BRAIN RESPONSES TO DIGITAL MUSIC: AN FNIRS STUDY
- LanguageENG
- URLhttp://db.koreascholar.com/Article/Detail/351222
pp.693-695
The young generation that was born in the digital age grew up with digital technologies; they listen to music online on web sites like YouTube, which provides access to music by artists from all over the world. We conducted a functional near-infrared spectroscopy (fNIRS) experiment with fifty-six young adults. The human brain generates electrical waves as long as it lives. Since the dynamic nature of brain rhythm is at work in all kinds of human brain function, neuroscientists have used brain rhythm to understand brain function. Since the work of Gerstein and Mandelbrot (1964), many attempts have been made to use random-walk analyses to account for brain responses like the spiking of neurons, cell migration, and motor variability. Like any other biological system, the brain pursues functional efficiency at all levels of operation—in the brain’s case, from the neuronal cell level to the neural network level. Before one can determine the presence of a periodic rhythm versus a random state in brain activation, one must determine whether external stimuli can shape the brain’s modulation pattern. Brain-wave patterns are affected by whether the neural circuit that governs a particular set of brain functions reaches a significant level of activation. The bottom-up processing of external stimuli can be affected by top-down processing; in other words, the execution of higher-order cognitive attention can affect the degree of randomness in the bottom-up processing of external sensory inputs like that of music. Unlike EEG signals, the rhythms of hemodynamic signals are not commonly calculated, possibly because hemodynamic signals are sluggish. The random-walk test on neural time series has been applied only recently to magnetoencephalography (MEG) data (Kipiński, König, Sielużycki, & Kordecki, 2011), and it has rarely been applied to hemodynamic signals measured with magnetic resonance imaging (MRI) or near-infrared spectroscopy (NIRS). However, since hemodynamic responses are the result of neurovascular coupling—a dynamic event among the brain’s neurons, glias, and vasculatures—it is possible to calculate the degree of randomness of hemodynamic signals as surrogates for neuronal activity. While brain activities are inherently random and noisy in their natural state, when the brain rhythm is modified by music that provides appropriate levels of sensory stimulation, the brain’s signals will begin to reflect the music’s rhythms. This reflection is called “attunement.” The effect of sensori-neural stimulation on hemodynamic responses measured by fNIRS has been reported in neuroscience research that found that auditory stimulation and music elevated the concentrations of oxygenated hemoglobin (HbO2) and total hemoglobin (HbT) in blood flow to certain regions of the brain (Hoshi & Tamura, 1993; Kotilahti et al., 2010; Sakatani, Chen, Lichty, Zuo, & Wang, 1999). However, studies have shown decreases (increases) in children’s (adults’) prefrontal cerebral volume after they play computer games. For example, one study suggested that the level of attention may modulate the directional changes in HbO2 and HbT concentrations (Nagamitsu, Nagano, Yamashita, Takashima, & Matsuishi, 2006). The brains of children who find that a game lacks adequate levels of perceptual stimulation do not require an additional supply of oxygen, but adults who find playing the same game a cognitive challenge require more effort to perform the same task, so they require elevated levels of oxygen in their brains (Ferreri et al., 2014). According to musical theorists, when the brain is entrained, the attention follows the music (London, 2012). When members of the digital generation listen to music, the perceptual stimulation level is likely to related to the degree of randomness in brain responses as well as the quality of the sensory experience. Drawing on the literature review, we predict that TBF is higher for a stimulus that is above OSL than the TBF for a counterpart stimulus that is below OSL. We also predict that the hemodynamic rhythm of related brain regions to music that is above OSL adopts a regular predictable pattern. Hence, we propose the following research hypotheses:
H1: Digital music that provides acoustic stimulation near the OSL creates brain responses in the form of higher TBF and lower randomness in HbO2 concentrations than does digital music that provides acoustic stimulation that is below the OSL.
Functional Near-infrared Spectroscopy
The transparency of biological tissue to light in the near-infrared wavelengths makes NIRS possible. NIRS is non-invasive and portable, and it has a cost advantage. The incident near-infrared light from a transmitting optode (source) is scattered through the tissues, and the reflected light is detected by a receiving optode (detector). The amount of the source light that a tissue absorbs depends on the light’s wave length, and the oxygenation status determines the brain’s absorption of the light. The loss of the intensity that is due to the absorption of the photons can be measured in units of optical density (Zaramella et al., 2001). The changes in [oxy-Hb] and [deoxy-Hb] can be calculated according to the modified Beer-Lambert law (Kocsis, Herman, & Eke, 2006) using two wave lengths of near-infrared light—in our case, 780 and 850nm. We used a 12-channel wireless fNIRS system (Biomedical Optics Lab, Korea University) with sampling rates of 8 ~10 Hz to measure the participants’ hemodynamic response while they watched the videos. The system consists of three light sources and five detectors (a 3×7 grid). The fNIRS probe was attached to each subject’s forehead. The detectors of the lowest line were set along the Fp1 and Fp2 electrode line according to the international 10/20 system. Measurements from channels 1, 2, 11, and 12, which contained noise from movements of the subjects’ heads, hair, and sweat, were excluded from further analysis. Neuroscience research has recorded acoustic stimulation in various regions of the brain, including the temporal brains of newborn babies (Hoshi & Tamura, 1993) and the frontal brains of adults (Sakatani et al., 1999). Since infrared light cannot penetrate hair, brain regions that are not covered by hair, such as the prefrontal cortex, are well-suited to an fNIRS study. When a member of the digital generation listens to music online, the motivation is usually enjoyment, so brain activity changes that are due to popular music should occur in brain areas that are associated with reward-related processing—that is, the medial pre-frontal cortex (mPFC) (Haber & Knutson, 2010). As the mental function of pleasurable experience that is modulated in the medial frontal cortex increases, TBF to this region increases. In particular, the processing of sounds is dominantly modulated by the brain’s right hemisphere (Kaiser, Lutzenberger, Preissl, Ackermann, & Birbaumer, 2000). We analyzed TBF to the right brain area of the mPFC at channel 5 using fNIRS. The research hypothesis predicts that songs that provide strong sensory stimulation above the OSL increase the TBFs of those in the digital generation more than do songs that present a sensory stimulation level that is much under the OSL. TBF can be directly obtained as a product of HbT (Wyatt et al., 1990). After the music began in the experiment, the subjects’ concentrations of HbT increased until HbT reached its peak at around five to eight seconds; then it decreased for the next thirty seconds. There was a divide of hemodynamic responses between the two songs that had more than a million hits per day (A and B) and the remaining three (C, D, and E). We conducted repeated measures of ANOVA on TBF, measured at forty seconds, for the five songs, since TBF at the end of each set of song segments can represent the digital generation’s level of sustained attention. The multivariate test for the model was significant, and the main effect of songs on TBF was significant (Wilk’s Lambda 0.75, F(4,51)=4.249, p=0.005, ). After the forty seconds of each song were over, the TBF levels remained at the highest level for song A, followed in order by Songs B, C, D, and E. Pairwise comparisons after the Bonferroni correction showed that there was a significant divide in the length of time that the TBF levels remained at the highest level between songs A/B and songs C/D/E (p<0.05). Other differences were not significant, possibly because the neural data contained large individual difference variances (between-subject F test: F(1,54) =38.501, .p<0.001, ). The results support hypothesis 1. Next, we examined the relationships between TBF and daily hits, and BORP and daily hits. The Pearson correlation coefficient between TBF and daily hits was 0.88 (p<0.05), and the correlation coefficient between BORP and daily hits was -0.96 (p<0.05). Pairwise comparisons after the Bonferroni correction showed that, there was a significant divide in the length of time that the TBF levels remained at the highest level between songs A/B and songs D/E (p<0.05). Other differences were not significant, possibly because neural data contains a large portion of individual variance (between-subject F test: F(1,54) =372.675, p<0.001, ). These results are consistent with our hypothesis 1b that pop music that presents stimulation above OSL can reduce the randomness in hemodynamic signals. The changes in the participants’ hemoglobin concentrations while they listened to popular songs show a mean-reverting tendency with low BORP—a “rhythm” such that a system recovers order and balance in due time. The brain’s response to less popular songs were random-walk processes, which represents a neural drain, a process in which brain fails to recover from oxygen depletion because of boredom. In conclusion, we found that total blood flow to the right medial prefrontal cortex increased less when the young adults were exposed to music that presented acoustic stimulation near the optimal sensory load (OSL) than it did when they were exposed to songs with a level of stimulation much below the OSL. The degrees of hemodynamic randomness decreased significantly while the participants listened to online music that provided near-OSL stimulation. Online popularity, recorded as the number of daily hits, was significantly positively correlated with the total blood flow and negatively correlated with hemodynamic randomness. These findings suggest that a new digital media strategy may be required that provides a sufficient level of sensory stimulation as an essential part of marketing to the digital consumer generation.