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Differential activation of a frontoparietal network explains population-level differences in statistical learning from speech [1]

['Joan Orpella', 'Department Of Psychology', 'New York University', 'New York', 'United States Of America', 'M. Florencia Assaneo', 'Institute Of Neurobiology', 'National Autonomous University Of Mexico', 'Juriquilla', 'Querétaro']

Date: 2022-07

People of all ages display the ability to detect and learn from patterns in seemingly random stimuli. Referred to as statistical learning (SL), this process is particularly critical when learning a spoken language, helping in the identification of discrete words within a spoken phrase. Here, by considering individual differences in speech auditory–motor synchronization, we demonstrate that recruitment of a specific neural network supports behavioral differences in SL from speech. While independent component analysis (ICA) of fMRI data revealed that a network of auditory and superior pre/motor regions is universally activated in the process of learning, a frontoparietal network is additionally and selectively engaged by only some individuals (high auditory–motor synchronizers). Importantly, activation of this frontoparietal network is related to a boost in learning performance, and interference with this network via articulatory suppression (AS; i.e., producing irrelevant speech during learning) normalizes performance across the entire sample. Our work provides novel insights on SL from speech and reconciles previous contrasting findings. These findings also highlight a more general need to factor in fundamental individual differences for a precise characterization of cognitive phenomena.

Funding: This work was supported by National Institutes of Health ( https://www.nih.gov/ ) grant 2R01DC05660 (D.P.), National Science Foundation ( https://www.nsf.gov/ ) grant 2043717 (P.R., D.P., J.O. and M.F.A.), European Research Council ( https://erc.europa.eu/ ) grant ERC-StG-313841 (TuningLang; RdD-B), and Spanish Ministry of Science and Innovation ( https://www.ciencia.gob.es/ ) grant BFU2017-87109-P (RdD-B), which is part of Agencia Estatal de Investigación (AEI) (Co-funded by the European Regional Development Fund. ERDF, a way to build Europe). We also thank the CERCA Program / Generalitat de Catalunya for the institutional support. DL-B was supported by a travel grant from the University of Málaga and the Ramón y Cajal program (RYC2020-029495-I) funded by the Spanish Ministry of Science and Innovation (MCIN/AEI/10.13039/50110001103) and FSE (invierte en tu futuro). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

To test this hypothesis, we used independent component analysis (ICA) of fMRI data in combination with a classic behavioral paradigm designed to interfere with the availability of the dorsal language stream for SWFL [ 20 ]. Specifically, the behavioral paradigm employed involves the contrast between passive listening (PL) and articulatory suppression (AS) conditions ( Fig 1B ). AS requires participants to repeat a nonsense syllable during (word) learning, which hampers SWFL performance [ 9 ]. ICA, on the other hand, is a data-driven neuroimaging approach well suited to identify spatially independent and temporally coherent brain networks that support specific cognitive processes [ 21 ]. Previous work using this approach has related SWFL to a network comprising auditory and superior pre/motor areas [ 14 ]. This earlier work, however, did not consider the high/low synchronizer distinction. In all, in the current experiment we investigate, in both high and low synchronizers, the brain networks engaged during SWFL under PL and AS conditions as well as the behavioral consequences of AS for performance ( Fig 1 ). We hypothesize that, if high synchronizers show better learning—putatively due to a greater reliance on the dorsal language stream—they should show a greater recruitment of this functional anatomic stream than low synchronizers during learning as well as a greater AS effect.

(A) During the SSS test, participants listen to an isochronous stream of random syllables (rate 4.5 syllables/sec) while concurrently whispering the syllable “tah.” Left panel: example of the perceived (upper panel) and produced (lower panel) signals. Green line, band-pass filtered envelope used to compute input-output synchrony. Middle panel: Synchrony between perceived and produced syllables yields a bimodal distribution, allowing the classification of participants into low (blue) and high (orange) synchronizers. While some participants spontaneously align the produced syllabic rate to the perceived one (high synchronizers), others show no modification of the produced rate due to the presence of the external rhythm (low synchronizers). Right panel: High synchronizers outperformed lows in a statistical word learning task. They also showed enhanced brain-to-speech synchronization over left frontal regions and a greater volume in the white-matter pathways connecting temporal and frontal areas [ 18 ]. (B) The word learning task consists of a learning phase wherein 4 trisyllabic pseudo-words are presented in a continuous stream. Learning is assessed post exposure. Left panel: Participants are instructed to repeat a nonsense syllable (AS condition) or to passively listen (PL condition) during the entire learning phase. Middle panel: predicted performance decreases due to AS [ 9 ]. Right panel: differences between high and low synchronizers are hypothesized at the cognitive and brain levels. AS, articulatory suppression; PL, passive listening.

In a recent study, we provided a first glance of how one might capitalize on individual differences to gain deeper mechanistic insights into SWFL: Individual listeners grouped by their spontaneous speech auditory–motor synchronization abilities turn out to differ in their SWFL performance [ 18 ]. Specifically, we showed there a behavioral task (the Spontaneous Speech Synchronization test, henceforth “SSS test”) that robustly classifies participants into high and low speech auditory–motor synchronizers. At the brain level, high synchronizers showed a greater brain-to-stimulus synchrony in the left IFG during passive speech listening as well as more volume in the white matter pathways underlying the dorsal language stream (i.e., the arcuate fasciculus) [ 19 ]. Critically, the high/low synchronizer distinction was predictive of SWFL performance ( Fig 1A ), but the relationship between auditory–motor synchrony, population-level brain differences, and SWFL remained elusive. Here, we hypothesized that the dorsal language stream, including the IFG, is not only linked to auditory–motor synchrony as previously reported but also gives high synchronizers the advantage in SWFL over low synchronizers.

There have been several experimental attempts to pinpoint the neural basis of SWFL, but the existing literature shows inconsistent results. Some studies report a correlation between SWFL performance and the activity of the superior temporal gyrus (STG) and dorsal pre/motor regions [ 12 – 15 ]. Other experiments instead implicate the left inferior frontal gyrus (IFG) [ 16 ] and its interaction with superior temporal areas [ 15 , 17 ]. We hypothesize that the inconsistency of results in the literature is a consequence of very specific individual differences in the neural resources allocated for SWFL.

Statistical learning (SL) is the capacity to use distributional information present in the environment to extract meaningful regularities. SL has been demonstrated across age groups from birth [ 1 , 2 ], sensory modalities (e.g., audition [ 3 , 4 ], vision [ 5 ], and touch [ 6 ]), representational domains [ 5 ] (temporal and spatial), and even species [ 7 , 8 ]. In the domain of speech and language processing, statistical word form learning (SWFL) is considered critical in the early stages of language acquisition as the ability to segment phonological word forms from continuous speech [ 3 , 9 ]. Segmented word forms are readily associated with meanings [ 10 ] and can also be used in subsequent stages to discover grammatical relationships [ 11 ]. While regarded fundamental to language learning by most contemporary theories, the precise neural substrates of this ubiquitous phenomenon are not well understood and remain controversial.

(A) The auditory network supports learning during PL. Upper panel: In red/yellow, the auditory network is shown over a canonical template, with MNI coordinates on the upper portion of each slice. Neurological convention is used with a p < 0.05 FWE-corrected threshold at the cluster level, with an auxiliary p < 0.001 threshold at the voxel level. Lower panel: Scatterplot displaying participants’ percentage of correct responses during PL as a function of the auditory network’s engagement. (B) The learning benefit during PL (correct answers in PLcorrect answers in AS) is related to the interplay between the time courses of the frontoparietal (red) and the auditory (green) networks. Left/right panel: a representative high synchronizer with a greater/smaller learning benefit. Lower panels: Time evolution of the XOR analysis. (C) Scatterplot displaying high synchronizers’ learning benefit as a function of the normalized XOR between the frontoparietal (red) and the auditory (green) networks. Red line: correlation of the data. Data for Fig 4B and 4C can be found in S4 Data . Data for Fig 4A can be found in S7 Data . FWE, family-wise error; MNI, Montreal Neurological Institute; PL, passive listening.

Next, we assessed whether the activity of any of the networks significantly engaged during the PL condition was predictive of SWFL. Replicating previous results [ 14 , 27 ], we found a network comprising mainly bilateral auditory regions and a small superior pre/motor cluster (henceforth, auditory network) whose activity positively correlated with learning performance in the whole sample ( Fig 4A ; Spearman correlation coefficient r = 0.42 and p = 0.032, FDR corrected). We found no significant correlations between learning and network activity in the AS condition. Since during PL (i) highs behaviorally outperformed lows; (ii) the frontoparietal network was only activated by high synchronizers; and (iii) the auditory network was related to learning performance, we next examined whether the learning benefit of high synchronizers over lows in SWFL was related to the interaction between the 2 networks (auditory and frontoparietal). Specifically, we explored the relationship between the time courses of these 2 networks at the individual listener level and the learning benefit. As illustrated in Fig 4B , high synchronizers with a greater learning benefit (defined as PL minus AS) appeared to show a distinct pattern with interweaving time courses between the auditory and frontoparietal networks. To quantify this observation, we employed an analysis typically used in electronics: XOR. Applied to our signals, this logical operation assigns a single value per time point: one [ 1 ] when a single network is above baseline activity, or zero (0) otherwise; that is, a one is assigned when one or the other network (but not both) is active ( Fig 4B , lower insets). For each high synchronizer, we averaged the XOR over time, and correlated this value with their learning benefit (PL-AS) (note that this analysis would be meaningless for low synchronizers, given the nonsignificant activation of their frontoparietal network). In line with the observed pattern, a positive correlation was found ( Fig 4C ; Spearman correlation coefficient r = 0.65, p < 0.005). This suggests that the learning benefit shown by high synchronizers over lows is related to a specific pattern of activity highlighted by the XOR rather than a perfect correlation between time courses of the networks.

(A) In red/yellow, the frontoparietal network is shown over a canonical template, with MNI coordinates on the lower portion of each slice. Neurological convention is used. A p < 0.05 FWE-corrected threshold at the cluster level with an auxiliary p < 0.001 threshold at the voxel is used. This is the only network showing significant differences in activity between high and low synchronizers during PL (see bar plots on the lower right; * p < 0.05, FDR corrected). ( B) Scatterplot displaying participants’ PLV during AS as a function of the frontoparietal network’s engagement. Black line represents the correlation of the data. Left panel: all participants. Right panel: high synchronizers. Data for Fig 3A (top bar plot) can be found in S7 Data . Data for Fig 3A (bottom bar plot) and for Fig 3B can be found in S3 Data . FDR, false discovery rate; FWE, family-wise error; MNI, Montreal Neurological Institute; PL, passive listening; PLV, phase locking value.

Similarly, during AS, only high synchronizers significantly engaged the frontoparietal network ( Fig 3A ; Mann–Whitney–Wilcoxon test, 2-sided p high < 0.005 and p low = 0.42, FDR corrected), again with moderate evidence in favor of the null hypothesis for the lows (BF 01 = 4.1). In this condition, however, the network’s activity did not differentiate between the groups. Given that groups were defined by their speech auditory–motor synchrony, we then correlated the engagement of the frontoparietal network with the synchronization (phase locking value, PLV) between the perceived and produced syllables during AS. Indeed, these measures were positively correlated in the entire sample as well as in the high synchronizers only ( Fig 3B ; Spearman correlation coefficient r all = 0.41 and r high = 0.56, p all = 0.009 and p high = 0.012), suggesting a link between spontaneous auditory–motor synchrony and frontoparietal network engagement.

Using the Group ICA of fMRI Toolbox (GIFT; see Methods ) [ 21 ], we identified 5 brain networks that were significantly recruited during SWFL in the PL and/or the AS condition ( S2 and S3 Figs). Critically, a frontoparietal network including bilateral inferior and middle frontal gyri, inferior parietal cortex, and the supplementary motor area distinguished between high and low synchronizers during the PL condition ( Fig 2A ; N high = 18, N low = 20, Mann–Whitney–Wilcoxon test, 2-sided p = 0.038, false discovery rate [FDR] corrected). Moreover, while the activity of this network during PL was statistically significant for high synchronizers, it was not for the lows (Mann–Whitney–Wilcoxon test, 2-sided p high < 0.005 and p low = 0.9, respectively, FDR corrected). Moreover, we found moderate evidence in favor of the null hypothesis that the network was not activated during PL for the lows (Bayes Factor BF 01 = 4).

Having established the expected behavioral differences between high and low synchronizers, we next acquired fMRI data from a new group of participants (N = 41) while they performed the same behavioral paradigm (see Methods ). The SWFL paradigm was optimized for fMRI testing. Specifically, we included both a rest block and a speech motor block as control conditions. During the speech motor block, participants were required to repeatedly whisper the syllable “tah” with no concurrent auditory input. The behavioral performance in the scanner showed the same trend as the learning pattern obtained with the first sample ( S1 Fig ; Condition (PL > AS): χ2 = 5.40, p < 0.05; Group (Highs > Lows): χ2 = 3.67, p = 0.055; Condition*Group: χ2 = 2.74, p = 0.098; Highs (PL > AS): zratio = 2.32, p < 0.05; Lows (PL versus AS): zratio = 0.08, p = 0.93; Highs versus Lows in PL: zratio = 1.92, p = 0.055; Highs versus Lows in AS: zratio = 0.05, p = 0.96). Even under notably adverse listening/learning conditions (i.e., during fMRI scanning), the detrimental effect of the AS condition was restricted to the high synchronizers.

A linear mixed-model analysis (N = 55; see Methods ) of the learning performance showed a significant decrement in AS relative to PL ( Fig 2C ; Main effect of Condition: χ2 = 15.4, p < 0.001). This result thus replicates previously reported AS effects on SWFL [ 9 ]. The analysis also showed a main effect of Group (Highs > Lows; χ2 = 9.11, p < 0.01) in line with our previous work [ 18 ] and, importantly, a significant interaction between the 2 factors (Condition*Group; χ2 = 4.22, p < 0.05). Critically, when the sample was next split into high and low synchronizers by estimating their corresponding marginal means (see Methods ), we observed the AS effect in the population of high synchronizers ( Fig 2D ; N high = 23, zratio = 3.92, p < 0.001) but not in the population of low synchronizers ( Fig 2D ; N low = 32, zratio = 1.63, p = 0.1); that is to say, the performance of low synchronizers was not modulated by the action of speaking during the learning phase. Additionally, in line with previously reported data [ 18 ], high synchronizers outperformed lows in the PL condition (zratio = 3.02, p < 0.01), but there was no difference in performance between groups in the AS condition (zratio = 0.64, p = 0.52). Importantly, for all groups and conditions, learning remained significantly above chance (signed rank tests against chance level, 2-sided: p high/AS < 0.001, p high/PL < 0.001, p low/AS < 0.001, p low/PL < 0.001).

(A) SSS test outcome. Histogram of the PLVs (measure of speech-to-speech synchrony; see Methods ) between the envelope of the perceived and produced speech signals, band-pass filtered at 3.5 to 5.5 Hz. Black line, critical value separating high and low synchronizers (N = 55; see Methods ). (B) Participants’ PLV during AS as a function of the PLV from the SSS test. Red line represents the correlation of the data. (C) Percentage of correct responses during PL and AS across the entire sample. (D) Percentage of correct responses during PL and AS for the low (blue) and the high (orange) synchronizers. *p < 0.05, **p < 0.01, ***p < 0.001 Linear mixed model results. Dots: model predicted group means. Bars: 95% confidence interval. Data for Fig 2A and 2B can be found in S1 Data . Data for Fig 2C and 2D can be found in S2 Data . AS, articulatory suppression; PL, passive listening; PLV, phase locking value; SSS test, Spontaneous Speech Synchronization test.

An initial cohort (N = 55, 34 females; mean age, 22; age range, 18 to 37) underwent behavioral testing. Participants completed 4 blocks of statistical word learning in 2 different experimental conditions, PL and AS, followed by the SSS test (Methods and Fig 1 ). In both tasks, the auditory stimuli were presented at a rate of 4.5 syllables per second, corresponding to the mean syllable rate across languages [ 22 – 24 ] and the natural frequency of speech motor regions [ 25 ]. The outcome of the SSS test showed the expected [ 18 , 26 ] bimodal distribution, allowing the classification of participants into high and low synchronizers ( Fig 2A ). Moreover, the synchrony between perceived and produced syllables in the SSS test was highly correlated with that in the AS blocks ( Fig 2B ; N = 55, Spearman correlation coefficient r = 0.75, p < 0.001). This demonstrates that speech-to-speech synchrony is not only reliable across time, as was previously demonstrated [ 18 ], but also across tasks, confirming that auditory–motor synchrony is a stable feature of each individual.

Discussion

The behavioral and neuroimaging data show that the neural substrates supporting SWFL vary across individuals in a systematic way. We arrived at this observation by splitting the population into 2 groups according to their spontaneous speech auditory–motor synchronization abilities (Fig 2), a classification that has now been shown to be robust in a number of experiments, both in-lab and online, in different languages, and with different experimental manipulations [18,26]. Specifically, we found 2 distinct networks related to SWFL performance. One network encompasses mainly auditory regions and a small superior pre/motor cluster (auditory network), appears to be universally or generically recruited, and directly correlates with learning. Another network, including inferior frontal, inferior parietal and supplementary motor areas (frontoparietal network), is neither necessary nor sufficient for learning, yet it boosts learning performance. This latter network, whose activity correlates with spontaneous auditory–motor synchrony, is exclusively recruited by high auditory–motor synchronizers during learning. These observations parsimoniously account for the apparently disparate results in previous SWFL literature and provide a new way to discuss SL in neural terms.

In terms of behavior, we find that the effects of AS are not universal. Typically, the execution of an AS task leads to performance deficits. We demonstrate—in 2 independent cohorts—that only the performance of participants with a high degree of auditory–motor synchronization is affected by AS. Low synchronizers, in contrast, remain unaltered in their word learning performance. Our results thus indicate that articulatory rehearsal is not necessary for SWFL but its additional recruitment confers a learning benefit: high synchronizers, who show robust AS effects, performed better than lows during PL. Note that these results are not discordant with the previous literature [9] since averaging across high and low synchronizers yields the expected overall AS effects.

At the neural level, we found an important distinction between high and low synchronizers with respect to the engagement of a frontoparietal network: Only high synchronizers engage this network during PL. While SWFL performance correlates with the activity of the auditory network across the entire sample—in line with previous literature [14]—a synergistic relationship between both networks boosts learning performance in the high synchronizer group. Importantly, the engagement of the frontoparietal network also predicted the degree of spontaneous synchronization of produced speech during the AS condition.

A relationship between auditory–motor synchronization and language skills has been previously reported in the literature [18,28,29]. For example, precision in tapping to a metronome has been argued to correlate with reading and spelling abilities in children with developmental dyslexia [30]. Similarly, synchronization to a beat correlates with phonological awareness and rapid naming in typically developing preschoolers [31]. Despite the cumulative evidence for a link between auditory–motor synchronization and these various language skills, the existence of a neural substrate shared among these apparently unrelated cognitive abilities remains an empirical question. With this question in mind, our results suggest that the reported frontoparietal cortical network subserves this shared role: On the one hand, the engagement of this network during PL confers a benefit in learning; on the other, during AS, the engagement of this network predicts the degree of speech auditory–motor synchronization.

Insofar as there are differences between high and low synchronizers at a structural level and differences in synchrony that are stable in time [18], we understand the high/low synchronizer differences reported in this and previous works as trait differences. We have also theorized on how structural connectivity differences can give rise to the synchrony differences between the groups under particular stimulation conditions (e.g., auditory stimulation within a specific frequency range [32]). Because of this, our reported group differences could also be understood as state differences, the precise functional significance of which remains an open question.

From a mechanistic and more neurophysiologically motivated perspective, we propose that enhanced syllable-level segmentation or parsing—a key prerequisite for SWFL—results from the coordinated activity between auditory and frontoparietal networks, ultimately leading to better SWFL performance. In line with this conjecture, we previously showed that the frontal—most component of the frontoparietal network, the IFG, aligns with the onset of passively perceived syllables in high synchronizers. This frontal region has also been shown to send top-down signals to auditory cortex (the main component of the auditory network) to better align its activity to the speech input [33,34]. A similar proposal has been advanced in the literature to account for the enhanced processing of phonology that results from improved auditory timing perception through auditory–motor training [35]. Broadly speaking, therefore, our results line up with recent theories of SL, which postulate the works of both learning systems (e.g., comprising auditory areas in the case of auditory input) and modulatory attentional/control systems (e.g., as supported by frontoparietal networks) underlying learning performance [36,37]. However, we add to these current views of SL by specifying the role of these modulatory systems in terms of timing operations critical to auditory–motor synchronization advantageous to learning.

On the other hand, there exist multiple and distinct frontoparietal networks associated with attention that overlap with our reported frontoparietal network in high synchronizers. Its ventral frontoparietal component (i.e., inferior prefrontal to inferior parietal cortex), for example, has been related to stimulus-driven attention [38], which may in turn be related to the salience network [39]. Note that the stimulus-driven attention network is mostly bilateral (e.g., [40]) but shows different patterns of lateralization, rightward for spatial attention and leftward for temporal attention [41]. Given the relationship between our frontoparietal network and auditory–motor synchronization (this paper and [18]), a possibility therefore is that high synchronizers’ frontoparietal engagement relates to a temporal attention mechanism.

Another possibility is that frontoparietal activity in high synchronizers relates to a control network (e.g., [42]) that flexibly interacts with other task-specific networks (e.g., [43,44]). This is possible given the activity in more dorsal frontal regions that also feature in our frontoparietal network. Interestingly, recent articles (e.g., [45,46]) show that a supra-modal frontoparietal network entrains to stimulation (sensory and via rhythmic TMS) in the theta band and that this entrainment (causally) enhances auditory working memory. This is very similar to our previous [18] and current findings, in which high synchronizers entrain to theta stimulation (higher behavioral PLV and brain-stimulus PLV during PL) and show a behavioral advantage over individuals that do not show this entrainment. The extent to which this and the aforementioned frontoparietal networks are one same network or different networks that interact for high synchronizers during the SL task cannot be answered by our current data and so remains an empirical question. However, our analysis (ICA for fMRI) indicates that, at the very least, these frontoparietal regions’ time-courses cohere in time.

There are also reasons to distinguish these frontoparietal networks from the dorsal network for goal-directed attention [47], despite a similar involvement of dorsal prefrontal regions. In contrast to research showing SL benefits from interfering with this network (e.g., [48,49]), we show that AS hinders learning. Moreover, frontoparietal involvement, which correlates with auditory–motor synchronization, confers a learning benefit during PL. It is therefore likely that the dorsal prefrontal regions we report, which are shown to cohere in time with other frontoparietal regions, perform a role different from goal-oriented attention within the context of our tasks. This is in line with the idea that the same region can undertake different roles depending on its interactions with other regions. It was not possible to determine the precise role of prefrontal regions alone from our data. On the other hand, we also show that the learning benefit relates to the way the frontoparietal network interacts with the auditory network. Another possibility, therefore, is that different kinds of dorsal prefrontal involvement during learning incur in either learning benefits or hindrance.

A possible reason for the lateralization discrepancies with Assaneo and colleagues [18] (bilateral engagement versus left lateralization) is the use of radically different measures and analyses (ICA of the BOLD signal versus a phase-locking value between an auditory stimulus and the MEG signals). Thus, although bilateral frontal and parietal regions may work together for synchrony (and learning benefits) in high synchronizers, as reflected in the ICA for fMRI analysis, each region may perform different computations to achieve that goal that are not captured by the PLV analysis, with entrainment in theta occurring only in left frontal regions. We similarly hypothesize that small structural differences (as those reported in [18], as captured by a particular method (diffusion-weighted MRI in that case) can give rise to large functional differences as appears to be the case in high synchronizers [32].

In sum, by considering individual differences in auditory–motor synchronization skills, our work sheds light onto the neural substrates of the SL of phonological word forms and shows that what appeared to be disparate results in the existing literature stems from pooling together fundamentally distinct populations. More specifically, we reveal that, beyond a universally recruited network for SWFL, an additional frontoparietal network that enables auditory–motor synchronization is selectively engaged by some individuals to produce a benefit in learning. The auditory–motor SSS test we use thus emerges, once more, as a useful tool to achieve a more nuanced characterization of speech related phenomena. This work, therefore, not only highlights the importance of considering individual differences in SL [41] but also sounds a note of caution about assuming the existence of monolithic mechanisms underlying such cognitive tasks.

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