Norman-Haignere Sam, McDermott Josh H
Department of Brain and Cognitive Sciences, MIT, United States.
Department of Brain and Cognitive Sciences, MIT, United States.
Neuroimage. 2016 Apr 1;129:401-413. doi: 10.1016/j.neuroimage.2016.01.050. Epub 2016 Jan 28.
Nonlinearities in the cochlea can introduce audio frequencies that are not present in the sound signal entering the ear. Known as distortion products (DPs), these added frequencies complicate the interpretation of auditory experiments. Sound production systems also introduce distortion via nonlinearities, a particular concern for fMRI research because the Sensimetrics earphones widely used for sound presentation are less linear than most high-end audio devices (due to design constraints). Here we describe the acoustic and neural effects of cochlear and earphone distortion in the context of fMRI studies of pitch perception, and discuss how their effects can be minimized with appropriate stimuli and masking noise. The amplitude of cochlear and Sensimetrics earphone DPs were measured for a large collection of harmonic stimuli to assess effects of level, frequency, and waveform amplitude. Cochlear DP amplitudes were highly sensitive to the absolute frequency of the DP, and were most prominent at frequencies below 300 Hz. Cochlear DPs could thus be effectively masked by low-frequency noise, as expected. Earphone DP amplitudes, in contrast, were highly sensitive to both stimulus and DP frequency (due to prominent resonances in the earphone's transfer function), and their levels grew more rapidly with increasing stimulus level than did cochlear DP amplitudes. As a result, earphone DP amplitudes often exceeded those of cochlear DPs. Using fMRI, we found that earphone DPs had a substantial effect on the response of pitch-sensitive cortical regions. In contrast, cochlear DPs had a small effect on cortical fMRI responses that did not reach statistical significance, consistent with their lower amplitudes. Based on these findings, we designed a set of pitch stimuli optimized for identifying pitch-responsive brain regions using fMRI. These stimuli robustly drive pitch-responsive brain regions while producing minimal cochlear and earphone distortion, and will hopefully aid fMRI researchers in avoiding distortion confounds.
耳蜗中的非线性现象会引入进入耳朵的声音信号中原本不存在的音频频率。这些额外的频率被称为失真产物（DPs），它们使听觉实验的解读变得复杂。声音产生系统也会通过非线性现象引入失真，这是功能磁共振成像（fMRI）研究特别关注的问题，因为广泛用于声音呈现的Sensimetrics耳机比大多数高端音频设备的线性度更低（由于设计限制）。在此，我们在音高感知的fMRI研究背景下描述耳蜗和耳机失真的声学及神经效应，并讨论如何通过适当的刺激和掩蔽噪声将其影响最小化。我们测量了大量谐波刺激下耳蜗和Sensimetrics耳机DPs的幅度，以评估电平、频率和波形幅度的影响。耳蜗DPs的幅度对DP的绝对频率高度敏感，在低于300Hz的频率处最为显著。因此，正如预期的那样，低频噪声可以有效掩蔽耳蜗DPs。相比之下，耳机DPs的幅度对刺激频率和DP频率都高度敏感（由于耳机传递函数中的显著共振），并且它们的电平随刺激电平增加的速度比耳蜗DPs的幅度增长得更快。结果，耳机DPs的幅度常常超过耳蜗DPs。通过fMRI，我们发现耳机DPs对音高敏感的皮质区域的反应有实质性影响。相比之下，耳蜗DPs对皮质fMRI反应的影响较小，未达到统计学显著性，这与它们较低的幅度一致。基于这些发现，我们设计了一组音高刺激，通过fMRI来优化识别对音高有反应的脑区。这些刺激能有力地驱动对音高有反应的脑区，同时产生最小的耳蜗和耳机失真，有望帮助fMRI研究人员避免失真干扰。
