Silver nanoparticles inhibit nitrogen fixation in soybean (Glycine max) root nodules.

Antimicrobial silver nanoparticles (AgNPs) are popular in consumer and industrial products, leading to increasing concentrations in the environment. We tested whether exposure to AgNPs could be detrimental to a microbe, its host plant, and their symbiotic relationship. When subjected to 10 µg/mL AgNPs, growth of Bradyrhizobium japonicum USDA 110 was halted. Axenic nitrogen-fertilized Glycine max seedlings were unaffected by 2.5 µg/mL of 30 nm AgNPs, but growth was inhibited with the same dose of 16 nm AgNPs. With 2.5 µg/mL AgNPs, biomass of inoculated plants was 50% of the control. Bacteroids were not found in nodules on plants treated with 2.5 µg/mL AgNPs and plants given 0.5-2.5 µg/mL AgNPs had 40-65% decreased nitrogen fixation. In conclusion, AgNPs not only interfere with general plant and bacterial growth but also inhibit nodule development and bacterial nitrogen fixation. We should be mindful of not releasing AgNPs to the environment or to agricultural land.

The contribution of individual differences in statistical learning to reading and spelling performance in children with and without dyslexia.

Using an individual differences approach in children with and without dyslexia, this study investigated the hypothesized relationship between statistical learning ability and literacy (reading and spelling) skills. We examined the clinical relevance of statistical learning (serial reaction time and visual statistical learning tasks) by controlling for potential confounds at the participant level (e.g., non-verbal reasoning, attention and phonological skills including rapid automatized naming and phonological short-term memory). A 100 Dutch-speaking 8- to 11-year-old children with and without dyslexia participated (50 per group), see also van Witteloostuijn et al. (2019) for a study with the same participants. No evidence of a relationship between statistical learning and literacy skills is found above and beyond participant-level variables. Suggestions from the literature that the link between statistical learning and literacy attainment, and therefore its clinical relevance, might be small and strongly influenced by methodological differences between studies are not contradicted by our findings.

School-Aged Children Learn Novel Categories on the Basis of Distributional Information.

Categorization of sensory stimuli is a vital process in understanding the world. In this paper we show that distributional learning plays a role in learning novel object categories in school-aged children. An 11-step continuum was constructed based on two novel animate objects by morphing one object into the other in 11 equal steps. Forty-nine children (7-9 years old) were subjected to one of two familiarization conditions during which they saw tokens from the continuum. The conditions differed in the position of the distributional peaks along the continuum. After familiarization it was tested how the children categorized the stimuli. Results show that, in line with our expectations, familiarization condition influenced categorization during the test phase, indicating that the frequency distribution of tokens in the input had induced novel object category formation. These results suggest that distributional learning could play an important role in categorizing sensory stimuli throughout life.

Assessing Visual Statistical Learning in Early-School-Aged Children: The Usefulness of an Online Reaction Time Measure.

Visual statistical learning (VSL) was traditionally tested through offline two-alternative forced choice (2-AFC) questions. More recently, online reaction time (RT) measures and alternative offline question types have been developed to further investigate learning during exposure and more adequately assess individual differences in adults (Siegelman et al., 2017b, 2018). We assessed the usefulness of these measures for investigating VSL in early-school-aged children. Secondarily, we examined the effect of introducing a cover task, potentially affecting attention, on children's VSL performance. Fifty-three children (aged 5-8 years) performed a self-paced VSL task containing triplets, in which participants determine the presentation speed and RTs to each stimulus are recorded. Half of the participants performed a cover task, while the other half did not. Online sensitivity to the statistical structure was measured by contrasting RTs to unpredictable versus predictable elements. Subsequently, participants completed 2-AFC (choose correct triplet) and 3-AFC (fill blank to complete triplet) offline questions. RTs were significantly longer for unpredictable than predictable elements, so we conclude that early-school-aged children are sensitive to the statistical structure during exposure, and that the RT task can measure that. We found no evidence as to whether children can perform above chance on offline 2-AFC or 3-AFC questions, or whether the cover task affects children's VSL performance. These results show the feasibility of using an online RT task when assessing VSL in early-school-aged children. This task therefore seems suitable for future studies that aim to investigate VSL across development or in clinical populations, perhaps together with behavioral tasks.

Statistical learning abilities of children with dyslexia across three experimental paradigms.

Statistical learning (SL) difficulties have been suggested to contribute to the linguistic and non-linguistic problems observed in children with dyslexia. Indeed, studies have demonstrated that children with dyslexia experience problems with SL, but the extent of the problems is unclear. We aimed to examine the performance of children with and without dyslexia across three distinct paradigms using both on- and offline measures, thereby tapping into different aspects of SL. 100 children with and without dyslexia (aged 8-11, 50 per group) completed three SL tasks: serial reaction time (SRT), visual statistical learning (VSL), and auditory nonadjacent dependency learning (A-NADL). Learning was measured through online reaction times during exposure in all tasks, and through offline questions in the VSL and A-NADL tasks. We find significant learning effects in all three tasks, from which we conclude that, collapsing over groups, children are sensitive to the statistical structures presented in the SRT, VSL and A-NADL tasks. No significant interactions of learning effect with group were found in any of the tasks, so we cannot conclude whether or not children with dyslexia perform differently on the SL tasks than their TD peers. These results are discussed in light of the proposed SL deficit in dyslexia.

Statistical Learning in Specific Language Impairment: A Meta-Analysis.

Purpose: The current meta-analysis provides a quantitative overview of published and unpublished studies on statistical learning in the auditory verbal domain in people with and without specific language impairment (SLI). The database used for the meta-analysis is accessible online and open to updates (Community-Augmented Meta-Analysis), which facilitates the accumulation and evaluation of previous and future studies on statistical learning in this domain. Method: A systematic literature search identified 10 unique experiments examining auditory verbal statistical learning in 213 participants with SLI and 363 without SLI, aged between 6 and 19 years. Data from qualifying studies were extracted and converted to Hedges' g effect sizes. Results: The overall standardized mean difference between participants with SLI and participants without SLI was 0.54, which was significantly different from 0 (p < .001, 95% confidence interval [0.36, 0.71]). Conclusion: Together, the results of our meta-analysis indicate a robust difference between people with SLI and people without SLI in their detection of statistical regularities in the auditory input. The detection of statistical regularities is, on average, not as effective in people with SLI compared with people without SLI. The results of this meta-analysis are congruent with a statistical learning deficit hypothesis in SLI. Supplemental Material: https://doi.org/10.23641/asha.5558074.

Visual artificial grammar learning in dyslexia: A meta-analysis.

BACKGROUND: Literacy impairments in dyslexia have been hypothesized to be (partly) due to an implicit learning deficit. However, studies of implicit visual artificial grammar learning (AGL) have often yielded null results. AIMS: The aim of this study is to weigh the evidence collected thus far by performing a meta-analysis of studies on implicit visual AGL in dyslexia. METHODS AND PROCEDURES: Thirteen studies were selected through a systematic literature search, representing data from 255 participants with dyslexia and 292 control participants (mean age range: 8.5-36.8 years old). RESULTS: If the 13 selected studies constitute a random sample, individuals with dyslexia perform worse on average than non-dyslexic individuals (average weighted effect size=0.46, 95% CI [0.14 … 0.77], p=0.008), with a larger effect in children than in adults (p=0.041; average weighted effect sizes 0.71 [sig.] versus 0.16 [non-sig.]). However, the presence of a publication bias indicates the existence of missing studies that may well null the effect. CONCLUSIONS AND IMPLICATIONS: While the studies under investigation demonstrate that implicit visual AGL is impaired in dyslexia (more so in children than in adults, if in adults at all), the detected publication bias suggests that the effect might in fact be zero.

Discriminating Non-native Vowels on the Basis of Multimodal, Auditory or Visual Information: Effects on Infants' Looking Patterns and Discrimination.

Infants' perception of speech sound contrasts is modulated by their language environment, for example by the statistical distributions of the speech sounds they hear. Infants learn to discriminate speech sounds better when their input contains a two-peaked frequency distribution of those speech sounds than when their input contains a one-peaked frequency distribution. Effects of frequency distributions on phonetic learning have been tested almost exclusively for auditory input. But auditory speech is usually accompanied by visual information, that is, by visible articulations. This study tested whether infants' phonological perception is shaped by distributions of visual speech as well as by distributions of auditory speech, by comparing learning from multimodal (i.e., auditory-visual), visual-only, or auditory-only information. Dutch 8-month-old infants were exposed to either a one-peaked or two-peaked distribution from a continuum of vowels that formed a contrast in English, but not in Dutch. We used eye tracking to measure effects of distribution and sensory modality on infants' discrimination of the contrast. Although there were no overall effects of distribution or modality, separate t-tests in each of the six training conditions demonstrated significant discrimination of the vowel contrast in the two-peaked multimodal condition. For the modalities where the mouth was visible (visual-only and multimodal) we further examined infant looking patterns for the dynamic speaker's face. Infants in the two-peaked multimodal condition looked longer at her mouth than infants in any of the three other conditions. We propose that by 8 months, infants' native vowel categories are established insofar that learning a novel contrast is supported by attention to additional information, such as visual articulations.

Observed effects of "distributional learning" may not relate to the number of peaks. A test of "dispersion" as a confounding factor.

Distributional learning of speech sounds is learning from simply being exposed to frequency distributions of speech sounds in one's surroundings. In laboratory settings, the mechanism has been reported to be discernible already after a few minutes of exposure, in both infants and adults. These "effects of distributional training" have traditionally been attributed to the difference in the number of peaks between the experimental distribution (two peaks) and the control distribution (one or zero peaks). However, none of the earlier studies fully excluded a possibly confounding effect of the dispersion in the distributions. Additionally, some studies with a non-speech control condition did not control for a possible difference between processing speech and non-speech. The current study presents an experiment that corrects both imperfections. Spanish listeners were exposed to either a bimodal distribution encompassing the Dutch contrast /ɑ/∼/a/ or a unimodal distribution with the same dispersion. Before and after training, their accuracy of categorization of [ɑ]- and [a]-tokens was measured. A traditionally calculated p-value showed no significant difference in categorization improvement between bimodally and unimodally trained participants. Because of this null result, a Bayesian method was used to assess the odds in favor of the null hypothesis. Four different Bayes factors, each calculated on a different belief in the truth value of previously found effect sizes, indicated the absence of a difference between bimodally and unimodally trained participants. The implication is that "effects of distributional training" observed in the lab are not induced by the number of peaks in the distributions.

Distributional vowel training is less effective for adults than for infants. A study using the mismatch response.

Distributional learning of speech sounds (i.e., learning from simple exposure to frequency distributions of speech sounds in the environment) has been observed in the lab repeatedly in both infants and adults. The current study is the first attempt to examine whether the capacity for using the mechanism is different in adults than in infants. To this end, a previous event-related potential study that had shown distributional learning of the English vowel contrast /æ/∼/ε/ in 2-to-3-month old Dutch infants was repeated with Dutch adults. Specifically, the adults were exposed to either a bimodal distribution that suggested the existence of the two vowels (as appropriate in English), or to a unimodal distribution that did not (as appropriate in Dutch). After exposure the participants were tested on their discrimination of a representative [æ] and a representative [ε], in an oddball paradigm for measuring mismatch responses (MMRs). Bimodally trained adults did not have a significantly larger MMR amplitude, and hence did not show significantly better neural discrimination of the test vowels, than unimodally trained adults. A direct comparison between the normalized MMR amplitudes of the adults with those of the previously tested infants showed that within a reasonable range of normalization parameters, the bimodal advantage is reliably smaller in adults than in infants, indicating that distributional learning is a weaker mechanism for learning speech sounds in adults (if it exists in that group at all) than in infants.

Fast phonetic learning occurs already in 2-to-3-month old infants: an ERP study.

An important mechanism for learning speech sounds in the first year of life is "distributional learning," i.e., learning by simply listening to the frequency distributions of the speech sounds in the environment. In the lab, fast distributional learning has been reported for infants in the second half of the first year; the present study examined whether it can also be demonstrated at a much younger age, long before the onset of language-specific speech perception (which roughly emerges between 6 and 12 months). To investigate this, Dutch infants aged 2 to 3 months were presented with either a unimodal or a bimodal vowel distribution based on the English /æ/~/ε/ contrast, for only 12 minutes. Subsequently, mismatch responses (MMRs) were measured in an oddball paradigm, where one half of the infants in each group heard a representative [æ] as the standard and a representative [ε] as the deviant, and the other half heard the same reversed. The results (from the combined MMRs during wakefulness and active sleep) disclosed a larger MMR, implying better discrimination of [æ] and [ε], for bimodally than unimodally trained infants, thus extending an effect of distributional training found in previous behavioral research to a much younger age when speech perception is still universal rather than language-specific, and to a new method (using event-related potentials). Moreover, the analysis revealed a robust interaction between the distribution (unimodal vs. bimodal) and the identity of the standard stimulus ([æ] vs. [ε]), which provides evidence for an interplay between a perceptual asymmetry and distributional learning. The outcomes show that distributional learning can affect vowel perception already in the first months of life.

Distributional training of speech sounds can be done with continuous distributions.

In previous research on distributional training of non-native speech sounds, distributions were always discontinuous: typically, each of only eight different stimuli was repeated multiple times. The current study examines distributional training with continuous distributions, in which all presented tokens are acoustically different. Adult Spanish learners of Dutch were trained on either a discontinuous or a continuous bimodal distribution of the Dutch vowel contrast /a/-/aː/. Both groups improved their perception of the contrast; this shows that continuous training works equally well as discontinuous training. Using the more natural continuous distributions is therefore recommended for future distributional learning experiments.

Context-specific acoustic differences between Peruvian and Iberian Spanish vowels.

This paper examines four acoustic properties (duration F0, F1, and F2) of the monophthongal vowels of Iberian Spanish (IS) from Madrid and Peruvian Spanish (PS) from Lima in various consonantal contexts (/s/, /f/, /t/, /p/, and /k/) and in various phrasal contexts (in isolated words and sentence-internally). Acoustic measurements on 39 speakers, balanced by dialect and gender, can be generalized to the following differences between the two dialects. The vowel /a/ has a lower first formant in PS than in IS by 6.3%. The vowels /e/ and /o/ have more peripheral second-formant (F2) values in PS than in IS by about 4%. The consonant /s/ causes more centralization of the F2 of neighboring vowels in IS than in PS. No dialectal differences are found for the effect of phrasal context. Next to the between-dialect differences in the vowels, the present study finds that /s/ has a higher spectral center of gravity in PS than in IS by about 10%, that PS speakers speak slower than IS speakers by about 9%, and that Spanish-speaking women speak slower than Spanish-speaking men by about 5% (irrespective of dialect).

A cross-dialect acoustic description of vowels: Brazilian and European Portuguese.

This paper examines four acoustic correlates of vowel identity in Brazilian Portuguese (BP) and European Portuguese (EP): first formant (F1), second formant (F2), duration, and fundamental frequency (F0). Both varieties of Portuguese display some cross-linguistically common phenomena: vowel-intrinsic duration, vowel-intrinsic pitch, gender-dependent size of the vowel space, gender-dependent duration, and a skewed symmetry in F1 between front and back vowels. Also, the average difference between the vocal tract sizes associated with /i/ and /u/, as measured from formant analyses, is comparable to the average difference between male and female vocal tract sizes. A language-specific phenomenon is that in both varieties of Portuguese the vowel-intrinsic duration effect is larger than in many other languages. Differences between BP and EP are found in duration (BP has longer stressed vowels than EP), in F1 (the lower-mid front vowel approaches its higher-mid counterpart more closely in EP than in BP), and in the size of the intrinsic pitch effect (larger for BP than for EP).

Spectral characteristics of three styles of Croatian folk singing.

This paper examines the differences between three Croatian folk singing styles, namely klapa, ojkanje, and tarankanje. In order to factor out singer-specific properties, each of the styles was performed by the same 12 professional male singers. The 36 performances were analyzed with a long-term average spectrum (LTAS) method from which direct effects of the pitch distribution were removed. After factoring out each singer's average, the 36 pitch-corrected LTAS contours were reduced to a two-dimensional representation in two ways: (1) a principal-component analysis and (2) a graphical plot of spectral slope versus speaker's formant strength. Both ways clearly separate the three styles. The spectrum of the klapa style turns out to be similar to that of speech. The ojkanje style is extremely loud and shows two spectral peaks: a sharp one tuned at twice the fundamental frequency and appropriate for long-distance communication on mountain slopes, and a broad one around 3.5 kHz, reminiscent of a speaker's formant. The tarankanje style has a very flat spectrum implemented by vocal pressedness and nasality, which is appropriate for blending into or imitating the timbral characteristics of the sopile folk instrument.