Forward-masking patterns produced by symmetric and asymmetric pulse shapes in electric hearing

Abstract

Two forward-masking experiments were conducted with six cochlear implant listeners to test whether asymmetric pulse shapes would improve the place-specificity of stimulation compared to symmetric ones. The maskers were either cathodic-first symmetric biphasic, pseudomonophasic (i.e., with a second anodic phase longer and lower in amplitude than the first phase), or "delayed pseudomonophasic" (identical to pseudomonophasic but with an inter-phase gap) stimuli. In experiment 1, forward-masking patterns for monopolar maskers were obtained by keeping each masker fixed on a middle electrode of the array and measuring the masked thresholds of a monopolar signal presented on several other electrodes. The results were very variable, and no difference between pulse shapes was found. In experiment 2, six maskers were used in a wide bipolar (bipolar+9) configuration: the same three pulse shapes as in experiment 1, either cathodic-first relative to the most apical or relative to the most basal electrode of the bipolar channel. The pseudomonophasic masker showed a stronger excitation proximal to the electrode of the bipolar pair for which the short, high-amplitude phase was anodic. However, no difference was obtained with the symmetric and, more surprisingly, with the delayed pseudomonophasic maskers. Implications for cochlear implant design are discussed.

Fig. 1

Predicted threshold difference obtained by Rattay et al. (2001) between several modeled neurons and neuron 7 for a monopolar electrode located in the scala tympani below neuron 7 (insertion of 180 degrees) for 100-μs anodic and cathodic monophasic pulses. Neurons are numbered from base to apex in 30-degrees step. These simulations correspond to the “short dendrite” case, replotted from Rattay et al. (2001, Table 2 p. 73).

Fig. 2

Schematic representation of the stimuli used in the forward masking experiments.

Fig. 3

Absolute and Masked thresholds of a signal presented on different channels and after the three different monopolar maskers (BI-C, PS-C and DPS-C). The error bars represent the standard errors, as in the following figures of the manuscript.

Fig. 4

Mean threshold shift (difference between masked and absolute thresholds) of a monopolar BI-C signal presented on different channels and after the three different monopolar maskers.

Fig. 5

Diagram showing the expected contributions of each phase of the BI-C, PS-C and DPS-C maskers to neural excitation of fibers proximal to electrode 4 and electrode 14. The maskers are in bipolar configuration and the leading polarity refers to the polarity relative to electrode 4. “S” and “w” correspond to expected strong and weak contributions, respectively. The predictions for BI-A, PS-A and DPS-A are not shown as they are symmetrical to the ones of BI-C, PS-C and DPS-C (by inverting electrodes 4 and 14).

Fig. 6

Each panel illustrates the threshold shift of a monopolar BI-C signal for each bipolar masker shape (left for BI, center for PS and right for DPS). Four conditions were performed for each masker shape: the signal was either presented on channel 4 (filled symbols) or 14 (open symbols) and the masker was either anodic-first (right part of each panel) or cathodic-first (left part) with reference to the most apical electrode (4) of the pair. Thick lines represent mean data of the four subjects.

Fig. 7

A, Mean pitch ranks and standard errors obtained by running the midpoint comparison with the six masker stimuli of Experiment 2. B, Threshold shift difference between electrode 14 and electrode 4 as a function of the mean pitch rank for the six maskers of Experiment 2. After standardization of the masking data, a correlation was performed on all the data points and turned out to be statistically significant (r=0.64, p=0.001).