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Interaural latency differences (ILDs) of wave V of the brainstem electric response (BSER) were studied (1) in 20 normal-hearing subjects at 90, 80, 60 and 40 dB HL click level, (2) in 22 patients with symmetrical cochlear hearing loss at 90 dB HL, corresponding to 45-80 dB SL, and (3) in 45 patients with asymetrical cochlear hearing loss with interaural recruitment, at 90 dB HL on one hand and at a click sensation level corresponding to 90 dB HL in the poorer ear (range 10-75 dB SL) on the other hand. In the normal hearing group the ILDs did not exceed 0.2 ms except in one case at 40 dB HL (0.3 ms). In the symmetrical cochlear loss group the ILD did not exceed 0.2 ms in any patient. In the asymmetrical cochlear loss group the ILD at 90 dB HL exceeded 0.2 ms in 12 cases, but only in four cases after correction for pure tone hearing loss at 4 kHz by 0.1 for each 10 dB above 50 dB HL; at equal sensation levels the ILDs were all zero or negative meaning that the wave V latency was shorter on stimulation of the poorer ear. Assuming ILD to be a valid discriminator between cochlear and retrocochlear lesions, the corrected ILD at equal click hearing levels (90 dB HL), with the critical value set at 0.2 ms, thus gave a 9% false-positive rate whereas the ILS at equal sensation levels, with the critical value set at zero, gave no false-positives. It still remains to be shown that the latter criterion will not increase the number of false-negative results in retrocochlear lesions.

The normal click-evoked BSER was studied in a total of 28 young (aged 12 to 40) normally hearing subjects in regard to wave latencies and amplitude ratios, considered to be (besides wave replicability) the characteristics of major interest from a clinical point of view. Latency means and standard deviations (S.D.), and peak-to-peak amplitude ratios between wave V on the one hand and wave I and III, respectively, on the other were determined in a sample of 20 subjects using unfiltered clicks at 20 to 90 dB SL. The latency averages were compared with those of other reports. Latency S.D.s were found to be generally less than 0.25 ms at levels above 50 dB SL. Medians of amplitude ratios (AV/AI and AV/AIII) were found to be significantly greater than unity. No significant latency differences concerning waves I, III, and V could be established on stimulation with condensation, rarefaction, and alternating clicks in 10 subjects at 80 and 60 dB SL, nor did the peak-to-peak amplitude predominance of wave V change with click polarity. With half-octave filtered clicks (2 and 4 kHz), latencies were found to increase significantly (and the responses to become less precise) at 80 and 60 dB SL in a sample of 12 subjects. Changing the interstimulus interval (ISI) from 80 to 60 to 40 ms (at 80 and 60 dB SL) hardly made any difference in regard to I, III, and V latencies, or wave V amplitude supremacy. Finally, a satisfactory test-retest reliability was established in a sample of 6 subjects, the intersession interval being 6 months.

Interpeak intervals of the auditory brainstem evoked electric response (ABR) were measured in 13 patients showing a characteristic slow rhythmic tremor and a more or less marked chronic cerebellar ataxia. Eleven of these patients were advanced alcoholics; two were less severe alcoholics. The tremor appeared primarily in the legs, and displayed distinctive features when examined in appropriate postures. Ten of the patients displayed a significant increase of the I-V interval of the ABR. The occurrence of the slow tremor with abnormal electric brainstem responses may provide objective evidence of lesions affecting extracerebellar regions of the CNS.

Brain stem electric responses (BSER) to clicks at 80, 60 and 40 dB SL (rate 22.5/sec) were recorded from 62 normally hearing subjects grouped according to sex and age. There were four groups: young females (mean age 26.8 years), old females (56.1 years), young males (29.6 years), and old males (59.3 years). The BSERs were measured in respect to peak latencies (I, III, V) and interpeak intervals (I-III, III-V, I-V). Wave replicability was seen to deteriorate with age. Concerning peak, latencies, highly significant differences were established between the group of young females on the one hand, and the other three groups on the other, young females exhibiting shorter latencies (of the order of 0.2 msec); differences between the other three groups were less significant or not significant at all. Concerning interpeak intervals, the differences between groups showed little or no significance. Furthermore, in old subjects--but not in young ones--the individual III-V interval exhibited a significant increase with reduction of click intensity from 80 to 60 dB SL of the order of 0.1 msec. Variances of peak latency and interpeak interval measures were not seen to differ significantly between groups.

Far-field recorded electric brain-stem responses on acoustic stimulation (BSER) were recorded in 29 patients with 31 verified cerebellopontine angle tumors. The responses were compared with the normal response and with responses in cochlear hearing loss by visual observation, considering reproducibility and (supra-liminal) configuration in the first place. There were established (a) obvious differences in response pattern between cochlear and retrocochlear lesions. (b) In the retrocochlear cases, BSER appeared to be more consistent that conventional audiometry, as demonstrated in ears ipsilateral to the lesion, with residual hearing and cochlear or inconsistent auditory findings. (c) BSER also proved able to offer additional diagnostic information by way of the ear contralateral to a retrocochlear lesion, not only in cases where the tumor caused a radiologically visible dislocation of the brain stem.

Auditory brainstem responses to monaural clicks and to binaural clicks delivered with interaural time differences of 0.5 and 1 ms (delayed clicks in left ear) and interaural intensity differences (right ear minus left ear) of 0, +/- 10, +/- 20, and +/- 30 dB were recorded bilaterally in 7 normal subjects. Latencies of wave III and wave V were studied as functions of click intensity difference for each of the two time-of-onset differences. As the intensity difference was gradually varied from +30 to -30 dB, the latencies were seen to shift (with constant III--V interval) from those of a monaural right-ear (non-delayed clicks) response to those of a monaural left-ear (delayed clicks) response by 0.5 and 1 ms. In all subjects this shift occurred in the 20-dB interval between equal intensity and 20-dB lagging-click dominance, and almost always most of the shift took place in either of the two 10-dB subintervals. Occasionally double-peaked waves appeared in the 20 dB-interval. Binaural ABRs may become useful for diagnosis in patients with signs of brainstem disorder but with normal-hearing and normal audiometric findings including monaural ABR, as such patients have been found to shift their latencies more slowly with varying interaural intensity difference.

The basis for the use of click-evoked BSER in neurotologic differential diagnosis would seem to be its sensitivity to retrocochlear but relative resistivity to cochlear lesions. The present study was performed in order to investigate the presumed robustness of BSER in cochlear hearing loss by recording BSER in 110 consecutive cochlear ears with click thresholds not in excess of 60 dB HL. There were eleven ears with rising, twenty-two ears with flat, and 77 ears with sloping audiograms. Click intensities were 80 dB HL and 60 dB SL, and the BSERs were observed for replicability and latencies of waves I, III and V, as well as for I-V and III-V interpeak intervals. Wave V latency increments (relative to normal values) were related to hearing thresholds at 4 kHz. Wave V was identifiable in all of the ears at 80 dB HL and/or 60 dB SL, whereas waves I and III often failed to appear at 80 dB HL and occasionally at 60 dB SL. At 80 dB HL click level, the wave V latency increment was related to the 4 kHz hearing loss (statistically significant at level 0.05 in the HF loss group), increasing by approximately 0.1 ms for each 10 dB, starting at 30 dB HL. Increasing click intensity to 60 dB SL tended to bring waves I and III into appearance and to offset the V latency increase (even inverting it in highly recruiting flat loss ears). Interpeak intervals were not significantly affected by cochlear hearing loss. Unfiltered alternating clicks were used after two initial experiments had shown that nothing would be gained in this study from using filtered clicks or uniphase unfiltered clicks.