Tiitinen Hannu, Mäkelä Anna Mari, Mäkinen Ville, May Patrick J C, Alku Paavo
Apperception & Cortical Dynamics, Department of Psychology, University of Helsinki, POB 9, FIN-00014, Finland.
BMC Neurosci. 2005 Oct 15;6:62. doi: 10.1186/1471-2202-6-62.
The cortical activity underlying the perception of vowel identity has typically been addressed by manipulating the first and second formant frequency (F1 & F2) of the speech stimuli. These two values, originating from articulation, are already sufficient for the phonetic characterization of vowel category. In the present study, we investigated how the spectral cues caused by articulation are reflected in cortical speech processing when combined with phonation, the other major part of speech production manifested as the fundamental frequency (F0) and its harmonic integer multiples. To study the combined effects of articulation and phonation we presented vowels with either high (/a/) or low (/u/) formant frequencies which were driven by three different types of excitation: a natural periodic pulseform reflecting the vibration of the vocal folds, an aperiodic noise excitation, or a tonal waveform. The auditory N1m response was recorded with whole-head magnetoencephalography (MEG) from ten human subjects in order to resolve whether brain events reflecting articulation and phonation are specific to the left or right hemisphere of the human brain.
The N1m responses for the six stimulus types displayed a considerable dynamic range of 115-135 ms, and were elicited faster (approximately 10 ms) by the high-formant /a/ than by the low-formant /u/, indicating an effect of articulation. While excitation type had no effect on the latency of the right-hemispheric N1m, the left-hemispheric N1m elicited by the tonally excited /a/ was some 10 ms earlier than that elicited by the periodic and the aperiodic excitation. The amplitude of the N1m in both hemispheres was systematically stronger to stimulation with natural periodic excitation. Also, stimulus type had a marked (up to 7 mm) effect on the source location of the N1m, with periodic excitation resulting in more anterior sources than aperiodic and tonal excitation.
The auditory brain areas of the two hemispheres exhibit differential tuning to natural speech signals, observable already in the passive recording condition. The variations in the latency and strength of the auditory N1m response can be traced back to the spectral structure of the stimuli. More specifically, the combined effects of the harmonic comb structure originating from the natural voice excitation caused by the fluctuating vocal folds and the location of the formant frequencies originating from the vocal tract leads to asymmetric behaviour of the left and right hemisphere.
元音识别背后的皮层活动通常通过操纵语音刺激的第一和第二共振峰频率（F1和F2）来研究。这两个源于发音的数值，对于元音类别的语音特征描述已经足够。在本研究中，我们探究了由发音引起的频谱线索与发声（语音产生的另一个主要部分，表现为基频（F0）及其谐波整数倍）相结合时，如何在皮层语音处理中得到体现。为了研究发音和发声的联合效应，我们呈现了具有高（/a/）或低（/u/）共振峰频率的元音，这些元音由三种不同类型的激励驱动：反映声带振动的自然周期性脉冲形式、非周期性噪声激励或音调波形。使用全头磁脑图（MEG）记录了十名人类受试者的听觉N1m反应，以确定反映发音和发声的脑事件是否特定于人类大脑的左半球或右半球。
六种刺激类型的N1m反应显示出115 - 135毫秒的相当大的动态范围，并且高共振峰/a/比低共振峰/u/引发反应的速度更快（约10毫秒），表明存在发音效应。虽然激励类型对右半球N1m的潜伏期没有影响，但音调激励的/a/引发的左半球N1m比周期性和非周期性激励引发的早约10毫秒。两个半球中N1m的幅度在自然周期性激励刺激下系统性地更强。此外，刺激类型对N1m的源位置有显著（高达7毫米）影响，周期性激励导致的源位置比非周期性和音调激励更靠前。
两个半球的听觉脑区对自然语音信号表现出不同的调谐，在被动记录条件下就已可观察到。听觉N1m反应潜伏期和强度的变化可追溯到刺激的频谱结构。更具体地说，由波动的声带引起的自然语音激励产生的谐波梳状结构与声道产生的共振峰频率位置的联合效应导致了左右半球的不对称行为。
