Leino Sakari, May Patrick J C, Alku Paavo, Liikkanen Lassi A, Tiitinen Hannu
Apperception & Cortical Dynamics (ACD), Department of Psychology, University of Helsinki, Finland.
BMC Neurosci. 2007 Sep 26;8:78. doi: 10.1186/1471-2202-8-78.
In the field of auditory neuroscience, much research has focused on the neural processes underlying human sound localization. A recent magnetoencephalography (MEG) study investigated localization-related brain activity by measuring the N1m event-related response originating in the auditory cortex. It was found that the dynamic range of the right-hemispheric N1m response, defined as the mean difference in response magnitude between contralateral and ipsilateral stimulation, reflects cortical activity related to the discrimination of horizontal sound direction. Interestingly, the results also suggested that the presence of realistic spectral information within horizontally located spatial sounds resulted in a larger right-hemispheric N1m dynamic range. Spectral cues being predominant at high frequencies, the present study further investigated the issue by removing frequencies from the spatial stimuli with low-pass filtering. This resulted in a stepwise elimination of direction-specific spectral information. Interaural time and level differences were kept constant. The original, unfiltered stimuli were broadband noise signals presented from five frontal horizontal directions and binaurally recorded for eight human subjects with miniature microphones placed in each subject's ear canals. Stimuli were presented to the subjects during MEG registration and in a behavioral listening experiment.
The dynamic range of the right-hemispheric N1m amplitude was not significantly affected even when all frequencies above 600 Hz were removed. The dynamic range of the left-hemispheric N1m response was significantly diminished by the removal of frequencies over 7.5 kHz. The subjects' behavioral sound direction discrimination was only affected by the removal of frequencies over 600 Hz.
In accord with previous psychophysical findings, the current results indicate that frontal horizontal sound localization and related right-hemispheric cortical processes are insensitive to the presence of high-frequency spectral information. The previously described changes in localization-related brain activity, reflected in the enlarged N1m dynamic range elicited by natural spatial stimuli, can most likely be attributed to the processing of individualized spatial cues present already at relatively low frequencies. The left-hemispheric effect could be an indication of left-hemispheric processing of high-frequency sound information unrelated to sound localization. Taken together, these results provide converging evidence for a hemispheric asymmetry in sound localization.
在听觉神经科学领域，许多研究都聚焦于人类声音定位背后的神经过程。最近一项脑磁图（MEG）研究通过测量源自听觉皮层的N1m事件相关反应，探究了与定位相关的大脑活动。研究发现，右半球N1m反应的动态范围（定义为对侧和同侧刺激之间反应幅度的平均差异）反映了与水平声音方向辨别相关的皮层活动。有趣的是，结果还表明，水平定位的空间声音中存在逼真的频谱信息会导致右半球N1m动态范围更大。由于频谱线索在高频时占主导地位，本研究通过低通滤波从空间刺激中去除频率，进一步探究了这个问题。这导致了方向特异性频谱信息的逐步消除。双耳间时间和强度差异保持不变。原始的未滤波刺激是从五个额部水平方向呈现的宽带噪声信号，并通过放置在每个受试者耳道中的微型麦克风进行双耳记录，供八名人类受试者使用。在MEG记录期间以及行为听力实验中向受试者呈现刺激。
即使去除所有高于600Hz的频率，右半球N1m振幅的动态范围也没有受到显著影响。去除高于7.5kHz的频率后，左半球N1m反应的动态范围显著减小。受试者的行为声音方向辨别仅受到去除高于600Hz频率的影响。
与之前的心理物理学研究结果一致，当前结果表明，额部水平声音定位及相关的右半球皮层过程对高频频谱信息的存在不敏感。先前描述的与定位相关的大脑活动变化，表现为自然空间刺激引起的N1m动态范围扩大，很可能归因于相对低频时就已存在的个性化空间线索的处理。左半球的效应可能表明左半球对与声音定位无关的高频声音信息进行了处理。综上所述，这些结果为声音定位中的半球不对称提供了一致的证据。
