Neural entrainment as a mechanism behind rate normalization in speech perception

Speech can be delivered at different rates and, as a consequence, listeners have to normalize the incoming speech signal for the rate at which it was produced. This perceptual process, known as rate normalization, is contrastive in nature: for instance, the perception of an ambiguous Dutch vowel in between short /ɑ/ and long /a:/ is biased towards hearing long /a:/ when preceded by a fast sentence context. Previously, rate normalization has (primarily) been explained in terms of durational contrast: the ambiguous vowel is perceived as longer following a fast context because the ambiguous vowel has a relatively long duration compared to the preceding shorter vowels in the fast context. However, durational contrast cannot easily account for findings of rate normalization induced by non-adjacent speech rate, or rate normalization triggered by speech rate calculated over longer periods of time. Therefore, neural entrainment of endogenous theta oscillations to the syllabic rate of the speech signal is considered as a novel mechanism behind rate normalization. Instead of contrasting the target sound to the duration of preceding sounds, it is hypothesized that listeners contrast the target sound to the entrained neural rhythm. In order to compare the two accounts of rate normalization (durational contrast vs. neural entrainment), a behavioral experiment was designed in which participants heard Dutch target words ambiguous between /ɑs/ “ash”and /a:s/ “bait”. These target words were preceded by four types of tone precursors, consisting of tone sequences with either short or long tones (71 vs. 125 ms), and presented at a fast or slow tonal rate (4 vs. 7 Hz). Categorization data show that the precursors’ tonal rate, not tonal duration, influenced listener’s perception towards one of either words. Thus, this finding challenges durational contrast, and supports neural entrainment, as the mechanism responsible for rate normalization