Electroencephalogram asymmetry and spectral power during sleep in the northern fur seal

Abstract

SUMMARY The fur seal (Callorhinus ursinus), a member of the Pinniped family, displays a highly expressed electroencephalogram (EEG) asymmetry during slow wave sleep (SWS), which is comparable with the unihemispheric sleep in cetaceans. In this study, we investigated the EEG asymmetry in the fur seal using spectral analysis. Four young (2-3 years old) seals were implanted with EEG electrodes for polygraphic sleep recording.In each animal, EEG spectral power in the frequency range of 1.2-16 Hz was computed in symmetrical cortical recordings over two consecutive nights. The degree of EEG asymmetry was measured by using thewhere L and R are the spectral powers in the left and right hemispheres, respectively]. In fur seals, EEG asymmetry, as measured by the percent of 20-s epochs with absolute AI > 0.3 and >0.6, was expressed in the entire frequency range (1.2-16 Hz). The asymmetry was significantly greater during SWS (25.6-44.2% of all SWS epochs had an absolute AI > 0.3 and 2.1-12.2% of all epochs had AI > 0.6) than during quiet waking (11.0-20.3% and 0-1.9% of all waking epochs, respectively) and REM sleep (4.2-8.9% of all REM sleep epochs and no epochs, respectively). EEG asymmetry was recorded during both low-and high-voltage SWS, and was maximal in the range of 1.2-4 and 12-16 Hz. As shown in this study, the degree of EEG asymmetry and the frequency range in which it is expressed during SWS in fur seals are profoundly different from those of terrestrial mammals and birds.

Fur Seals Display a Strong Drive for Bilateral Slow-Wave Sleep While on Land

“…Fur seals display two fundamentally different patterns of sleep. The first is bilaterally symmetrical slow-wave sleep (BSWS) and rapid eye movement (REM) sleep (Mukhametov et al, 1985;Lyamin et al, 2008a). This is the predominant pattern when fur seals sleep on land and it is similar to that seen in terrestrial mammals.…”

“…Surgical procedures have been fully described in our previous publications (e.g., Lapierre et al, 2007;Lyamin et al, 2008a). Briefly, under general anesthesia (isoflurane 1-3%) fur seals were implanted with three pairs of electrodes (stainless steel screws, 1 mm in diameter) for EEG recording from symmetrical fronto-occipital or fronto-parietal derivations of the right and left hemispheres.…”

“…Sleep stages were scored as in our previous studies (Lyamin et al, 2004(Lyamin et al, , 2008aLapierre et al, 2007). EEG from symmetrical fronto-occipital derivations was scored in 20 s epochs for each hemisphere independently and subdivided into 3 stages: desynchronization (waking EEG), low-amplitude EEG synchronization (SWA of maximal amplitude occupies Ͻ50% of the epoch time), or high-amplitude EEG synchronization (SWA of maximal amplitude occupies Ͼ50% of the epoch).…”

“…In this study, unihemispheric and ASWS were added together as a combined amount and will be referred to as ASWS. REM sleep was scored according to conventional criteria (bilateral desynchronization in the EEG, muscle hypotonia, occasional muscle jerks and rapid eye movements) which are generally similar in seals (Lyamin and Mukhametov, 1998;Lyamin et al, 2008a) and terrestrial mammals (e.g., Zepelin et al, 2005).…”

Sleeping on the wing

“…Using criteria established in northern fur seals (Callorhinus ursinus) [82], SWS can be categorized as bihemispheric SWS (BSWS) or asymmetric SWS (ASWS) based on the degree of interhemispheric asymmetry in SWA. ASWS can be further categorized as unihemispheric SWS (USWS) when the asymmetry is particularly pronounced [8].…”