The K-complex: a 7-decade history.
ABSTRACT The K-complex was first described by Loomis et al 67 years ago in a paper that was one of a series of seminal studies of sleep conducted in Loomis' private laboratory. The study of the K-complex was almost immediately taken up by many notable figures in early electroencephalography research, such as Robert Schwab, Mary Brazier, and W. Gray Walter. More than 200 papers have been published in the years since these early studies, including major reviews in 1956 by Roth et al and in 1985 by Peter Halász. More recently, K-complex study has been taken up by event-related potentials researchers such as Ken Campbell and animal neurophysiologists such as Florin Amzica and Mircea Steriade. The present paper provides a historical and thematically based review of the K-complex literature and attempts to integrate the various theoretical positions and neurophysiologic data. Specifically, K-complexes are discussed in terms of their relationship to other electroencephalographic phenomena, their relationship to autonomic activation, their role in the study of information processing during sleep, and what is understood of their underlying neurophysiology.
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ABSTRACT: Sleep staging is a critical step in a range of electrophysiological signal processing pipelines used in clinical routine as well as in sleep research. Although the results currently achievable with automatic sleep staging methods are promising, there is need for improvement, especially given the time-consuming and tedious nature of visual sleep scoring.Journal of Neuroscience Methods 01/2015; DOI:10.1016/j.jneumeth.2015.01.022 · 1.96 Impact Factor
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ABSTRACT: Even modest sleep restriction, especially the loss of sleep slow wave activity (SWA), is invariably associated with slower electroencephalogram (EEG) activity during wake, the occurrence of local sleep in an otherwise awake brain, and impaired performance due to cognitive and memory deficits. Recent studies not only confirm the beneficial role of sleep in memory consolidation, but also point to a specific role for sleep slow waves. Thus, the implementation of methods to enhance sleep slow waves without unwanted arousals or lightening of sleep could have significant practical implications. Here we first review the evidence that it is possible to enhance sleep slow waves in humans using transcranial direct-current stimulation (tDCS) and transcranial magnetic stimulation. Since these methods are currently impractical and their safety is questionable, especially for chronic long-term exposure, we then discuss novel data suggesting that it is possible to enhance slow waves using sensory stimuli. We consider the physiology of the K-complex (KC), a peripheral evoked slow wave, and show that, among different sensory modalities, acoustic stimulation is the most effective in increasing the magnitude of slow waves, likely through the activation of non-lemniscal ascending pathways to the thalamo-cortical system. In addition, we discuss how intensity and frequency of the acoustic stimuli, as well as exact timing and pattern of stimulation, affect sleep enhancement. Finally, we discuss automated algorithms that read the EEG and, in real-time, adjust the stimulation parameters in a closed-loop manner to obtain an increase in sleep slow waves and avoid undesirable arousals. In conclusion, while discussing the mechanisms that underlie the generation of sleep slow waves, we review the converging evidence showing that acoustic stimulation is safe and represents an ideal tool for slow wave sleep (SWS) enhancement.Frontiers in Systems Neuroscience 10/2014; 8:208. DOI:10.3389/fnsys.2014.00208
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ABSTRACT: To test whether humans can encode words during sleep we played everyday words to men while they were napping and assessed priming from sleep-played words following waking. Words were presented during non-rapid eye movement (NREM) sleep. Priming was assessed using a semantic and a perceptual priming test. These tests measured differences in the processing of words that had been or had not been played during sleep. Synonyms to sleep-played words were the targets in the semantic priming test that tapped the meaning of sleep-played words. All men responded to sleep-played words by producing up-states in their electroencephalogram. Up-states are NREM sleep-specific phases of briefly increased neuronal excitability. The word-evoked up-states might have promoted word processing during sleep. Yet, the mean performance in the priming tests administered following sleep was at chance level, which suggests that participants as a group failed to show priming following sleep. However, performance in the two priming tests was positively correlated to each other and to the magnitude of the word-evoked up-states. Hence, the larger a participant's word-evoked up-states, the larger his perceptual and semantic priming. Those participants who scored high on all variables must have encoded words during sleep. We conclude that some humans are able to encode words during sleep, but more research is needed to pin down the factors that modulate this ability.Frontiers in Psychology 11/2014; 5:1319. DOI:10.3389/fpsyg.2014.01319 · 2.80 Impact Factor