Lack of effect of pulsed ultrasound on the mammalian EEG

Anesthetized rhesus monkeys and rabbits were exposed to pulsed ultrasound, from a 1.8 cm diameter transducer placed against the head. Each ultrasonic exposure lasted 4 min; it consisted of a series of 1-2 mu sec pulses, with a frequency spectrum broadly centered about the transducer resonant frequency of 1.5 MHz, repeated at a rate of 950 Hz. The time averaged ultrasonics power was 12.6 mW, with an estimated peak power of ca. 15 watts. No changes were observed in the EEG or its power spectrum during or immediately after the exposure. This contrasts to a previous report of significant changes in the EEG of squirrel monkeys during comparable exposures to pulsed ultrasound. Analysis of the earlier reported results suggests that some of the "effects" were due to aliasing artifacts.