1.- Introduction

1. 4.- Estimation of the average distance between the active electrode and the neural ends (b) • The theoretical model provides information about how the thresholds are affected by a modification of the distance between electrodes: • This can be obtained for different values of d and b. • An estimation of the average b level can be obtained by comparing: • the theoretical variations of the THR levels with d for different values of b • and the variation of the THR levels for the different stimulation modes observed for the patients. • This comparison provides an average distance between the active electrode and the neural ends b close to 4 mm (Figure 5). b = 1 mm Figure 4: Electrical threshold measured for different configurations of the stimulation. The vertical bars represent mean ± standard deviation. Extra-cochlear reference electrode b = 2 mm monopolar common ground bipolar b = 4 mm b = 6 mm inactive active reference b = 8 mm Figure 1: Different configurations of the cochlear implant electrodes. bipolar + 1 bipolar +2 bipolar +3 b = 10 mm • We study an electrical model to describe the electrical phenomenons induced by the cochlear implant. • Several approaches are considered (for an analytical treatment): • Spherical electrodes of radius a: • Homogeneous medium with constant conductivity s: • Electrical stimulation considered a stationary process: • Active and reference electrodes separated by a distance d. • Bipolar if d < 10 mm • Monopolar if d > 20 mm • Distance between active electrode and neural ends b: it depends on the state of the neural ends, alocation of the electrode carrier, etc. • Stimulation voltage: N22 BP+5 N22 BP+3 N22 BP y functional neural ends C40+ b active electrode reference electrode a Figure 6: Influence of the parameter b over the efficiency of the electrical stimulation (top) and the width of the density of current curves (bottom). Figure 5: Experimental fitting of the average distance between active electrode and functional neural ends. Theoretical values provided by the model (red) and experimental measurements (blue). x d MP BP+5 Figure 2: Model of the electrical stimulation and parameters involved in the geometry of the electric (E) and the density of current (J) fields. Radius of electrode (a); distance between active electrode and neural ends (b); distance between electrodes (d). BP+3 BP MP BP BP+3 BP+5 BP+3 BP V=+0V b = 1 mm V=+0.005V b = 2 mm V=+0.01V b = 4 mm V=+0.02V b = 6 mm V=+0.05V b = 8 mm V=+0.1V b = 10 mm V=+0.2V V=+0.5V - + V= - 0.5V V= - 0.2V Figure 7: Interaction among channels for b=2mm for the N22 (in BP and BP+3 configurations) and C40+ devices. V= - 0.1V V= - 0.05V V= - 0.02V V= - 0.01V V= - 0.005V b = 1 mm b = 2 mm V=+0.05V b = 4 mm V=+0.1V b = 6 mm V=+0.2V b = 8 mm V=+0.5V b = 10 mm + Figure 8: Interaction among channels as a function of the parameter b for the different electrical configurations. Figure 3: (Left side) Surfaces V=constant (red) and current lines (blue) for bipolar configuration (d=2mm) and monopolar configuration (d=100mm). (Right side) Variation of the density of current J along the x axis for different values of the b parameter, for bipolar configuration (d=2mm) and monopolar configuration (d=100mm). INFLUENCE OF THE PLACEMENT AND CONFIGURATION OF THE ELECTRODES OVER THE PERCEPTION USING COCHLEAR IMPLANTSA. de la Torre (1), M. Sainz (2,3), C. Roldán (2)(1) Dpto. Electrónica y Tecn. Computadores, Universidad de Granada, 18071 Granada (Spain) (2) Servicio ORL, Hospital Universitario S. Cecilio, 18012 Granada (Spain)(3) Dpto. Cirugía y sus Especialidades, Universidad de Granada, 18071 Granada (Spain) • 1.- Introduction • We study the electric (E) and density of current (J) fields for different electrode configurations of cochlear implants. • We propose a model to describe the spatial distribution of the currents provided by the cochlear implant. • The model considers several parameters to take into account the technical features of the implants and the allocation of the electrodes with respect to the neural ends. • The model describing the interaction between the electrodes and the cochlear nerve is a useful tool: • to evaluate the interaction among the channels, • to understand the relationship between the electrical thresholds and the state of the neural ends and • to select the best processor set-up for a given placement of the electrode carrier and state of the cochlear nerve. • 3.- Influence of the distance between electrodes (d) • The different electrical configurations cause different spatial distributions of the electric field and the density of current field. • Bipolar configuration confines most of the current between the electrodes. • When d << b , the density of current is very low. For this reason, as d is smaller, a higher stimulation level is necessary in order to obtain the same hearing sensation. • We have measured the perception thresholds for 20 patients, 10 implanted with the COMBI 40+ (C40+) and 10 implanted with the NUCLEUS 22 (N22). The C40+ can be configured only for monopolar stimulation while the N22 can be only configured for bipolar modes (BP, BP+1,....,BP+5 and common ground). • The average thresholds are shown in Figure 4. The thresholds are expressed in charge units (nC) taking into account the intensity and the duration of the stimulation pulses. • As could be expected, bipolar like stimulations is less efficient and higher stimulation levels are necessary: the electric field is confined near the electrodes and a higher level is necessary to obtain the same density of current at the neural ends. • 2.- Electrical model of interaction • The different electrical configurations cause different spatial distributions of the electric field and the density of current field. • The model provides: • The electrical potential field V(x,y) • The electrical field E(x,y) • The density of current field J(x,y) • The distribution of the current lines around the electrodes • The density of current at the neural ends situated at a distance b from the active electrode • The variation of the density of current J along the x axis (along the cochlea axis) • The spatial distribution of currents essentially depends on 2 parameters: • b (distance active electrode – neural ends) • d (distance between active and reference electrodes) • 6.- Interaction among channels • Taking into account the separation between adjacent channels and the width of the density of current curves, the channel interaction can be compared for the different electrical configurations of the different devices (Figures 7 and 8). • 5.- Selecting an stimulation mode • The parameters b and d affects the efficiency of the electrical stimulation, (since they determine the density of current provided by the stimulation at the neural ends, and the width of the curves of density of current: • As the density of current is higher, the electrical stimulation is more efficient and lower stimulation levels are necessary. • As the width of the curves of density of current is smaller, the interaction among channels is smaller. • As b is greater, the efficiency falls very fast and the interaction among channels is more important (Figure 6). • For the BP+N modes, a trade-off should be considered for a given value of b: the efficiency of the stimulation is better as N is greater, but this increases the channel interaction. • The best trade-off for any given value of b is always provided by the monopolar stimulation mode. • Conclusions • The distance between the active electrodes and the functional neural ends (the parameter b) plays a very important role in cochlear implants with respect to: • efficiency of the electrical stimulation • and channel interaction. • Monopolar stimulation present advantages with respect to bipolar stimulation modes in all the situations. Bipolar stimulation should be specially avoided in those cases when a high value of b is expected (important degradation of the cochlear nerve, ossification, etc.). • The dispersion of the density of current field (even for low values of b) implies that no significant benefit with respect to spectral resolution is obtained from the use of a high number of channels. This would explain the fact that no significant degradation is observed in the speech perception tests when the number of active channels is reduced to 6. • Deep insertion of the electrode carrier is very important in cochlear implantation in order to: • make use of all the length of the cochlea for the stimulation of the whole auditory nerve • stimulate the apical partition of the cochlea, which is the best preserved and therefore with shortest distance between active electrode and functional neural ends. • The proposed theoretical model is useful to understand the mechanism involved in the stimulation using cochlear implants. However, a numerical model should be developed in order to obtain more precise results and to take into account aspect not considered in the studied model (stimulation is not stationary, the model should consider radiation, electrodes are not spherical, conductivity is not constant, different particles are involved in the charge transport...)