Sensitivity to the sonority sequencing principle in rats (Rattus norvegicus)

Abstract

Albeit diverse, human languages exhibit universal structures. A salient example is the syllable, an important structure of language acquisition. The structure of syllables is determined by the Sonority Sequencing Principle (SSP), a linguistic constraint according to which phoneme intensity must increase at onset, reaching a peak at nucleus (vowel), and decline at offset. Such structure generates an intensity pattern with an arch shape. In humans, sensitivity to restrictions imposed by the SSP on syllables appears at birth, raising questions about its emergence. We investigated the biological mechanisms at the foundations of the SSP, testing a nonhuman, non-vocal-learner species with the same language materials used with humans. Rats discriminated well-structured syllables (e.g., pras) from ill-structured ones (e.g., lbug) after being familiarized with syllabic structures conforming to the SSP. In contrast, we did not observe evidence that rats familiarized with syllables that violate such constraint discriminated at test. This research provides the first evidence of sensitivity to the SSP in a nonhuman species, which likely stems from evolutionary-ancient cross-species biological predispositions for natural acoustic patterns. Humans' early sensitivity to the SSP possibly emerges from general auditory processing that favors sounds depicting an arch-shaped envelope, common amongst animal vocalizations. Ancient sensory mechanisms, responsible for processing vocalizations in the wild, would constitute an entry-gate for human language acquisition.

Figure 1

Changing sonority across syllables. (A) Schematic representation of the SSP. Left: from, example of a well-structured syllable matching the SSP. The intensity of /f/ and /r/ rises until /o/ is reached, and subsequently falls. Sonority degrees (s) of such phonemes are: /f/ s = 1, /r/ s = 3, /o/ s = 5, /m/ s = 2. The highest the value, the most sonorant the phoneme; vowels are the most sonorant of all. Right: lbug, example of an ill-structure syllable violating the SSP. Sonority degrees (s) are: /l/ s = 3, /b/ s = 1, /u/ s = 5, /g/ s = 1. The drop of intensity is caused by the second phoneme /b/ having a lower intensity than the initial phoneme /l/. (B) Syllables used in the three experiments, during Familiarization (above) and Test (below). Syllables are retrieved from. (C) Examples of intensity contours of syllables used in the three experiments (one per category: Rise, Fall, Plateau). Contours are plotted in Praat (version 6.1.53). Y axis indicates intensity (dB), X axis indicates time, the vertical dotted line indicates the mid of the temporal window. Note that Plateau syllables are not completely flat, but show a flatter intensity contour at the onset (before the mid line) with respect to Rise and Fall syllables.

Figure 2

Test results. Number of nose-poking responses to syllables at test for Rise, Plateau and Fall experiments. Dots indicate mean responses, error bars indicate standard error of the mean. Grey lines connect individual responses to syllables presented at test in each experiment.

Figure 3

Familiarization results. Experiments: nose-poking responses during the last 5 sessions of familiarization of Rise (green), Plateau (orange) and Fall (purple) experiments. Dots indicate mean responses during a session, error bars indicate standard error of the mean. Faded lines indicate rats’ individual performance. Summary: mean number of responses across sessions for each experiment.