Abstract
BackgroundThe 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.ResultsThe N1m responses for the six stimulus types displayed a considerable dynamic range of 115–135 ms, and were elicited faster (~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.ConclusionThe 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.
Highlights
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
The latency behavior of the N1m was asymmetric across the two hemispheres: In the right hemisphere, N1m latency was determined by articulation, whereas the latency of the left-hemispheric N1m depends on both phonation and articulation
The present study suggests that in human auditory cortex, categorization of speech sounds takes place irrespective of attentional engagement and is based on cues provided by both phonation and articulation which, lead to hemispheric asymmetries as indexed by the auditory N1m response
Summary
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. The vibrating vocal folds produce a periodic excitation, termed the glottal flow Due to this inherent periodicity, the spectra of vowels produced by normal phonation are characterized by a harmonic comb structure, i.e., distribution of energy at the fundamental frequency (F0, ranging from 100 Hz in males up to 400 Hz in infants) and its harmonic integer multiples (2 × F0, 3 × F0, etc.) located regularly in frequency [2]. This comb structure is locally weighted in frequency by the resonances caused by the vocal tract. The F0 and its harmonics are the primary acoustical cues underlying pitch perception and the lowest two formants are regarded as the major cues in vowel categorization [1]
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