Effect of electrode site on the ELAR latency

Neither the latency of N1, P2 nor the interval N1-P2 showed statistically significant differences between stimulation by electrodes 3, 7, 11, or 15. This is consistent with data found at the corresponding frequencies (500, 1000, 2000, and 4000 Hz) by Verkindt et al. (1995) in normal-hearing subjects. The fact that electrical N1 and P2 latencies were not tonotopically organized is also coherent with findings in implantees by Firszt et al. (2002a) with electrodes coding for frequency bands centered on 420, 1185, and 5349 Hz, for which latencies were shown not to vary as a function of electrode site for various stimulation intensities. The influence of electrode stimulation site on EABR latency, which increases for basal stimulation (e.g., Allum et al., 1990; Miller et al., 1993; Guiraud et al., in press), is thus lost when upper parts of the auditory pathway are reached and there is no frequency-related differences in ELAR latency in experienced implant users as in normal-hearing subjects for tones above 500 Hz.

This could enlighten an adaptation of the tonotopic organization of the auditory pathway to electrical stimulation of various cochlear sites. This adaptation would enable the neural information resulting from stimulation of the auditory nerve with various frequencies to reach the upper parts of the auditory pathway at the mean time as in normal hearing subjects. This seems indeed to be an important factor in auditory rehabilitation since better performers were found to have less variable latencies across electrodes by Groenen et al. (1997). Such an adjustment of transmission delays as a function of the stimulation frequency may also occur in normal hearing subjects as, while brainstem response latency is longer for lower pitched sounds (e.g., Stapells et al., 1990; Gorga et al., 1988), cortical response latencies are equal according to Picton et al. (1976) and Verkindt et al. (1995). In any case, stimulation site may not interfere with the potential relationship between latency and auditory pathway integrity, as it does not have any effect on ELAR latency.