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

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.