Spatial and Temporal Effects of Interleaved Masking in Cochlear Implants

Abstract

Modern cochlear implants utilize interleaved presentation of pulses on different electrodes to avoid physical interference among multiple current fields, yet neural interaction still exists. In the present study, masking was examined with four Nucleus24 users with the banded electrode array in an interleaved masking paradigm, where a probe stimulus was interleaved with a masker stimulus. Spatial and temporal aspects of masking were addressed by fixing the masker at the middle of the electrode array and changing the location of the probe and by testing various stimulation rates: 125, 500, 2,000, and 6,410 Hz. In addition, growth of masking (GOM) was assessed by changing the masker level in six steps. Results indicated that masking patterns were generally much wider, regardless of stimulation rate, than those in acoustic hearing. The amount of masking decreased from the peak at the rate of approximately 0.5 dB/mm even at the highest masker level. The pattern of GOM with the rates higher than 500 Hz was different from that observed in previous masking studies, characterized by markedly shallow growth at low masker levels or overall shallow growth. A facilitating effect of the masker (lowering the threshold) was suspected, except for the 125-Hz condition, due to the fibers that were subliminally excited, but not discharged, by the masker with local perturbations of membrane potentials, and were subsequently discharged easily by a lower level probe when the temporal gap between masker and probe was sufficiently short. These results suggest that both refractory characteristics of neurons and neural summation be considered in interleaved stimulation of pulses at high, but clinically relevant, stimulation rates. Overall, the present masking study might provide a basis for models in psychophysics and speech understanding in current cochlear implant systems utilizing high-rate interleaved stimulation.

Procedural Factors That Affect Psychophysical Measures of Spatial Selectivity in Cochlear Implant Users

“…For interleaved masking, our results suggest that, to obtain a measure of selectivity that is uncontaminated by such effects, and with a single masker, it is necessary to use a pulse rate lower than about 1,000 pps, which would allow for an MPG of 500 µs. This is roughly consistent with the conclusions of Kwon and van den Honert (2009) , who argued that charge summation had a substantial effect on the growth of loudness functions measured with interleaved masking at rates above, but not below, 500 pps. Certainly, in everyday situations, multiple electrodes will be stimulated concurrently, and the interpulse intervals between successive pulses will often be considerably less than 500 µs.…”

“…The majority of experiments on spatial selectivity in CI users has used forward-masking paradigms. An alternative is to present the probe pulses interleaved with the masker pulses ( Azadpour et al., 2013 ; Kwon & van den Honert, 2009 ). A potential limitation of this approach is suggested by evidence from both psychophysical and physiological experiments that the response to, or detection of, a single pulse can be enhanced by the presence of a preceding pulse ( Bierer & Middlebrooks, 2004 ; Nelson & Donaldson, 2001 ; Stypulkowski & van den Honert, 1984 ).…”

Sentence recognition in noise promoting or suppressing masking release by normal-hearing and cochlear-implant listeners

“…The effect can be further complicated by broadened auditory filters in SNHL (Moore and Glasberg, 1986), which would cause masking to occur in the spectral domain as well. These same arguments can be extended to CI listeners, who can exhibit longer recovery times for forward masking (Nelson and Donaldson, 2001) and wide excitation patterns and abnormal growth of masking (Kwon and van den Honert, 2009). A third, more recent suggestion is that a limited coding of fundamental frequency (F0) might also result in reduced masking release in the SNHL population (Summers and Leek, 1998) and in CI users (Stickney et al, 2004).…”

Place specificity measured in forward and interleaved masking in cochlear implants

“…The reason is lateral suppression, a basilar membrane mechanism that suppresses excitation by an off-frequency simultaneous probe and influences measurement of spread of masker excitation in simultaneous masking (Moore, 1978;Oxenham and Plack, 1998). The electrical counterpart of acoustic simultaneous masking is interleaved masking in which the detection threshold of a probe stimulus is obtained in the presence of interleaved pulses of a masker (Kwon and van den Honert, 2009). The effects of interleaved masking are relevant to speech perception with the existing cochlear implant processing strategies that interleave pulses on simultaneously activated electrodes to avoid physical interference of electrical fields.…”

“…Place specificity was estimated from the normalized spatial masking patterns obtained from forward and interleaved masking. The normalized patterns eliminate the influence of absolute amount of masking (McKay, 2012) and the effects of masking decay, which may differ between forward and interleaved masking (Kwon and van den Honert, 2009). Table 1 shows the information of the nine users of Freedom Nucleus implants (Cochlear Ltd.) with the ACE processing strategy who participated in this study.…”