Kristoffer Appel’s research while affiliated with Osnabrück University and other places

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Publications (13)


Electrophysiological correlates of lucid dreaming
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April 2024

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275 Reads

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1 Citation

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Kristoffer Appel

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Lucid dreaming (LD) is a state of conscious awareness of the current dream state, predominantly associated with REM sleep. Research progress in uncovering the neurobiological basis of LD has been hindered by low sample sizes, diverse EEG setups, and specific artifact issues like saccadic eye movements and signal non-stationarity. To address these matters, we developed a multi-stage preprocessing pipeline that integrates standardized early-stage preprocessing, artifact subspace reconstruction, and signal-space projection. This approach enhanced data quality by precisely removing saccadic potential effects even in setups with minimal channels. To robustly identify the electrophysiological correlates of LD, we applied this methodology to LD data collected across laboratories (pooled N = 44) and explored sensor- and source-level markers hypothesized to underlie LD. Compared to non-lucid REM sleep, we observed few robust differences on the EEG sensor level, which is in line with recent findings. In contrast, on the source level, gamma1 power (30-36 Hz) showed increases during LD in left-hemispheric temporal areas, which might reflect verbal insight processes. Gamma1 power also increased around the onset of LD eye signaling in right temporo-occipital regions including the right precuneus, in line with its involvement in self-referential thinking. Reductions in beta power (12-30 Hz) during LD in right central and parietal areas including the temporo-parietal junction are potentially associated with a conscious reassessment of the veridicality of the currently perceived reality. Notably, functional connectivity in alpha band (8-12 Hz) increased during LD, in contrast to the reductions typically seen in psychedelic states, highlighting enhanced self-awareness. Taken together, these findings illuminate the electrophysiological correlates of LD state, and may serve as a basis to uncover neural mechanisms at the time point of lucid dream insight.

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Citizen neuroscience: Wearable technology and open software to study the human brain in its natural habitat

February 2024

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124 Reads

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4 Citations

European Journal of Neuroscience

Citizen science allows the public to participate in various stages of scientific research, including study design, data acquisition, and data analysis. Citizen science has a long history in several fields of the natural sciences, and with recent developments in wearable technology, neuroscience has also become more accessible to citizen scientists. This development was largely driven by the influx of minimal sensing systems in the consumer market, allowing more do‐it‐yourself (DIY) and quantified‐self (QS) investigations of the human brain. While most subfields of neuroscience require sophisticated monitoring devices and laboratories, the study of sleep characteristics can be performed at home with relevant noninvasive consumer devices. The strong influence of sleep quality on waking life and the accessibility of devices to measure sleep are two primary reasons citizen scientists have widely embraced sleep research. Their involvement has evolved from solely contributing to data collection to engaging in more collaborative or autonomous approaches, such as instigating ideas, formulating research inquiries, designing research protocols and methodology, acting upon their findings, and disseminating results. In this article, we introduce the emerging field of citizen neuroscience, illustrating examples of such projects in sleep research. We then provide overviews of the wearable technologies for tracking human neurophysiology and various open‐source software used to analyse them. Finally, we discuss the opportunities and challenges in citizen neuroscience projects and suggest how to improve the study of the human brain outside the laboratory.


Citizen neuroscience: wearable technology and open software to study the human brain in its natural habitat

March 2023

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173 Reads

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2 Citations

Citizen science allows the public to participate in various stages of scientific research, including study design, data acquisition, and analysis of the resulting data. Citizen science has a long history in several fields of the natural sciences, and with recent developments in technology, neuroscience has also become more accessible to citizen scientists. This development waslargely driven by the development of minimal sensing systems for the consumer market, allowing for do‐it‐yourself (DIY) or quantified‐ self (QS) investigations of an individual's brain. While most subfields of neuroscience require sophisticated monitoring devices in the laboratory, the study of sleep characteristics has been widely embraced by citizen neuroscientists, likely due to the strong influence of sleep quality on waking life and an increasingly broad accessibility of relevant non‐invasive consumer devices. Here, we introduce into the emerging field of citizen neuroscience, illustrating examples of citizen neuroscience projects in the field of sleep research. We then give an overview on wearable technologies for tracking human neurophysiology, and on open software to run them, each with unique capabilities and intended purposes. Finally, we discuss chances and challenges in citizen neuroscience research, and suggest how to improve studying the human brain outside the laboratory.


Figure 3. Interactive dreaming (German group) The participant was stimulated during REM sleep with red and green LED light flashes to convey Morse-coded math problems. (A) Hypnogram of the night. (B) An awake period (left) and a period of interactive dreaming during REM sleep (right), corresponding to the times indicated by the gray and red arrows in (A), respectively. The question ''4 minus 0'' was presented, as shown in green. The resulting answer ''4'' produced by the dreamer was apparent in the EOG signals. Upon awakening, the participant recalled the problem almost correctly. Dream report: ''A medical practice, maybe for physiotherapy. I was alone in the room and there was a large doctor's couch in the middle of the room, shelves, sideboards. The couch was strange. The room seemed solid and steady, when the lights started flickering. I recognized this as the flashing signal [Morse code] from the outside (4 plus 0, ) and reported the answer '4' with eye signals. I looked for a tool that could flash, and I found a round bowl full of water. The water flashed (like a fish tank light that one turns on and off). I again saw a signal, but was not able to identify it. The bowl broke because I accidentally let it fall while trying to decode the flashes. I left the room, trying to find something else that could flash, and went outside and looked up to the clouds. There was yellow sunlight and light gray clouds. I saw variations in the brightness, clouds drifting past quickly, but again, unfortunately, I could not decipher a flashing signal. It was too fast to decode, but I knew that these were math problems.''
Figure 4. Interactive dreaming (French group) (A) Hypnogram showing a daytime nap in a participant with narcolepsy. The red arrow indicates the beginning of a yes-no question period. Before sleep, the participant was instructed to contract zygomatic muscles twice to signal ''YES'' and corrugator muscles twice to signal ''NO.'' (B) Polysomnographic results documenting periods of wake (left) and REM sleep from the beginning of a yes-no question period (right). The first question was answered correctly (NO signal). The next question was answered, but the answer was judged as ambiguous. Three further questions were asked. In total, four of these five questions were answered; negligible facial EMG activity was observed after one question. Two answers were rated as correct and two as ambiguous. There was no facial EMG activity outside of the stimulation periods. The dream report upon waking was as follows: ''In my dream, I was at a party and I heard you asking questions. I heard your voice as if you were a God. Your voice was coming from the outside, just like a narrator of a movie. I heard you asking whether I like chocolate, whether I was studying biology, and whether I speak Spanish. I wasn't sure how to answer the last one, because I am not fluent in Spanish, but I have some notions. In the end, I decided to answer 'NO' and went back to the party.''
Summary of data collection from each team
Observed responses during two-way communication attempts in REM sleep periods with signal-verified lucid dreaming
Real-time dialogue between experimenters and dreamers during REM sleep

February 2021

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2,153 Reads

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110 Citations

Current Biology

Dreams take us to a different reality, a hallucinatory world that feels as real as any waking experience. These often-bizarre episodes are emblematic of human sleep but have yet to be adequately explained. Retrospective dream reports are subject to distortion and forgetting, presenting a fundamental challenge for neuroscientific studies of dreaming. Here we show that individuals who are asleep and in the midst of a lucid dream (aware of the fact that they are currently dreaming) can perceive questions from an experimenter and provide answers using electrophysiological signals. We implemented our procedures for two-way communication during polysomnographically verified rapid-eye-movement (REM) sleep in 36 individuals. Some had minimal prior experience with lucid dreaming, others were frequent lucid dreamers, and one was a patient with narcolepsy who had frequent lucid dreams. During REM sleep, these individuals exhibited various capabilities, including performing veridical perceptual analysis of novel information, maintaining information in working memory, computing simple answers, and expressing volitional replies. Their responses included distinctive eye movements and selective facial muscle contractions, constituting correctly answered questions on 29 occasions across 6 of the individuals tested. These repeated observations of interactive dreaming, documented by four independent laboratory groups, demonstrate that phenomenological and cognitive characteristics of dreaming can be interrogated in real time. This relatively unexplored communication channel can enable a variety of practical applications and a new strategy for the empirical exploration of dreams.


Inducing signal-verified lucid dreams in 40% of untrained novice lucid dreamers within two nights in a sleep laboratory setting

August 2020

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241 Reads

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14 Citations

Consciousness and Cognition

Dreams in which the dreamer is aware of the dream state (lucid dreams, LD) are difficult to induce in naïve subjects in-laboratory. Recently, Stumbrys and Erlacher (2014) used a combination of existing induction techniques together with a self-developed experiment protocol and achieved comparatively high LD induction rates. In this study, we simplified their methodology slightly and repeated their experiment with twenty naïve subjects who spent one or two nights in our sleep laboratory. After about six hours of sleep, they were woken up during REM sleep and engaged in a series of cognitive tasks before going back to bed. Ten subjects reported a LD during the following period of sleep in one of the nights. Eight of these subjects gave a predefined eye signal, which was clearly visible in the electrooculogram during REM sleep. In summary, we replicated Stumbrys and Erlacher’s results using a simplified version of their induction protocol.


Combining Presleep Cognitive Training and REM-Sleep Stimulation in a Laboratory Morning Nap for Lucid Dream Induction

April 2020

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590 Reads

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45 Citations

Psychology of Consciousness: Theory, Research, and Practice

Previous experiments combining cognitive techniques and sleep disruption have been relatively successful in inducing at-home lucid dreams (LD) over training periods of 1 week or more. Here, we induce LD in a single laboratory nap session by pairing cognitive training with external stimulation. Participants came to the laboratory at 7:30 a.m. or 11:00 a.m. and during polysomnography setup were provided with information about lucid dreaming. For 20 min prior to sleep the experimenter played alternating audio and visual cues at 1-min intervals. Participants were instructed to practice a mental state of critical self-awareness, observing their thoughts and experiences each time they noticed a cue. This procedure associated the cues with the trained mental state. Subsequently, participants were allowed 90 min to nap, and the audio and visual cues were presented during REM sleep to activate self-awareness in dreams and elicit lucidity. A control group followed the same procedure but was not cued during sleep. All participants were instructed to signal their lucidity by looking left and right 4 times (LR signal). Signal-verified lucid dreams (SVLDs) qualified as dreams in which the LR signal was observed and the participant reported becoming lucid. Across the 2 nap times, this protocol induced SVLDs in 50% of cued participants. In the absence of cueing during sleep, participant SVLD rate was 17%. Of note, 3 successful participants had never before experienced a LD, suggesting this protocol may be effective across the general population. Implications of this Targeted Lucidity Reactivation protocol for nightmare treatment are discussed.



The public perception of lucid dreaming and its research

October 2018

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156 Reads

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5 Citations

International Journal of Dream Research

A Lucid dream (LD) is a dream in which the dreaming person knows that he or she is dreaming. Being neglected by scientific researchers and viewed as esoteric or paranormal for many decades, nowadays LD is an acknowledged research field, which also has practical clinical implications. However, the public's perception of LD has not yet been studied. This online study investigates the public's perception and evaluation of LD and its research with respect to eventual paranormal features. The underlying research question was whether the popular view reflects the change from scientific ignorance to scientific acceptance. 270 participants took part, 55 of which were members in LD internet forums. Main findings: 1. Lucid dreaming is generally viewed as a positive, non-paranormal phenomenon. Lucid dreaming forum members have an especially positive view. 2. LD research is accepted as being scientific and no longer seen as esoteric. 3. Regarding exceptionality, LD is classified as a phenomenon comparable to hypnosis, and more exceptional than normal dreaming and meditation, but less exceptional than paranormal abilities such as telepathy. 4. Applications of LD are estimated differently regarding their possibility and provability. Generally, fewer applications are evaluated as being possible than LD research has already been able to show. Several significant effects of demographic variables such as age on several dependent variables were found. The present findings show a generally positive view of the public on LD and its research and support the use of LD for scientific, personal or therapeutic purposes.


Investigating consciousness in the sleep laboratory – an interdisciplinary perspective on lucid dreaming

November 2017

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506 Reads

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29 Citations

Interdisciplinary Science Reviews

During dreaming, we experience a wake-like hallucinatory reality, however with restricted reflective abilities: in the face of a bizarre dream environment, we do not realize that we are actually dreaming. In contrast, during the rare phenomenon of lucid dreaming, the dreamer gains insight into the current state of mind while staying asleep. This metacognitive insight often enables the dreamer to control own dream actions and the course of the dream narrative. Lucid dreaming allows for radically new methodological and theoretical approaches and has led to new insights in diverse scientific disciplines beyond classical sleep and dream research, including neuroscience, psychotherapy, philosophy, art, and sports sciences. Here, we review past research and the current knowledge on lucid dreaming. We present insights into the scientific work in a sleep laboratory and describe how lucid dreams can be induced through methodologies from diverse academic backgrounds including psychology, electrical engineering and pharmacology.


How to induce lucid dreams in beginners in a sleep laboratory setting with high success rate

November 2017

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155 Reads

Several methods exist to help novices experience a lucid dream, i.e. a dream in which the dreamer is aware of the dream state. However, when validated polysomnographically in a sleep laboratory, these techniques proved to be only of limited success in beginners. Recently, Stumbrys and Erlacher developed a protocol, which combines several lucid dream induction methods, and achieved a high success rate with about 50% of their subjects having a lucid dream within one or two nights. In this study, we simplified and formalized their protocol and replicated their experiment with twenty subjects inexperienced in lucid dreaming. The subjects underwent one or two nights of polysomnographic in-lab recordings. After about six hours of undisturbed sleep, the subjects were woken up during rapid eye movement sleep and engaged in a series of cognitive tasks. They then went back to bed and were instructed to move their eyes according to a predefined signal (three times left-right) when reaching lucidity, and to conduct the task of “flying around” in their dream. Ten subjects reported a lucid dream during the following period of sleep. Eight of these subjects gave the predefined eye signal, which could be detected clearly and distinctly in the electrooculogram during rapid eye movement sleep. Furthermore, five subjects reported being able to perform the “flying” task. Our results confirm Stumbrys and Erlacher’s results and demonstrate the feasibility of inducing lucid dreams in-laboratory with a high success rate using the simplified protocol presented here.


Citations (10)


... For instance, Baird et al. (2022) argued that this phenomenon might be an artifact caused by saccadic spike potentials, which are linked to the heightened REM density observed during LDs. Conversely, a more recent study reported augmented low-gamma activity during LDs, leaving the role of low-gamma inconclusive 7 . ...

Reference:

Electrophysiological Correlates of Conscious Experiences During Sleep: Lucid Dreams, Sleep Paralysis, Out-Of-Body Experiences, and False Awakenings.
Electrophysiological correlates of lucid dreaming

... EEG headbands have become more present in sleep research in the last decade [94,95]. The Lite version [96] of Zmax used in this study has two gel-electrode-based frontal EEG channels, a tri-axial accelerometer, and PPG at a sampling rate of 256 Hz. ...

Citizen neuroscience: Wearable technology and open software to study the human brain in its natural habitat

European Journal of Neuroscience

... This approach will allow for the pooling of datasets, resulting in the largest experimental study of lucid dream induction with EEG data collection to date, showcasing robust methodological rigor, replicability, and effective collaboration. Furthermore, the protocol was designed to be cost-effective and rely on minimal technical requirements by using commercially available, ready-to-use portable EEG devices and opensource dream engineering software Esfahani, Sikder, et al., 2023). The pooled data will also be openly shared in public repositories, in compliance with current scientific best practices. ...

Citizen neuroscience: wearable technology and open software to study the human brain in its natural habitat

... Such dream control ranges from simple tasks like performing reality checks or flying to more complex actions, such as executing prearranged tasks like clenching their hands, performing mathematical calculations, or even communicating with researchers during the lucid dream (Erlacher et al., 2014;Konkoly et al., 2021;Peters et al., 2024). In contrast, non-lucid dreams almost lack this kind of volitional dream control (Voss et al., 2013). ...

Real-time dialogue between experimenters and dreamers during REM sleep

Current Biology

... Previous attempts to induce LD have explored a wide array of approaches, including cognitive training (Adventure-Heart, 2020; Adventure-Heart et al., 2017; Appel et al., 2020;Baird, Riedner, et al., 2019;Dyck et al., 2017Dyck et al., , 2018Saunders et al., 2017;Schredl et al., 2020;Taitz, 2011), external sensory stimulation during REM sleep (Erlacher, Schmid, Bischof, et al., 2020;Erlacher, Schmid, Schuler, et al., 2020;Kumar et al., 2018;Paul et al., 2014;, pharmacological interventions (Kern et al., 2017;LaBerge et al., 2018), brain stimulation (Blanchette-Carrière et al., 2020;Stumbrys et al., 2013;Voss et al., 2014), and combinations of different methods (Adventure-Heart, 2020; Carr, Konkoly, et al., 2020;Erlacher, Schmid, Bischof, et al., 2020;Erlacher, Schmid, Schuler, et al., 2020;Saunders et al., 2017;. Comprehensive reviews of these induction techniques are available elsewhere (Stumbrys et al., 2012;Tan & Fan, 2023). ...

Inducing signal-verified lucid dreams in 40% of untrained novice lucid dreamers within two nights in a sleep laboratory setting
  • Citing Article
  • August 2020

Consciousness and Cognition

... After such adaptation, the proposed hybrid nature of lucid dreaming as a state between non-lucid REM sleep and wakefulness is less obvious. Considering recent findings on the possibility of two-way communication with the real world during lucid dreaming (Konkoly et al., 2021), lucid REM sleep arguably inhibits a more medium position on the 'I' axis of the model, though. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) ...

Real-Time Dialogue between Experimenters and Dreamers During rem Sleep
  • Citing Article
  • January 2020

SSRN Electronic Journal

... The ability to manipulate oneiric experiences holds the promise of substantial scientific breakthroughs, ranging from understanding the origin and function of dreams to developing new treatments for clinical conditions associated with dream alterations. Consequently, the recent surge in dream engineering is unsurprising (e.g., Carr, Haar, et al., 2020;Carr, Konkoly, et al., 2020;Haar Horowitz et al., 2018;Kamal et al., 2012). The utilisation of sensory stimuli is particularly relevant among the various dream engineering techniques due to its reliance on well-established physiological pathways and functions, ease of implementation, and extensive history of anecdotal and empirical observations. ...

Combining Presleep Cognitive Training and REM-Sleep Stimulation in a Laboratory Morning Nap for Lucid Dream Induction

Psychology of Consciousness: Theory, Research, and Practice

... connecting with Divine (Bogzaran, 1990), while being lucid in a dream. In such a way, lucid dreaming may serve as a genuine spiritual practice, especially within a more secular context of self-spirituality (Heelas, 1996), as lucid dreaming has a rather positive and credible profile within the general population (Lüth et al., 2018). ...

The public perception of lucid dreaming and its research
  • Citing Article
  • October 2018

International Journal of Dream Research

... By one estimate, 55% of the general population has experienced at least one lucid dream and 23% claim to experience lucid dreaming at least once a month (Saunders et al., 2016). Although the scientific literature on lucid dreaming is limited due to the challenges of inducing lucid dreams (LaBerge et al., 1981;Hearne, 1978;Tan & Fan, 2022;Van Eeden, 1913), the extant evidence suggests that lucid dreams can be used to enhance creativity, promote motor learning, and treat nightmares (Appel et al., 2018). Furthermore, lucid dreams allow for novel studies of human consciousness. ...

Investigating consciousness in the sleep laboratory – an interdisciplinary perspective on lucid dreaming
  • Citing Article
  • November 2017

Interdisciplinary Science Reviews

... Previous attempts to induce LD have explored a wide array of approaches, including cognitive training (Adventure-Heart, 2020; Adventure-Heart et al., 2017; Appel et al., 2020;Baird, Riedner, et al., 2019;Dyck et al., 2017Dyck et al., , 2018Saunders et al., 2017;Schredl et al., 2020;Taitz, 2011), external sensory stimulation during REM sleep (Erlacher, Schmid, Bischof, et al., 2020;Erlacher, Schmid, Schuler, et al., 2020;Kumar et al., 2018;Paul et al., 2014;, pharmacological interventions (Kern et al., 2017;LaBerge et al., 2018), brain stimulation (Blanchette-Carrière et al., 2020;Stumbrys et al., 2013;Voss et al., 2014), and combinations of different methods (Adventure-Heart, 2020; Carr, Konkoly, et al., 2020;Erlacher, Schmid, Bischof, et al., 2020;Erlacher, Schmid, Schuler, et al., 2020;Saunders et al., 2017;. Comprehensive reviews of these induction techniques are available elsewhere (Stumbrys et al., 2012;Tan & Fan, 2023). ...

No effect of alpha‑GPC on lucid dream induction or dream content

Somnologie - Schlafforschung und Schlafmedizin