Infant cortical electrophysiology and perception of vowel contrasts

Abstract

Cortical auditory evoked potentials (CAEPs) were obtained for vowel tokens presented in an oddball stimulus paradigm. Perceptual measures of vowel discrimination were obtained using a visually-reinforced head-turn paradigm. The hypothesis was that CAEP latencies and amplitudes would differ as a function of vowel type and be correlated with perceptual performance. Twenty normally hearing infants aged 4-12 months were evaluated. CAEP component amplitudes and latencies were measured in response to the standard, frequent token /a/ and for infrequent, deviant tokens /i/, /o/ and /u/, presented at rates of 1 and 2 tokens/s. The perceptual task required infants to make a behavioral response for trials that contained two different vowel tokens, and ignore those in which the tokens were the same. CAEP amplitudes were larger in response to the deviant tokens, when compared to the control condition in which /a/ served as both standard and deviant. This was also seen in waveforms derived by subtracting the response to standard /a/ from the responses to deviant tokens. CAEP component latencies in derived responses at 2/s also demonstrated some sensitivity to vowel contrast type. The average hit rate for the perceptual task was 68.5%, with a 25.7% false alarm rate. There were modest correlations of CAEP amplitudes and latencies with perceptual performance. The CAEP amplitude differences for vowel contrasts could be used as an indicator of the underlying neural capacity to encode spectro-temporal differences in vowel sounds. This technique holds promise for translation to clinical methods for evaluating speech perception.

Keywords: Auditory evoked potential; Infant; Speech perception.

Figure 1

Vowel Spectra- Each of the tokens was recorded using an Etymotic ER-7c probe microphone inserted into an insert phone sound tube. The spectra were calculated using SpectraPlus v XYZ software. Note that /o/ and /u/ stimuli have a more limited bandwidth compared to the /a/ and /i/ stimuli, with limited energy for frequencies about 4 kHz.

Figure 2

CAEP waveforms- The waveforms shown are from a representative infant participant. All waveforms were averaged over a 500 ms epoch initiated at the onset of the speech token. A) Rate =1/s. The onset responses, P1-N1-P2, are shown for each vowel token. The token /a/ was the standard stimulus at a probability of 75% and the contrasting vowel was present with a probability of 25%. The /a/ deviant at 25% is the control condition. CAEP components P1, N1, P2, N2 are labeled for the response to the /i/ deviant token. B) Rate = 2/s. Responses to the standard and deviant /a/ tokens are of low amplitude owing to adaptation. C) Derived waveforms, rate= 1/s. The waveforms were derived by subtracting the response to standard from the response to the deviant stimulus. The waveform for /a/-/a/ is the control condition. D) Derived waveforms, rate = 2/s. As in the original waveforms, the derived responses for the control condition, /a/-/a/, are of low amplitude, and individual components are not evident.

Figure 3

CAEP component amplitudes. The amplitudes of CAEP components P1-N1, N1-P2, and P2-N2 are graphed for each vowel token, for Rate 1 = 1/s and Rate 2 = 2/s. Error bars indicate standard error of the mean. In general, amplitudes for contrasting vowels, /i/, /o/ and /u/ are larger than those measured in the control condition when /a/ was used for both standard and deviant.

Figure 4

CAEP derived response component amplitudes. The amplitudes of CAEP components P1-N1, N1-P2, and P2-N2 are graphed for each derived wave, for Rate 1 = 1/s and Rate 2 = 2/s. Error bars indicate standard error. At 2/s, derived waveforms for contrast conditions are larger than those for the control condition.

Figure 5

CAEP component latencies. The latencies of CAEP components P1, N1, P2, and N2 are graphed for each vowel token, for Rate 1 = 1/s and Rate 2 = 2/s. Latencies for each component do not vary significantly for vowel type nor for control vs. contrast conditions. Error bars indicate standard error.

Figure 6

derived response component latencies. The latencies of CAEP components P1, N1, P2, and N2 are graphed for each derived wave, for Rate 1 = 1/s and Rate 2 = 2/s. Latencies were prolonged for the contrast conditions relative to the control conditions, which were most apparent for the later components, P2 and N2. Error bars indicate standard error.