We examined the changes in electromyographic reaction time (EMG-RT) with increases in the number of auditory stimuli that were administered at regular intervals, and confirmed that the factor of forecast influenced EMG-RT when stimulation was carried out at regular intervals. In the present study, we examined whether the factor of forecast influences EMG-RT on the basis of the difference in interstimulus interval (ISI). Ten healthy subjects (eight males, two females; mean age 26.5 ± 5.6 years) participated in this study. The characteristics of the auditory stimulus study were a stimulus frequency of 900 Hz with 15 consecutive stimuli per trial. The subjects were requested to raise their right ankle in response to each auditory stimulus. A total of three different ISIs (900, 2,700, and 4,500 ms) were used in this study. The influence of the increase in the number of stimuli on EMG-RT was examined. At each ISI, EMG-RT showed a tendency to decrease at the 2nd and 3rd stimulus compared with 1st and 2nd stimulus. At an ISI of 900 ms (n=6), EMG-RT observed at the 8th-15th stimuli were significantly decreased compared with that observed at the 1st stimulus, whereas EMG-RT observed at the 2nd-7 th stimuli were not. At an ISI of 2,700 ms, EMG-RT observed at the 3rd-15th stimuli were significantly decreased compared with that at the 1st stimulus. At an ISI of 4,500 ms, EMG-RT observed at the 3rd-6th and 8th stimuli were significantly decreased compared with that at the 1st stimulus, but EMG-RT at 7th and 9th, 15th stimuli were not. Four subjects, whose muscle activities were recorded before the input of the auditory stimulus, were excluded from the analysis at 900-ms ISI. In the remaining six subjects, forecast was initiated following stimulus by recognition of ISI at the 3rd stimulus and was adjusted to the frequency during the 3rd-7th stimuli. We consider that the results at the 8th or subsequent stimuli may be affected by the factor or automation and the results at 2,700-ms ISI have been affected by continuation of attention to the stimulation as well as the forecast of the following stimulation. At the 4,500-ms ISI, the forecast of the following stimulation is difficult because recognition of periodic rhythm is difficult; therefore, attention should be paid to the ISI. We hypothesize that the attention continued up to the 8th stimulus and attention at the 8th or subsequent stimuli was diverted, which may have affected the results.
