## 4 Discussion  
Given that emotions modify the characteristics of the phonatory apparatus influencing different speech parameters [15; 16], and several studies in the field of sonobiology have demonstrated the capacity of AAWs to induce changes in cell behavior [34; 35], this investigation sought to identify whether the physical-acoustic changes produced by emotions in speech production could have any effect on cellular dynamics. Among all available emotions, gratitude was chosen because there is an effective method to induce feelings of gratitude in a laboratory setting: gratitude letters [30; 31]. At the level of the brain, reading a letter of gratitude written by the same participant following the protocol of [PERSON] and [PERSON] produced a change in the participant's brain activity. While we cannot conclude that changes in EEG power are uniquely due to gratitude, there were significative differences between reading aloud a fragment of _Don Quixote_ and a personal gratitude letter in the theta, alpha, and beta bands. However, the results obtained are in concordance with several studies that reported neural correlates of gratitude in ACC [27; 28]. Furthermore, the gratitude letter analysis revealed notable differences in valence between reading aloud a modern version of _Don Quixote_ and a personal gratitude letter. Combining EEG measurements and affective text analysis, we showed that the participant's emotional system was positively altered when he was in a state of gratitude. The participant reported to the researcher that he felt \"calmer and  
Figure 6: The impact of emotional, non-emotional, and synthesized voice radiation on the 661W cell line: proliferation assay and Ki-67 protein expression. Confocal microscopy representative image depicts the Ki-67 control condition (**a**) and the Ki-67 emotional human voice condition (**b**). A passive vibro-acoustic treatment (PVT) was installed in the CO2 incubator test. 661W cells seeded at a density of 20,000 cells/well and incubated for 72 h with the electro-acoustic system turned off (control group) and gratitude voice (emotional group). All acoustic stimuli were radiated using a loudspeaker-based system with an equivalent level of 80 dB. Cells were imaged with a laser-scanning confocal microscope at 40\(\times\) magnification. Scale bar, 20 \(\upmu\)m. (**c**) Absorbance values at 620 nm for crystal violet staining of 661W cells, seeded at a density of 2000 cells/well, and incubated for 72 h without acoustic stimuli (control group), with gratitude voice (emotional group), with _Don Quixote_ text voice (non-emotional group), and with gratitude synthesized voice (synthesized group). Results are the mean \(\pm\) standard deviation of three independent experiments (12 replicates per plate). **\(p\)**-value = 0.007. AU, arbitrary units. (**d**) Nuclear luminance values per pixel analyzed on a cell-by-cell basis for Ki-67 expression in the control and emotional conditions (n = 127 cell nuclei samples were analyzed in the control condition, and n = 172 in the emotional condition). Results are the mean \(\pm\) standard deviation of three independent experiments. ****\(p\) = 0.000007. AU, arbitrary units. happier\" after writing and reading the gratitude letter. As a result, we support [PERSON] and [PERSON]'s hypothesis that writing a gratitude letter can promote a positive emotional state. Before performing the biological assays, a PVT was installed in the CO\({}_{2}\) incubator, which provided better acoustic control of the environment and reduced background noise from electro-mechanical systems and actuators. The signal-to-noise ratio was significantly improved, reaching values of 10 dB at 250 Hz, 5 dB at 1000 Hz, and 20 dB at 5 KHz. Biological assays showed a significant increase in the proliferation of 661W cells after exposure to the gratitude voice compared to the control group. This result is consistent with the findings from the crystal violet and the Ki-67 (well-known proliferation marker [56]) immunocytochemistry assays.