Influence of electrode site on EABR latency

The latencies of the various EABR responses were compared using 2-way ANOVAs for electrode site versus duration of deafness. This enabled us to investigated whether the possible relationship between latency and electrode site was influenced by pathological factors. The parameters cited above were separated into groups in such a way that each group contained the same number of data or subjects. For the stimulation site factor, four groups with four electrodes each, were defined: group 1 consisted of electrodes 1, 2, 3 and 4; group 2 of electrodes 5, 6, 7, and 8; group 3 of electrodes 9, 10, 11, and 12; and group 4 of electrodes 13, 14, 15, and 16. Two groups of subjects were also defined according to the duration of auditory deprivation before implantation: subjects whose deafness had lasted less than 7 years were in group 1 and subjects with more than 7 years of deafness in group 2. Those two groups were already presented in Table I. For waves IIIe and Ve, latencies were significantly influenced by the stimulation site (respectively F = 17.8, df = 3, p < 0.001 and F = 8.4, df = 3, p < 0.001). The mean latency value for all electrodes increased from apex to base by 0.49 ms for wave IIIe and by 0.43 ms for wave Ve, with standard errors being respectively ± 0.15 and ± 0.12. However, for interval IIIe-Ve latency, there was no effect of stimulation site (F = 1.4, df = 3, p > 0.1). When averaged, it remained roughly equal at 1.8 ms, with ± 0.03 of standard error. There was no significant interaction between the duration of deafness and the electrode site for latency of wave IIIe (F = 1.3, df = 3, p = NS), wave Ve (F = 0.7, df = 3, p = NS), and interval IIIe-Ve (F = 0.1, df = 3, p = NS). ANOVAs’ normality test was passed for analysis about wave IIIe (p > 0.05), wave Ve (p > 0.05), and interval IIIe-Ve (p > 0.05), while the equal variance test failed for waves IIIe (p ≤ 0.001) and Ve (p ≤ 0.001) and passed for interval IIIe-Ve (p > 0.05).Post hoc comparisons (Student-Newman-Keuls test) for electrode site were significant for electrode site group 4 versus 1, 2, and 3 and electrode site group 1 versus 2 and 3 for latencies of wave IIIe (p < 0.01); for wave Ve, latencies were significantly different for electrode site group 4 versus 1, 2, and 3 (p < 0.05). Figure 4 illustrates across subject averages of latency as a function of stimulation site for wave IIIe (4a), wave Ve (4b), and interval IIIe-Ve (4c). The slope estimated by linear regression of latency plotted against the stimulation site was 26 µs per mm of cochlea (p < 0.05) for wave IIIe, 27 µs per mm of cochlea (p < 0.05) for wave Ve, and 0.4 µs per mm of cochlea (p = NS) for interval IIIe-Ve. Nevertheless, EABR latency did not vary the same way according to electrode site in all subjects. Figure 5 shows trends in the effects of stimulation site in Subject 2 (5a) in whom latencies of waves IIIe and Ve did not change along the array and in Subject 4 (5b) in whom the stimulation site had some effect on waves IIIe and Ve latencies, which shortened at the apex. Latencies of Subjects 6 and 8 did not vary in a monotonic way according to the stimulation site either.

Figure 4. Mean latencies of waves IIIe, Ve, and interval IIIe-Ve obtained from all the subjects as a function of the stimulation site. Vertical bars show the standard error around each mean. Linear regression was applied on mean values: the equation of the resulting line is y = 0.026x + 2.03 with R² = 0.79 (p < 0.005) for wave IIIe, y = 0.027x + 3.82 with R² = 0.84 (p < 0.005) for wave Ve, and y = 0.0004x + 1.79 with R² = 0.00 (p = NS) for interval IIIe-Ve. * represents significant differences among groups of electrodes (p < 0.05).
Figure 4. Mean latencies of waves IIIe, Ve, and interval IIIe-Ve obtained from all the subjects as a function of the stimulation site. Vertical bars show the standard error around each mean. Linear regression was applied on mean values: the equation of the resulting line is y = 0.026x + 2.03 with R² = 0.79 (p < 0.005) for wave IIIe, y = 0.027x + 3.82 with R² = 0.84 (p < 0.005) for wave Ve, and y = 0.0004x + 1.79 with R² = 0.00 (p = NS) for interval IIIe-Ve. * represents significant differences among groups of electrodes (p < 0.05).
Figure 5. EABR waveforms for all electrodes stimulated at a comfortably loud intensity in two subjects. The stimulation site seems to have no effect on waves IIIe and Ve latencies of Subject 2 (figure a). In Subject 4 (figure b), the latencies of waves IIIe and Ve decrease for more apical stimulation sites. The etiologies were perinatal asphyxia for Subject 2 whose deafness last 17 years and stroke for Subject 4 whose deafness last 5 years. The intensities of stimulation expressed in clinical units (CU) are indicated next to the electrode of stimulation.
Figure 5. EABR waveforms for all electrodes stimulated at a comfortably loud intensity in two subjects. The stimulation site seems to have no effect on waves IIIe and Ve latencies of Subject 2 (figure a). In Subject 4 (figure b), the latencies of waves IIIe and Ve decrease for more apical stimulation sites. The etiologies were perinatal asphyxia for Subject 2 whose deafness last 17 years and stroke for Subject 4 whose deafness last 5 years. The intensities of stimulation expressed in clinical units (CU) are indicated next to the electrode of stimulation.