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Individual differences in the responses to repeated bell sound bursts of heart rate and plethysmogram from the index finger.

To investigate the possible individual differences in response pattern of the autonomic functions to sound stimulation, simultaneous measurements of the heart rate (HR) and plethysmogram (PLT) from the index finger were performed when bell-sound bursts were presented irregularly as well as regularly.The subjects were 3 healthy male students, ages 19, 19 and 22 years. They were exposed to 3s long bell sound bursts (91.5dBA) presented regularly with intervals of 150s and irregularly with random from 90 to 150s in an insulated room with the background sound level under 30dBA inside. Both of the regular and irregular sessions consisted of 5 blocks of 5 exposures as above. The intervals between blocks were 5min. The irregular session was conducted one week after the regular one.ECG (chest lead, V4) records from -3 to 30s after each 3s long burst were divided into eleven 3s records, and mean HR was calculated from the mean R-R intervals included completely within each 3s record. In the same manner, mean finger-top plethysmogram amplitude for all the complete waves within each time period of 3s before, during and after each bell sound burst were calculated. However, all the responses were examined as percent changes to the pre-exposure 3 s-long values in all the cases.The results of HR responses to sounds showed such great intra- and inter-individual variations that any consistent trends like habituation (gradual decrement of responses with repetition of stimuli) could not be observed. Only in block 1 of the regular session in Subject 1, consistent and typical HR responses were found, which showed rapid tachycardia followed by bradycardia. On the other hand, PLT showed consistent and significant responses especially in the regular session as shown in the figures. Decremental process of the minimum peaks or habituation with repetitive stimulations from block 1 to 5 in the regular session was obvious in Subject 1 and 3, whereas in Subject 2, on the contrary, the minimum peaks increased with repetition of the stimulations.Thus, individual differences in response patterns of the autonomic nervous system functions were confirmed to be possible, which may cause great intra- and inter-individual variation in the autonomic functions especially when only one measurement like HR is adopted as a parameter. However, one cannot deny that a subject, who does not respond in HR to sound, may be significantly susceptible to the same sound in another aspect like peripheral circulation. Therefore, further study is needed to clarify such subtypes of the autonomic functions to investigate the physiological effects of noise as well as to use sound stimulation in a test of the autonomic functions. It should also be noted that habituation or sensitization could be significantly observed in some of the autonomic functions, only if sounds are presented regularly.