Here is a new video explaining the our new stimulation mode, the Virtual Tripole (in blue) from our new paper (Padilla et al., 2017). The Virtual Tripole (or VTP) is a current focused virtual channel that can increase the sites of stimulation beyond the number of physical electrodes (like a Monopolar Virtual Channel used in Fidelity 120) and hopefully also reduce the spread of excitation (like a Tripole).
The text from the video is as follows:
In this video, we illustrate the Virtual Tripole mode introduced in Padilla et al. (2017) and compare it to typical virtual channels (i.e. Monopolar virtual channels). Both the Virtual Tripole and the Monopolar Virtual Channel are current steered stimulation modes used to provide more sites of stimulation than there are physical electrodes. In the top right panel, you can see the modeled spread of excitation from a Monopolar Virtual Channel (in black) and a Virtual Tripole in Blue as the peak of stimulation shifts across the cochlea. Both modes successfully provide a continuum in peaks of stimulation across the array. However, like a traditional tripole, the virtual tripole should provide a narrower spread of excitation. Hopefully, when implemented in a sound coding strategy, this will limit channel interaction and improve the perception of spectral information.
The bottom two panels on the right side illustrate how the stimulation modes are generated. Each red point indicates an electrode and each arrow indicates the amplitude (and phase) of stimulation at each location. The black arrows illustrate current steering using a traditional Monopolar Virtual Channels. The blue arrows illustrate current steering using the Virtual Tripole. As can be seen, the virtual tripole is effectively a traditional tripole generated by three virtual channels instead of three physical electrodes.
On the left, a monopolar virtual channel is compared to Quadrupolar Virtual Channels. Quadrupolar virtual channels are an early attempt to create a current focused virtual channel that were first introduced in Landsberger and Srinivasan (2009). The Monopolar virtual channel is indicated in black while the Quadrupolar Virtual Channel is indicated in magenta. Unlike the Virtual Tripole, the Quadrupolar Virtual Channel uses fixed location out-of-phase stimulation for the current focusing. The result is that the distribution moves asymmetrically as it is steered across the array. Perhaps even more troublesome is that when stimulation shifts from between one electrode pair to the next, the shape of the distribution jumps and can cause pitch reversals.
For further details on the Virtual Tripole stimulation mode, consult Padilla et al. (2017). Thank you to Leo Litvak for providing his model from Litvak et al. (2007) which was used to generate this animation and to Chi-young Kim for writing the earliest version of the code to generate these animations.
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