A study on the emotional responses on hearing laughter from brain activity measurements using near-infrared spectroscopy

doi:10.21203/rs.3.rs-5318371/v1

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A study on the emotional responses on hearing laughter from brain activity measurements using near-infrared spectroscopy

https://doi.org/10.21203/rs.3.rs-5318371/v1

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Background: Studies have recognized the beneficial effects of hearing laughter. However, its mechanism has not been sufficiently investigated. Therefore, we examined how laughter influenced affect and emotions and evaluated brain activity upon hearing laughter via near-infrared spectroscopy (NIRS).

Methods: An experiment was conducted with seven healthy university students and used a block design with laughter and relaxing music as interventions. A portable brain activity measurement device (HOT-2000; NeU Corporation) was used to capture neural activity in the lateral prefrontal cortex, based on the degree of changes in total hemoglobin (Hb).

Results: Comparison of the blood flow volume between the right and left prefrontal lateral cortices after each intervention revealed a significant difference in total Hb; however, this was only at rest. No differences were observed during either the laughter or music intervention. Furthermore, comparison of the cerebral blood flow volume between the three experimental conditions (rest period, laughter, music) revealed a significant difference (p = 0.016) between rest period and laughter. However, no significant difference was observed between the other factors.

Discussion: The results did not provide substantial insights into the affect and emotions during laughter interventions based on the degree of change in total Hb between the right and left lateral prefrontal cortices. However, hearing laughter brought about changes in mental states, such as feelings of pleasure and calm, which suggested that hearing laughter relaxed brain activity.

Emotions

Laughter

Near-infrared spectroscopy

Total Hb

Brain activity

The recent COVID-19 pandemic damaged people’s health worldwide and also significantly impacted the global economy. Economic activities were halted to prevent the spread of the infection. Consequently, people’s incomes became unstable, their lifestyles were disrupted, and restrictions were imposed on various daily activities, such as going out. Thus, the pandemic had an immeasurable impact on people’s mental well-being. Approximately 121 million people worldwide are estimated to be suffering from depression, with health problems and occupational stress cited as major causes [1]. Therefore, formulating appropriate mental health measures is an urgent task.

Improving the workplace environment is a common measure to cope with occupational stress [2, 3]. However, this is insufficient for some employees. Other effective mental health measures include cognitive behavioral therapy [4], education [5], and other methods. However, professional knowledge and skills are required to implement them.

Therefore, identifying a mental health management method that enables primary prevention, requires no professional knowledge or skills, and is cost-effective is necessary. Laughter, previously reported to reduce stress, is a noteworthy method [6, 7]. Human relationships in the workplace increase the rate of incidence of mental illness [8]. Laughter contributed to maintaining and enhancing personal relationships, thereby acting as a means to alleviate stress. Studies on laughter can be classified into several categories [9, 10]; however, defining these clearly is difficult. Therefore, we focused on “hearing laughter,” a type of behavior related to laughter, and investigated whether the effect of hearing laughter alleviated stress to the same degree as laughing itself. Fujiwara et al. [11] revealed that hearing laughter effectively alleviated subjective stress in psychiatric nurses. Furthermore, it effectively reduced salivary amylase level, an objective indicator of autonomic nervous system activity, among individuals with high-level depression. Another study that used heart rate variability suggested that hearing laughter increased parasympathetic nerve activity and brought about relaxation effects [12]. Although the beneficial effects of hearing laughter are becoming better known, its mechanism has not been sufficiently investigated. Clarifying the emotional responses/reactions to hearing laughter is necessary to examine its stress alleviation effects.

We employed near-infrared spectroscopy (NIRS), a method of measuring brain activity. When neural activities occur inside the brain, the surrounding blood vessels expand and supply arterial blood. Blood flow increases in the active neural tissues, and the ratio between oxyhemoglobin (oxy-Hb) and deoxyhemoglobin (deoxy-Hb) in the blood changes [13], serving as an indicator of brain activity. Past research on affect and emotions evaluated brain activity via NIRS. Hoshi et al. reported that when feelings of pleasure occurred, oxy-Hb rose in the left hemisphere of the frontal lobe as it activated [14]. Furthermore, when feelings of pleasure occurred owing to auditory stimulation, the left hemisphere of the frontal lobe was similarly activated [15]. Another study observed a negative correlation between the logHF of heart rate variability and oxy-Hb during stress interventions [16]. These findings indicate the potential for quantitatively evaluating brain activity upon hearing laughter via NIRS. Therefore, this study investigated how laughter influenced affect and emotions and used NRIS to evaluate brain activity upon hearing laughter.

Criteria for participant selection

The participants were healthy students enrolled at universities in Japan. We requested voluntary participation through an advertisement on the university’s message board site. To eliminate the influence of drugs and mental conditions, we excluded pregnant women and individuals who regularly visited hospitals for treatment.

Experimental conditions

We conducted the experiments inside a soundproof room to eliminate the effects of external sounds. Participants were instructed to not drink alcohol or engage in vigorous physical activity from the day before the experiment, and sleep for either their customary number of hours or approximately 7 hours, regarded as adequate sleep. On the day of the experiment, participants were prohibited from smoking or ingesting caffeine (coffee, green tea, oolong tea, chocolates, and carbonated drinks/energy drinks, among others), and instructed to not eat or drink anything, except water starting from 2 hours before the experiment.

The average temperature, humidity, and Equivalent Continuous Noise Level (LAeq) inside the room at the beginning of the experiment were 23.9 ± 1.2˚C (digital thermohygrometer CR-1180W; Crecer Co., Ltd.), 45.6 ± 7.4%, and 22.7 ± 1.3 db (resonance vibration measurement device NL-21; Rion Co., Ltd.), respectively.

Experimental procedures

Figure 1 illustrates the experimental procedure. A portable brain activity measurement device was employed, and the location of measurement was set at Fp1 and Fp2 in the frontal pole region of the lateral prefrontal cortex via the 10/20 method. To unify the participants’ sense of familiarity with wearing the device and their physical conditions, we asked them to sit on a chair for 10 minutes after they put on the device and maintain a resting posture. We began the experiment by asking participants to sit on a chair for another 10 minutes as a “rest period,” and measured cerebral blood flow in this condition. Subsequently, we set up laughter and relaxation music as interventions, and designated them as Task 1* and Task 2*, respectively. The rest periods between the interventions were termed “intervals.” Each task and interval lasted 30 seconds. The interventions were conducted via a randomized block design, with three each for Tasks 1 and 2. The two rest were provided alternately and comprised three blocks each, with intervals.

* Task 1: Different types of laughter were played from a CD (product name: A Complete Collection of Sound Effects 16, by King Records Co., Ltd.; product name: The Best Sound Effects, by King Records Co., Ltd.; product name: A Collection of New Sound Effects, by King Records Co., Ltd.; product name: A Complete Collection of New Sound Effects 43, by King Records Co., Ltd.).

*Task 2: A CD of relaxing music was used (product name: Stabilize the Autonomic Nervous System—Music that Invites You to a Relaxing Time, by Della Inc.).

Measurements

Basic information

We used a self-administered questionnaire to obtain participants’ age and sex.

Brain activity measurement

A portable brain activity measurement device (HOT-2000; NeU Corporation) was used. We used near-infrared light with a wavelength of approximately 800 nm and measured the degree of change in total hemoglobin (Hb) between two points on the dorsolateral prefrontal cortex. Normally, oxy-Hb and total Hb rose when neural activity was stimulated in the brain [17, 18]. Total Hb is not readily susceptible to effects from the vein and has high spatial specificity [19]. Moreover, total Hb has sufficiently high sensitivity, as evidenced by numerous studies on hypers-canning, a technique for measuring brain activity in multiple persons and investigating their interactions, via changes in total Hb [20–23]. We used this setup to indirectly capture neural activity in the lateral prefrontal cortex.

Analysis

To examine the degree of change in total Hb between the two dorsolateral prefrontal cortex points, a specialized analysis tool (NeU Analytics; NeU Corporation) was used to remove noise and make baseline corrections. Afterward, for the total Hb values, we calculated the average data covering a period of 10 minutes, at rest, and for both the right and left cortices. For laughter and relaxation music, we calculated the sum and average data of the three interventions for each task and both the right and left lateral cortices, and used the results as indicators of the brain’s physiological activity.

A Wilcoxon signed-rank test was performed to compare the activity between the right and left cortices during each intervention. A Friedman test was performed to compare the total Hb during the rest period, laughter, and relaxation music.

For all tests, p-values were two-sided. Significance was set at p < 0.05. We used IBM SPSS Statistics version 28 for all statistical analyses.

Study participants

This study included seven participants (one male and six female) who provided their consent. Their average age was 21.2 ± 0.4 years.

Table 1 presents the comparison of the right and left lateral prefrontal cortices during each intervention.

Comparison of the cerebral blood flow volume between the right and left cortices during each intervention revealed a significant difference between total Hb; however, this was observed only during rest. No differences were observed during either the laughter or music intervention.

Table 2 presents the comparison of the right and left lateral prefrontal cortices during each condition.

Comparison of the cerebral blood flow volume between the three experimental conditions via the Friedman test revealed a significant difference in the left dorsolateral prefrontal cortex (p = 0.049). However, no difference was observed in the right dorsolateral prefrontal cortex. In the left dorsolateral prefrontal cortex, a significant difference was observed between the rest period and laughter (p = 0.016); however, no differences were observed between the other conditions.

When neural activity occurs in the brain, the surrounding blood vessels expand and supply arterial blood; consequently, the volume of blood flow increases in the active nervous tissues [13]. When feelings of pleasure occur owing to auditory stimulation, oxy-Hb increases in the left hemisphere of the frontal lobe [15]. In this study we used total Hb as an indicator of brain activation, which also increased when the brain was activated [17]. Results revealed that total Hb in the left lateral prefrontal cortex was significantly lower during laughter intervention than during the rest period.

Significant reduction in total Hb during the intervention compared with that during the rest period is evidence of the quieting of brain activity. Park et al. [24] found that as total Hb in the prefrontal cortex decreased while individuals were engaged in “shinrin-yoku (forest bathing),” or taking in the atmosphere of the forest, an activity with stress-reduction effects, brain activity revealed the tendency to relax. Furthermore, relaxation effects brought about a reduction in brain activity. Researchers have reported the potential relaxation effects of hearing laughter [12]; therefore, as in previous studies, the reduction in total Hb in this study, attributable to auditory stimulation in the form of laughter, may have been caused by the quieting of brain activity owing to relaxation effects.

Conversely, Park et al. [24] reported that based on the measurements of total Hb in the prefrontal cortex, when individuals were in a state of displeasure/discomfort or excitement, brain activity did not relax, instead it was activated.

Therefore, hearing laughter brought about changes in mental state, such as feelings of pleasure and calm. Whereas, when activation was observed during rest, the brain may have been in a state of displeasure/discomfort and/or excitement. This study was conducted under circumstances that differed from the everyday environment, that is, the participants were enclosed in a soundproof room from the beginning till the end. Therefore, the 10 minutes of rest period during which participants sat on a chair may not have created a psychologically calm state, which may have led to activation of the brain even during rest.

This study has some limitation. First, the small number of participants. Moreover, most were female university students in their 20s, which reflected an imbalance in terms of age and sex. Second we did not evaluate the participants’ subjective psychological states during the interventions. Hence, we were unable to evaluate their brain activity by cross-checking it with their affect and emotions. Third, no estimate of affect or emotions could be made from brain activity, as neural activity represented multiple meanings. Therefore, a protocol to detect subjective psychological states, as well as clarification of the definitions of affect and emotion, are required. However, our findings confirmed that hearing laughter brought about changes in mental state, such as feelings of pleasure and calm, and could quieten brain activity.

Ethical approval and informed consent to participate

This study was approved by the Ethics Review Committee of Kawasaki University of Medical Welfare. Interventions were implemented only for participants who provided written informed consent.

Publication consent

Not applicable.

Availability of data and materials

Data sharing is not applicable.

Competing Interests

The authors declare no conflict of interests.

Funding

This study received a research grant from the Health Science Center, a general foundation.

Authors’ contributions

YF and HO designed the research; YF collected the data; YF and HO analyzed and interpreted the data; YF made a significant contribution to writing the manuscript. All the authors read and approved the final manuscript.

Acknowledgments

None.

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Tables 1 and 2 are available in the Supplementary Files section.

No competing interests reported.

Table.docx

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Version 1

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You are reading this latest preprint version

A study on the emotional responses on hearing laughter from brain activity measurements using near-infrared spectroscopy

Status:

Version 1

Abstract

Figures

Introduction

Methods

Criteria for participant selection

Experimental conditions

Experimental procedures

Measurements

Basic information

Brain activity measurement

Analysis

Results

Discussion

Declarations

References

Tables

Additional Declarations

Supplementary Files

Status:

Version 1