In situations of stress, the momentary loss of the ability to perceive the sounds of the environment is common. Scientists at the Paulista State University (Unesp) in Brazil, in collaboration with colleagues at Oxford Brookes University in the United Kingdom, have made a discovery that may help to understand this phenomenon.
According to this study, published in the journal Scientific Reports, brain activity related to hearing attention follows the rhythm of the heart. Thus, the change in the heart rate induced by stress compromises the auditory perception. This finding opens new perspectives for the treatment of attention and communication disorders.
This research was supported by FAPESP – Foundation for Scientific Research Support of the State of São Paulo and with the collaboration of Brazilian scientists linked to the University of São Paulo (USP) and the Faculty of Medicine of ABC.
"We found that even low levels of stress are capable of altering the heart rate and thus compromising auditory attention," said Vitor Engrácia Valenti, a professor at the Unesp campus in Marília and coordinator of the research. .
In situations of stress, the momentary loss of the ability to perceive the sounds of the environment is common. (Photo: Viviane B. de Góes)
According to Engrácia Valenti, in studies published in recent years, it has been stated that the auditory stimuli are capable of inducing oscillations in the heart rate and that the driver of these alterations is the vagus nerve.
This nerve, which crosses a large part of the body, going from the brain to the abdomen, and which performs motor and sensory functions, decreases the heart rate when activated. In the same way, it participates in the activity of the parasympathetic nervous system, in charge of stimulating actions like the deceleration of the heart beat.
In previous animal studies, it was observed that the activity of the vagus nerve increases during the relaxation of the auditory stimulation and activates the expression of a protein called c-Fos in the auditory cortex. This finding indicated an association between sound processing in the cerebral cortex and the parasympathetic nervous system, explained Engrácia Valenti.
"However, the influence of sound stimuli on heart rate control by the vagus nerve and whether there is an interaction between heart rate control and cortical cerebral activity related to auditory attention in humans was unclear," he said.
To elucidate these questions, the researchers conducted an experiment with 49 women in whom the heart rate regulation was subjected to an overload induced by a mild stress test.
This test consisted of saying as many words as possible in Portuguese that began with the letter A, without repeating them or using them in augmentative or diminutive, in 60 seconds. The test was limited at that time so there was no interference of the sympathetic nervous system – which stimulates actions to respond to stressful situations, such as the acceleration of the heart rate, the effects of adrenaline and the release of cortisol. in the volunteers' brain activity.
The heart rate and brain activity variability of the participants was evaluated using the test known as long-latency auditory evoked potential (P300) before and after the test.
The variability of the heart rate allows to measure the autonomic control of the heart rhythm at varying levels of stress. On the other hand, the long-latency auditory evoked potential allows the analysis of the level of auditory attention to a sound stimulus, monitoring the activity of the prefrontal cortex and the auditory cortex, with electrodes placed in the region of the frontal bone and the joints of the parietal and frontal bones.
The results of the tests indicated that the low level of stress to which the volunteers were submitted was sufficient to alter the heart rhythm, and this occurred simultaneously to the attenuation of the auditory attention measured by the long auditory evoked potential. latency
Statistical analysis of correlation and linear regression showed that the autonomic control of the heart through the activity of the vagus nerve and the cerebral processing of the auditory stimuli work in harmony.
"This indicates that in situations of stress, the auditory information is processed worse than if the person were in a calmer state," said Engrácia Valenti.
"Thus, in a stressful situation, breathing slower, for example, it is possible to activate the parasympathetic nervous system and thereby decrease the heart rate and improve the perception of auditory information," he suggested.
In the opinion of the researcher, this finding opens new perspectives for the treatment of cases related to attention and communication disorders based on activation of the vagus nerve by means of electrical stimuli in the auricular region in order to control the rhythm of the heart.
Studies conducted by researchers from the Department of Speech and Hearing Therapy at Unesp de Marília with autistic children and applying this method showed promising results.
"Data from the studies showed that children with autism experienced a significant improvement in their symptoms through the application of this method of treatment," said Engrácia Valenti. (Source: Agência FAPESP / DICYT)