N1 Repetition-Attenuation for Acoustically Variable Speech and Spectrally Rotated Speech

Abstract

The amplitude of the event-related N1 wave decreases with repeated stimulation. This repetition-attenuation has not previously been investigated in response to variable auditory stimuli, nor has the relative impact of acoustic vs. perceptual category repetition been studied. In the present study, N1 repetition-attenuation was investigated for speech and spectrally rotated speech with varying degrees of acoustic and perceptual category variation. In the speech condition, participants (n = 19) listened to stimulus trains consisting of either the same vowel exemplar (no variability condition), different exemplars of the same vowel (low variability condition), or different exemplars of two different vowels (high variability condition). In the rotated speech condition, the spectrally rotated counterparts of the vowels were presented. Findings show N1 repetition-attenuation in the face of acoustic and perceptual category variability, but no impact of the degree of variability on the degree of N1 attenuation. Speech stimuli resulted in less attenuation than the acoustically matched non-speech stimuli, which is in line with previous findings. It remains unclear if the attenuation of the N1 wave is reduced as a result of stimuli being perceived as belonging to perceptual categories or as a result of some other characteristic of speech.

Repetition-attenuation of auditory N1 is modulated by the phonological representations of spoken word-forms

“…Since the representational difference between real and pseudo word-forms lies in the existence of lexical-level representations for real wordforms but only sublexical-level representations for pseudo word-forms, the current result indicates that N1 repetition-attenuation can capture the difference of representational levels of spoken word-forms. The N1-attenuation's sensitivity to stimuli with contrastive representational levels has also been revealed by previous studies finding differential N1 repetition-attenuation patterns between acoustically matched speech and nonspeech sounds, that is, a higher representational level for speech sounds and a lower level for nonspeech sounds (Marklund et al, 2020;Teismann et al, 2004;Woods & Elmasian, 1986).…”

“…For example, Woods and Elmasian (1986) found more attenuated N1 responses to repeated speech sounds (synthesised vowels) than to nonspeech sounds, which are matched with major psychophysical features, such as loudness, duration, and formant frequencies. In contrast, two more recent studies reported less attenuated N1 to repeated speech sounds than non-speech sounds, with careful controls for the acoustic features (Teismann et al, 2004;Marklund et al, 2020). Despite the opposite attenuation directions in speech and non-speech sounds, these studies seem to suggest that the repetition-attenuation of N1 is modulated according to stimuli's representational properties, instead of their basic acoustic parameters.…”

“…First, N1 repetition-attenuation is not likely a totally bottom-up, refractory process. Otherwise, no systematic difference should be observed during perceiving spoken word-forms that are acoustically similar but phonologically distinctive (Marklund et al, 2020).…”

“…According to this refractory account, the N1 repetition-attenuation between acoustically similar stimuli should be alike, because of the highly overlapping neural representations of these stimuli (Marklund et al, 2020).…”

“…Furthermore, the repetition-attenuation of N1 has been found to differ between speech and non-speech stimuli (e.g., Marklund et al, 2020;Teismann et al, 2004;Woods & Elmasian, 1986). For example, Woods and Elmasian (1986) found more attenuated N1 responses to repeated speech sounds (synthesised vowels) than to nonspeech sounds, which are matched with major psychophysical features, such as loudness, duration, and formant frequencies.…”