Implant systems attempt to mimic or reproduce this “tonotopic” encoding by stimulating basally situated electrodes (first turn of the cochlea and lower part of the drawing) to indicate the presence of high-frequency sounds, and by stimulating electrodes at more apical positions (deeper into the ST and ascending along the first and second turns in the drawing) to indicate the presence of sounds with lower frequencies. As described above, neurons at different positions along the length of the cochlea respond to different frequencies of acoustic stimulation in normal hearing. This array was used in the UCSF/Storz and Clarion 1.0 devices (Reproduced from Leake and Rebscher (2004) with permission of Springer Science + Business Media)ĭifferent electrodes in the implanted array may stimulate different subpopulations of neurons. Only four of the bipolar pairs are visible in the drawing, as the others are “hidden” by cochlear structures. That array includes eight pairs of bipolar electrodes, spaced at 2 mm intervals and with the electrodes in each pair oriented in an “offset radial” arrangement with respect to the neural processes peripheral to the ganglion cells in the intact cochlea. Illustrated is the electrode array developed at the University of California at San Francisco ( Loeb et al., 1983). Cutaway drawing of the implanted cochlea. Techniques to improve current focusing using simultaneous stimulation of multiple sites are undergoing clinical trials ( Van den Honert and Kelsall, 2007 Saoji et al., 2009).įigure 58.2.

Because channel interaction is unpredictable, often resulting in undesirable perceptual effects including pain, current pulses are typically delivered sequentially to a single electrode at a time ( Seligman and Shepherd, 2004). Both monopolar and bipolar stimulations produce significant channel interaction during simultaneous stimulation of two or more electrodes ( Black et al., 1981 Shannon, 1983 Townshend and White, 1987). As a consequence, stimulating channels overlap considerably so that any one SGN can be activated by multiple electrodes. Neural excitation via an electrode within the scala tympani is spatially broad due to the conductive nature of the fluid filled inner ear ( Black et al., 1981 Van den Honert and Stypulkowski, 1987 Snyder et al., 2004). James Fallon, in Handbook of Clinical Neurophysiology, 2013 16.3.6 Sequential versus simultaneous stimulation

Scala tympani