Keywords

1 Introduction

In this paper, we propose a training method to improve the stammering symptom, which automatically adjusts the rhythm of speech using vibrational stimulation linked to the heart rate through a smart watch. A disorder in which a person is not able to speak smoothly is called stammering. There are training methods that use delayed auditory feedback (DAF), a hooked metronome, and other mechanisms to improve the symptoms of stammering [1]. In addition, tactile stimulation training is proposed as a method to improve stammering using something other than hearing [2]. We focus on the rhythm control effect through vibration confirmed by tactile stimulation training, and propose a training method to improve the symptoms of stammering while automatically adjusting the rhythm of the utterance based on heartbeat-linked vibration stimuli. In addition, a system using the proposed method is constructed, and the effects of providing vibration stimulation to stutterers on the heart rate and stammering symptoms are investigated. It is examined whether stammering improvement training through vibration stimuli is effective.

Stammering symptoms include repeating a particular sound involuntarily during speech, or uttering with a stammer or timid hesitancy. In these symptoms, speech fluency is compromised. This condition is said to affect approximately 1% of adults, regardless of nationality and language. Symptoms can be broadly divided into three types: problems with words and phrases that are difficult to start with, prolongation of repetitive sounds, and repeated occurrences of the same sound. Stuttering occurs most often occurs in childhood at elementary school and is often repeated at first. However, as a child grows, the block reaction to the utterance gradually becomes stronger due to anxiety after being pointed out by others, and an uncomfortable feeling about their utterance. The rate of departure increases; therefore, stuttering in adults is difficult. Because stutterers are more likely to have social anxiety disorder (SAD) or depression due to insecurity about interpersonal relationships, which may hinder social life, effective improvement measures are desired.

Training methods to improve or reduce stammering include DAF that enables speaking while listening to the uttered voice with a momentary delay, shadowing (repetition), and sound emitted at a constant tempo from a hearing aid-type device. There are ear-hung metronomes that speak together. However, DAF is difficult to use in situations where first voice is difficult to emit in the first place, shadowing is not used in situations where speech is actually required, and ear-hung metronomes inhibit the feedback of a speaker’s own voice by voice. In addition, similar to DAF, there is a problem that metronomes have to be placed on the ears; hence, young people are particularly resistant to its daily use.

In this study, we focus on the symptoms that are common in adults, and propose a training method that solves the above problems. In addition to the degree of stuttering symptoms, using the heart rate as biological information related to the state of speech and considering large individual differences, we show the efficacy of the oscillator in improving stuttering, and at the same time, we conduct an oscillator training in a more practical way. Furthermore, we conduct an experiment in which not only vibration is generated at a fixed tempo but also the tempo of vibration is changed by the fluctuation of heart rate, and confirmed whether the heart rate can be stabilized through this.

2 Proposed Method

Several methods have been proposed to improve the symptoms of stammering. In [1], stammering training using an ear-hung metronome was conducted for one subject for 100 days. In this study, stuttering symptoms began to decrease about one month after use. However, a comparison between the cases of not wearing and wearing an ear-hung metronome suggests that the effect of wearing a metronome is obtained from the beginning of use; therefore, in this study (which was a short-term experiment), it is observed even when using vibration stimulation. Similarly, it is important to compare the non-mounted state with the mounted state to obtain the effect. Other experimental conditions differ when a plurality of subjects is considered. The utterance is considered a speech instead of a telephone response, and heart rate information is used in addition to stuttering symptoms.

In [3], improvements of stuttering symptoms by conducting a short-term shadowing training are discussed. This study analyzed the effects on the stuttering symptoms and psychological aspects of 16 adult stutterers who listened to model sounds that were continuously heard and played back orally in parallel. The short-term stuttering symptoms can be expected to improve and alleviate, and the frequency of stuttering in shadow-reading tasks after shadowing is lower than that before shadowing. This differs from this study in that it uses speech tasks that are considered more practical than reading-aloud tasks.

The relationship between heart rate variability (HRV) and mental stress has also been studied. Stuttering symptoms are thought to be greatly affected by psychological pressure. Therefore, the result of measuring heart rate in a speech situation is important. The researchers in [4] investigated the correlation between the intensity of mental stress and the magnitude of HRV at rest, at the time of presentation practice, research presentation, and question-and-answer sessions in subjects involved in research. In any of the factors representing the magnitude of the fluctuation of the heart rate, a negative correlation is shown with the intensity of mental stress. In other words, the fluctuation of the heartbeat reduces as the mental stress increases, and the fluctuation increases as the mental stress reduces.

In this study, we focus on the symptoms that are common in adults, and propose a training method that solves the above problems. Figure 1 shows the overview of the proposed method. The system proposed in this paper consists of a sensor that acquires heart rate information, and a vibrator that generates a vibration stimulus. The acquired heart rate information is linked to the vibration stimulation interval, and the users are provided with a vibration at a constant rhythm to improve stuttering. The heart rate information is obtained using the Android app “Heart Rate Monitor Ware”. In addition, the smartwatch “HUAWEI WATCH 2” is used as a device. The proposed system generated a vibration stimulus at a constant tempo or automatically changed the tempo of a vibration stimulus according to the change in heart rate from a preset reference value. The vibration tempo of the latter type was designed to decrease by 1 bpm as the heart rate increased by 1 bpm. The vibration tempo range is the vibration reference value ±10 bpm. The heart rate I at this time is represented by the following equation:

$$ I = k - \left( {HB - i} \right) $$

where i is the reference heart rate, k is the reference vibration tempo, and HB is the current heart rate.

Fig. 1.
figure 1

Overview of the method of a system in which vibration tempo changes automatically.

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3 Experiment

An experiment was conducted to evaluate whether the proposed system is effective in improving stuttering, which included eight subjects who stutter (five men and three women).

3.1 Experimental Method

The experiment was conducted in a closed conference room. There were occasions when external sounds were heard, but there was no problem with recording. The experimenter and the subject were seated equally in a rectangular conference room consisted of multiple tables.

The subject reads aloud a manuscript prepared by the experimenter for about 1 min. At this time, we examined the effect of the heart rate-linked vibration stimulus generated by the proposed system on the generation of stuttering. First, each subject introduced himself/herself to other subjects to get accustomed to the speech. Subsequently, the following three experimental tasks were performed sequentially. For speech to be read aloud, the subjects selected different speech for each task from multiple speech manuscripts prepared by the experimenter.

  • Task 1: Read the selected speech aloud.

  • Task 2: Generate a vibration stimulus at a constant tempo from a smartwatch, and read aloud according to the tempo.

  • Task 3: Generate a vibration stimulus that changes the tempo according to the change in heart rate, and read aloud according to the tempo.

During this, the heart rate of the subject was measured using a smartwatch, and at the same time as the beginning of the measurement, a timer was used to record the heart rate data and the start time of the utterance. Simultaneously, the heart rate was measured, and the time was measured using a stopwatch. Next, the time of the stopwatch was noted at the same time as the beginning of the utterance to know the point of the heart rate data at which the utterance started. In Tasks 1 and 2, the utterance started 20 s after the beginning of the heart rate measurement, but in Task 3, the utterance started after about 1 min for preparation, as described later. The utterance content was selected by the subject from each of the 15 speech themes prepared by the experimenter. The reason why we chose speech as a task was that we thought that it was an important scene in social life, and it was easy to prepare an experimental environment, especially for stutterers who struggled with this scenario. Although the speech time was set to be 1 min as a guide, it varied in the experimental results because the subject gave priority to giving speech from beginning to end rather than time. Table 1 shows the prepared speech themes. All utterances in these experimental tasks were recorded using an IC recorder.

Table 1. List of speech themes.
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Furthermore, after performing three types of tasks, all subjects were asked to respond to an experiment evaluation questionnaire distributed in advance. Table 2 shows the contents of the questionnaire. We asked them to respond to the stuttering situation when not wearing an ear-hung metronome, evaluate this device using the 7-step semantic differential (SD) method [5], and freely give their specific impressions and hopes for the experiment in the free description column.

Table 2. Contents of the questionnaire of SD method.
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3.2 Experiment Results

The stuttering frequency was calculated by dividing the number of stutters in the recorded speech content by the total number of phrases. Symptoms were classified into the following four groups based on the stuttering method [6]:

  1. 1)

    Rarely-onset symptoms (prevention, discontinuity, preparation, duration, withdrawal)

  2. 2)

    Repeated symptoms (sound repetition, partial word repetition)

  3. 3)

    Insertion symptoms (insertion, repeated words)

  4. 4)

    Other non-fluid symptoms (excluding the other three groups of symptoms)

In addition, repeated words were not added as phrases, and when two or more symptoms appeared at the same time, the one with the strongest intensity was selected. For the heart answers of each subject to the questions described in the questionnaire of the SD method rate data, the average and variance of the heart rate during the utterance of each subject in each task were obtained, and then compared for the three tasks.

For the heart rate data, the average heart rate and standard deviation during utterance in each subject for all subjects were obtained, and then compared for the three tasks. Figure 2 shows the results.

Fig. 2.
figure 2

Average heart rate and standard deviation of each subject’s task.

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Table 3 shows the answers of each subject to the questions described in the questionnaire of the SD method.

Table 3. Answers of each subject to the questions described in the questionnaire of the SD method.
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4 Discussion

As a result of the experiment, the frequency of stuttering in Task 2 and Task 3 was lower than in Task 1 among five of the eight subjects (Subjects 1, 2, 5, 6, and 8). Four of them (subjects 1, 5, 6, and 8) showed a decrease in stuttering frequency as the task progressed. In Task 3, the frequency of stuttering decreased in all seven subjects, except Subject 7, who failed to record. In the stuttering system with stuttering as the core symptom, and in the continuous system, at least seven out of eight subjects recorded the lowest symptom incidence in Task 2 or Task 3 with vibration. In particular, in the refractory system in which adult stutterers tended to have the highest symptom, six out of seven subjects, excluding subject 7, showed the lowest symptom incidence in Task 3.

As shown in Fig. 2, the average heart rate of six subjects out of eight is lower in Task 3 than in Task 2, and three subjects show a decrease in the heart rate as the task progresses. It was suggested that the height of the number may be related to the frequency of stuttering, and in the standard deviation of heart rate, Task 3 showed the highest value (the heart rate fluctuated greatly). Many subjects showed less mental stress in Task 3 because the variation was large. As mentioned above, by changing the vibration tempo during speech, it is suggested that there are two effects of a greater decrease in the mental stress relaxation of the stuttering frequency.

As shown in Table 3, many subjects feel it difficult to control the speed of utterance. Many subjects answered that there was not much discomfort at the time of wearing. In the open-ended description, the points that were popular in this experiment were those when they used a wristwatch, unlike the ear-hung metronome, and that they could relax by attaching the device. On the other hand, there are many criticisms and points of improvement. It can be observed from the results in Table 3 that it is difficult to adjust to the tempo because subjects were not familiar with the experiment method. Hence, more training is required, and it is better to shorten the experiment.

Effectiveness of Vibration Stimulation:

Five subjects showed that the frequency of stuttering in Tasks 2 and 3 with vibration was lower than that in Task 1 without vibration, suggesting that vibration stimulation has the effect of reducing stuttering. It is possible that the effect of the habit of having performed a number of speeches may be involved, but it is considered that the influence of habituation was suppressed to some extent during the self-introduction by each subject before starting the experiment. The two subjects whose stuttering frequency in Task 2 exceeded the stuttering frequency in Task 1 had the highest stuttering frequency among the subjects, and the effect of vibration stimulation may differ depending on the severity of stuttering. Was suggested. However, in both cases, the frequency of stuttering in Task 3 was clearly lower than that in Task 1, and it is conceivable that either the stuttering effect was slightly slower than that of mild stutterers or the effect was specific to the condition of Task 3. In any case, the utterance at the tempo of the vibration stimulus is expected to have an effect of improving the stuttering symptoms of many stutterers.

Effect of Changes in Vibration Tempo:

Compared to Task 2 in which the vibration tempo was constant, Task 3 in which the vibration tempo changed was more effective for difficult-to-treat symptoms. The frequency was lower, and six out of eight subjects showed a lower average heart rate in Task 3 than in Task 2, so that changing the vibration tempo was more effective in reducing the effect. This possibility was suggested. In addition, many subjects showed a high standard deviation (variation) of heart rate in Task 3. A previous study [5] showed that the lesser the mental stress, the greater the fluctuation of the heart rate. It can be said that there were many subjects who had less mental stress in Task 3. In addition, although the standard deviation of the heart rate of subject 4 in Task 3 was small, the average value of the heart rate showed a considerably low value, indicating that the heart rate in Task 3 was always low and stable. It is determined that Task 3 was also more effective mentally in Task 3 than in Task 2. These results suggest that changing the vibration tempo during speech has two effects: a greater reduction in stuttering frequency and a reduction in mental stress.

Relationship Between Stammering Frequency and Heart Rate:

Although no direct comparison between stuttering frequency and heart rate has been found, a previous study [7] that examined the relationship between stuttering and anxiety found that stuttering frequency increased during higher anxiety situations, that is, greater mental stress. A previous study [8] that examined the relationship between high heart rate and mental stress showed that higher the mental stress, the higher the heart rate. Together, these studies predicted that higher the heart rate, the higher the frequency of stuttering. Based on these facts, in this experiment, five out of eight subjects showed similar trends in the transition of stuttering frequency between tasks and the transition of the average value of heart rate. It was suggested that this might be related to the frequency of stuttering. The correlation between the heart rate standard deviation and stuttering frequency was not as high as the average heart rate (only two people showed similarity). Considering that the target stress is less, it is difficult to say that there exists a relationship between the magnitude of heart rate variability and the likelihood of stuttering.

Practicality of Vibration Stimulation Training Based on Subject Impressions:

Finally, we will evaluate the practicality of vibration stimulation training for stuttering symptoms based on the results of the experimental evaluation questionnaire of subjects. According to the results of the experimental evaluation questionnaire, many subjects felt it difficult to speak according to the vibration tempo. However, it was found that stuttering frequency reduced by speaking according to the rhythmic vibration. Therefore, as one becomes accustomed to the utterance timing through long-term training, further improvement can be expected, as shown in a previous study [2]. In addition, because many subjects did not feel discomfort from the vibration stimulus, their mental discomfort was less and they could be used for long-term training without any problems. Furthermore, some subjects were interested in using a wristwatch as a device. Therefore, it was suggested that it could be a tempo stimulator instead of the ear-hung metronome. All three subjects who responded favorably to this vibration stimulus in the free description column (Subjects 3, 5, and 8) had a large standard deviation of the heart rate in Task 3; therefore, they were subjective and objective. It can be observed that the effect was also obtained in from the above. It is considered that vocal training in which the vibration stimulus tempo is changed by a change in the heart rate is an effective and practical method for improving stuttering.

5 Conclusion

In this study, we aimed to improve stuttering and conducted an experiment to utter at the tempo of vibration stimulation. Consequently, it was confirmed that stuttering decreased by changing the vibration tempo according to the change in heart rate during an utterance. This means that providing a vibration stimulus to the stuttering person based on the heart rate information is more effective in reducing stuttering symptoms than simply providing vibration at a constant tempo.

In future studies, it will be necessary to consider its use in long-term training. In this study, although it was shown that changing the vibration tempo according to the change in heart rate was more effective in reducing stuttering than a constant tempo, it was not possible to examine the appropriate tempos for individuals. A tendency was observed in the subjective evaluation of subjects and the stuttering reduction effect of the vibration stimulus with the changing tempo. Therefore, if a subject adjusts to an appropriate tempo by subjective judgment during long-term training, a more appropriate experimental design can be realized. Consequently, it can be used as effective stuttering improvement training that replaces the ear-hung metronome.