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

Abstract

Behavioral measures of spatial selectivity in cochlear implants are important both for guiding the programing of individual users’ implants and for the evaluation of different stimulation methods. However, the methods used are subject to a number of confounding factors that can contaminate estimates of spatial selectivity. These factors include off-site listening, charge interactions between masker and probe pulses in interleaved masking paradigms, and confusion effects in forward masking. We review the effects of these confounds and discuss methods for minimizing them. We describe one such method in which the level of a 125-pps masker is adjusted so as to mask a 125-pps probe, and where the masker and probe pulses are temporally interleaved. Five experiments describe the method and evaluate the potential roles of the different potential confounding factors. No evidence was obtained for off-site listening of the type observed in acoustic hearing. The choice of the masking paradigm was shown to alter the measured spatial selectivity. For short gaps between masker and probe pulses, both facilitation and refractory mechanisms had an effect on masking; this finding should inform the choice of stimulation rate in interleaved masking experiments. No evidence for confusion effects in forward masking was revealed. It is concluded that the proposed method avoids many potential confounds but that the choice of method should depend on the research question under investigation.

Spatial Selectivity in Cochlear Implants: Effects of Asymmetric Waveforms and Development of a Single-Point Measure

“…The probe had a total duration of 10 ms and consisted of three pulses at a rate of 344 pps. This low rate was chosen to be substantially lower than that used for the masker (see below) so as to avoid “confusion” effects that may occur when the masker and probe are presented to the same electrode and are otherwise identical, leading to the probe being mistaken for a continuation of the masker (Neff 1985 ; Cosentino et al 2015 ).…”

“…McKay 2012 ). However, as Cosentino et al ( 2015 ) have pointed out, this method assumes that Weber’s law holds at each place along the cochlea, so that the threshold for a probe on a given electrode can be taken as an accurate measure of the amount of excitation produced by the masker near that electrode. There is evidence to suggest that, for electric stimulation, this is not the case, with smaller changes in current being detectable at high overall excitation levels (e.g.…”

Effect of stimulus polarity on detection thresholds in cochlear implant users: relationships with average threshold, gap detection, and rate discrimination

“…Such effects could lead to a correlation that does not reflect any common processing of the two tasks, except at very central levels. A more rigorous approach is to partial out between-subject effects, and to correlate the relative pattern of scores across electrodes (Bierer, 2007;Cosentino et al, 2015;Zhou and Pfingst, 2016). This approach is immune to between-listener cognitive differences.…”

Using spectral blurring to assess effects of channel interaction on speech-in-noise perception with cochlear implants

“…The manipulations that we performed were limited to the effects of spectral blurring, and did not include distortions such as those caused by cross-turn stimulation (Frijns et al, 1995;Cosentino et al, 2015a), ectopic stimulation (Finley et al, 1990), and neural dead regions (Bierer and Faulkner, 2010;Cosentino et al, 2015b). These distortions manifest as place-pitch reversals (Kenway et al, 2015) or double-peaked excitation patterns, rather than the simple reduction in spectral contrast that is produced by blurring.…”