. However, both rhythms overlap considerably regarding their cortical distribution. The high high frequency delta-activity usually ranges from 1.5-2.9 Hz with a maximum at 2Hz; the one with low frequency ranges from 0.1-1.4 Hz with a maximum at 0.9 Hz. The interhemispheric coherence of the delta-activities is high with 0.7-0.9; for the anterior regions it is slightly higher than for the posterior ones. The intrahemispheric coherence is low, as was to be expected, and it is less than 0.3.