Effects of pure-tone sound, impulse noise, and vibration on visual orientation

Eye movements and the electroencephalogram (EEG) were recorded in intact rabbits during an optokinetic test when the animals were exposed to pure-tone sound (85 dB at 4,000 Hz), impulse noise (159 dB), and vibration directed to the abdomen (at an amplitude of 0.9 mm at frequencies of 40 to 140 Hz). The frequency and velocity of optokinetic nystagmus significantly increased in response to these stimuli. The increase seen with vibration was greater than that resulting from sound, and the response was strongest when sound and vibration were combined. The increase of optokinetic nystagmus seen with induced vibration was progressive and dependent on the frequency. The increase was weakest during vibration at 40 Hz and strongest during vibration at 140 Hz. Electroencephalograms (EEGs) of the amygdaloid complex, dorsal hippocampus, midbrain reticular formation, and frontal motor cortex all were activated during exposure to sound and vibration, but activation of the hippocampal EEG was most closely related to the increase of optokinetic nystagmus. During optokinetic tests, impulse noise regularly triggered nystagmic beats. When the rabbits were not in the test apparatus, nystagmus was produced in response to about 18 per cent of the presentations of impulse noise, while activation of the EEG was constant. Thus, vibration and noise, when excessive, may interfere with visual orientation and hence disturb equilibrium. These findings can be related to the nonspecific dizziness that occurs in aerospace or industrial workers exposed to excessive noise and vibration.