The relation between spontaneous k-complexes during stage 2 and delta waves

Contrasting results have been provided on the existence of an inverse relationship between sigma and delta activity by means of the analysis of power spectra after total sleep deprivation: in humans, power spectra analysis and transient patterns detection algorithms revealed that sigma activity was reduced in the recovery night after 40 h of wakefulness, while delta activity was enhanced [I]; in the cat, total sleep deprivation enhanced slow-wave activity (SWA) but did not reduce sigma activity in the cortical EEG, while EEGs derived from thalamic structures showed an inverse relationship between these two activities [2]. This dissociation between cortical and thalamic EEGs in the cat has been explained by an enhancement of background activity in the frequency range of sleep spindles which FFf can not discriminate from organized sleep spindles [2]. Due to the intrinsic limitations of FFT analysis in distinguishing phasic activity from background EEG activity, a more direct evaluation of phasic spindle activity after sleep deprivation is nccdcd. The aim of the present study was to assess spindle activity, visually scored in multichannel EEG recordings, after a selective SWS deprivation that caused a strong rebound of stagc 4 in the recovery sleep. Spindle activity changes across consecutive NREM cycles have been also evaluated. METHODS Ten normal right-handed male students were selected as paid volunteers for the study. Participants slept for 6 consecutive nights in a sound-proof, temperature controlled room: I) Adaptation; 2) Baseline (BSL); 3) Baseline with awakenings (BSL-A); 4) SWS Deprivation-I (DEP-I); 5) SWS Deprivation-2 (DEP- 2); 6) Recovery (REe). Spindles during BSL-A and REC nights were scored by two blind scorers. Cases in which they were in disagreement were solved by a third blind scorer. The following criteria were applied to score spindles: minimum frequency 12 Hz, maximum frequency IS Hz, minimum amplitude 10 uv, maximum amplitude 80 uv, minimum duration 0.5 s. Spindle density was computed during stage 2 of each sleep cycle as expressed by the ratio: number of spindles/duration of stage 2. A 4x2x5 repeated measure ANOV A, Sleep cycle (1st, 2nd, 3rd, 4th) x Night (BSL-A, REC) x Derivation (Fpz, Fz, Cz, Pz, Oz) was carried out on the spindle density of stage 2 sleep. RES ULTS The comparison between BSL-A (M=3.60; S.E.M.=0.37) and REC night (M=3.14; S.E.M.=O.22) was not significant (F(I,9)=1.30; p=O.28), even though the significant Night x Sleep cycle interaction (F(3,27)=4.36; p=O.OI) indicated that thc decrease of spindle density during REC night as compared to BSL-A was present in the first sleep cycle and progressively vanished in the course of the night. In fact, the comparison between the two nights was significant in the first sleep cycle (F(I,9)=6.28; p=O.03), approached significance in second one (F(I,9)=3.33; p=O.IO), and was not significant in the other sleep cycles. Spindle density showed a close to significance global increase across sleep cycle (F(3,27)=2.69; p=O.07) and a significant main effect for the factor Derivation (F(4,36)=33.78; p Fpz=3.38 > Fz = 2.54, Oz=2.53). The Sleep cycle x Derivation interaction was also significant (F(l2,108)=6.34; p