Forward Masking in Cochlear Implant Users: Electrophysiological and Psychophysical Data Using Pulse Train Maskers

Abstract

Electrical stimulation of auditory nerve fibers using cochlear implants (CI) shows psychophysical forward masking (pFM) up to several hundreds of milliseconds. By contrast, recovery of electrically evoked compound action potentials (eCAPs) from forward masking (eFM) was shown to be more rapid, with time constants no greater than a few milliseconds. These discrepancies suggested two main contributors to pFM: a rapid-recovery process due to refractory properties of the auditory nerve and a slow-recovery process arising from more central structures. In the present study, we investigate whether the use of different maskers between eCAP and psychophysical measures, specifically single-pulse versus pulse train maskers, may have been a source of confound.In experiment 1, we measured eFM using the following: a single-pulse masker, a 300-ms low-rate pulse train masker (LTM, 250 pps), and a 300-ms high-rate pulse train masker (HTM, 5000 pps). The maskers were presented either at same physical current (Φ) or at same perceptual (Ψ) level corresponding to comfortable loudness. Responses to a single-pulse probe were measured for masker-probe intervals ranging from 1 to 512 ms. Recovery from masking was much slower for pulse trains than for the single-pulse masker. When presented at Φ level, HTM produced more and longer-lasting masking than LTM. However, results were inconsistent when LTM and HTM were compared at Ψ level. In experiment 2, masked detection thresholds of single-pulse probes were measured using the same pulse train masker conditions. In line with our eFM findings, masked thresholds for HTM were higher than those for LTM at Φ level. However, the opposite result was found when the pulse trains were presented at Ψ level.Our results confirm the presence of slow-recovery phenomena at the level of the auditory nerve in CI users, as previously shown in animal studies. Inconsistencies between eFM and pFM results, despite using the same masking conditions, further underline the importance of comparing electrophysiological and psychophysical measures with identical stimulation paradigms.

Neural Adaptation of the Electrically Stimulated Auditory Nerve Is Not Affected by Advanced Age in Postlingually Deafened, Middle-aged, and Elderly Adult Cochlear Implant Users

“…These single-pulse evoked eCAP results do not provide information about the neural representation of long-duration pulsatile stimulation or the effects of prior neural activity on response properties of the AN (e.g., Wilson et al 1997; Matsuoka et al 2000a, 2020b). Using different stimulation paradigms to compare results between electrophysiological and psychophysical measures has been shown to lead to wrong conclusions (Adel et al 2017).…”

The Electrically Evoked Compound Action Potential: From Laboratory to Clinic

“…Recovery from neural adaptation at the level of the auditory nerve can be evaluated by measuring eCAP amplitude in response to the probe pulse at different time points after the masker-pulse-train ceases. Two stimulation paradigms have been used for this purpose (Dhuldhoya, 2013 ; He et al, 2016b ; Adel et al, 2017 ). A schematic illustration of the first paradigm is shown in Figure 9 .…”

“…In addition to this paradigm, the modified alternating polarity paradigm has recently been used to derive ARFs in human CI users. For details of this paradigm, please see Adel et al ( 2017 ).…”

“…The literature related to recovery from neural adaptation of the auditory nerve in CI users is relatively scarce. To date, only three studies have evaluated this specific issue (Dhuldhoya, 2013 ; He et al, 2016b ; Adel et al, 2017 ). Overall, these studies showed that ARFs could consist of up to three components with an initial rapid increase (fast recovery) followed by a rapid decrease (adaptation enhancement) and a second slower increase (slow recovery) in eCAP amplitude (Dhuldhoya, 2013 ; He et al, 2016b ).…”

“…High masker level or low probe level yields longer adaptation recovery in both adult and pediatric CI users (Dhuldhoya, 2013 ). At a fixed current level, increasing pulse rate yields long recovery from neural adaptation (He et al, 2016b ; Adel et al, 2017 ). Preliminary data reported by He et al ( 2016b ) indicated that auditory nerve fibers in older CI users might have slower adaptation recovery than those of young CI patients.…”