Department of Communication Sciences and Disorders, East Carolina University, Greenville, NC, 27834, USA.
Department of Communication Sciences and Disorders, East Carolina University, Greenville, NC, 27834, USA.
Hear Res. 2020 Apr;389:107921. doi: 10.1016/j.heares.2020.107921. Epub 2020 Feb 15.
The goal of the present study was to compare forward masking patterns by stimulation of low and high rates in cochlear implant users. Postlingually deafened Cochlear Nucleus® device users participated in the study. In experiment 1, two maskers of different rates (250 and 1000 pulses per second) were set at levels that produced equal masking for a probe presented at the same electrode as the maskers. This aligned the two masking functions at the on-site probe location. Then their forward masking patterns for the far probes were compared. Results showed that slope of the masked probe-threshold decay as a function of probe-masker separation was steeper for the high-rate than the low-rate masker. A linear model indicated that this difference in spread of neural excitation (SOE) was accounted for by two factors that were not correlated with each other. One factor was that the low-rate masker required a considerably higher current level to be equally effective in masking as the high-rate masker. The second factor was the effect of stimulation rate on loudness, i.e., integration of multiple pulses. This was consistent with our hypothesis that if an increase in stimulation rate does not result in an increased total neural response, then it is unlikely that the change in rate would change spatial distribution of the neural activity. Interestingly, the difference in masking effectiveness of the maskers predicted subjects' speech recognition. Poorer performers were those who showed more comparable masking effects by maskers of different rates. The difference in the masking effectiveness may indirectly measure the auditory neurons' excitability, which predicts speech recognition. In experiment 2, SOE of the high-rate and low-rate maskers were compared at a level that is clinically relevant, i.e., equal loudness. At equal loudness, high-rate stimulation not only produced an overall greater amount of forward masking, but also a shallower decay of masking with probe-masker separation (wider SOE), compared to low rate. The difference in SOE was the opposite to the findings from experiment 1. Whether the maskers were calibrated for equal masking or loudness, the absolute current level was always higher for the low-rate masker, which suggests that the SOE patterns cannot be explained by current spread alone. The fact that high-rate stimulation produced greater masking and wider SOE at equal loudness may explain why using high stimulation rates has not produced consistent benefits for speech recognition, and why lowering stimulation rate from the manufacturer's default sometimes results in improved speech recognition for subjects.
本研究的目的是比较不同刺激率（低率和高率）对人工耳蜗使用者的前向掩蔽模式的影响。研究对象为人工耳蜗植入后失聪的 Cochlear Nucleus® 设备使用者。在实验 1 中，两个掩蔽器的刺激率分别为 250 脉冲/秒和 1000 脉冲/秒，其强度设定为在同一电极处产生与掩蔽器相同的掩蔽作用的探针。这将两个掩蔽函数在现场探针位置对齐。然后，比较它们对远探针的前向掩蔽模式。结果表明，高率掩蔽器的掩蔽探针阈值衰减斜率比低率掩蔽器的斜率更陡。线性模型表明，这种神经兴奋扩展（SOE）的差异是由两个不相关的因素引起的。一个因素是，低率掩蔽器需要更高的电流水平才能与高率掩蔽器具有相同的掩蔽效果。第二个因素是刺激率对响度的影响，即对多个脉冲的整合。这与我们的假设一致，如果刺激率的增加没有导致总神经反应的增加，那么改变刺激率不太可能改变神经活动的空间分布。有趣的是，掩蔽器的掩蔽效果差异预测了受试者的言语识别能力。表现较差的受试者在不同刺激率的掩蔽器中表现出更可比的掩蔽效果。掩蔽效果的差异可能间接测量听觉神经元的兴奋性，这预测了言语识别能力。在实验 2 中，在临床相关的水平（即响度相等）下比较了高率和低率掩蔽器的 SOE。在响度相等的情况下，与低率相比，高率刺激不仅产生了更大的整体前向掩蔽，而且随着探针-掩蔽器分离的增加，掩蔽的衰减也更浅（SOE 更宽）。SOE 的差异与实验 1 的结果相反。无论掩蔽器是根据等掩蔽还是等响度进行校准，低率掩蔽器的绝对电流水平总是更高，这表明 SOE 模式不能仅用电流扩散来解释。在等响度下，高率刺激产生更大的掩蔽和更宽的 SOE 的事实可能解释了为什么使用高刺激率没有为言语识别带来一致的益处，以及为什么降低制造商默认的刺激率有时会导致受试者的言语识别能力提高。
