Predicting developmental shifts in perceptual weighting schemes

Recent models of developmental changes in speech perception suggest that the weights assigned to acoustic properties change as children gain experience with a native language. Empirical evidence supports this position, but few suggestions have been offered as to what guides this shift. These three experiments were designed to improve our ability to predict how perceptual weighting schemes change with development. The specific hypothesis explored was twofold: (1) the weight assigned by adults to any one acoustic property differs across phonetic environments according to how informative that property is in each environment; and (2) the weight assigned by children to any one acoustic property differs less across phonetic environments because children have not fully learned the patterns of covariation between phonetic informativeness and environment for each property. Experiment 1 replicated previous findings of age-related differences in the weights assigned to noise spectra and formant transitions in labeling of syllable-initial fricatives (/s/ or /[symbol: see text]/). In experiment 2 the variation in F3-onset frequency associated with place of fricative constriction was eliminated. This property differs more (i.e., is more informative) in /u/ than in /a/. Accordingly adults' transition effect was reduced more for /u/ than for /a/ from experiment 1. Children's transition effect was similarly reduced across vowel environments. In experiment 3, F3-onset frequency was appropriately manipulated for both vowels, and adults transition effect increased more for /u/ than for /a/ from experiment 2. The increase in children's transition effect was more similar across vowels. We conclude that the children had not fully learned how information provided by F3 transitions varies across /a/ and /u/ environments, and suggest that developmental weighting shifts may be guided by children learning the relation between phonetic informativeness and environment.     

210Pb geochronology of flood events in large tropical river systems

Floodplain sedimentation removes particles from fluvial transport and constructs stratigraphic records of flooding, biogeochemical sequestration and other aspects of the environmental history of river basins-insight that is enhanced by accurate geochronology. The natural fallout radionuclide (210)Pb, often employed to date lacustrine and marine sediments, has previously been used to determine floodplain accumulation rates over decadal-to-century time scales using the assumption that both input concentration and sediment accumulation rates are constant. We test this model in approximately 110 cores of pristine floodplains along approximately 2000 km of the Rios Beni and Mamore in northern Bolivia; over 95 per cent of the (210)Pb profiles depict individual episodic deposition events, not steady-state accumulation, requiring a revised geochronological methodology. Discrete measurements of down-core, clay-normalized adsorbed excess (210)Pb activity are coupled with a new conceptual model of (210)Pb input during floods: constant initial reach clay activity, unknown sedimentation (CIRCAUS). This enhanced methodology yields (210)Pb dates that correspond well with (i) dates determined from meteoric caps, (ii) observed dates of river bar formation, (iii) known flood dates, and (iv) dates from nearby cores along the same transect. Similar results have been found for other large rivers. The CIRCAUS method for geochronology therefore offers a flexible and accurate method for dating both episodic (decadal recurrence frequency) and constant (annual recurrence) sediment accumulation on floodplains.     

Children discover the spectral skeletons in their native language before the amplitude envelopes.

Much of speech perception research has focused on brief spectro-temporal properties in the signal, but some studies have shown that adults can recover linguistic form when those properties are absent. In this experiment, 7-year-old English-speaking children demonstrated adultlike abilities to understand speech when only sine waves (SWs) replicating the 3 lowest resonances of the vocal tract were presented, but they failed to demonstrate comparable abilities when noise bands amplitude-modulated with envelopes derived from the same signals were presented. In contrast, adults who were not native English speakers but who were competent 2nd-language learners were worse at understanding both kinds of stimuli than native English-speaking adults. Results showed that children learn to extract linguistic form from signals that preserve some spectral structure, even if degraded, before they learn to do so for signals that preserve only amplitude structure. The authors hypothesize that children’s early sensitivity to global spectral structure reflects the role that it may play in language learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)