EFFECTS OF BIPOLAR & MONOPOLAR CURRENT SPREAD ON COCHLEAR IMPLANT USERS

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 人工耳蜗(CI)是一种帮助聋人感知声音的假体；其主要组件是麦克风、将声音转换为电脉冲的处理器，以及刺激耳蜗神经节细胞的电极阵列。CI处理器将语音信号分解成多个频段，提取频段时间包络，这些频段时间包络进而调制施加到神经节细胞的双相电脉冲的幅度。电极配置可以是单极(MP)或双极(BP)。 在MP配置中，有源电极和返回电极分别位于耳蜗内和耳蜗外，而在BP配置中，两个电极都在耳蜗内。由于有源电极和返回电极之间的空间间隔较小，BP中的电流扩散比MP配置中的更局部化，从而能够沿基底膜刺激特定的神经节细胞。从理论上讲，BP刺激应该达到更高的空间(谱)分辨率，然而，许多CI用户更喜欢MP而不是BP配置。 在心理物理掩蔽实验中估计了MP和BP构型的有效电流扩散(Chatterjee和Shannon 1998,2004，Kwon 2006)；两种构型的掩蔽水平相同，表明两者的有效电流扩散也是相同的。然而，使用相同MP和BP电流水平的生理实验表明，MP配置的听觉皮质中的兴奋传播更大(Hughes 2006，Bierer和Middlebrooks 2002，Dingemanse and Frijns 2006)。这种差异可能是因为心理物理实验测量的是响度，而要获得相同的响度，MP配置需要比BP配置小得多的电流水平。相比之下，生理学实验测量了听觉皮质神经元对施加到神经节细胞的电流的反应；两种配置的电流水平是相同的。 在这项研究方案中，我们提出了一种新的前向掩蔽范式来确定MP和BP配置中的电流扩散。为了测量MP屏蔽器和BP屏蔽器的有效电流扩散，在这两种情况下，探头的电极配置应该相同。这与通常的前向掩蔽实验不同，前向掩蔽实验对探头和掩蔽器使用相同的电极配置模式，见图4。通过将探头配置固定为BP，我们消除了探头电流扩散的影响。通过固定探头配置，我们只比较掩模电流扩散的影响，它可以是MP或BP。考虑到MP需要较少的电流才能产生与BP相同的响度，我们将MP和BP屏蔽器电流幅度设置为产生相同响度的电平。我们假设MP掩蔽将大于BP，这意味着MP的当前价差更大。如果我们的假设是正确的，它将与耳蜗内电流传播的生理测量和听觉皮质兴奋神经元的反应模式相一致。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. A cochlear implant (CI) is a prosthesis that helps deaf people perceive sounds; its main components are a microphone, a processor that converts sounds into electrical pulses, and an electrode array that stimulates the cochlear ganglion cells. The CI processor decomposes the speech signal into a number of spectral bands, extracts the bands temporal envelopes, which in turn modulate the amplitudes of biphasic electrical pulses applied to the ganglion cells. The electrode configuration can be monopolar (MP) or bipolar (BP). While in MP configurations the active and return electrodes are, respectively, inside and outside the cochlea, both electrodes are inside the cochlea in BP configurations. Because of the smaller spatial separation between active and return electrodes, the current spread is more localized in BP than in MP configurations, thereby enabling stimulation of specific ganglion cells along the basilar membrane. In theory, BP stimulation should achieve greater spatial (spectral) resolution, however, many CI users prefer MP to BP configurations. The effective current spread of MP and BP configurations has been estimated in psychophysical masking experiments (Chatterjee and Shannon 1998, 2004, Kwon 2006); the masking levels are the same for both configurations, suggesting that the effective current spread is also the same for both. However, physiological experiments using the same MP and BP current levels have shown that the spread of excitation in the auditory cortex is larger for MP configuration(Hughes 2006, Bierer and Middlebrooks 2002, Dingemanse and Frijns 2006). This discrepancy may arise because the psychophysical experiment measures loudness, and to achieve the same loudness a much smaller current level is needed with MP than BP configurations. In contrast, the physiological experiments measure the response of auditory cortex neurons to the electrical current applied to the ganglion cells; the current levels are the same for both configurations. In this research proposal we propose a new paradigm for the forward masking to determine the current spread in the MP and BP configurations. In order to measure the effective current spread of the MP masker and BP masker, the electrode configuration of the probe should be the same in both cases. This is different from the customarily forward masking experiments which use the same electrode configuration mode for probe and masker see Fig. 4. By setting the probe configuration to be fixed as BP then we eliminate the effect of the current spread at the probe. By fixing the probe configuration then we are comparing only the effect of masker current spread which could be MP or BP. To take into consideration the fact that MP requires less current to generate the same loudness as BP we will set the MP and BP masker current amplitude at levels that generate the same loudness. We hypothesis that the MP masking will be greater than for BP meaning that the current spread is larger for MP. If our hypothesis is correct, it will agree with the physiological measurements of the current spread inside the cochlea and the response pattern of the excited neurons in the auditory cortex.