In a previous study it was found that highly sensitive individuals considered relaxation in a flotation tank as being more important and beneficial than did those with lower sensitivity (Kjellgren, Lindahl, & Norlander, 2009). The highly sensitive persons also experienced more nonordinary (or altered) states of consciousness than did those with lower sensitivity when in a state of relaxation in in the flotation tank. In the present study we aimed to further explore these observations.

Flotation tank therapy, or flotation-REST (restricted environmental stimulation technique), is a method used to induce deep relaxation through minimization of sensory input. During flotation-REST a person lies in a supine position inside a quiet and dark tank filled with water saturated with salt (magnesium sulfate) and maintained at outer skin temperature (35–36°C). Buoyancy is high, which makes it possible to float comfortably on one’s back inside the tank. Earplugs are used to further reduce audio input, and the duration of the session is generally 45 minutes.

In a meta-analysis van Dierendonck and Te Nijenhuis (2005) showed that flotation-REST effectively induced stress reduction, as well as increasing well-being and enhanced performance in sports and the military, for example, archery and flight performance of pilots. The method has also been used successfully as a treatment for chronic pain conditions (e.g., Bood, 2007) and burnout syndrome (e.g., Bood, Kjellgren, & Norlander, 2009). The most common effects of this treatment are stress reduction, alleviation of depression and anxiety, and increases in optimism and sleep quality (e.g., Bood, Sundequist, Kjellgren, Nordstrom, & Norlander, 2005). Several other effects have also been noted in research on flotation-REST, such as mild euphoria (Schulz & Kaspar, 1994), increased creativity (e.g., Forgays & Forgays, 1992), lower blood pressure (e.g., Turner, Fine, Ewy, Sershon, & Freundlich, 1989), and altered states of consciousness (Kjellgren, 2003).

Altered states of consciousness are usually referred to as transient changes in conscious experience, subjective to each individual’s normal waking state (Farthing, 1992; Tart, 2000). ASC can be voluntarily induced by various techniques, such as meditation, hypnosis, physical exercise, and psychoactive drugs, but also occur naturally every day through dreaming and daydreaming. The altered states that occur during flotation-REST are usually mild and consist of experiences of weightlessness, altered perception of time, mental imagery, changes in body perception, and emotional expression (Kjellgren, 2003). During relaxation in a flotation tank attention and thoughts are also more directed towards mindfulness or the intuitively and visually “here-and-now” state. The type and degree of the ASC can vary greatly between individuals and treatment sessions. Our experience is that the degree of ASC seems to affect how beneficial the treatment is for the participants. A few people experience stronger ASC, such as complex inner scenery, experiences of hearing music, out-of-body experiences, and so forth. Individuals who do experience these strong ASC often have increased benefits from the treatment in terms of alleviation of depression, anxiety, and stress. For a more detailed discussion of the ASC concept and its limitations see Dietrich (2007).

Background variables, such as age, gender, stress levels, depression, anxiety, and optimism do not explain why some people experience ASC to a greater extent than others do during flotation-REST (Kjellgren et al., 2009), nor do prior experiences of ASC or expectations of ASC during flotation-REST (Norlander, Kjellgren, & Archer, 2001). An interesting avenue to explain this phenomenon is differences in personality traits, as demonstrated in a pilot study at Karlstad University, in which highly sensitive individuals were shown to experience significantly more deviations from normal state during flotation-REST (Kjellgren et al., 2009) than persons with lower sensitivity. This was the first study in which some light was shed on factors capable of predicting a high degree of ASC during flotation-REST.

That some individuals are more sensitive than others has been recognized since the inception of psychological research. Jung (1921) proposed that innate sensitivity makes individuals more vulnerable to negative conditions during childhood, and Aron (2004) further proposed that sensitive individuals are prone to deeper processing of information before taking action and, therefore, to being viewed as inhibited or introverted.

Various theories regarding sensitivity have been proposed and Aron and Aron (1997) conceptualized the term sensory processing sensitivity (SPS). The concept refers to individual innate differences in relation to depth of sensory processing and general responsivity. Aron and Aron also developed the Highly Sensitive Person Scale (HSPS), used to identify individuals with high SPS, whom these authors suggest comprise about 25% of any given population. In comparison with less sensitive individuals, highly sensitive individuals are characterized by exhibiting more inhibition of behavior, especially in novel situations, giving them a more reflective response style; by being more sensitive to sensory stimuli, making them more prone to notice subtleties, but also increasing their risk for overstimulation; by processing information at a deeper level, giving them a complex and rich inner life; and by being more reactive emotionally, both positively and negatively (Aron, Aron, & Jagiellowicz, 2012). In earlier studies these individuals, as defined using the HSPS, have also been shown to be more sensitive to pain, medications, caffeine, and to have a higher rate of somatic problems, like migraine, headaches, chronic pain, and chronic fatigue (Jawer, 2005), as well a psychological problems, such as anxiety, depression, and social phobia (Neal, Edelmann, & Glachan, 2002).

There might also be other aspects that differentiate these highly sensitive individuals from the rest of the population. Aron et al. (2012) have suggested that, compared with those who are less sensitive, highly sensitive individuals could also have more highly developed unconscious processing abilities and intuition, as well as more meaningful dreams and increased suggestibility (Aron et al., 2012). This was also elaborated by Jawer (2005), who discussed sensitivity from the perspective of different conceptualizations, which he suggested overlapped with SPS and indicated a shared common ground in neurobiology. One aspect of sensitivity highlighted in these conceptualizations was the tendency for the sensitive person to merge with experience. This propensity is best described by Tellegen and Atkinson’s (1974) concept of absorption, which implies a disposition to fuse with one’s ongoing experience; that is, absorption involves the individual’s responsiveness to engaging stimuli, which can be an experience outside of, or within oneself (e.g., fantasies, imaginations). Absorption also implies a tendency to lose oneself in these engaging stimuli, because of intense engagement resulting in heightened perception and temporarily blurred boundaries between the experiencer and the object of attention, which can result in states labeled mystical or transcendent.

In an earlier study at Karlstad University Kjellgren et al. (2009) found that a high degree of SPS (measured using the HSPS) could predict a high degree of ASC during flotation-REST. These researchers also observed that people with high SPS considered flotation-REST as more important and beneficial than those with low SPS did, probably because of those with high SPS experiencing some alleviation of stress. Further study is required of the relationship between SPS and the other background variables, such as depression, anxiety, optimism, absorption, and mystical experiences.

Our main research hypothesis in the present study was that, in comparison with people low in sensitivity, people high in sensitivity would exhibit a higher degree of ASC during flotation-REST. We also explored how SPS was related to the psychological background variables of depression, anxiety, optimism, mystical experiences, and absorption.

Method

Participants

The sample comprised 57 psychology students at Karlstad University, Sweden (19 men and 37 women) with a mean age of 31.93 years (SD = 12.44, range = 19 to 63). Participants were allocated to one of two groups (high and low sensitivity) depending on their degree of SPS as measured using the Highly Sensitive Person Scale (see Measure subsection below). It has been suggested that between about 10% and 35% of any given population is highly sensitive (Aron & Aron, 1997; Woodward, Lenzenweber, Kagan, Snidman, & Arcus, 2000), and psychology students have been observed to lie at the high end of this proportion (Aron, Aron, & Davies, 2005). Based on this, the high sensitivity group (n = 20) comprised individuals scoring in the top 35% on the Highly Sensitive Person Scale and the remaining participants were allocated to the low sensitivity group (n = 36). An independent samples t test (with significance set at p < .05) was conducted, the results of which showed no age difference between the groups. In the low sensitivity group, a gender difference was found in regard to sensitivity (p < .05), showing women to be more sensitive than were men, but no such difference was found in the high sensitivity group. A chi-square test (goodness-of-fit, with significance set at p < .05) did indicate a gender difference in the high sensitivity group, which included more women than men, but the low sensitivity group did not display any gender differences.

Instruments and Measures

Participants completed all measures in the Swedish language.

Flotation tanks. Flotation tanks measuring 270 cm × 150 cm × 130 cm were used. The tanks were filled with water saturated with Epsom salt (magnesium sulfate) to approximately 0.3 m in depth. The water temperature was constantly maintained at skin temperature (35°C). The tanks were insulated to keep out light and sound and earplugs were used to further minimize sensory input.

Experienced Deviation from Normal State (EDN). This is a 29-item self-report measure specifically designed to assess experiences of ASC during flotation tank therapy (Kjellgren, Sundequist, Norlander, & Archer, 2001). Respondents answer a series of statements by indicating the degree to which each applies to their experiences during the flotation session, using a visual analog scale ranging from 0 (not more than usual) to 100 (much more than usual). Example statements are: “My thoughts slowed down,” “With closed eyelids I saw colors,” and “I got original ideas.” The EDN builds on the psychometrics APZ and OAVAV, which have shown good validity and reliability in several previous studies (Dittrich, 1998). Cronbach’s alpha was .90 in the present study.

Hospital Anxiety Depression Scale (HAD). This is a 14-item self-report measure used to assess the degree of anxiety and depression of respondents (Zigmond & Snaith, 1983). The scale has two subscales (seven items each), to create separate values for anxiety and depression. Respondents use a 4-point Likert scale, ranging from 0 (not at all) to 3 (most of the time), to rate a series of questions regarding how they estimate their experience of the last week. Example questions are: “I can laugh and see the funny side of life,” “Worrying thoughts go through my mind,” and “I can sit and feel relaxed.” The instrument has shown good validity and reliability as a screening instrument for anxiety and depression in clinical practice (Herrmann, 1997). In a review of 747 papers in which the HAD scale was used with both clinical and nonclinical samples, the Cronbach’s alpha was found to be within acceptable ranges (depression: .68–.93; anxiety: .67–.90) and the measure correlated moderately (.49–.83) with other commonly used questionnaires for assessing anxiety and depression (Bjelland, Dahl, Haug, & Neckelmann, 2002).

Life Orientation Test (LOT). The LOT (Scheier & Carver, 1985) is a 10-item self-report measure used to assess dispositional optimism. The respondent rates how well statements apply to him/her, using a 5-level scale ranging from A (I agree a lot) to E (I disagree a lot). Example questions are: “In uncertain times, I usually expect the best,” “I enjoy my friends a lot,” and “I rarely count on good things happening to me.” The parallel test reliability has been reported as .76 (Scheier & Carver, 1985) and internal consistency as .76 (Norlander, Bergman, & Archer, 2002). Sufficient levels of convergent and discriminant validity have also been reported using LISREL version 6 (r = 0.64). For an updated review see Carver, Scheier, and Segerstrom (2010).

Mystical Experience Scale (MES). The MES (Hood, 1975) is a 32-item self-report measure of the degree and type of mystical experiences a person has had throughout his/her life. Respondents answer a series of statements by indicating how well each applies to them, using a 5-level ordinal scale ranging from 1 (not true at all) to 5 (very true). In previous studies the MES has been shown to have good validity and reliability (Hood, 1975; Hood et al., 2001). In the present study the total value of the MES was used as an indication of overall experiences of mystical states experienced. Cronbach’s alpha in the present study was .96.

Tellegen Absorption Scale (TAS). The TAS (Tellegen & Atkinson, 1974) is a 34-item multidimensional self-report measure used to assess involvement in imagination and the propensity to become mentally absorbed in everyday experience. Respondents rate a series of statements by indicating how well each applies to them, using a 4-level ordinal scale ranging from 1 (I disagree a lot) to 4 (I agree a lot). The TAS has been shown to correlate with psychological constructs such as hypnotizability, imagery, daydreaming, and ASC, which have been reviewed elsewhere (Roche & McConkey, 1990). In the present study the total value of the TAS was used as an indication of the degree to which the participants had the trait of absorption. Cronbach’s alpha was .97 in the present study.

Highly Sensitive Person Scale (HSPS). The HSPS (Aron & Aron, 1997) is a 27-item self-report measure of SPS. Respondents answer a series of questions, indicating how much the situation described in each applies to them, using a 7-point Likert scale ranging from 1 (not at all) to 7 (extremely). Example questions are: “Are you easily affected by other people’s moods?”, “Do you find loud noises uncomfortable?”, and “Are you aware of subtle changes in your surroundings?” The scale gives a total value, which is then used to create a dichotomous variable representing two groups (high sensitivity and low sensitivity), but the total value has also been used as a continuous variable in some studies (e.g., Jagiellowicz et al., 2011). The HSPS has shown good content validity and reliability in several studies (Aron & Aron, 1997; Smolewska, McCabe, & Woody, 2006). Cronbach’s alpha in the present study was .86.

Procedure

The participants in this study were recruited from among psychology students at Karlstad University. Students were asked during lectures if they would like to participate in a study on flotation therapy. Those who were interested were handed a compendium containing self-report measures (background information, HAD, LOT, MES, TAS, and HSPS) and instructed to fill out the compendium and bring it to the flotation laboratory. Criteria for inclusion in the study were that the participant was a full-time student and that he/she did not have any previous experience of flotation therapy. Participation was voluntarily and students were informed that they could terminate their participation at any time. When the participants arrived at the laboratory, they were informed about the flotation-REST procedure and underwent a 45-minute flotation session. Afterwards, they anonymously filled out the EDN to assess their experience during the flotation session.

Results

Self-Report Measures Before Flotation-REST

The initial data collected using the self-report measures before flotation-REST were analyzed through a series of one-way analyses of variance (ANOVA). The independent variable was degree of SPS as measured using the HSPS (high sensitivity, low sensitivity), and the dependent variables were Anxiety and Depression (HAD), Optimism (LOT), Absorption (TAS), and Mystical Experiences (MES). The analysis indicated significant differences for Anxiety, F(1, 54) = 9.95, p < .01, η² = .15; Absorption, F(1, 54) = 10.80, p < .01, η² = .16; and Mystical Experiences, F(1, 54) = 7.99, p < .01, η² = .13, whereby the high sensitivity group had higher scores for all variables in comparison with the low sensitivity group (Anxiety: M = 6.14, SD = 2.96; Absorption: M = 52.75, SD = 27.92; Mystical Experiences: M = 105.06, SD = 29.13). These significant results all have medium effect sizes (Cohen, 1988). There were no other significant effects, as can be seen in Table 1.

Altered States of Consciousness

Examination of differences in ASC during flotation-REST was conducted using a one-way ANOVA with degree of SPS as measured using the HSPS (high sensitivity, low sensitivity) as the independent variable and degree of ASC (as measured with the EDN) as the dependent variable. As can be seen in Table 1, the test indicated there was a significant effect, F(1, 56) = 5.56, p < .05, η² = 0.12, with medium effect size (Cohen, 1988), whereby the high sensitivity group had a higher degree of ASC during flotation in comparison with the low sensitivity group.

Discussion

In the present study we investigated whether or not an individual’s degree of SPS had an effect on his or her experience of flotation-REST. The statistical analysis indicates that among our participants, compared with those who were less sensitive, highly sensitive individuals had significantly more experiences of ASC during flotation-REST, which is in line with the findings of Kjellgren et al. (2009). In the current study highly sensitive individuals also reported having had significantly higher degrees of anxiety and absorption, as well as experiencing a mystical state more frequently prior to the flotation session.

Earlier researchers (e.g., Aron et al., 2012) have suggested that SPS may predict a higher degree of ASC during sensory deprivation because highly sensitive individuals are more easily overwhelmed by sensory input than others are, as well as being more aware of subtleties in their environments (e.g., light, noise, others’ moods). This may relate to a higher level of anxiety as it puts these individuals in a position where they are constantly aware of a plethora of information that they need to process and integrate, thus creating a higher overall mental activity workload. Considering the fact that contemporary society can be characterized as performance- and high-tempo-oriented, it is reasonable to assume that sensitive individuals are more vulnerable than others are, and exhibit a higher degree of anxiety, because of the stressful environment. This could mean that highly sensitive persons are in greater need of having a break from everyday reality and reconnecting with their inner worlds, which could explain why this group also reported that they had mystical experiences more frequently and a higher degree of absorption prior to the flotation session.

Mindfulness is increasingly being embraced in psychological therapy (e.g., Bakker & Moulding, 2012). This is an approach in which individuals are encouraged to enhance their attention and awareness of the present experience together with an open and accepting attitude. The positive effects of mindfulness practices on, for example, anxiety, have been confirmed in several studies (e.g., Hofman, Sawyer, Witt, & Oh, 2010). A higher degree of mindfulness has, above all, been found to be associated with an improved ability to ignore distracting information (Moore & Malinowski, 2009), which makes it an excellent approach for highly sensitive persons who are easily overwhelmed by sensory impressions. The state induced in the flotation tank, characterized by “here-and-now-thinking” is likely to be similar to a high degree of mindfulness. This indicates that it would be an effective treatment option for highly sensitive persons. In the flotation tank sensory input is minimized, temporarily removing the need to process information from the environment, and creating a safe and comfortable space with reduced stress and anxiety, which might, potentially, free up inner mental resources. For the highly sensitive individual this might result in a heightened degree of contact with his or her more complex and rich inner world, as manifested in a higher degree of ASC, and possible mindfulness. This corresponds with results in the earlier study conducted by Kjellgren et al. (2009) showing that high-sensitivity persons thought that flotation-REST was of greater importance than did low-sensitivity persons.

Highly sensitive persons are more prone than others are to having negative health issues, but the sensitivity trait also has positive aspects, for example, being more conscientious and better at avoiding errors than others are, as well as being more in tune with their thoughts and emotions (Aron, 2010). The price of these advantages is being more prone to negative health issues, so it is reasonable to assume that there is also a capacity or restoring mechanism to cope with this increased sensitivity. Such a mechanism could be an increased ability to experience mindfulness and ASC, thereby restoring the system. Kjellgren et al. (2009) suggested that highly sensitive individuals do have a less robust stabilizing system than that of less sensitive individuals for maintaining a normal state of consciousness and are, therefore, more disposed to experience ASC during flotation-REST. A less robust stabilization system might be explained by our finding in the present study that the highly sensitive individuals scored significantly higher on the trait of absorption, as well as on reported earlier experiences of a mystical state. This finding implies that highly sensitive persons are more predisposed to be absorbed in such processes. The tendency for absorption might also make it harder for highly sensitive individuals to focus their attention, especially in stressful and demanding situations, which further underlines the usefulness of mindfulness for this group, because it strengthens the ability to focus attention voluntarily, resulting in feelings of an increased sense of control.

There is reason to continue conducting research on highly sensitive persons to further establish what differentiates this group from the general population, and to investigate more thoroughly whether this group benefits more from mindfulness or ASC, where awareness of the present moment is the distinguishing effect. Because highly sensitive people tend to exhibit more physiological and psychological problems than others do, it is essential to find adequate treatment options for this group, and flotation-REST could be a useful tool that is free from side effects.

Limitations

A limitation in the present study is that the high sensitivity sample had an overrepresentation of women and was, therefore, skewed. Aron and Aron (1997) discussed gender differences in relation to SPS and observed that women in general score higher on the HSPS, although they pointed out that after partialing out gender, SPS correlations to other variables in their initial study did not change, which indicates that the sample in the present study is sufficient. The gender skewedness of the sample does, however, raise questions regarding generalizability.

Filling out a relatively large number of questionnaires might also have been a burden for the participants, and could possibly have resulted in their losing concentration and, thus, reducing the level of care taken when they were rating the final items. However, in this regard, it is worth noting that the participants were full-time students and probably had sufficient mental resources to cope with the tasks required.

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This study was supported by grants from the County Council of Vä

rmland (Landstinget i Vä

rmland)

Sweden. The research was carried out in compliance with the World Medical Association Declaration of Helsinki on the ethical principles of medical research involving human participants.

Kristoffer Jonsson, Department of Psychology, Karlstad University, SE-651 88 Karlstad, Sweden. Email: [email protected]