As a part of the study to elucidate the influence of rhythmical sound on the central nervous system activity, the effect on the threshold of arousal reaction and evoked muscular discharge was investigated in the rabbit and the following observed.
1) The threshold of the arousal reaction, appearing in the cerebral cortex EEG and the hippocampus EEG and the muscular discharge in the fore- and hind limbs, due to stimulati on of the midbrain reticular formation (RF) with 100 Hz, was increased by the rhythmical sound of 60/min beat frequency and 500 Hz pitch.
2) The threshold of the arousal reac tion and the evoked muscular discharge was decreased by the rhythmical sound of 450/min beat frequency and 1000 Hz pitch.
3) The decrease or increase in the threshold of the arousal rea ction and evoked muscular discharge following sonic stimuli (500 Hz 60/min or 1000 Hz 450/min), was abolished by destruction of the nucleus ventralis anterior or nucleus reticularis, which belong to the diffuse thalamocortical projection system.
From these results, it w as concluded that excitability of the brain stem reticular formation is inhibited by rhythmical sound of a low frequency and low pitch (500 Hz 60/min) and facilitated by rhythmical sound of a high frequency and high pitch (1000 Hz 450/min). These findings suggest that the decrease in excitability of the RF by rhythmical sound of a low frequency and low pitch and the increase in excitability of the RF by rhythmical sound of a high freguency and high pitch can be attributed to a rise in indirect inhibition and a decrease in indirect inhibition of the RF, respectively, via the diffuse thalamocortical projection system.
