Infant-directed speech becomes less redundant as infants grow: Implications for language learning.

Do speakers use less redundant language with more proficient interlocutors? Both the communicative efficiency framework and the language development literature predict that speech directed to younger infants should be more redundant than speech directed to older infants. Here, we test this by quantifying redundancy in infant-directed speech using entropy rate - an information-theoretic measure reflecting average degree of repetitiveness. While IDS is often described as repetitive, entropy rate provides a novel holistic measure of redundancy in this speech genre. Using two developmental corpora, we compare entropy rates of samples taken from different ages. We find that parents use less redundant speech when talking to older children, illustrating an effect of perceived interlocutor proficiency on redundancy. The developmental decrease in redundancy reflects a decrease in lexical repetition, but also a decrease in repetitions of multi-word sequences, highlighting the importance of larger sequences in early language learning.

Intonation Units in Spontaneous Speech Evoke a Neural Response.

Spontaneous speech is produced in chunks called intonation units (IUs). IUs are defined by a set of prosodic cues and presumably occur in all human languages. Recent work has shown that across different grammatical and sociocultural conditions IUs form rhythms of ∼1 unit per second. Linguistic theory suggests that IUs pace the flow of information in the discourse. As a result, IUs provide a promising and hitherto unexplored theoretical framework for studying the neural mechanisms of communication. In this article, we identify a neural response unique to the boundary defined by the IU. We measured the EEG of human participants (of either sex), who listened to different speakers recounting an emotional life event. We analyzed the speech stimuli linguistically and modeled the EEG response at word offset using a GLM approach. We find that the EEG response to IU-final words differs from the response to IU-nonfinal words even when equating acoustic boundary strength. Finally, we relate our findings to the body of research on rhythmic brain mechanisms in speech processing. We study the unique contribution of IUs and acoustic boundary strength in predicting delta-band EEG. This analysis suggests that IU-related neural activity, which is tightly linked to the classic Closure Positive Shift (CPS), could be a time-locked component that captures the previously characterized delta-band neural speech tracking.SIGNIFICANCE STATEMENT Linguistic communication is central to human experience, and its neural underpinnings are a topic of much research in recent years. Neuroscientific research has benefited from studying human behavior in naturalistic settings, an endeavor that requires explicit models of complex behavior. Usage-based linguistic theory suggests that spoken language is prosodically structured in intonation units. We reveal that the neural system is attuned to intonation units by explicitly modeling their impact on the EEG response beyond mere acoustics. To our understanding, this is the first time this is demonstrated in spontaneous speech under naturalistic conditions and under a theoretical framework that connects the prosodic chunking of speech, on the one hand, with the flow of information during communication, on the other.

The Impact of Information Structure on the Emergence of Differential Object Marking: An Experimental Study.

Many languages exhibit differential object marking (DOM), where only certain types of grammatical objects are marked with morphological cases. Traditionally, it has been claimed that DOM arises as a way to prevent ambiguity by marking objects that might otherwise be mistaken for subjects (e.g., animate objects). While some recent experimental work supports this account, research on language typology suggests at least one alternative hypothesis. In particular, DOM may instead arise as a way of marking objects that are atypical from the point of view of information structure. According to this account, rather than being marked to avoid ambiguity, objects are marked when they are given (already familiar in the discourse) rather than new. Here, we experimentally investigate this hypothesis using two artificial language learning experiments. We find that information structure impacts participants' object marking, but in an indirect way: atypical information structure leads to a change in word order, which then triggers increased object marking. Interestingly, this staged process of change is compatible with documented cases of DOM emergence. We argue that this process is driven by two cognitive tendencies. First, a tendency to place discourse given information before new information, and second, a tendency to mark noncanonical word order. Taken together, our findings provide corroborating evidence for the role of information structure in the emergence of DOM systems.

Inferring recent evolutionary changes in speech sounds.

In this paper, we investigate evolutionarily recent changes in the distributions of speech sounds in the world's languages. In particular, we explore the impact of language contact in the past two millennia on today's distributions. Based on three extensive databases of phonological inventories, we analyse the discrepancies between the distribution of speech sounds of ancient and reconstructed languages, on the one hand, and those in present-day languages, on the other. Furthermore, we analyse the degree to which the diffusion of speech sounds via language contact played a role in these discrepancies. We find evidence for substantive differences between ancient and present-day distributions, as well as for the important role of language contact in shaping these distributions over time. Moreover, our findings suggest that the distributions of speech sounds across geographic macro-areas were homogenized to an observable extent in recent millennia. Our findings suggest that what we call the Implicit Uniformitarian Hypothesis, at least with respect to the composition of phonological inventories, cannot be held uncritically. Linguists who would like to draw inferences about human language based on present-day cross-linguistic distributions must consider their theories in light of even short-term language evolution. This article is part of the theme issue 'Reconstructing prehistoric languages'.

Sequences of Intonation Units form a ~ 1 Hz rhythm.

Studies of speech processing investigate the relationship between temporal structure in speech stimuli and neural activity. Despite clear evidence that the brain tracks speech at low frequencies (~ 1 Hz), it is not well understood what linguistic information gives rise to this rhythm. In this study, we harness linguistic theory to draw attention to Intonation Units (IUs), a fundamental prosodic unit of human language, and characterize their temporal structure as captured in the speech envelope, an acoustic representation relevant to the neural processing of speech. IUs are defined by a specific pattern of syllable delivery, together with resets in pitch and articulatory force. Linguistic studies of spontaneous speech indicate that this prosodic segmentation paces new information in language use across diverse languages. Therefore, IUs provide a universal structural cue for the cognitive dynamics of speech production and comprehension. We study the relation between IUs and periodicities in the speech envelope, applying methods from investigations of neural synchronization. Our sample includes recordings from every-day speech contexts of over 100 speakers and six languages. We find that sequences of IUs form a consistent low-frequency rhythm and constitute a significant periodic cue within the speech envelope. Our findings allow to predict that IUs are utilized by the neural system when tracking speech. The methods we introduce here facilitate testing this prediction in the future (i.e., with physiological data).

Advances in Polyimide-Based Materials for Space Applications.

The space environment raises many challenges for new materials development and ground characterization. These environmental hazards in space include solar radiation, energetic particles, vacuum, micrometeoroids and debris, and space plasma. In low Earth orbits, there is also a significant concentration of highly reactive atomic oxygen (AO). This Progress Report focuses on the development of space-durable polyimide (PI)-based materials and nanocomposites and their testing under simulated space environment. Commercial PIs suffer from AO-induced erosion and surface electric charging. Modified PIs and PI-based nanocomposites are developed and tested to resist degradation in space. The durability of PIs in AO is successfully increased by addition of polyhedral oligomeric silsesquioxane. Conductive materials are prepared based on composites of PI and either carbon nanotube (CNT) sheets or 3D-graphene structures. 3D PI structures, which can expand PI space applications, made by either additive manufacturing (AM) or thermoforming, are presented. The selection of AM-processable engineering polymers in general, and PIs in particular, is relatively limited. Here, innovative preliminary results of a PI-based material processed by the PolyJet technology are presented.

3D Graphene-Infused Polyimide with Enhanced Electrothermal Performance for Long-Term Flexible Space Applications.

Polyimides (PIs) have been praised for their high thermal stability, high modulus of elasticity and tensile strength, ease of fabrication, and moldability. They are currently the standard choice for both substrates for flexible electronics and space shielding, as they render high temperature and UV stability and toughness. However, their poor thermal conductivity and completely electrically insulating characteristics have caused other limitations, such as thermal management challenges for flexible high-power electronics and spacecraft electrostatic charging. In order to target these issues, a hybrid of PI with 3D-graphene (3D-C), 3D-C/PI, is developed here. This composite renders extraordinary enhancements of thermal conductivity (one order of magnitude) and electrical conductivity (10 orders of magnitude). It withstands and keeps a stable performance throughout various bending and thermal cycles, as well as the oxidative and aggressive environment of ground-based, simulated space environments. This makes this new hybrid film a suitable material for flexible space applications.

Atomic-Oxygen-Durable and Electrically-Conductive CNT-POSS-Polyimide Flexible Films for Space Applications.

In low Earth orbit (LEO), hazards such as atomic oxygen (AO) or electrostatic discharge (ESD) degrade polymeric materials, specifically, the extensively used polyimide (PI) Kapton. We prepared PI-based nanocomposite films that show both AO durability and ESD protection by incorporating polyhedral oligomeric silsesquioxane (POSS) and carbon nanotube (CNT) additives. The unique methods that are reported prevent CNT agglomeration and degradation of the CNT properties that are common in dispersion-based processes. The influence of the POSS content on the electrical, mechanical, and thermo-optical properties of the CNT-POSS-PI films was investigated and compared to those of control PI and CNT-PI films. CNT-POSS-PI films with 5 and 15 wt % POSS content exhibited sheet resistivities as low as 200 Ω/□, and these resistivities remained essentially unchanged after exposure to AO with a fluence of ∼2.3 × 10(20) O atoms cm(-2). CNT-POSS-PI films with 15 wt % POSS content exhibited an erosion yield of 4.8 × 10(-25) cm(3) O atom(-1) under 2.3 × 10(20) O atoms cm(-2) AO fluence, roughly one order of magnitude lower than that of pure PI films. The durability of the conductivity of the composite films was demonstrated by rolling film samples with a tight radius up to 300 times. The stability of the films to thermal cycling and ionizing radiation was also demonstrated. These properties make the prepared CNT-POSS-PI films with 15 wt % POSS content excellent candidates for applications where AO durability and electrical conductivity are required for flexible and thermally stable materials. Hence, they are suggested here for LEO applications such as the outer layers of spacecraft thermal blankets.

Liquid phase deposition of a space-durable, antistatic SnO2 coating on Kapton.

Polyimides are widely used in thermal blankets covering the external surfaces of spacecrafts due to their space durability and their thermo-optical properties. However, they are susceptible to atomic oxygen (AO) erosion, the main hazard of low Earth orbit (LEO), and to electrical charging. This work demonstrates that liquid phase deposition (LPD) of 100 nm of tin oxide creates a protective coating on Kapton polyimide that has good adherence and is effective in preventing AO-induced surface erosion and in reducing electrical charging. The as-deposited tin oxide induces no significant changes in the original thermo-optical properties of the polymer and is effective in preventing electrostatic discharge (ESD). The durability of the oxide coating under AO attack was studied using oxygen RF plasma. The AO exposure did not result in any significant changes in surface morphology, thermo-optical, mechanical, and electrical properties of the tin oxide-coated Kapton. The erosion yield of tin oxide-coated Kapton was negligible after exposure to 6.4 × 10(20) O atoms·cm(-2) of LEO equivalent AO fluence, indicating a complete protection of Kapton by the LPD deposited coating. Moreover, the tin oxide coating is flexible enough so that its electrical conductivity stays within the desired range of antistatic materials despite mechanical manipulations. The advantages of liquid phase deposited oxides in terms of their not being line of site limited are well established. We now extend these advantages to coatings that reduce electrostatic discharge while still providing a high level of protection from AO erosion.

Reinforced carbon nanotubes as electrically conducting and flexible films for space applications.

Chemical vapor deposition (CVD)-grown entangled carbon nanotube (CNT) sheets are characterized by high electrical conductivity and durability to bending and folding. However, since freestanding CNT sheets are mechanically weak, they cannot be used as stand-alone flexible films. In this work, polyimide (PI) infiltration into entangled cup-stacked CNT (CSCNT) sheets was studied to form electrically conducting, robust, and flexible films for space applications. The infiltration process preserved CNTs' advantageous properties (i.e., conductivity and flexibility), prevented CNT agglomeration, and enabled CNT patterning. In particular, the CNT-PI films exhibited ohmic electrical conductance in both the lateral and vertical directions, with a sheet resistivity as low as 122 Ω/□, similar to that of as-grown CNT sheets, with minimal effect of the insulating matrix. Moreover, this high conductivity was preserved under mechanical and thermal manipulations. These properties make the reported CNT-PI films excellent candidates for applications where flexibility, thermal stability, and electrical conductivity are required. Particularly, the developed CNT-PI films were found to be durable in space environment hazards such as high vacuum, thermal cycling, and ionizing radiation, and hence they are suggested as an alternative for the electrostatic discharge (ESD) protection layer in spacecraft thermal blankets.

A novel method for on-orbit measurement of space materials degradation.

The low Earth orbit (LEO) environment is considered hazardous to spacecraft, resulting in materials degradation. Currently, in order to evaluate the degradation of materials in LEO, a retrieval of space exposed samples is required. In this study, a novel approach is proposed to evaluate degradation of materials in LEO without the need of retrieval. The method is utilizing photovoltaic cells (PVCs), an existing component onboard of any satellite. The PVCs are coated by various materials which are sensitive to different LEO constituents, such as atomic oxygen (AO) or ultra-violet (UV) radiation. The method's acronym is ORMADD (on-ORbit MAterials Degradation Detector). The ORMADD's principle of operation is based on measuring the PVC output power which depends on the cell coating material's optical transmission. Erosion of the coating by AO or coloring due to UV radiation affects its optical transmission and, accordingly, the PVC output. The ORMADD performance was tested using different coatings, such as polyimide and amorphous carbon (sensitive to AO), and siloxane based coating which is sensitive to UV radiation. The proposed ORMADD reveals sensitivity to different LEO components and can be used either as material degradation detector or as an AO monitor.