In order to investigate the reliability of 40Hz event related potential (40Hz ERP) in audiometry, discrepancy between the 40Hz ERP and the behavioral threshold were compaired in normal adults, normal children, adults with hearing disorders and children with hearing disorders. Though 40Hz ERP measurements for adults were performed in awake state, the measurements in children were performed in asleep. For the acoustic stimuli, 0.5, 1 and 2kHz tone pips were given with repetition rate of 40/sec. A total of 1024 sweeps were averaged in each series with a window length of 50msec. For smoothing wave form, 80-120Hz bandpass analog filter and 20-80Hz bandpass digital filter by means of FFT algorithm were used. The results were as follows; (1) The mean differences between 40Hz ERP thresholds and behavioral thresholds were 0-30dB in adults and 10-50dB in children. (2) The correlation coefficients between 40Hz ERP thresholds and behavioral thresholds in patients with hearing disorders were 0.77-0.8 in adults and 0.63-0.67 in children. (3) These results shows that the 40Hz ERP is useful for hearing evaluation in adults, but test reliability is not enough for determining accurate hearing in children.
The author studied the influence of brain ischemia on ABR, the author used Mongolian gerbils which have an insufficient Willis'brain circulation. At first, the right cochlea was destructed but the left auditory pathway was kept intact. The right common carotid artery (CA) was ligated to produce ischemia of the right brain hemisphere. The unilateral auditory pathway was confirmed by the evoked potentials of the inferior colliculus. The unilateral brain ischemia was determined by the abnormal movement, dyeing (2% Evans blue) of brain artery, and pathological specimens. Finally, the ABR of 8 animals was recorded successively from the ligation of the right CA to death. During 3 hours of ischemia, III-V waves of ABR were slightly delayed, but the total pattern of ABR did not change greatly. After 10 hours the I wave remained and the other waves disappeared. The change of ABR seemed not directly influenced by brain ischemia, but inffluenced by the increased intracranial pressure due to brain edema.
Following ABR measurement, tympanogram was recorded and analysed in infants and children during sleep induced by sedatives. In this study, the middle ear cavity pressure in most of the subjects was increased in course of sleep, and we called this type as type +A tympanogram (positive pressure tympanogram). The acoustic reflex threshold on type +A tympanogram was compared with that on type A. The results were as follows. There was no significant difference in threshold between both acoustic reflex of normal hearing subjects. When ear canal pressure was not matched to the point of maximal compliance on tympanogram, acoustic reflex thereshold was elevated or absent. Therfore, on measurement of acoustic reflex threshold, we emphasized that ear canal pressure must be matched to the point of maximal compliance of tympanogram.