Disentangling the effects of phonation and articulation: hemispheric asymmetries in the auditory N1m response of the human brain

Abstract

Background: 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.

Results: 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.

Conclusion: 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.

Figure 1

The spectra of the stimuli for the vowels /a/ (upper row) and /u/ (lower row), representing how articulation modifies stimulus structure. The stimuli were created using three different types of phonation: the natural periodic glottal pulseform (sounds /a/_per and /u/_per in the left column), the aperiodic noise sequence (/a/_aper and /u/_aper, center column), and tonal excitation (/a/_tone and /u/_tone, right column). The vowels excited by the natural periodic glottal pulseform are characterized by a harmonic comb structure, that is, distribution of sound energy at multiple integers of the fundamental frequency. This regular spectral fine structure is absent from the spectra of the vowels produced by the aperiodic excitation. The spectra of the sounds generated by tonal excitation are further impoverished, comprising only two spectral components. The spectral characteristics of the stimuli of all three excitation types are affected by the formant structure of the underlying vowel. Due to this, the vowel /a/ comprises more high frequencies than the vowel /u/.

Figure 2

Grand-averaged (N = 10) responses elicited by periodic (thick line), aperiodic (dashed line), and tonal (dotted line) excitation of the vowel /a/ and /u/, calculated over the pair of planar gradiometers displaying N1m response maxima above the left and right hemisphere. In all cases, the stimuli comprising natural periodic structure elicited a prominent N1m response peaking at around 120 ms after stimulus onset.

Figure 3

The grand-averaged latency of the left- and right-hemispheric N1m for the vowels /a/ and /u/ with three different types of phonation (periodic, aperiodic & tonal). In both hemispheres, the N1m for the vowel /a/ was elicited, on the average, 10 ms earlier than that for /u/. The latency behavior of the N1m was asymmetric across the two hemispheres: In the right hemisphere, N1m latency was determined by articulation (vowel category), whereas the latency of the left-hemispheric N1m depends on both phonation and articulation. Notably, in the left hemisphere, there were no significant latency differences between the N1m responses elicited by the periodic vowels /a/_per and /u/_per. Bars indicate standard error of the mean.

Figure 4

The grand-averaged amplitude of the N1m elicited by the vowels /a/ and /u/ with periodic, aperiodic, and tonal excitation (due to hemispheric symmetry, the left- and right-hemispheric data has been averaged). The vowels with periodic glottal excitation (/a/_per & /u/_per) elicited the most prominent N1m responses, and the amplitude difference between the two was statistically significant. In all cases, the vowels with aperiodic (/a/_aper & /u/_aper) and tonal (/a/_tone & /u/_tone) excitation resulted in N1m responses with significantly smaller amplitudes than did vowels with periodic excitation. Bars indicate standard error of the mean.

Figure 5

The ECD locations of the N1m responses in the anterior-posterior and superior-inferior dimensions. These were located in a restricted cortical area in both the left and the right hemisphere. The ECDs for the vowels with periodic (/a/_per & /u/_per) excitation were anterior to those for aperiodic (/a/_aper & /u/_aper) and tonal (/a/_tone & /u/_tone) excitation. Bars indicate standard error of the mean.