*4.3. Pulse Duration and Shape*

Altering the grounding scheme modifies the spatial distribution of current. While certain configurations might reduce this spread, predicting current pathways in individual anatomies is challenging. Specific grounding configurations, like bipolar or multipolar schemes—which are presumed to be more focused—typically require higher charge levels to reach equivalent loudness percepts vs. MP grounding [12]. This could, in turn, lead to a broader current spread again.

OM differs from most CI manufacturers in its encoding of loudness; it uses pulse duration rather than current amplitude. Consequently, the current is consistently set at a relatively low level, even for intense sounds. It has been shown that this approach can lead to a more focused area of excitation, especially at higher stimulation levels [16].

The OM pulse shape is also different than the standard biphasic one. It begins with an active rectangular phase, but rather than being followed by a symmetric shape, the charge return is via passive (capacitive) discharge, leading to an exponential decay (Figure 1). This unique pulse waveform requires only half the stimulation power needed for generating symmetric biphasic pulses. While the amplitude of the second phase varies based on the duration and current of the initial active phase, it is typically much smaller, creating a pulse shape akin to a pseudo-monophasic one. Such pseudo-monophasic (or asymmetric) pulses are known to be charge-efficient, activating nerve fibers with lower charge levels than symmetric biphasic pulses [17,18]. Mathematical modeling by Frijns et al. [19] also suggests that asymmetric pulses like these might act to reduce current spread to some extent compared to their symmetric counterparts.

In essence, employing pulse duration coding and pseudo-monophasic pulse shapes appears to limit current spread within the cochlea, potentially decreasing the likelihood of FNS.

#### *4.4. Pulse Polarity*

Not only does the OM device have a unique grounding scheme and pulse shape, but it also has opposite polarity to the standard clinical biphasic pulses. The majority of CI manufacturers initiate their biphasic pulses with a cathodic phase, while OM devices begin with an anodic phase. This alternative polarity, combined with the pseudo-monophasic pulse shape, seems to significantly impact the occurrence of FNS, as observed in our two subjects. Specifically, when using the OM's clinical pulses, both subjects experienced no side effects. However, when traditional biphasic active stimuli with MP grounding were applied, side effects were evident. To better understand the effect of the polarity alone, we inverted the polarity of the stimuli in the OM stimulation mode for subject S2. We found that those pseudo-monophasic cathodic-leading pulses (using DAP grounding) did induce FNS, even at low loudness levels and in the absence of any auditory loudness growth.

Notably, when using the clinical anodic leading pulses, the subject reached her maximum tolerable loudness (level 10) at a charge of 32 nC without any side effects. However, when cathodic-leading pulses of the same type were used, reported loudness plateaued at level 2, while side effects continued to escalate as charge levels were increased. This striking contrast between these two conditions suggests that, for this subject, anodic stimulation primarily excited auditory nerve structures, while cathodic stimulation was effective at activating other neural structures, such as the facial nerve.
