Feeling the beat: a window on the brain mechanisms enabling grammar acquisition
During language acquisition, infants display the remarkable ability to unravel which linguistic units are related to each other. Scaffolding hardwired exogenous attentional mechanisms, infants can extract statistical regularities between adjacent units, something that enables the segmentation and recognition of words. However, infants’ capacity to detect dependencies across units that are distant in time, e.g., UNbelieveABLE, remains poorly understood. In fact, brain circuits sustaining the temporal orientation of attention are still not fully mature in the early stages of development.
Crucially, speech constitutes a quasi-rhythmic entity with specific temporal regularities related to different levels of speech structure (syllable rate: 4-5 Hz, phrase rate: 1-2 Hz) (Ding et al., 2016). Furthermore, evidence has consistently shown a link between language processing efficiency and language rhythm in both adults (Assaneo et al., 2019) and infants (Martinez-Alvarez et al., 2023).
In humans, exposure to periodic acoustic stimuli induces the endogenous perception of beat (Povel & Essens, 1985). Underpinned by the phenomenon of neural entrainment (Large, 2008), beat perception enables organized processing of temporal events, fostering the detection of stimuli unfolding at specific long-distanced moments in time (Jones, 1976), as evidenced by its effect in improving perception and memory ( Hilton & Goldwater, 2021).
Therefore, bzeat perception in rhythm constitutes an endogenous effect triggered by exogenous cues presenting a patterned organization. Crucially, beat perception in rhythm is observed in infants as young as 7mo (Cirelli et al., 2016). Therefore, our hypothesis is that beat perception in speech may play a crucial role in helping infants extract non-adjacent dependencies.
To first test this hypothesis in the adult population, we designed a new experimental paradigm involving rhythmic artificial languages (ALs). ALs were designed based on rhythmic acoustic stimuli used in Nozaradan et al., ( 2011, 2017), which are shown to consistently induce neural entrainment at their beat rate. Nozaradan’s stimuli consist of repeating chains of twelve tone sounds and silences (e.g., “ xxx s xxx s xx ss” – where ‘x’ is a tone and ‘s’ a silence) at a rate of 5 Hz. If continuously listened for at least 30s, these patterns induce the automatic organization of events by groups of four (e.g., Xxx s Xxx s Xx ss – where X is a group-border tone), resulting into listener’s perception of a beat on these boundaries.
Two of Nozaradan’s rhythms were used: (i) un unsyncopated rhythm (U: Xxx s Xxx s Xx ss), where tones occur on beat; (ii) a syncopated rhythm, (S: Xxxx S xxx S x ss) where beat mostly falls on silence events. Differently from Nozaradan’s stimuli, we replaced tone sounds with syllables (i.e., xxx s xxx s xx ss → ginoli s finoto s lino ss). For both U and S rhythms two ALs were obtained. In the R+ version, non-adjacent contingencies (e.g., syllable PU- always predicts syllable KI) were placed on rhythmic beats (e.g., UR+: PUlifo KIgona fonu / SR+: PUginoKI limiso fo); Conversely, in R- condition syllables were displaced without respecting any specific rule structure. Additionally, an arhythmic AL with rules (AR+) was obtained by inserting unpredictable silences in the chain of syllables. This resulted into five ALs conditions: UB+, UB-, SB+, SB- and AR+.
The experimental paradigm consisted in a first phase of passive exposure of each AL condition. After exposure to each AL, participants underwent an implicit and explicit learning test for that specific AL. In the implicit test participants were asked to detect four alternating target syllables during a further 6.4 min exposure to the AL. Two of the targets were rule syllables, two were no-rule syllables. Response reaction times (RTs) were obtained. In the explicit test participants had to overtly recognize rule and rule-violation sequences of the AL. D-prime indexes were extracted.
These preliminary results suggest that the new experimental paradigm isolates the targeted mechanism, showing that rhythmic organization of speech facilitates the detection of non-adjacent dependencies.
ZOOM LINK: https://ub-edu.zoom.us/j/99683492311
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