High-resolution audio has recently emerged in the digital music market due to recent advances in information and communications technologies. Because of a higher sampling frequency and a greater bit depth than conventional low-resolution audio such as compact disks (CDs), it provides a closer replication of the real analog sound waves. Sampling frequency means the number of samples per second taken from a sound source through analog-to-digital conversion. Bit depth is the number of possible values in each sample and expressed as a power of two. A higher sampling frequency makes the digitization of sound more accurate in the time-frequency domain, whereas a greater bit depth increases the resolution of the sound. What kind of advantage does the latest digital audio have for human beings? This question has not been sufficiently discussed. The present investigation used physiological, behavioral, and subjective measures to provide evidence that high-resolution audio affects human psychophysiological state without conscious awareness.

The higher sampling frequency enables higher frequency sound components to be reproduced, because one-half of the sampling frequency defines the upper limit of reproducible frequencies (as dictated by the Nyquist–Shannon sampling theorem). However, in conventional digital audio, sampling frequency is usually restrained so that sounds above 20 kHz are cut off in order to reduce file sizes for convenience. This reduction is based on the knowledge that sounds above 20 kHz do not influence sound quality ratings (Muraoka et al., 1981) and do not appear to produce evoked brain magnetic field responses (Fujioka et al., 2002).

In contrast to this conventional digital recording process in which inaudible high-frequency components are cut off, high-resolution music that retains such components has been repeatedly shown to affect human electroencephalographic (EEG) activity (Oohashi et al., 2000, 2006; Yagi et al., 2003a; Fukushima et al., 2014; Kuribayashi et al., 2014; Ito et al., 2016). This effect is often called “hypersonic” effect. In these studies, only the presence or absence of inaudible high-frequency components is manipulated while the sampling frequency and the bit depth are held constant. Interestingly, this effect appears with a considerable delay (i.e., 100–200 s after the onset of music). However, it remains unclear what kind of psychological and cognitive states are associated with this effect. These studies also suggest that it is difficult to distinguish in a conscious sense between sounds with and without inaudible high-frequency components (full-range vs. high-cut). Some studies have shown that full-range audio is rated as better sound quality (e.g., a softer tone, more comfortable to the ears) than high-cut audio (Oohashi et al., 2000; Yagi et al., 2003a). Another study has shown that participants are not able to distinguish between the two types of digital audio, with no significant differences found for subjective ratings of sound qualities (Kuribayashi et al., 2014). The feasibility of discrimination seems to depend on the kinds of audio sources and individuals (Nishiguchi et al., 2009). Regarding behavioral aspects, it has been shown that people listen to full-range sounds at a higher level of sound volume than high-cut sounds (Yagi et al., 2003b, 2006; Oohashi et al., 2006).

Previous studies have examined the effect of inaudible high-frequency components on EEG activity while listening to music under resting conditions. It has been shown that EEG alpha-band (8–13 Hz) frequency power is greater for high-resolution music with high-frequency components than for the same sound sources without them (Oohashi et al., 2000, 2006; Yagi et al., 2003a). The effect appears more clearly in a higher part of the conventional alpha-band frequency of 8–13 Hz (10.5–13 Hz: Kuribayashi et al., 2014; 11–13 Hz: Ito et al., 2016). Ito et al. (2016) reported that low beta-band (14–20 Hz) EEG power also showed the same tendency to increase as high alpha-band EEG power.

A study using positron emission tomography (PET) revealed that the brainstem and thalamus areas were more activated when hearing full-range as compared with high-cut sounds (Oohashi et al., 2000). Because such activation may support a role of the thalamus in emotional experience (LeDoux, 1993; Vogt and Gabriel, 1993; Blood and Zatorre, 2001; Jeffries et al., 2003; Brown et al., 2004; Lupien et al., 2009) and also in filtering or gating sensory input (Andreasen et al., 1994), Oohashi et al. (2000) speculated that the presence of inaudible high-frequency components may affect the perception of sounds and some aspects of human behavior.

Another line of research suggests a link between cognitive function and alpha-band as well as beta-band EEG activities. Alpha-band EEG activity is thought to be associated not only with arousal and vigilance levels (Barry et al., 2007) but also with cognitive tasks involving perception, working memory, long-term memory, and attention (e.g., Basar, 1999; Klimesch, 1999; Ward, 2003; Klimesch et al., 2005). Higher alpha-band activity is considered to inhibit task-irrelevant brain regions so as to serve effective disengagement for optimal processing (Jensen and Mazaheri, 2010; Foxe and Snyder, 2011; Weisz et al., 2011; Klimesch, 2012; De Blasio et al., 2013).

Beta-band power is broadly thought to be associated with motor function when it is derived from motor areas (Hari and Salmelin, 1997; Crone et al., 1998; Pfurtscheller and Lopes da Silva, 1999; Pfurtscheller et al., 2003). Moreover, beta power has been shown to increase with corresponding increases in arousal and vigilance levels, which indicates that participants get engaged in a task (e.g., Sebastiani et al., 2003; Aftanas et al., 2006; Gola et al., 2012; Kamiñski et al., 2012).

What kind of advantage does high-resolution audio with inaudible high-frequency components have for human beings? What remains unclear is what kind of psychophysiological states high-resolution audio induces, along with the corresponding increase in alpha- and beta-band EEG activities. To monitor listeners’ arousal level, we asked participants to listen to a musical piece while performing a visual vigilance task that required sustained attention in order to continuously respond to specific stimuli. Two types of high-resolution audio of the same musical piece were presented using a double-blind method: With or without inaudible high-frequency components.

EEG was recorded along with other psychophysiological measures: Heart rate (HR), heart rate variability (HRV), and facial electromyograms (EMGs). The former two measures index autonomic nervous system activities. HRV contains two components with different frequency bands: High frequency (HF; 0.15-0.4 Hz), and low frequency (LF; 0.04-0.15 Hz). HF and LF activities are mediated by vagal and vagosympathetic activations, respectively (Malliani et al., 1991; Malliani and Montano, 2002). The LF/HF power ratio is sometimes used as an index parameter that shows the sympathetic activities. Facial EMGs in the regions of the corrugator supercilii and the zygomaticus major muscles have been used as indices of negative and positive affects, respectively (Larsen et al., 2003). Decrements in vigilance task performance such as longer reaction times (RTs) and higher error rates are interpreted as reflecting the decrease in arousal level, which is also reflected in the electrical activity of the brain (Fruhstorfer and Bergström, 1969). Besides ongoing EEG activity, event-related potentials (ERPs) are associated with vigilance task performance. When vigilance task performance decreases, the amplitude of P3, a positive ERP component observed dominantly at parietal recording sites between 300 and 600 ms after stimulus onset, decreases and its latency increases (Fruhstorfer and Bergström, 1969; Davies and Parasuraman, 1977; Parasuraman, 1983). The P3 outcomes are thought to be modulated not only by overall arousal level but also by attentional resource allocation (Polich, 2007). P3 amplitude has been shown to be larger when greater attentional resources are allocated to the eliciting stimulus. It is thus thought that P3 amplitude can serve as a measure of processing capacity and mental workload (Kok, 1997, 2001).

In the present study, physiological, behavioral, and subjective measures were recorded to examine what kind of advantage high-resolution audio with inaudible high-frequency components has. Specifically, we were interested in how the increase in alpha- and beta-band EEG activities is associated with listeners’ arousal and vigilance level. Using a double-blind method, two types of high-resolution audio of the same musical piece (with or without inaudible high-frequency components) were presented while participants performed a vigilance task in the visual modality.

High-resolution audio with inaudible high-frequency components is a closer replication of real sounds than similar and indistinguishable sounds in which these components are artificially cut off. It remains unclear what kind of advantages high-resolution audio might have for human beings. Previous research in which participants listened to high-resolution music under resting conditions have shown that alpha and low-beta EEG powers were larger for an excerpt with high-frequency components as compared with an excerpt without them (Oohashi et al., 2000, 2006; Yagi et al., 2003a; Fukushima et al., 2014; Kuribayashi et al., 2014; Ito et al., 2016). The present study asked participants to listen to two types of high-resolution audio of the same musical piece (with or without inaudible high-frequency components) while performing a vigilance task in the visual modality. Although the effect size is small, the overall results support the view that the effect of high-resolution audio with inaudible high-frequency components on brain activity reflects a relaxed attentional state without conscious awareness.

We found greater high-alpha (10.5–13 Hz) and low-beta (13–20 Hz) EEG powers for the excerpt with high-frequency components as compared with the excerpt without them. The effect appeared in the latter half of the listening period (200-400 s) and during the 100-s period after music presentation (post-music epoch). Furthermore, for full-range sounds compared with high-cut sounds, Go trial P3 amplitude increased, and subjective relaxation scores were greater. Because task performance did not change across musical excerpts, with no difference in self-reported arousal, the effects of high-resolution audio with inaudible high-frequency components on brain activities should not reflect a decrease of listeners’ arousal level. These findings show that listeners seem to experience a relaxed attentional state when listening to high-resolution audio with inaudible high-frequency components compared to similar sounds without these components.

It has been shown that listening to musical pieces increases EEG powers of theta, alpha, and beta bands (Pavlygina et al., 2004; Jäncke et al., 2015), and that the enhanced alpha-band power holds for approximately 100 s after listening (Sanyal et al., 2013). Therefore, high-resolution audio with inaudible high-frequency components would be advantageous compared to a similar digital audio in which these components are removed, in terms of the enhanced brain activity. Kuribayashi and Nittono (2014) have localized the intracerebral sources of this alpha EEG effect using standardized low-resolution brain electromagnetic tomography (sLORETA). The analysis revealed that the difference between full-range and high-cut sounds appeared in the right inferior temporal cortex, whereas the main source of the alpha-band activity was located in the parietal-occipital region. The finding that the alpha-band activity difference was obtained in specific but not whole regions is suggestive that this increase may reflect an activity related to task performance rather than a global arousal effect (Barry et al., 2007).

The present study shows that not only high-alpha and low-beta EEG powers but also P3 amplitude increased in the last half of the listening period (200-400 s). Alpha-band EEG activity and P3 amplitude have been shown to be positively correlated, in such a way that prestimulus alpha directly modulates positive potential amplitude in an auditory equiprobable Go/NoGo task (Barry et al., 2000; De Blasio et al., 2013). P3 amplitude is larger when greater attentional resources are allocated to the eliciting stimulus (Kok, 1997, 2001; Polich, 2007). Alpha power is increased in tasks requiring a relaxed attentional state such as mindfulness and imagination of music (Cooper et al., 2006; Schaefer et al., 2011; Lomas et al., 2015). Increased alpha power is thought to be a signifier of enhanced processing, with attention focused on internally generated stimuli (Lomas et al., 2015). Beta power has been shown to increase when arousal and vigilance level increase (e.g., Sebastiani et al., 2003; Aftanas et al., 2006; Gola et al., 2012; Kamiñski et al., 2012). Taken together, the EEG and ERP results support the idea that listening to high-resolution audio with inaudible high-frequency components enhances the cortical activity related to the attention allocated to task-relevant stimuli. Although the effect was not observed in behavior, the gap between behavioral and EEG and ERP results is probably due to the ceiling effect of the vigilance task performance. Such a gap is often observed in other studies. For example, Okamoto and Nakagawa (2016) similarly reported that event-related synchronization in the alpha band during working memory task was increased 20–30 min after the onset of the exposure to blue (short-wavelength) light, as compared with green (middle-wavelength) light, while task performance was high irrespective of light colors.

As a mechanism underlying the effect of inaudible high-frequency sound components, we speculate that the brain may subconsciously recognize high-resolution audio that retains high-frequency components as being more natural, as compared with similar sounds in which such components are artificially removed. A link between alpha power and ratings of ‘naturalness’ of music has been reported. When listening to the same musical piece with different tempos, alpha-band EEG power increased for excerpts that were rated to be more natural, the ratings of which were not directly related to subjective arousal (Ma et al., 2012; Tian et al., 2013). As high-resolution audio replicates real sound waves more closely, it may sound more natural (at least on a subconscious level) and facilitate music-related psychophysiological responses.

Our findings have some limitations. First, because we used only a visual vigilance task, it is unclear whether high-resolution audio can improve performance on tasks that involve working memory and long-term memory. Because a vigilance task is relatively easy, our participants were able to sustain high performance. Other research using an n-back task requiring memory has shown that high-resolution audio also enhances task performance (Suzuki, 2013). Future research will benefit from using other tasks requiring various cognitive domains and processes.

Second, the underlying mechanism of how inaudible high-frequency components affect EEG activities cannot be revealed by the current data. It is noteworthy that presenting high-frequency components above 20 kHz alone did not produce any change in EEG activities (Oohashi et al., 2000). Therefore, the combination of inaudible high-frequency components and audible low-frequency components should be a key factor that causes this phenomenon. A possible clue was obtained by a recent study of Kuribayashi and Nittono (2015). Recording sound spectra of various musical instruments, they found that high-frequency components above 20 kHz appear abundantly during the rising period of a sound wave (i.e., from the silence to the maximal intensity, usually less than 0.1 s), but occur much less after that. Artificially cutting off the high-frequency components may cause a subtle distortion in this short period. It will take some time to accumulate these small, short-lasting differences until they produce discernible psychophysiological effects. This explanation is consistent with the fact that the effect of high-frequency components on EEG activities appears only after a 100–200-s exposure to the music (Oohashi et al., 2000, 2006; Yagi et al., 2003a; Fukushima et al., 2014; Kuribayashi et al., 2014; Ito et al., 2016).

Third, it remains unclear why there was a time lag until the effects of high-resolution audio on brain activity show up, and why this effect was maintained for 100 s after music stopped. A possible reason is that, as mentioned above, sufficiently long exposure is needed for the effects of inaudible high-frequency components. Another possibility is that listening to music has psychophysiological impact through the engagement of various neurochemical systems (Chanda and Levitin, 2013). Humoral effects are characterized by slow and durable responses, which might be underlying the lagged effect of high-resolution audio with inaudible high-frequency components. Although the present study did not reveal this effect on autonomic nervous system (HR and HRV) indices during music listening, participants reported greater relaxation scores after listening to high-resolution music with inaudible high-frequency components. It is a task for future research to determine the time course of the effect more precisely.

Fourth, the present study did not manipulate the sampling frequency and the bit depth of digital audio. High-resolution audio is characterized not only by the capability of reproducing inaudible high-frequency components but also by more accurate sampling and quantization (i.e., a higher sampling frequency and a greater bit depth) as compared with low-resolution audio. If the naturalness derived by a closer replication of real sounds affects EEG activities, the sampling frequency and the bit depth would do too regardless of whether the real sounds feature high-frequency components. This idea would be worth examining in future research.

In summary, high-resolution audio with inaudible high-frequency components has some advantages over similar and indistinguishable sounds in which these components are artificially cut off, such that the former type of digital audio induces a relaxed attentional state. Even without conscious awareness, a closer replication of real sounds in terms of frequency structure appears to bring out greater potential effects of music on human psychophysiological state and behavior.

This study was carried out in accordance with the recommendations of The Research Ethics Committee of the Graduate School of Integrated Arts and Sciences in Hiroshima University. All participants gave written informed consent in accordance with the Declaration of Helsinki. The protocol was approved by The Research Ethics Committee of the Graduate School of Integrated Arts and Sciences in Hiroshima University.

RK and HN planned the experiment, interpreted the data, and wrote the paper. RK collected and analyzed the data.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.