This paper discusses the influence of genetic factors on speech encoding at the subcortical level. In particular, it focuses on the effects of the serotonin transporter gene. The discussion of the whole topic is briefly outlined. The findings obtained by the authors are pointed out along with the methods used to receive and analyze related information. Finally, the authors’ contribution to the field is considered, and questions for future research are outlined.
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People’s success in music and foreign languages depends significantly on their capacity to perceive sounds and speech. The better the neural representation of the sound signal is, the better results are likely to be achieved (Ansari & Rangasayee, 2016). Thus, high quality of encoding of acoustic features is vital for everyone, and the subcortical auditory system is believed to ensure it.
The inferior colliculus (IC) is the main sound center that receives major serotonergic innervation, which allows presupposing that “serotonin [5- hydroxytryptamine (5-HT)] is crucial for the modulation of information processing in the ascending auditory pathway” (Selinger, Zarnowiec, Via, Clemente, & Escera, 2016, p. 10783). In this way, serotonin can change the peculiarities of the neural response to the perceived sound. In particular, it affects the number, latency, and precision of spikes and the timing of their trains. Thus, it can be stated that serotonin affects the response and encoding of sensory stimuli.
Purpose of the Study
As the authors emphasize the value of serotonin for the accurate subcortical speech encoding, they also pay attention to the serotonin transporter [5-HT transporter (5-HTT)]. It is believed to regulate the availability of serotonin, which proves that its functioning can affect the whole process of encoding, defining people’s auditory capacity. In this way, the serotonin transporter gene is likely to be involved in multiple functions. Based on this information, the authors developed a hypothesis of its involvement in subcortical auditory processing. All in all, Selinger et al. (2016) wanted to find out if “the 5-HTT linked polymorphic region (5-HTTLPR) is involved in speech encoding at subcortical stages” (p. 10783).
Questions Addressed in the Reviewed Paper
In the framework of their research, Selinger et al. (2016) addressed several questions to reveal how the serotonin transporter gene influences the accuracy of subcortical speech encoding. First of all, they revealed their interest in the normal frequency following response (FFR) and cortical responses to the presentation of a syllable /ba/ on the background of a speech bubble. In addition to that, they were interested in the different levels of serotonin transporter expression. The authors also wondered how they are connected with the allelic variation of the 5-HTTLPR. They were interested in the meaning of the higher and lower signal-to-noise ratio (SNR). Selinger et al. (2016) investigated how the periodic response signal differed from its time-shifted part. They were interested in the characteristics of long-latency auditory-evoked potentials. The authors found out the grand average FFRs for responses. The influence of alterations in the consonant-vowel transition and language or musical training was questioned. In addition to that, they wondered if the timing features of stimulus onset asynchrony (SOA) affected sound encoding significantly. All in all, answers to these questions and further analysis of the received data provided the researchers with an opportunity to discuss the influence of the serotonin transportation gene.
A quantitative research study was conducted including a sample of 79 young adults who were mostly psychology students. All of them were from 18 to 31 years old. The majority of participants were females (59 individuals). The authors used opportunistic sampling to gather them, as they represent a part of volunteers who agreed to be engaged in a clinical interview. This selection was not maintained randomly, and the sample remained limited because of the initially identified group of people. In this way, the inclusion criteria were the previous participation in the interview and normal vision and audition. The exclusion criteria were diagnosed with neurological and psychiatric illnesses and drug intake. As a result, 21 individuals did not participate. The representatives of the sample received compensation and gave their informed consent. The research study was approved, and the participants’ anonymity was ensured.
Buccal cell cheek swabs were used to extract DNA and identify short (S) and long (L) alleles for genetic variants of the SLC6A4 gene. “PCR amplification, digestion with the restriction enzyme Mspl, and migration in an agarose gel” were used to identify the 5-HTTLPR and A/G single nucleotide polymorphism (SNP) (Selinger et al., 2016, p. 10784). Based on their genotypes, participants were divided into three groups: those who have low, medium, and high levels of 5-PEE expression.
The Klatt speech synthesizer was used to generate a syllable the brainstem responses to which was collected. Stimuli lasted for 170ms. It was repeated five times using different frequencies. To make the laboratory environment more similar to the real one, speech babble was played in the background. SOA was present at the beginning of the syllable presentation. Randomly, some participants listened to constant SOA and others to jittered one. EEG recordings were made to gather data regarding responses. The electrooculogram (EOG) was also considered.
EEGlab version 7 toolbox was used to analyze data. With the help of component analysis, the signal activity was obtained. The second-order blind identification algorithm was used to identify components in reactions, and the SNR was used to calculate them. Further, responses were averaged.
The main findings of the research study conducted by Selinger et al. (2016) are that “individuals with lower serotonin transporter expression, compared with individuals with medium to high expression had higher signal-to-noise ratios in the frequency following response to the vowel of the syllable (/ba/), as well as higher pitch strength for their subcortical neuronal responses, both pointing toward a sharper speech signal extraction in the subcortical auditory pathway” (p. 10787). The authors claim that the 5-HTTLPR affects the quality of speech encoding. It becomes more robust and accurate already at subcortical stages. In this way, they also believe that the HTTLPR can determine the extent of experience’s influences on the subcortical auditory pathway. In particular, both its structure and function can be affected. Unfortunately, the authors failed to identify whether the HTTLPR manages to be rather influential only due to people’s genetic predisposition or its combination with other genes and experiences matters. Even though the researchers have divided the sample into the low-, medium- high-expressing groups, differences in results between them were not critical enough to consider them. Nevertheless, they believed that low 5-HTT is a benefit for those individuals who are occupied in music or/and languages.
It is also interesting that the time spent under the influence of musical training did not provide any significant difference in the SNR. Thus, even though this experience is considered to be extremely beneficial for the representatives of the general public and their ability of speech encoding, it is not critical for the study and its outcomes. The authors also indicated that the alterations of the pitch strength did not provide any important influence on the research results. The authors emphasize the connection between 5-HTT and neural representation but underline the necessity to interpret the obtained findings with caution.
Role of the New Study
Neurophysiology discusses the way the human nervous system functions, and responses to various sounds are also considered by it. A lot of professionals have already paid attention to the encoding of speech sounds, including Cooper, Brouwer, and Bradlow (2015) for instance. They emphasize the fact that is accuracy plays an enormous role in human communication because it determines their capacity to understand one another. The researchers identified that this process takes place in the subcortical auditory nervous system (Bellier, Bouchet, Jeanvoine, Valentin, & Thai-Van, 2015). If a person has some disorders connected with encoding through hearing or speaking, one is proved to have altered speech and auditory processing. Nevertheless, researchers underline that the ability of encoding is not static. It can be improved with time due to the hard work associated with language and music experience (Krizman, Marian, Shook, Skoe, & Kraus, 2012; Schoof & Rosen, 2016). As a result, it can be concluded that the influence of developmental changes and the environment affects speech encoding significantly.
However, there is also an assumption that people’s capacity to perceive sounds is associated with genetics. Even though several research studies were conducted to reveal this connection, the role of genetic factors in speech encoding is not properly discussed yet. Nevertheless, Selinger et al. (2016) reveal their interest in this unresolved topic. In their article, the authors focus on the serotonin transporter gene and its involvement in speech encoding. Professionals manage to identify their polymorphism and association with the high-quality neural tracking of speech stimuli. In this way, the authors conclude that 5-HTT can limit the influence of people’s experiences (such as those associated with language and music) on the subcortical encoding of speech. However, they also point out that this eventual effect can be provided only in combination with polymorphisms.
In addition to that, Selinger et al. (2016) indicate that the previous studies they used as the basis of their research have already emphasized that serotonin plays a critical role in the auditory perception. However, the value of the 5-HTTLPR for language and music perception was not pointed out (Krizman et al., 2012). Its functions at subcortical stages remained unrevealed, as well as the association between generics and auditory processing. Nevertheless, the connection between auditory and some psychiatric disorders and serotonin was discussed by other professionals. This knowledge was used for exclusive criteria when collecting the sample for the study. All these links were aligned with auditory processing at the cortex while the subcortical area remained uninvolved (Bellier et al., 2015). Similarly, the implication of the 5-HTTLPR in speech encoding has not been considered. Only the impact of serotonin on auditory processing in the IC received enough attention from the researchers. This neuromodulator was said to affect the brain response to auditory stimuli.
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Alternative Explanations for the Conclusions
In many cases, the application of serotonin suppressed the amplitude of responses, which provided the authors with the opportunity to presuppose that it leads to another clear response and the decrease of the representation of noise. However, previous studies do not provide an opportunity to make any claims regarding this issue, because their findings remain rather vague. There is a possibility that the outlined outcomes are just spontaneous activities that happen in some humans but are not generally observed. Thus, there is a possibility that the conclusions made by researchers cannot be generalized and can be applied only to particular populations. Moreover, it was revealed that people’s responses at the cortical level do not differ in two groups. On the one hand, there is a possibility that the effects of 5-HTT can be observed only at the subcortical level, and the authors of the article have accepted this idea. However, on the other hand, it is possible that genetic influences on the 5-HTT expression are not that critical to consider them along with the extraction of cortical potentials. Individual subcortical differences may be observed even without the effects of 5-HTT. They can be developed based on purely environmental factors or condition of their combination with genetic ones, as subcortical processing of sound can be changed under the influence of people’s experience and training. Thus, it is vital to take into consideration the fact that the authors of the discussed article based their research on the layering hypothesis, according to which, both cortical and subcortical processing can be affected by previous experiences and have altered auditory functions.
Questions to Be Discussed
For now, the topic lacks well-grounded information and facts that is why it would be advantageous to conduct additional research studies that discuss associated questions. The authors reveal that 5-HTTLPR can influence the quality of speech encoding. However, the scope of this impact is not yet revealed. In this way, it would be beneficial to conduct a study that discloses this aspect of speech encoding. In addition to that, both previous and recent studies fail to discuss the role of other genes and environmental factors, as only some of them have been already considered by researchers. Moreover, the focus can be made on the early developmental stage of the accuracy of encoding. In particular, attention should be paid to the way people with genetic difficulties for auditory processing. The extent to which their condition can be improved due to music and language training should be discussed.
Thus, it cannot be denied that Selinger et al. (2016) made an enormous contribution to the field of neurophysiology with the help of their research. Even though it was based on assumptions and hypotheses, the authors managed to reach a clear conclusion that provides an opportunity to discuss this topic further. The professionals did not prove the value of purely genetic factors involved in the speech encoding, but their impact in combination with experiences and training cannot be denied. Moreover, they indicate that every person might have a possibility to affect his/her genetic expression with the help of plasticity that depends on experience. In this way, a step forward to the understanding of speech encoding at cortical and subcortical levels is made.
Ansari, M., & Rangasayee, R. (2016). Construction of Hindi speech stimuli for eliciting auditory brainstem responses. Indian Journal of Otolaryngology & Head and Neck Surgery, 68(4), 496-507.
Bellier, L., Bouchet, P., Jeanvoine, A., Valentin, O., & Thai-Van, H. (2015). Topographic recordings of auditory evoked potentials to speech: Subcortical and cortical responses. Psychophysiology, 52(4), 594-599.
Cooper, A., Brouwer, S., & Bradlow, A. (2015). Interdependent processing and encoding of speech and concurrent background noise. Attention, Perception, and Psychophysics, 77(4), 1342-1357.
Krizman, J., Marian, V. Shook, A., Skoe, E., & Kraus, N. (2012). Subcortical encoding of sound is enhanced in bilinguals and relates to executive function advantages. Proceedings of the National Academy of Sciences of the United States of America, 109(20), 7877.
Schoof, T., & Rosen, S. (2016). The role of age-related declines in subcortical auditory processing in speech perception in noise. Journal of the Association for Research in Otolaryngology, 17(5), 441-460.
Selinger, L., Zarnowiec, K., Via, M., Clemente, I., & Escera, C. (2016). Involvement of the serotonin transporter gene in accurate subcortical speech encoding. The Journal of Neuroscience, 36(42), 10782–10790.