Relaxing E ﬀ ect Induced by Forest Sound in Patients with Gambling Disorder

: The number of people addicted to gambling has increased worldwide. They often su ﬀ er from debilitating medical conditions associated with stress or depression. This study examined the physiological and psychological reactions of gambling disorder (GD) patients while listening to high-deﬁnition forest or city sounds using headphones. In total, 12 Japanese male GD patients were exposed to high-deﬁnition forest or city sound waves for 1 min via headphones. Near-infrared spectroscopy of the prefrontal cortex was used to examine oxyhemoglobin (oxy-Hb) concentrations. Heart rate and heart rate variability are indicators of autonomic nervous function. We performed subjective evaluation via the modiﬁed version of the semantic di ﬀ erential (SD) method with the proﬁles of the mood states (POMS). Experiencing forest sounds led to substantial di ﬀ erences as opposed to listening to city sounds: (1) oxy-Hb levels of the bilateral prefrontal cortices were lower (2) the modiﬁed SD method resulted in increased comfortable and relaxed feelings, (3) the negative POMS subscale scores were signiﬁcantly lower, indicating that negative emotions diminished markedly when patients listened to forest sounds. This is the ﬁrst study to show that sounds of forest relaxed individuals physiologically and psychologically to minimize GD.


Introduction
The integrated resorts (IR) maintenance law was passed in July 2018 to enable the opening of IR in Japan [1]. IR is defined as a resort with a hotel and casino. In response to this movement, the headquarters for the promotion of gambling disorder (GD) countermeasures were established in October 2018 [2]. In Japan, many people are addicted to gambling because of easily accessible game machines such as Pachinko and slot machines. GD is widely accepted worldwide as a significant public health issue that causes heavy personal and social losses, high psychiatric comorbidity, poor physical health, and increased suicide rates [3]. The government has determined that it is urgent to establish an appropriate medical system for the increase in patients with GD expected after the release of the IR bill [4]. Therefore, the Health, Labor and Welfare Ministry created a policy allowing patients with GD to obtain public medical insurance for the treatment of gambling in several venues including the new casinos, existing horse racing, and Pachinko in December 2019 [4]. The abovementioned details indicate that Japanese people have become attentive toward gambling addiction.
The 5th edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) defines GD as a pattern of repeated gambling associated with great distress and disability [5]. Studies have shown that depressive moods can cause GD to exacerbate and interfere with healing [6,7]. Furthermore, a study has reported that patients with comorbidities accounted for 38% of pathological gamblers who are at a high suicide risk [8]. While the exact cause and effect association between depressive illnesses and GD remains unelucidated, some researchers have concluded that GD could facilitate depression [9]. Furthermore, the severity of GD positively correlates with the seriousness of depression, stress, and anxiety [7]. Therefore, both improving depressive symptomatology and preventing depression may serve as part of the key strategies for alleviating GD symptoms.
The relaxing and restorative effects of shinrin-yoku or forest therapy have gradually gained attention in recent years [10][11][12]. It refers to the healing effects imparted to the physical and psychological health of humans via a sensory experience, including "five senses," when we spend time with nature, especially a forest environment [13,14]. However, while it is ideal to go out into the forest, it is difficult for city dwellers to visit forests on a daily basis. Therefore, research into bringing the forest to city dwellers began, and in 2019, Jo et al. [15] indicated that female university students when exposed to high-resolution forest sounds compared with city sounds were induced with physiological and psychological relaxation, and the profiles of the mood states (POMS) depression scale scores were also significantly lower. Furthermore, some studies have reported several distinct structural differences in the brains of patients with GD [16,17], resulting in unpredictable reactions. This study determined the physiological and psychological reactions of GD patients to high-resolution forest sounds compared with city sounds and evaluated the improvement in depression-related symptoms using this therapy.

Participants
Twelve male patients attending a psychiatric clinic for GD treatment, aged 19-58 years (mean age ± standard deviation, 36.9 ± 11.5 years), participated in this experiment. The prerequisites for inclusion were a Japanese man without a hearing impairment, who had been diagnosed with GD by a psychiatrist, a person without respiratory diseases such as asthma and chronic rhinitis, and a person without arrhythmia. At the beginning of this experiment, the subjects in this study were gathered in a waiting room and were informed of the purpose and methods of this experiment in detail. Subjects who agreed to participate provided a signed consent form. The protocol for the experiment was certified by the Ethics Committee of the Center for Environment, Health and Field Science, Chiba University (project identification number 37), in accordance with the Declaration of Helsinki. The study was registered at the University Hospital Medical Information Network of Japan (UMIN ID: UMIN000037637).

Auditory Stimulation
High-resolution audio has a higher sampling frequency and a greater bit depth. [18]. We adopted this type of sound for stimulation because the sound is more natural than conventional low-resolution audio. A murmuring creek in the forest of Togakushi village in Nagano Prefecture was selected for the forest auditory stimulation (forest sound). The traffic sound of the Shibuya intersection in Tokyo was used as the second auditory stimulation (city sound). A high-resolution sound recorder taped these sounds at a sampling frequency of 96 kHz and a 24-bit quantization. The subjects listened to these sounds using headphones, which were set at 48.4 and 51.3 dB for the forest and city sounds, respectively.

Study Protocol
The experiment was conducted in an isolated room for the physiological measurements maintained at approximately 60% relative humidity, 150 lux illuminance, and a temperature of 25 • C in the Oishi clinic ( Figure 1A; psychiatric clinic). Study participants were provided with a description of the purpose and an outline of the study while in a waiting room. The study was able to be performed for 2 patients simultaneously because the auditory stimulus was provided while the participants were in a seated position with closed eyes. After exposure, they entered the experimental room where sensors for the physiologic measurements were fitted while they received a description of the measurement procedure. The present experiment was conducted after the participants first practiced listening to the sound of ocean waves, which served as a dummy auditory stimulus. The set-up for measuring auditory stimulation physiologically is shown in Figure 1B. Devices for heart rate variability (HRV) and near-infrared spectroscopy (NIRS) and the headphones producing the sounds were affixed to the participants. We performed a within-participant experiment. First, each participant rested with their eyes closed for 1 min; then, an auditory stimulus was played for 1 min. Each participant was provided with the forest and city auditory stimuli once each, only on the day of the experiment. Furthermore, these sounds were played according to a balanced order; thus, we ensured that the order of sound did not affect the physiological response. The bioactivity was continually measured between the resting period and hearing stimulation. Subsequent to measuring physiological activity, they took off the headphones and we conducted the modified version of the semantic differential (SD) method together with the profiles of the mood states (POMS) for approximately 2 min.
Sustainability 2020, 12, x FOR PEER REVIEW 3 of 9 The experiment was conducted in an isolated room for the physiological measurements maintained at approximately 60% relative humidity, 150 lux illuminance, and a temperature of 25°C in the Oishi clinic ( Figure 1A; psychiatric clinic). Study participants were provided with a description of the purpose and an outline of the study while in a waiting room. The study was able to be performed for 2 patients simultaneously because the auditory stimulus was provided while the participants were in a seated position with closed eyes. After exposure, they entered the experimental room where sensors for the physiologic measurements were fitted while they received a description of the measurement procedure. The present experiment was conducted after the participants first practiced listening to the sound of ocean waves, which served as a dummy auditory stimulus. The set-up for measuring auditory stimulation physiologically is shown in Figure 1B. Devices for heart rate variability (HRV) and near-infrared spectroscopy (NIRS) and the headphones producing the sounds were affixed to the participants. We performed a within-participant experiment. First, each participant rested with their eyes closed for 1 min; then, an auditory stimulus was played for 1 min. Each participant was provided with the forest and city auditory stimuli once each, only on the day of the experiment. Furthermore, these sounds were played according to a balanced order; thus, we ensured that the order of sound did not affect the physiological response. The bioactivity was continually measured between the resting period and hearing stimulation. Subsequent to measuring physiological activity, they took off the headphones and we conducted the modified version of the semantic differential (SD) method together with the profiles of the mood states (POMS) for approximately 2 min.

NIRS
We measured a change in the concentration of oxyhemoglobin (oxy-Hb) on the surface of the prefrontal cortices using an NIRS device with two channels (PocketNIRS Duo, DynaSense, Shizuoka, Japan). NIRS probes were affixed symmetrically on each participant's bilateral forehead [19], and alterations in oxy-Hb concentration of the bilateral prefrontal cortex were recorded at 1 s intervals when the participants were resting and receiving auditory stimulation. The baseline was set to 0 before each measurement and then the relative change from the baseline was evaluated.

HRV and Heart Rate
HRV and HR (heart rate) were used as activity of ANS (autonomic nervous system) parameters [20]. The participants were assessed using a portable type of electrocardiogram (Activtracer AC-301A; GMS, Tokyo, Japan). Of the HRVs, power levels in the range of 0.15-0.40 Hz for the high

NIRS
We measured a change in the concentration of oxyhemoglobin (oxy-Hb) on the surface of the prefrontal cortices using an NIRS device with two channels (PocketNIRS Duo, DynaSense, Shizuoka, Japan). NIRS probes were affixed symmetrically on each participant's bilateral forehead [19], and alterations in oxy-Hb concentration of the bilateral prefrontal cortex were recorded at 1 s intervals when the participants were resting and receiving auditory stimulation. The baseline was set to 0 before each measurement and then the relative change from the baseline was evaluated.

HRV and Heart Rate
HRV and HR (heart rate) were used as activity of ANS (autonomic nervous system) parameters [20]. The participants were assessed using a portable type of electrocardiogram (Activtracer AC-301A; GMS, Tokyo, Japan). Of the HRVs, power levels in the range of 0.15-0.40 Hz for the high frequency component (HF) and 0.04-0.15 Hz for the low frequency component (LF) were analyzed (MemCalc/win; GMS, Tokyo, Japan) [21]. The HF component indicated the activity of the parasympathetic nervous system (PSNS), and the LF/HF ratio revealed the activity of the sympathetic nervous system (SNS). In this study, HRV recording was analyzed every 60 seconds to investigate acute physiologic responses to sound stimulation.

Psychological Measurements
The emotions evoked by each auditory stimulation were evaluated using the modified version of the SD method and POMS. A 13 point scale between three pairs of indicators, "comfortable to uncomfortable", "relaxed to aroused", and "natural to artificial", was used as a modified version of the SD method [22]. The POMS score was generated on the following six subscales: "tension-anxiety (T-A)", "depression-dejection (D)", "anger-hostility (A-H)", "vigor (V)", "fatigue (F)", and "confusion (C)". By using a brief version of POMS that contained only thirty queries, the participants' effort was reduced [23]. The score of "TMD (total mood disturbance)" was determined by calculation using a determined formula: . A high score of TMD indicates a negative state of psychology.

Statistical Analyses
We performed paired t-tests of the physiologic index (NIRS, HRV, and heart rate) to compare the effect of the auditory sound stimulation of forest and city sounds for 1 min. To analyze the results of the psychological measurements and interpret the physical responses to two types of auditory stimulation, we conducted a Wilcoxon signed rank test. To test the hypothesis that humans relax by listening to forest sounds, we adopted a one-side test. The SPSS (statistical package for the social sciences) software version 21.0 (IBM, NY, USA) was used and p < 0.05 was considered statistically significant. Figure 2 shows the per-second changes of the concentration level of oxy-Hb in the left (A) and right (B) prefrontal cortices when the participants were listening to forest and city sounds. The oxy-Hb concentration of the forest sound gradually decreased from the start in the bilateral prefrontal cortex and reached a minimum at 25 s. Following this time point, the concentration gradually returned to the original value over 1 min. Conversely, the level of oxy-Hb in the prefrontal cortices for city sounds was static, and then gradually increased after 30 s (A-1, B-1).

HRV and HR
There were no marked differences in HF and LF/HF ratio during the 1 min auditory experience of forest and city sounds.

Psychological Effects
The results of the modified version of the SD method were shown in Figure 3A. It was revealed that participants were overwhelmingly aware of "comfortable (p < 0.01)", "relaxed (p < 0.01)", and "natural (p < 0.05)" feelings when listening to forest sounds compared to city sounds (p < 0.01) by the subjective evaluation. Figure 3B shows the marked decrease in the subscale scores of POMS for "D (depressiondejection, p < 0.05)", "T-A (tension-anxiety, p < 0.01)", "C (confusion, p < 0.01)", "A-H (angerhostility, p < 0.01)", and "F (fatigue, p < 0.01)" which are negative subjects, when listening to forest sounds compared to when listening to city sounds. The general "TMD (total mood disturbance)" scores declined thoroughly while listening to forest sounds compared when listening to city sounds (p < 0.01). Furthermore, negative feelings were noticeably less prevalent during the experience of patients listening to forest sounds. The data have been presented as the mean ± standard error (n = 11). * A one sided paired t-test was carried out with significance at p < 0.05.

HRV and HR
There were no marked differences in HF and LF/HF ratio during the 1 min auditory experience of forest and city sounds.

Psychological Effects
The results of the modified version of the SD method were shown in Figure 3A. It was revealed that participants were overwhelmingly aware of "comfortable (p < 0.01)", "relaxed (p < 0.01)", and "natural (p < 0.05)" feelings when listening to forest sounds compared to city sounds (p < 0.01) by the subjective evaluation. Figure 3B shows the marked decrease in the subscale scores of POMS for "D (depression-dejection, p < 0.05)", "T-A (tension-anxiety, p < 0.01)", "C (confusion, p < 0.01)", "A-H (anger-hostility, p < 0.01)", and "F (fatigue, p < 0.01)" which are negative subjects, when listening to forest sounds compared to when listening to city sounds. The general "TMD (total mood disturbance)" scores declined thoroughly while listening to forest sounds compared when listening to city sounds (p < 0.01). Furthermore, negative feelings were noticeably less prevalent during the experience of patients listening to forest sounds.  -3)), according to a regulation of a modified semantic differential method, following the experience of the forest or city sounds. (B) The subjectivity evaluation of feelings as documented with six items of the questions and TMD score on the questionnaire evaluating feeling states due to the experience of the forest or city sounds. Tension-anxiety (T-A), depression-dejection (D), anger-hostility (A-H), fatigue (F), confusion (C), vigor (V), and total mood disturbance (TMD). Data are shown as the mean ± standard error (n = 12). ** p < 0.01, * p < 0.05 as generated by the Wilcoxon signed-rank test (one sided).

Discussion
In this study, we monitored patients with GD listening to high-resolution sounds from forest and city environments to determine their responses to auditory stimuli under laboratory conditions. The rich and dynamic forest environment comprises a near-infinite number of sensory and auditory components. To simplify this diverse environment for targeted assessment within this study design, the number of forest bathing inputs was reduced to one. We therefore focused on the "murmuring creek" sound as one of the environmental elements of the forest, and we observed the responses by gamblers to this specific forest sound alone. The results revealed the following: (1) listening to forest sounds markedly reduced the levels of oxy-Hb in the bilateral prefrontal cortices. Increase in oxygen consumption, topical cerebral blood flow, and oxy-Hb delivering have been reported in strongly stimulated neural areas, resulting in decreased oxy-Hb concentrations of prefrontal cortex activity during pleasant feelings [24] and a relaxed state of being [25,26]. It is reported that NIRS-determined cerebral oxy-Hb may be affected by changes in forehead skin blood flow during dynamic exercise [27]. However, in this study, the auditory stimulation was provided in a seated position without exercise or physical movement. (2) The modified SD method results revealed that forest-related stimuli significantly elevated "comfortable", "relaxed", and "natural" feelings. The negative POMS subscales also declined substantially when listening to forest sounds compared with city sounds.  -3)), according to a regulation of a modified semantic differential method, following the experience of the forest or city sounds. (B) The subjectivity evaluation of feelings as documented with six items of the questions and TMD score on the questionnaire evaluating feeling states due to the experience of the forest or city sounds. Tension-anxiety (T-A), depression-dejection (D), anger-hostility (A-H), fatigue (F), confusion (C), vigor (V), and total mood disturbance (TMD). Data are shown as the mean ± standard error (n = 12). ** p < 0.01, * p < 0.05 as generated by the Wilcoxon signed-rank test (one sided).

Discussion
In this study, we monitored patients with GD listening to high-resolution sounds from forest and city environments to determine their responses to auditory stimuli under laboratory conditions. The rich and dynamic forest environment comprises a near-infinite number of sensory and auditory components. To simplify this diverse environment for targeted assessment within this study design, the number of forest bathing inputs was reduced to one. We therefore focused on the "murmuring creek" sound as one of the environmental elements of the forest, and we observed the responses by gamblers to this specific forest sound alone. The results revealed the following: (1) listening to forest sounds markedly reduced the levels of oxy-Hb in the bilateral prefrontal cortices. Increase in oxygen consumption, topical cerebral blood flow, and oxy-Hb delivering have been reported in strongly stimulated neural areas, resulting in decreased oxy-Hb concentrations of prefrontal cortex activity during pleasant feelings [24] and a relaxed state of being [25,26]. It is reported that NIRS-determined cerebral oxy-Hb may be affected by changes in forehead skin blood flow during dynamic exercise [27]. However, in this study, the auditory stimulation was provided in a seated position without exercise or physical movement. (2) The modified SD method results revealed that forest-related stimuli significantly elevated "comfortable", "relaxed", and "natural" feelings. The negative POMS subscales also declined substantially when listening to forest sounds compared with city sounds. These results coincide with data from studies that reported physiological and psychological responses to forest stimuli in healthy adults [12,15,25,28,29]. "Forest therapy" has become increasingly popular as a therapeutic activity worldwide. It relies on scientifically proven results of walking in and viewing forest areas [15,25,27,28]. On the basis of research on forest therapy, the natural environment, such as forests, is well known as a suitable place for making full use of the five senses; as a result, it leads to increased PSNS activity, decreased SNS activity, decreased cortisol levels, and cerebral blood flow in the prefrontal cortex [12,25,28,30,31].
In Japan, GD is the most frequently observed behavioral addiction, and it is widespread among general family office workers, housewives, and their children. At present, there is no approved drug available for GD treatment; however, cognitive behavioral therapy (CBT) has been the treatment approach relied on most often to help individuals with gambling problems [32]. In addition, Sung et al. created a CBT using a forest therapy program that incorporated meditation and relaxation techniques in forest environments [33]. GD patients usually visit psychiatric hospitals and receive treatment alongside working, similar to healthy people. Therefore, we chose the method of listening to forest sounds as a practical way to relieve stress during work and daily life. Listening to the high-resolution forest sounds can be easily performed in daily life, it is potentially a viable option for patients with GD to induce relaxing effects whenever they are stressed by simply listening to the recorded sounds. We consider it a valuable finding that study participants with GD, as well as the healthy subjects, showed a relaxation response to the sounds of the forest elements.
Moreover, decreased activation of the ventral striatum and ventral medial part of the prefrontal cortex in morbid gamblers was assessed using magnetic resonance imaging [34]. Despite this phenomenon, the physiological and psychological reactions of the patients with GD showed a relaxation effect similar to that of the healthy subjects reported by Jo et al. [15].
In this study, the primary limitation is the possibility of a selection bias because participants were selected by a psychiatrist who determined that they exhibited calm symptoms. Although they underwent psychiatric treatment prior the experiment, the impact of previous psychiatric treatment was not evaluated, potentially complicating the evaluation of specific responses for each GD.
Furthermore, it was difficult to recruit individuals with GD who were willing to participate in the experiment, and ultimately only 12 participants could be included; therefore, robust statistical evidence could not be provided. Therefore, a larger pool of participants is warranted for future studies. The influence of skin blood flow on NIRS measurements remains an ongoing debate [27,35,36]; therefore, for future studies, we believe that we should measure NIRS signals and skin blood flow simultaneously to show the correct evaluation of prefrontal activities. Another limitation was that we recruited only male participants. As a result, it remains unclear whether our observations can be applied to females with GD. Additional research, including females, is warranted. The final limitation of this study was the short exposure time to the stimulus. The degree of relaxation was compared before exposure and after only 1 minute of exposure to forest and to city auditory stimulation; therefore, these results serve as only preliminary findings for exposure time. The relationship between specific stimuli and biologic responses remains complex to study and a difficult topic to fully understand, and more extensive investigation is required to examine the effective degree of, and time for, exposure to stimuli; the frequency of exposure to stimuli; and the type of stimulation.

Conclusions
This study examined differences in reactions caused by listening to a high-resolution sound (forest and city) in patients with GD. It showed that forest auditory stimulation created physiological and psychological relaxation. It is noteworthy that individuals with GD can experience a relaxation effect through exposure to forest-related stimuli, which suggests that this result may be effective against the progression of symptoms.