Basic cuts revisited: Temporal segmentation of speech into phone-like units with statistical learning at a pre-linguistic level

Basic cuts revisited: Temporal segmentation of speech into phone-like units with statistical learning at a pre-linguistic level Okko Rasanen (okko.rasanen@aalto.fi) Department of Signal Processing and Acoustics, Aalto University PO Box 13000, 00076 Aalto, FINLAND Abstract Considerable effort has been put to understand how infants may utilize statistical regularities of speech in early word segmentation. Some studies suggest that infants are able to discover word boundaries at the points of high unpredictability across subsequent linguistic units such as phonemes or syllables. Meanwhile, the possible role of the statistical regularities in the temporal organization of the speech at a pre-linguistic acoustic level has not been widely addressed. The current work examines how the short-term temporal predictability of the acoustic speech signal correlates with linguistically motivated phone-, syllable-, and word-level units. The results indicate that the points of low predictability correlate mainly with the boundaries between phone-like segments. This suggests that the same statistical learning mechanisms hypothesized to operate at the word level can also aid in temporal organization of the speech stream into phone-like temporal segments before knowing the phonemic or syllabic units of the language. Keywords: distributional learning; language acquisition; phone segmentation; speech segmentation; statistical learning Introduction Segmentation of continuous speech into linguistically relevant units is essential for successful language acquisition (LA). Segmentation can take place at a number of levels, as the speech can be linguistically characterized in terms of units such as phones, syllables, and words, and with the latter always consisting of the former. In the early LA research, infants’ ability to segment words from speech has received a large amount of attention as the words are the main functional units of the language, standing for entities, events, actions, and states of the surrounding world. In the word segmentation studies, one of the major findings is that the infants can use statistical regularities in the speech input in order to discover boundaries between words (Saffran, Aslin & Newport, 1996). Also, these statistical learning mechanisms do not seem to be specific to words or even language faculty but operate across many levels of representation and perceptual domains (see, e.g., Romberg & Saffran, 2010, for a recent review). Importantly, a large body of the existing work on statistical word learning assumes that the infants are capable of representing speech input in terms of linguistically relevant units such as phones or syllables. Given the representational units, the infants are supposedly tracking transitional probabilities (TPs) between these units across time and use low-probability transitions as indications for word boundaries while the high-probability regions form representational units (Saffran et al., 1996). This strategy is valid as long as the TPs within words are higher than the TPs across word boundaries. However, the infant’s access to linguistic units such as phones or syllables and their statistics cannot be taken for granted. It is still unclear whether early adaptation to phonetic units drives lexical learning (c.f., NLM-e theory by Kuhl et al., 2008) or whether early lexical learning actually precedes, or at least parallels, the acquisition of sub-word representation of spoken language (e.g., Werker & Curtin, 2005). The “sub- word units –first” approach is challenged by the fact that the bottom-up organization of speech signal into temporally and categorically discrete units is far from trivial. Learning a phonetic or syllabic representation of the spoken language includes both the segmentation problem (division of the signal in time) and the categorization problem (assigning context-, talker-, and speaking style-dependent acoustic observations into a correct number of linguistic categories). Importantly, infants do not have access to any ground truth in either of the two tasks while learning the native language, suggesting that some speech-external factors such as feedback from lexical level or social interaction are required for successful learning. Still, it seems that even the basic problem of segmenting speech into sub-word units has been largely overlooked in the existing LA research. For example, it is unclear how well natural co-articulated speech can be segmented into sub-word units before learning the phonetic or lexical units of the language, and whether infants actually do such segmentation. Possibly the most concrete reference to early sub-word segmentation in the existing literature is the Kuhl’s concept of basic cuts: a perceptual mechanism that provides an initial low-level chunking of the speech stream into primitive phone-like units and which then gradually improves towards native language phone system through language exposure (Kuhl, 2004, and references therein). Segmentation into syllabic units is also central to many theories of LA (e.g., Jusczyk, 1993) although explicit and well-controlled studies on the segmentation process itself are few. In the speech engineering community, both phone- and syllable-level segmentation have been widely studied. The general finding is that the spectral changes (or “jumps”) in speech are good candidates for phone boundaries as they correlate with the changes in articulator positions (e.g., Almpanidis & Kotropulos, 2008; Esposito & Aversano, 2005; ten Bosch & Cranen, 2007; Scharenborg et al., 2007). On the other hand, it is known that syllabic segmentation