Threshold adaptation of hearing and vibratory sensation

Bekesy tracings were obtained in three normal subjects for vibratory sense of the midfinger before and after cooling by ethyl chloride, and for hearing levels after pure tone stimulation of the ear. After a mild cooling was applied on the finger, there were no observable differences in Bekesy tracing amplitude width or pulsed stimulus threshold when compared to before-cooling tracings. For steady stimuli, however, there was reduction in the amplitude width only at the top portion of Bekesy spikes without affecting the bottom portion. After a profound cooling, there was an elevation of thresholds for both pulsed and steady stimuli, and the reduction in amplitude widith for steady stimuli. The reduction in amplitude width seemed to appear before threshold shift. During the recovery, these changes returned to original conditions in the reverse order. Similar results were found in hearing after stimulation of the ear by a pure tone. The above findings are explained on the basis of perstimulatory fatigue of rapid form.