Comparisons of ELAR latency and M levels

The M levels were assumed to reflect the state of degeneration of the part of the auditory pathway related to cochlear regions used to initiate the ELARs. The current would indeed need to be higher to reach a comfortable level for more damaged parts of the auditory pathway. The M levels increasing significantly at the base as shown by figure 4 may thus reflect the fact that the perception of high pitch sounds is generally lost first and that there may be a baso-apical gradient of neural degeneration resulting in greater degradation of distal processes of primary afferent fibres toward the base (Abbas and Brown, 1991). Auditory fibers were indeed shown to be less intact at the base in animals (Ylikoski et al., 1974; Leake and Hradek, 1988; Miller, 1992).

However, figures 5 and 6 show that latencies of ELARs did not increase significantly with higher M levels as was found for EABR latency (Guiraud et al., in press). The comparison between EABR latency and M levels from recipients’ programs at first fitting was possible because stimulation parameters were almost the same resulting in approximately the same intensities of stimulation and the same pattern of neural excitation. It is indeed known that psychophysics are influenced by stimulation parameters such as pulse width (Pfingst et al., 1997). However, the parameters used to initiate the ELARs may have been too different from those of recipients’ programs and may only reflect a global effect of intensity on latency rather than an effect of fiber degeneration. The slight tendency of latencies to increase with lower M levels may then be due to the fact that lower intensities of stimulation lead to less neural synchronization. They may also activate more peripheral places, which results in more synaptic relays than when higher current activates directly more central places, as it has been described in the guinea-pig (Nicolas-Puel et al., 1996).