Donald K. Eddington
Jay T. Rubinstein
Scott B. C. Dynes
Dept. of Otology and Laryngology, Harvard Med. School, Massachusetts Eye and Ear Infirmary, 243 Charles St., Boston, MA 02114
the Res. Lab. of Electronics, MIT, Cambridge, MA
Interactions between nonsimultaneous stimuli presented on the same or different electrodes may affect the performance of cochlear implant systems. Single-unit models based on nonlinear nodes of Ranvier include three mechanisms whereby a subthreshold masker (STM) could affect the spike threshold of a following probe pulse: (1) stored charge, (2) sodium channel activation, and (3) sodium channel inactivation. Physiological (single-unit) recordings in cat show that the threshold for a single, monophasic cathodic probe is decreased with cathodic STMs and increased with anodic STMs. This anodic/cathodic asymmetry is qualitatively consistent with the model's linear mechanism of stored charge. However, nonlinear sodium channel activation/inactivation are required to predict the effects quantitatively. Model, physiological and human psychophysical studies using biphasic (charge-balanced) stimuli also show asymmetries in the maskers' effect on probe threshold. In human, (a) the maximum effect of a cathodic-phase-last masker is stronger than that of an anodic-phase-last (APL) masker, (b) the APL masker decreases probe threshold (in contrast to the model's prediction), and (c) the APL masker's effect remains significant after 3 ms while the model and physiological results show effects lasting less than 0.5 ms. Taken together, these studies suggest that the human psychophysical results cannot be explained solely by response characteristics of the auditory nerve. [Work supported by NIDCD.]