Latency of MEG M100 response indexes first formant frequency

Abstract

Magnetoencephalography recordings from the auditory cortex of subjects listening to synthetic vowels show a close correlation between the timing of the evoked M100 response and the first formant frequency (F1). These results are consistent with evoked magnetic field latencies elicited by tone stimuli, which show 100 to 300-Hz tones associated with latencies up to 30 ms longer than 500 to 3000-Hz tones. In experiment 1, three-formant vowels /u,a,i/ were presented at two fundamental frequencies (F0=100 Hz, 200 Hz). The M100 latency was a function of the vowel identity and not F0: M100 was significantly shorter for /a/ than /u/, consistent with the spectral center of gravity being at a higher frequency for /a/. In experiment 2, single-formant vowels (/u/:F1=330 Hz, /a/:F1=720 Hz) were covaried with two F0 values. M100 latencies were shorter for /a/ (high F1) than for /u/ (low F1), at both fundamentals. In experiment 3, subjects listened to pure-tone complexes with frequencies and amplitudes matching the F0 and F1 energy peaks of the stimuli in experiment 2. M100 latencies showed the same pattern: latency covaried with the energy peak corresponding to F1, suggesting that the sensitivity to the energy in the F1 range is not speech-specific. Finally, an experiment involving two-formant, amplitude-varying vowels will be presented.

Auditory sensitivity to formant ratios: Toward an account of vowel normalisation

“…Given our hypothesis regarding the algorithm that is (at least partly) responsible for vowel normalization, combined with previous MEG findings on vowel perception (Diesch et al 1996; Govindarajan et al 1998; Poeppel et al 1997; Roberts et al 2000; Roberts et al 2004; Tiitinen et al 2005), we propose that the M100 is actually sensitive to the ratio of the first formant (F1) against the third (F3), instead of F1 alone. In order for us to test this representational and normalization hypothesis with the M100, the M100 must be able to index more complex auditory operations performed on the input and not solely reflect surface properties of the stimulus.…”

“…Poeppel, et al (1997) synthesized three English vowels (/i/, /u/, /a/) and also report a reliable difference in the evoked M100 latency between /a/ and /u/, with /a/ eliciting a shorter M100 latency, and do not report a difference between /i/ and /u/. This finding was replicated in Govindarajan, et al (1998), who also found that both one and three formant synthesized tokens of /a/ elicit reliably faster M100 evoked latencies in English listeners than one and three formant synthesized tokens of /u/, respectively. Moreover, Tiitinen, et al (2005) replicated these findings in Finnish speakers, showing again that /a/ elicits faster M100 latencies than /u/ using semi-synthetic speech.…”

“…Relevant to the current work, the M100 response properties to vowels have been fairly well characterized. In particular, the M100 seems to be sensitive to F1 (Diesch et al 1996; Govindarajan et al 1998; Poeppel et al 1997; Roberts et al 2000; Roberts et al 2004; Tiitinen et al 2005) independent of differences in fundamental frequency (Govindarajan et al 1998; Poeppel et al 1997). Diesch, et al (1996) compared the evoked latencies of the M100 to four different synthesized German vowels (/a/, /i/, /u/, /æ/) and found that /a/ and /æ/, having higher F1 values, elicited reliably shorter latencies than /u/.…”

“…Moreover, Tiitinen, et al (2005) replicated these findings in Finnish speakers, showing again that /a/ elicits faster M100 latencies than /u/ using semi-synthetic speech. The interpretation for the directionality of these effects, namely that /a/ elicits reliably shorter M100 evoked latencies than /u/, is that the spectral energy in F1 is driving the M100 response (Govindarajan et al 1998; Poeppel et al 1997; Roberts et al 2000), and /a/ elicits shorter latencies because the F1 in /a/ (~ 700 Hz) is considerably closer to 1 KHz than the F1 in /u/ (~ 300 Hz), consistent with the sinusoidal data (Roberts and Poeppel 1996). This effect does not seem to be speech specific, however (Diesch et al 1996; Govindarajan et al 1998).…”

Levels of representation in the electrophysiology of speech perception

“…Building upon the finding that M100 latency varies with changes in the pitch of a pure tone , it has been shown that the latency of M100 responses to vowels is sensitive to variation in vowel category (/a, i, u/), specifically to variation in vowel height (F1), and is relatively insensitive to variation in speaker, that is, male versus female voice (Poeppel et al, 1997;Govindarajan, Phillips, Poeppel, Roberts, & Marantz, 1998). Studies of this kind show that the M100 is sensitive to certain important cues to vowel categorization, but do not show how vowel categories are represented.…”