Friday, 8 May 2015

Lucid Dream Induction Techniques: A Review

Introduction - What is 'Lucid Dreaming'?
A lucid dream is a dream in which the dreamer is consciously aware that they are dreaming. During a lucid dream, the dreamer can often control, manipulate or influence the content of the dream. Some lucid dreamers are able to naturally (i.e. spontaneously) lucid dream, but it is possible to learn techniques and methods to induce lucid dreaming. 

There have been academic literature reviews which have analysed the effectiveness of various lucid dreaming induction techniques (Stumbrys et al, 2012). This literature review concentrated on 35 separate studies into lucid dreaming - 11 sleep laboratory studies and 24 field studies. Within these studies, 26 used cognitive techniques, 11 used external stimulus and one used the administration of drugs. Stumbrys (2012) state that the methodological quality of the studies was relatively low and none of the techniques proved to be very effective at reliably inducing lucid dreams consistently.

Some lucid dream academics dispute whether awareness of dreaming is enough for lucid dreaming (Gillespie, 1984 & Tart, 1984 & 1985). Tart (1984) states that there is a distinction between dream-awareness dreams and lucid dreams. He argues that lucid dreaming involved the added criterion that the dreamer retains the clarity of their waking consciousness within the dream state. 

Tholey (1983) describes 7 aspects of lucidity:
  1. Clarity about the state of consciousness (awareness that the dreamer is dreaming)
  2. Clarity about freedom of choice
  3. Clarity of consciousness
  4. Clarity about waking reality
  5. Clarity of perception
  6. Clarity about the meaning of the dream
  7. Clarity in remembering the dream
Tholey states that 1 - 4 are pre-requisites for lucid dreaming, although many lucid dream experts argue that a basic definition - conscious awareness of dreaming - is sufficient. Lucidity is not an absolute state, but can be better described as a 'continuum' with different degrees of lucidity (Barrett, 1992 & Moss, 1986).

Lucid dreaming has been a known phenomenon since the era of Aristotle, although it was at recently as 1978 that it was scientifically proven by Hearne, and subsequently LaBerge (1980). Since then there have been numerous studies into REM sleep, dreaming and lucid dreaming (i.e. Erlacher & Schredl, 2008a). 

REM sleep is characterised by muscle atonia (paralysis) to prevent the dreamer from acting out their dreams (Hobson, Pace-Schott & Stickgold, 2000). This atonia is controlled by neural structures in the brain, and the exception is eye movements which happen during this stage of sleep, a fact which has been confirmed in studies (Roffwarg et al, 1962). 

Lucid dreamers have access to their waking memories (Erlacher, 2009), it has been possible for them to pre-arrange a pattern of eye movements in advance of entering a lucid dream. When performed in a lucid dream state, these eye movements can be monitored on an EOG, meaning that the dreamer can 'communicate' with a researcher from within the lucid dream state (LaBerge, 1981). When awake, the dreamer can provide a dream report which should correspond the recorded eye movement data  with their dreamed gaze shifts. This means that lucid dreaming can be verified by subjective dream reports and objective EOG data (Dane, 1984; Dresler et al, 2012; Fenwick et al, 1984; Hickey, 1988; Kueny, 1985; Oligivie et al, 1983, Vos et al, 2009 & Watanabe, 2003).

The most frequent types of lucid dreams are DILDs (Dream Initiated/Induced Lucid Dreams), although of course, they can also occur via WILDs (Wake Initiated/Induced Lucid Dreams) (LaBerge, Levitan & Dement, 1986). 

Physiologically, lucid dreams are associated with elevated levels of automatic nervous system activity and higher H-reflex suppression (Brylowski, Levitan & LaBerge, 1989). Recent findings, comparing lucid dreams with non-lucid dreams have found that lucidity is associated with increased EEG 40 HZ activity in the brain, particularly in the frontal and frontolateral regions (Voss et al, 2009). Another recent fMRI study discovered that several areas of the brain (including the pre-frontal cortex, precneous and occipito-temporal cortices) are more strongly activated during lucid dreaming (Dressler et al). This specific activation pattern may explain the higher-order cognitive processes and skills which are involved in lucid dreaming, such as self-assessment, executive function, attention-regulation and behavioural controls (Arnsten & Li, 2005; Fernandez-Duque et al, 2000; Miller & Cohen, 2001 & Schmitz et al, 2004). Lucid dreamer have also been shown to display brain activity in regions connected to first-person perspective, the experience of agency and other self-processing operations (Cavanna & Trimble, 2006).

In a lucid dream, the dreamer must observe and evaluate their present experience to recognise that they are in a dream state. They are then able to take first-person perspective, agency and control to guide their attention and behaviour intentionally in order to control the dream (Kahan & LaBerge, 1994). 

Frequent lucid dreaming is regarded as a rare skill, although studies show that approximately 50% of people will experience at least one lucid dream in their lifetimes and about 1/5 people experience them regularly - i.e. at least once a month (Snyder & Gachanbach, 1988 & Stepansky et al, 1998). 

Children and adults seem to share the same prevalence of lucid dreaming, although children appear to have lucid dreams more frequently (Schredl et al 2012). There are also cross-cultural differences - for example lucid dreaming is less prevalent in Japanese persons according to one study (Erlacher et al, 2008).

Lucid dreaming is a very learnable skill - as demonstrated by the work of LaBerge and a number of practical ways of inducing lucid dreams have been proposed (LaBerge, 1980 & Saint-Denys, 1867/1982). For example, lucid dreaming has been harnessed as a therapeutic tool in many nightmare treatments (Abromovitch, 1995 & Spoormaker et al, 2003). A controlled trial (Spoormaker & van den Bout, 2006) demonstrated that lucid dreaming can decrease the frequency and intensity of nightmares. 

It has also been found that lucid dreaming can be used to enhance and improve motor and cognitive skills and promote creative problem-solving (Erlacher & Schredl, 2010 & Strumbrys & Daniels, 2010). Lucid dreaming has also proven to be a useful way for neuroscientists to explore the mind-body relationship during REM sleep and arguably, should be treated as a discreet and special component of consciousness research (Hobson, 2009). 

In order to utilise the advantages and delights of lucid dreaming and render them more accessible to both academics and the general population, effective and reliable induction techniques need to be established which will serve the increase the frequency of lucid dreaming. Finding genuine frequent lucid dreamers who are willing and able to take part in lucid dream research remains one of the biggest obstacles to furthering empirical studies and academic enquiry into this phenomenon. 

Classification of Lucid Dream Induction Techniques
Lucid dream induction basically means the techniques used to cause or increase frequency of lucid dreaming.

Gackenbach (1985/1986)
Gackenbach classified lucid dreaming induction techniques into 2 main categories:
  1. Pre-sleep induction
  2. Sleep induction
Pre-sleep induction involves intentional techniques and 'unintentional considerations'. This category of lucid dream induction uses intentional techniques to focus on the present moment (reality checks to reflect on whether the individual is currently awake or dreaming, engaging in activities such as meditation or alpha feedback training etc); or the future (auto-suggestion, post-hypnotic suggestion or dream recall affirmations). Some techniques combine both present and future aspects - i.e. Tholey's combined technique (Tholey, 1983) which includes elements of reflection (present focusing - also known commonly as reality checks) and auto-suggestion (future focusing). 

Unintentional considerations include situations during the waking day (interpersonal interactions, emotions) and individual propensities (field independence, creativity) which are not specifically linked to the phenomenon of lucid dreaming or directly related to the induction of a lucid dream, but increase the likelihood of experiencing a lucid dream. 

Sleep induction involves 2 subcategories: internal and external cues. Internal cues are unusual events or inconsistencies within a dream (dreamsigns) or spontaneous insight which occurs within a dream, alerting the dreamer to the fact they are experiencing a dream and not waking reality. External cues are the various environmental stimuli (auditory, olfactory, tactile) which can be applied during REM sleep, and experienced by the dreamer while they are within a dream state. These 'cues' are incorporated into the dream and allow the dreamer to become aware that they are dreaming.

Prince & Cohen (1988)
Prince & Cohen developed 3 main categories for classifying lucid dream induction techniques:
  1. Lucid-awareness training
  2. Intention & suggestion techniques
  3. Cue REM-minding techniques
Lucid-awareness training involves cultivating a proper waking attitude to promote lucid dreaming, such as critically reflecting on whether one is awake or dreaming (reality checks), heightening perception awareness, alpha feedback or waking fantasy training. 

Intention & suggestion techniques involve triggering lucidity by setting the will or intention to do so. These methods can also be used for improving dream recall or incubating a chosen dream theme, action or narrative. 

REM-minding techniques are similar to Gackenbach's 'external cues' - and involve sensory stimuli applied during REM sleep with the aim of triggering lucidity in the dreamer. 

Some other lucid dream induction methods - such as Tholey's combined method - do not fit into these 3 categories, and neither does hypnagogic techniques (where the dreamer enters the lucid dream at the onset of sleep rather than becoming lucid within a non-lucid dream) and WILD (Wake Initiated/Induced Lucid Dreaming) by which the individual enters a lucid dream state directly from the waking state without any lapse in consciousness. 

The above systems of classification are useful, but fragmentary and non-exhaustive as they fail to cover all types of lucid dream induction technique. Since they were developed, empirical research into lucid dreaming have expanded our knowledge about induction techniques and new methods have emerged (Noreika et al, 2010). Another criticism of the above classification models is that they seem to be based on subjective personal or anecdotal evidence and lacked empirical validation. Some of the categories tend to overlap also - such as Tholey's combined technique.

The following categories may therefore be more useful:
  1. Cognitive techniques - lucid awareness training, intention, suggestion, hypnagogic methods
  2. External stimuli - acoustic, light, vestibular, olfactory electrical, brain stimulation which are applied during REM sleep too trigger lucidity
  3. Miscellaneous techniques - diverse methods not covered  by the first 2 categories - such as ingestion of lucid dream substances

Review of Lucid Dream Induction Techniques
Overall, the methodological quality of studies into lucid dreaming induction techniques has been criticised as 'poor'. This includes both laboratory and field studies. The main criticisms of methodology are that the samples sizes are small and the exact conditions of the studies have significant variability.

Cognitive Techniques
There have been 27 studies (77% of all studies overall) which focus on cognitive lucid dream induction techniques. Within this group of studies, 22 were field studies and 5 were laboratory studies. The following cognitive lucid dream induction techniques were used:
  1. MILD Technique (Mnemonic/Memory Induced Lucid Dreaming)
  2. Reality Checking/Reflection 
  3. Tholey's Combined Technique
  4. Auto-suggestion
  5. Dream Re-entry
  6. Post-hypnotic Suggestion
  7. Alpha Feedback
  • MILD Technique
Developed by LaBerge in 1980, the MILD Technique requires the dreamer to use memory aids to set an intention to experience a lucid dream. This is the lucid dream induction technique which has been tested empirically the most often - it was applied in 10 studies, 9 of which were field studies, and one of which was undertaken in a laboratory. In the laboratory study (Kueny, 1985), the MILD Technique was used as a control condition. The field studies were conducted by LaBerge, Levitan and their colleagues and displayed low reporting and methodological quality.

However, it does appear that use of the MILD Technique can increase the frequency of lucid dreams (LaBerge, 1988 & Levitan et al, 1992). Despite this fact, statistical analysis shows that the relationship between MILD practice and lucid dreaming to be weak, yet significant. LaBerge and Levitan - in a number of studies - have argued that when using the MILD Technique in the early morning, lucid dreaming tends to occur more frequently following naps. This is known as the Wake-Back-to-Bed method (abbreviated as WBTB or WB2B). They suggest that it is favourable to wake up 30 - 120 minutes before the individual would normally get out of bed, stay awake for the period of 30 - 120 minutes and practice the MILD Technique before taking a nap. It was found that this method had better chances of inducing lucid dreams than when the dreamer woke for 10 minutes and then went back to sleep (so shorter waking periods between sleep and nap) or went back to sleep immediately after waking. The WBTB method (using 30 - 120 minutes of wakefulness) also proved to be more effective than a longer period of wakefulness i.e. 4 hours or wakefulness and then an afternoon nap. 

It was found that the MILD Technique was slightly more effective than using light stimulus during REM sleep, although the combination of both light stimulation and MILD Technique appeared to be more effective than either taken in isolation (LaBerge, 1988 & Levitan & LaBerge, 1994).

  • Reality Checking/Reflection
Reality checking/reflection requires the individual to frequently ask themselves whether they are actually dreaming during the waking day. The individual also examines their environment for possible incongruences or inconsistencies which should confirm whether or not they are awake or dreaming (Tholey, 1983).

Reality checking was used in one laboratory study (Dane, 1984) but was not used as an experimental condition. Reality checking was also used in 8 field studies (including: LaBerge, 1988; Levitan & LaBerge, 1994; Malamud, 1979; Purcell, 1988; Reis, 1989 & Schlag-Gies, 1992). However, in one study (Levitan & LaBerge, 1994) there were no reports of relevant findings and in another (Reis, 1989), reality checking was combined with the use of external stimuli, so the data has to be excluded.

It appears that reality checking does increase the likelihood of inducing a lucid dream (Purcell, 1988), although LaBerge (1988) did not find any positive correlation between reality checking and lucid dream frequency. The studies tend to indicate that reality checking may be more effective as a lucid dream induction technique than other cognitive techniques, such as auto-suggestion (Levitan 1989), post-hypnotic suggestion (Purcell et al, 1986) or intention (Schlag-Gies, 1992). However, the comparison between the effectiveness of reality checking and MILD Technique is ambiguous - LaBerge (1988) found that MILD Technique was the more effective lucid dream induction technique, while Levitan (1989) discovered the opposite. 

  • Intention
Intention requires the dreamer to intensely imagine themselves within a dream scene just before they fall asleep. They must imagine that they are able to recognise that they are dreaming (Tholey, 1983). Intention as a lucid dream induction technique is quite similar to the MILD Technique, although it does not require the mnemonic/memory component, which is central to the MILD Technique. In MILD, the emphasis is on the ability of the dreamer to remember, whereas in intention techniques, the key is for the dreamer to recognise they are dreaming.

Intention was used in 4 field studies, although 3 of them were not specifically focused on lucid dream induction, but rather on the use of lucid dreaming as a therapeutic tool for sufferers of nightmares. The study which was concerned with lucid dream induction (Schlag-Gies, 1992) compared it with other lucid dream induction techniques.

In the 'nightmare studies', it was found that almost 50% of nightmare sufferers who were taught to lucid dream using the intention technique experienced a lucid dream within 1 - 3 months (Spoormaker & van den Bout, 2006 & Zadra & Pihl, 1997). It was also found that intention is about as effective as auto-suggestion, but less effective than reality checking (Schlag-Gies, 1992).

  • Auto-suggestion
Auto-suggestion involves the individual suggesting to themselves that they will experience a lucid dream. This is done while the individual is on the verge of sleep and in a relaxed state (Tholey, 1983). 

There have only been two studies which have assessed the effectiveness of auto-suggestion in lucid dream induction (Levitan, 1989 & Schlag-Gies, 1992). In the 1992 study, auto-suggestion seemed to increase the likelihood of lucid dreaming, but no such effect was found in the earlier 1989 one. Auto-suggestion appears to be less effective than reality checking/reflection techniques, but has similar levels of effectiveness as intention (Schlag-Gies, 1992). Levitan (1989) found that auto-suggestion may be more effective for frequent lucid dreamers who experience at least one lucid dream per month (compared with amateurs/beginners). 

  • Tholey's Combined Technique
Tholey's combined technique (Tholey, 1983) incorporates elements of reality checking/reflection, intention and auto-suggestion and involves developing a reflective frame of mind (reflection); imagining being in a dream and recognising the dream state (intention); and suggesting that one will become lucid when they fall asleep (auto-suggestion). This lucid dream induction technique was analysed in 2 field studies (Paulsson & Parker, 2006 & Zadra et al, 1992). These studies were rated as 'moderate' in terms of their methodological quality and they showed that Tholey's combined technique for lucid dream induction can significantly increase the frequency of lucid dreaming, particularly for individuals who have previous experience in lucid dreaming. However, even individuals who did not have any experience lucid dreaming (amateurs/beginners) were found to have more lucid dreams than dreamers who were not exposed to Tholey's combined method - i.e. those in control groups (Zadra et al, 1992). 

  • Post-hypnotic Suggestion
Post-hypnotic suggestion involves a hypnotist suggesting that the individual will experience a lucid dream when they next sleep, while that person is under a hypnotic trance. The individual is usually told that they will experience a lucid dream the next night. Post-hypnotic suggestion was explored in 2 laboratory experiments (Dane, 1984 & Galvin, 1993). The overall methodological quality of these studies was rated as 'fair'. In the study by Dane (1984), 14 out of 15 female subjects (who were susceptible to hypnosis) reported experiencing a lucid dream during one night spent in a sleep laboratory, although Galvin's study (1993) failed to replicate these findings.

The results from field studies are also variable and inhomogenous. In one study, it was reported that post-hypnotic suggestion helped to increase self-reflectiveness in dreams and subjects were able to experience at least one lucid dream within a period of 9 weeks (Galvin, 1993). However, a further study did not find any effect during a 3 week experiment (Purcell et al, 1986).

It is noteworthy that in Dane's successful 1984 study, post-hypnotic suggestion resulted in a greater number of NREM (non-REM) lucid dreams than REM lucid dreams.

  • Alpha Feedback
There was one study which used EEG biofeedback training (using alpha activity) before lucid dream induction (Ogilvie et al, 1982). This study was conducted on the premise that lucid dreams are associated with high levels of EEG alpha frequency synchronisation. This is the theory behind use of binaural beats for lucid dream induction. The study found that alpha feedback training had no effect on lucid dream induction or REM alpha levels.

  • Dream Re-entry
In one study (Levitan, 1991), dream re-entry was explored as a lucid dream induction technique. This is where the dreamer aims to re-enter the dream state immediately after waking from a dream. The individual remains still and focuses their mind on a particular activity (i.e. counting) while falling asleep again. Using this method, the individual should re-enter the dream state without losing conscious awareness - an idea from Tibetan Dream Yoga (Wangyal, 1998), and the ethos of the WILD Technique (Wake Initiated/Induced Lucid Dreams).

The study used two methods of focusing: counting and body awareness. The results showed that dream re-entry was quite successful as a lucid dream induction technique. 43 out of 191 attempts (23%) resulted in a lucid dream and counting was shown to be slightly more effective than body awareness. In the failed attempts, subjects using counting were less likely to fall asleep again, while subjects using body awareness were most likely to fall asleep without dream recall. However, this study was rated as being methodologically 'low'. 

Other Techniques
One study (Hickey, 1988) which was a mixture of field study and laboratory experiments used a combination of methods (MILD Technique, reality checking, dream re-entry etc) to promote lucid dreaming in children aged 10 - 12 years. During a 6 week training programme, 12 out of 13 children reported at least one lucid dream in their home environment (24 lucid dreams in total). In addition 2 out of 4 children experienced a verified lucid dream in a sleep laboratory setting (6 lucid dreams were recorded across 16 nights). However, due to the eclectic, combined approach used, it is impossible to determine exactly which aspects of the lucid dream induction were successful or more effective than others. 

  • External Stimulation
There have been 11 studies which have tested the effect of external stimulation to trigger lucid dreaming during REM sleep. There were 7 laboratory studies and 4 field studies, and external stimuli used ranged from light, acoustic, vibro-tactile, electro-tactile, vestibular bodily stimulation and water.

  • Light Stimulation (DreamLight, DreamLink & NovaDreamer)
There have been 4 studies into light stimulation as a lucid dream induction techniques - all conducted by LaBerge and his colleagues. One of the studies was a laboratory experiment (LaBerge et al, 1988), while the remaining 3 were field studies. 

The 3 field studies used commercially available specialist devices for producing light stimulation during REM sleep (LaBerge, 1988; LaBerge & Levitan, 1995 & Levitan & LaBerge, 1994). 

In the 1995 study, the methodological quality was rated as 'fair', whereas the other 3 were considered to be 'poor'. It was found that light cues can be successfully incorporated into REM sleep and trigger lucidity (LaBerge & Levitan, 1995 & LaBerge et al, 1988), but there were indications that this lucid dream induction technique may be slightly less effective than MILD, although the combination of both seemed to yield more positive results (LaBerge, 1988 & Levitan & LaBerge, 1994). 

  • Acoustic Stimulation
Acoustic stimulation is generally a voice, buzzer or musical tone and this method has been applied in 3 laboratory studies (Kueny, 1985; LaBerge et al, 1981 & Ogilvie et al, 1983) as well as one field study (Reis, 1989). The methodological quality was rated as 'average'. There has been some indication that acoustic stimulation may be effective as a lucid dream induction technique (LaBerge et al, 1981), but this finding was not conclusive (Kueny, 1985 & Reis, 1989). 

One study did not find any difference in using a voice or a musical tone, although it was found that gradually increasing the volume is more effective than using a constant volume (Kueny, 1985). 

It was also found that using acoustic stimulation during REM sleep which has little alpha activity in the EEG may be more effective than during REM when there is high alpha activity (Ogilvie et al, 1983) although an earlier study by Ogilvie et al (1982) found that lucidity may be associated with high alpha EEG activity. 

  • Vibro-tactile Stimulation
There has been one field study into vibro-tactile stimulation as a lucid dream induction technique (Reis, 1989). However, the results of the study which showed some effectiveness in inducing lucid dreams when vibro-tactile stimulation was used in combination with acoustic stimulation and/or reality checking/reflection methods was difficult to generalise due to variations in duration of training etc.

  • Electro-tactile Stimulation
Electro-tactile stimulation is applied to the wrist and was used in one laboratory study (Hearne, 1983), yielding fairly good results. The study showed that 6 out of 12 subjects who spent one night in the sleep laboratory experienced a lucid dream. A further 2 subjects experienced lucidity, but woke up at signalling and another one became lucid after falsely perceiving stimulation. 

  • Vestibular Stimulation
There has been one study into vestibular stimulation (Leslie & Ogilvie, 1996). This involved subjects being rocked at a constant speed while laying in a hammock during REM sleep. The results from the study are not conclusive but it was found that vestibular stimulation may increase dream reflectiveness in early versus late morning REM sleep stages.

  • Water Stimulation
There has been one laboratory study into the use of water stimulation as a lucid dream induction technique (Hearne, 1978). Water was splashed onto the face or hand of the subject, but there was no effect on lucid dreaming found. 

  • Application of Drugs
In one study (LaBerge, 2004), acetylcholine esterase inhibitors (Donepezil) was administered to subjects to enhance lucid dreaming. There were 2 doses administered - 5mg and 10mg with a placebo used as a control condition. During this study, 9 out of 10 subjects reported one or more lucid dreams in 2 nights when using the drug, and only one subject reported a lucid dream while using the placebo. Donepezil was found to significantly enhance lucidity rate, frequency of sleep paralysis and increased estimated time spent awake during the night. The higher dose was associated with stronger effects, but also seemed to produce some adverse effects (insomnia, nausea and vomiting).

The lucid dream induction techniques used in the above studies can be split into 3 classes: cognitive, external stimulation and drug application. The only studies which have been included in this review are those which were available in publication - there may be other forms of research in these areas which was not published or accessible.

Out of the cognitive techniques, Tholey's combined technique (Tholey, 1983) seemed to be the most promising. Additionally, the MILD Technique - with or without external light stimulation - when practiced in the early morning during a 30 - 120 minute wake-back-to-bed also seemed to be effective at lucid dream induction. Although there was limited empirical study, dream re-entry showed a good success rate in lucid dream induction. The low or moderate methodological quality of these studies should be borne in mind when considering their reliability. 

The effectiveness of auto-suggestion and post-hypnotic suggestion as lucid dreaming induction techniques remains unclear. These may rely on the susceptibility of the individual to hypnosis or suggestion and success in studies may be due to selection criteria.

The association between alpha activity in the EEG during REM sleep and lucid dreaming is interesting, but also unclear and alpha feedback is a complex method for inducing lucid dreaming outside a sleep laboratory. 

Regarding external stimulation, the results of the studies into these forms of lucid dream induction techniques is also ambiguous. Some success in inducing lucid dreams were found when using light or electrical stimulation, but these findings should be treated with caution as the studies led to the development of commercially available lucid dream induction devices and therefore may be biased (i.e. unsuccessful studies not published). In terms of acoustic stimulation, a gradual increase in volume was found to be more effective than using a constant tone and there was some limited success when using vibro-tactile and vestibular stimulation, although not when using water stimulation and these methods of lucid dream induction should be studied further. It was found that in most cases the subject is able to experience a lucid dream after perceiving a pre-arranged external stimulus during REM sleep, but also on occasions when the external stimulation was applied, but not recognised by the subject. It should be noted that when using external stimulation for lucid dream induction, some forms of cognitive preparation may be necessary for the dreamer to recognise the cue. 

While only one form of drug application (Donepezil) was studied, there is indication that other drugs/substances which alter the cholinergic system (i.e. causing an increase of acetylcholine in the brain) such as DMAE (2-dimethylaminoethanol); rivastigmin; galantamine; huperzine etc may be effective in lucid dream induction (Sergio, 1988 & Yuschak, 2006)

In developing a taxonomy of lucid dream induction techniques, cognitive techniques should be broken down into 2 further sub-categories: DILD (Dream Initiated/Induced Lucid Dreams) and WILD (Wake Initiated/Induced Lucid Dreams) (LaBerge & Rheingold, 1990) as these two methods represent very different approaches to lucid dream induction. In DILD, lucidity is initiated from within the dream state (i.e. the dream becomes lucid during a non-lucid dream), whereas in WILD, the dreamer directly enters the lucid dream state from the waking state, from dream re-entry (Levitan, 1991) or after a short period of wakefulness (Tholey, 1983). 

The WBTB method has only been empirically tested in combination with the MILD Technique, but it is thought to perhaps be an effective method for inducing lucid dreaming on its own and is likely to have some degree of success when combined with other lucid dream induction techniques. 

There are a number of lucid dream induction methods which have not been empirically tested, but should be further analysed to rate their effectiveness and reliability, Amongst cognitive techniques, WILD Technique and concentration on hypnagogic imagery or active visualisation (LaBerge & Rheingold, 1990 & Tholey, 1983) warrant further investigation. 

Also, neuroscientific investigation into how lucid dreaming may be induced by exciting various structures or activity of the brain may produce interesting results (Hobson et al, 2000; Karim, 2010 & Noreika et al, 2010).

Overall, it is unfortunate that the methodological quality of many studies into lucid dream induction techniques was poor or low. Variability, selection criteria, effect sizes etc may all have an impact and it is impossible to perform a meta-analysis on the available data to ascertain precise degrees of effectiveness for the lucid dream induction techniques which have been subjected to empirical analysis in these studies. It is very difficult to make comparisons between the lucid dream induction techniques using the available information. 

All studies reviewed lacked external validation. Most subjects were self-selected frequent/experienced lucid dreamers or university students which makes it impossible to generalise the findings of the studies out to the larger population. Indeed, it may be very difficult to conduct a study which is representative of the general population. 

There were also issues with internal validity. The majority of the studies were not blinded for the subjects or those measuring the outcomes and in field studies compliance with procedure was not always strictly observed and therefore unreliable. 

Outcome validity and reliability of outcome measures presented further problems in many of the studies, some of which relied on the subjective judgement of the dreamer as to whether they experienced a lucid dream and some of the reports could be fallacious or exaggerated (Synder & Gackenbach, 1988). Some for of external measures to counteract this would be useful in future studies,

One of the biggest problems for research into lucid dream induction is ascertaining the criterion for successful lucid dream induction. In sleep laboratories, the strict criterion is unambiguous, pre-determined eye movement signalling on the EOG during REM or NREM sleep stages and a dream report received immediately after the subject wakes. In field experiments, there is a dream report, but no ploysomnographic sleep recordings are carried out.

While external (blinded) dream report judges can be used in studies, the validation of lucid dreams is still complicated by the fact that recollection of the dream may be affected by sleep inertia, which is a transitional state between sleep and wakefulness during which the dreamer's cognitive abilities are impaired. This presents problems if the subject does not record the dream immediately upon waking. While eye signalling is an objective test of lucidity, dream reports alone are entirely subjective and leaves the question of verification open-ended. Some critics of lucid dream induction research have pointed out that in the absence of confirmatory dream reports, eye signalling may not always be reliable as  eye movements during REM sleep may coincidentally correspond with the pre-determined eye movements used for signalling. The necessity of confirmatory dream reports is also highlighted by the criticism that it may be ambiguous as to whether the subject is experiencing a lucid dream by eye signals alone given that researchers may have encouraged the subject to signal, even if they are not completely aware of dreaming. It may be that future studies devise a more sophisticated way for the dreamer to communicate with the researcher while experiencing a lucid dream.

Lucidity is often considered an all-or-nothing phenomenon - the dreamer either knows they are dreaming and are lucid, or they do not know they are dreaming and are not lucid. This ignores the fact that there are different levels of lucidity and lucidity is more of a continuum than a binary experience (Barrett, 1992). 

The different levels of lucidity are not accounted for in the lucid dream induction studies, although Purcell (1988) devised a dream self-reflectiveness scale which involves 2 categories: lucidity and control. 

Some researchers use a more specific definition of the minimum criterion for lucid dreaming. Schlag-Gies (1992) only considers a dream to be a lucid dream if some consequence occurs as a result of the realisation that the dreamer is dreaming (i.e, an intention to change the setting of the dream). It is therefore necessary for more complex lucidity scales to be devised in order to discriminate between the different degrees of lucidity and their association with various lucid dream induction techniques. This would allow the comparison of lucid dream induction techniques on both a quantitative and qualitative basis. 

It is also necessary to analyse the differences between laboratory and field study experiments - it has been noted that the environment of the sleep laboratory may act as an additional motivation for the subject to experience a lucid dream. On the other hand, pressure to experience a lucid dream may be counter-productive and cause sleep disruption. 

Researchers should also put a time factor into consideration, given that the studies have shown that the MILD Technique is most likely to be effective when performed in the early morning. This would allow them to explore whether certain technique are more effective at specific times.

The stage of sleep during which the lucid dream occurs should also be taken into consideration. Although the majority of lucid dreams occur during REM sleep and are largely considered to be a form of REM phenomena, they can also occur during NREM sleep. In one study (Dane, 1984) a number of dreams were recorded in NREM1 and NREM2 sleep, but none on NREM3 sleep; and it has been found that self-reflection may even be possible in the deep sleep stage (Mason et al, 1997).

Hobson (2009) proposes that lucid dreaming is a dissociative state which has elements of both waking and dreaming, while LaBerge (2010) states that REM sleep is capable of supporting reflective consciousness. Future research should focus on lucid dreaming during REM and NREM stages of sleep and compare the findings.

Some lucid dream induction techniques work better for some people. For example auto-suggestion was found to be most effective for frequent, experienced lucid dreamers, but have little effect for infrequent lucid dreamers (Levitan, 1989). This may explain why post-hypnotic suggestion may work for those who are susceptible to hypnosis or why MILD may work well for persons with good prospective memories. Therefore, individual difference and level of lucid dreaming experience should be taken into account in future studies. 

It seems that the 'Golden Age of lucid dream research' was during the 1980s and 1990s - and since then scientific or academic interest in dreaming and lucid dreaming has declined dramatically, although it appears that we are experiencing a newly emerging reawakening in interest in the subject as a result of  contemporary neuroscientific research and a rise in cultural popularity. With the assistance of new brain-imaging technology (Dressler et al, 2011), lucid dreaming may represent an invaluable tool for understanding how the brain works during sleep and the wider issue of the nature of our consciousness.

In order to progress lucid dreaming research and make it available to greater populations, effective and reliable lucid dream induction techniques must be established - and this is likely to be assisted by an increasing public interest in lucid dreaming. 

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