Human equilibrium during acoustic stimulation by discrete frequencies. AMRL-TR-68-7.

The ability of subjects to balance on narrow rails was measured during exposure to pure tones of 100. 260, 500, 1500, and 2500 Hz and a control condition. One group of subjects was presented the test stimulus at intensity levels of 95 dB (re 0.0002 dyne/cm) in the left ear and 75 dB in the right ear (asymmetrical exposure). The other group was presented the tones at a level of 95 dB in both ears (symmetrical exposure), A decrement in rail performance occurred at 1500 Hz for the asymmetrical exposure group and at 590 Hz for the symmetrical exposure group. The decrement determined with symmetrical exposure was less than the decrement found with asymmetrical exposure. This result supports previous findings that asymmetrical exposure has more adverse effects on human equilibrium than symmetrical exposure. Both frequencies at which decrements occirrod have been found in previous studies to have the lowest threshold for vestibular stimulation as determined by a slight shifting of the visual field in normal hearing subjects and by nystagmus measures in deaf individuals. Therefore, the results of the present study are interpreted as a possible demonstration of the dirtct effects of noise on the vestibular system. 1