Complementary hemispheric lateralization of language and social processing in the human brain

Cognitive independence and interactions between cerebral hemispheres

The two cerebral hemispheres can often operate independently. But interactions between them are critical for cognition and have been implicated in a number of neuropsychiatric disorders. Neurophysiological studies have long focused on a single hemisphere. Here, we review recent studies showing neurophysiological evidence showing both independence and interactions between the hemispheres during complex behavior.

Innate network mechanisms of temporal pole for semantic cognition in neonatal and adult twin studies

What are the innate neural mechanisms scaffolding the protracted development of sophisticated human cognition observable later in life? We investigate this question by focusing on the putative hub of the human semantic memory system-the temporal pole. Combining infant- and twin-based imaging analyses, we examine the ontogenetic mechanisms and network characteristics of the functional subdivisions within the temporal pole that are specialized for semantic processing of different types in adults. Our findings reveal topologically similar temporal pole parcellations in the adult and neonatal brains. Notably, the specific functional connectivity of the dorsal and ventrolateral subdivisions with semantic-related networks are evident in neonates, significantly heritable, and associated with semantic functions in adult twins. These results demonstrate the neonatal emergence of genetically programmed functional connectivity characteristics in the temporal pole parcellations that underlie its crucial role in semantic processing, highlighting the innate network mechanisms that support semantic cognition in humans.

Do Children's Brains Function Differently During Book Reading and Screen Time? A fNIRS Study

Previous research suggests that book reading and screen time have contrasting effects on language and brain development. However, few studies have explicitly investigated whether children's brains function differently during these two activities. The present study used functional near-infrared spectroscopy (fNIRS) to measure brain response in 28 typically developing preschool-aged children (36-72 months old) during two conditions-a book reading condition, in which children listened to a story read by a live experimenter while viewing words and pictures in a book, and a screen time condition, in which children listened to a story that was played via an audio recording while viewing words and pictures on a screen. Analyses revealed significant activation in the right temporal parietal junction (TPJ) during the book reading condition only. Across regions of interest (ROIs), including the inferior and middle frontal gyrus (IMFG), the superior and middle temporal gyrus (SMTG), and the TPJ, brain response during the book reading condition was greater in right-lateralized ROIs than left-lateralized ROIs, while brain response during the screen time condition was similar across left and right ROIs. Findings suggest that the lateralization of preschool-aged children's brain function within these ROIs differs during book reading and screen time, which provides a possible neurobiological explanation for why book reading and screen time impact language development in such different ways. Findings provide important insights into how children's brains function during different types of activities (dyadic vs. solitary) and when using different types of media (print vs. digital).

Individual variation in the functional lateralization of human ventral temporal cortex: Local competition and long-range coupling

The ventral temporal cortex (VTC) of the human cerebrum is critically engaged in high-level vision. One intriguing aspect of this region is its functional lateralization, with neural responses to words being stronger in the left hemisphere, and neural responses to faces being stronger in the right hemisphere; such patterns can be summarized with a signed laterality index (LI), positive for leftward laterality. Converging evidence has suggested that word laterality emerges to couple efficiently with left-lateralized frontotemporal language regions, but evidence is more mixed regarding the sources of the right lateralization for face perception. Here, we use individual differences as a tool to test three theories of VTC organization arising from (1) local competition between words and faces driven by long-range coupling between words and language processes, (2) local competition between faces and other categories, and (3) long-range coupling with VTC and temporal areas exhibiting local competition between language and social processing. First, in an in-house functional MRI experiment, we did not obtain a negative correlation in the LIs of word and face selectivity relative to object responses, but did find a positive correlation when using selectivity relative to a fixation baseline, challenging ideas of local competition between words and faces driving rightward face lateralization. We next examined broader local LI interactions with faces using the large-scale Human Connectome Project (HCP) dataset. Face and tool LIs were significantly anti-correlated, while face and body LIs were positively correlated, consistent with the idea that generic local representational competition and cooperation may shape face lateralization. Last, we assessed the role of long-range coupling in the development of VTC lateralization. Within our in-house experiment, substantial positive correlation was evident between VTC text LI and that of several other nodes of a distributed text-processing circuit. In the HCP data, VTC face LI was both negatively correlated with language LI and positively correlated with social processing in different subregions of the posterior temporal lobe (PSL and STSp, respectively). In summary, we find no evidence of local face-word competition in VTC; instead, more generic local interactions shape multiple lateralities within VTC, including face laterality. Moreover, face laterality is also influenced by long-range coupling with social processing in the posterior temporal lobe, where social processing may become right lateralized due to local competition with language.

White-matter controllability at birth predicts social engagement and language outcomes in toddlerhood

Social engagement and language are connected through early development. Alterations in their development can have a prolonged impact on children's lives. However, the role of white matter at birth in this ongoing connection is less well-known. Here, we investigate how white matter at birth jointly supports social engagement and language outcomes in 642 infants. We use edge-centric network control theory to quantify edge controllability, or the ability of white-matter connections to drive transitions between diverse brain states, at 1 month. Next, we used connectome-based predictive modeling (CPM) to predict the Quantitative Checklist for Autism in Toddlers (Q-CHAT) for social engagement risks and the Bayley Scales of Infant and Toddler Development (BSID-III) for language skills at 18 months from edge controllability. We created the social engagement network (SEN) to predict Q-CHAT scores and the language network (LAN) to predict BSID-III scores. The SEN and LAN were complex, spanning the whole brain. They also significantly overlapped in anatomy and generalized across measures. Controllability in the SEN at 1 month partially mediated associations between Q-CHAT and BSID-III language scores at 18 months. Further, controllability in the SEN significantly differed between term and preterm infants and predicted Q-CHAT scores in an external sample of preterm infants. Together, our results suggest that the intertwined nature of social engagement and language development is rooted in an infant's white-matter controllability.

Significance Statement

During infancy and toddlerhood, social engagement and language emerge together. Delays are often observed in both simultaneously. These interactions persist into later childhood, potentially affecting life quality. We reveal that the interplay between social engagement and language milestones in toddlerhood is rooted in the infant's structural connectivity, which may assist in early risk identification of developmental delays. Insights into the early brain foundations for emerging social engagement and language skills may open opportunities for individualized interventions to improve developmental outcomes.

Individual variation in the functional lateralization of human ventral temporal cortex: Local competition and long-range coupling

The ventral temporal cortex (VTC) of the human cerebrum is critically engaged in high-level vision. One intriguing aspect of this region is its functional lateralization, with neural responses to words being stronger in the left hemisphere, and neural responses to faces being stronger in the right hemisphere; such patterns can be summarized with a signed laterality index (LI), positive for leftward laterality. Converging evidence has suggested that word laterality emerges to couple efficiently with left-lateralized frontotemporal language regions, but evidence is more mixed regarding the sources of the right-lateralization for face perception. Here, we use individual differences as a tool to test three theories of VTC organization arising from: 1) local competition between words and faces driven by long-range coupling between words and language processes, 2) local competition between faces and other categories, 3) long-range coupling with VTC and temporal areas exhibiting local competition between language and social processing. First, in an in-house functional MRI experiment, we did not obtain a negative correlation in the LIs of word and face selectivity relative to object responses, but did find a positive correlation when using selectivity relative to a fixation baseline, challenging ideas of local competition between words and faces driving rightward face lateralization. We next examined broader local LI interactions with faces using the large-scale Human Connectome Project (HCP) dataset. Face and tool LIs were significantly anti-correlated, while face and body LIs were positively correlated, consistent with the idea that generic local representational competition and cooperation may shape face lateralization. Last, we assessed the role of long-range coupling in the development of VTC lateralization. Within our in-house experiment, substantial positive correlation was evident between VTC text LI and that of several other nodes of a distributed text-processing circuit. In the HCP data, VTC face LI was both negatively correlated with language LI and positively correlated with social processing in different subregions of the posterior temporal lobe (PSL and STSp, respectively). In summary, we find no evidence of local face-word competition in VTC; instead, more generic local interactions shape multiple lateralities within VTC, including face laterality. Moreover, face laterality is also influenced by long-range coupling with social processing in the posterior temporal lobe, where social processing may become right-lateralized due to local competition with language.

Functional architecture of cerebral cortex during naturalistic movie watching

Characterizing the functional organization of cerebral cortex is a fundamental step in understanding how different kinds of information are processed in the brain. However, it is still unclear how these areas are organized during naturalistic visual and auditory stimulation. Here, we used high-resolution functional MRI data from 176 human subjects to map the macro-architecture of the entire cerebral cortex based on responses to a 60-min audiovisual movie stimulus. A data-driven clustering approach revealed a map of 24 functional areas/networks, each explicitly linked to a specific aspect of sensory or cognitive processing. Novel features of this map included an extended scene-selective network in the lateral prefrontal cortex, separate clusters responsive to human-object and human-human interaction, and a push-pull interaction between three executive control (domain-general) networks and domain-specific regions of the visual, auditory, and language cortex. Our cortical parcellation provides a comprehensive and unified map of functionally defined areas in the human cerebral cortex.

Processing Language Partly Shares Neural Genetic Basis with Processing Tools and Body Parts

Language is an evolutionarily salient faculty for humans that relies on a distributed brain network spanning across frontal, temporal, parietal, and subcortical regions. To understand whether the complex language network shares common or distinct genetic mechanisms, we examined the relationships between the genetic effects underlying the brain responses to language and a set of object domains that have been suggested to coevolve with language: tools, faces (indicating social), and body parts (indicating social and gesturing). Analyzing the twin datasets released by the Human Connectome Project that had functional magnetic resonance imaging data from human twin subjects (monozygotic and dizygotic) undergoing language and working memory tasks contrasting multiple object domains (198 females and 144 males for the language task; 192 females and 142 males for the working memory task), we identified a set of cortical regions in the frontal and temporal cortices and subcortical regions whose activity to language was significantly genetically influenced. The heterogeneity of the genetic effects among these language clusters was corroborated by significant differences of the human gene expression profiles (Allen Human Brain Atlas dataset). Among them, the bilateral basal ganglia (mainly dorsal caudate) exhibited a common genetic basis for language, tool, and body part processing, and the right superior temporal gyrus exhibited a common genetic basis for language and tool processing across multiple types of analyses. These results uncovered the heterogeneous genetic patterns of language neural processes, shedding light on the evolution of language and its shared origins with tools and bodily functions.

Artistic turns: laterality in paintings of kisses and embraces

Many lateral biases exist in human behavior, often implicit and not deliberated. Romantic kissing and embracing received experimental attention in the last three decades. We investigated laterality in paintings depicting these social interactions using two methodologies to assess whether painters depicted such biases and whether these biases could be due to observers' aesthetic preferences or painters' ability in portraying naturalistic interactions. In Study 1, we inspected about 190,000 artworks available online to classify leftward and rightward biases in romantic kisses and embraces. The comparison of 103 paintings depicting clearly lateralized interactions revealed a significant rightward bias in romantic kissing (66%) and a trend toward a leftward bias (62%) for embraces, aligning with naturalistic studies of human interactions. In Study 2, 128 participants expressed their aesthetic preference between the paintings selected in Study 1 and their vertically mirrored versions. A preference for the original paintings over their mirrored versions emerged, especially when presented in the upper portion of the screen, but no significant preference for the typical lateralization of kissing and embracing was found. These findings suggest that artists' alignment with naturalistic biases could be due to familiarity and exposure to asymmetric interactions rather than observers' aesthetic preferences.

Precision fMRI reveals that the language network exhibits adult-like left-hemispheric lateralization by 4 years of age

Left hemisphere damage in adulthood often leads to linguistic deficits, but many cases of early damage leave linguistic processing preserved, and a functional language system can develop in the right hemisphere. To explain this early apparent equipotentiality of the two hemispheres for language, some have proposed that the language system is bilateral during early development and only becomes left-lateralized with age. We examined language lateralization using functional magnetic resonance imaging with two large pediatric cohorts (total n=273 children ages 4-16; n=107 adults). Strong, adult-level left-hemispheric lateralization (in activation volume and response magnitude) was evident by age 4. Thus, although the right hemisphere can take over language function in some cases of early brain damage, and although some features of the language system do show protracted development (magnitude of language response and strength of inter-regional correlations in the language network), the left-hemisphere bias for language is robustly present by 4 years of age. These results call for alternative accounts of early equipotentiality of the two hemispheres for language.

Left-Hemisphere Cortical Language Regions Respond Equally to Observed Dialogue and Monologue

Much of the language we encounter in our everyday lives comes in the form of conversation, yet the majority of research on the neural basis of language comprehension has used input from only one speaker at a time. Twenty adults were scanned while passively observing audiovisual conversations using functional magnetic resonance imaging. In a block-design task, participants watched 20 s videos of puppets speaking either to another puppet (the dialogue condition) or directly to the viewer (the monologue condition), while the audio was either comprehensible (played forward) or incomprehensible (played backward). Individually functionally localized left-hemisphere language regions responded more to comprehensible than incomprehensible speech but did not respond differently to dialogue than monologue. In a second task, participants watched videos (1-3 min each) of two puppets conversing with each other, in which one puppet was comprehensible while the other's speech was reversed. All participants saw the same visual input but were randomly assigned which character's speech was comprehensible. In left-hemisphere cortical language regions, the time course of activity was correlated only among participants who heard the same character speaking comprehensibly, despite identical visual input across all participants. For comparison, some individually localized theory of mind regions and right-hemisphere homologues of language regions responded more to dialogue than monologue in the first task, and in the second task, activity in some regions was correlated across all participants regardless of which character was speaking comprehensibly. Together, these results suggest that canonical left-hemisphere cortical language regions are not sensitive to differences between observed dialogue and monologue.

No evidence of theory of mind reasoning in the human language network

Language comprehension and the ability to infer others' thoughts (theory of mind [ToM]) are interrelated during development and language use. However, neural evidence that bears on the relationship between language and ToM mechanisms is mixed. Although robust dissociations have been reported in brain disorders, brain activations for contrasts that target language and ToM bear similarities, and some have reported overlap. We take another look at the language-ToM relationship by evaluating the response of the language network, as measured with fMRI, to verbal and nonverbal ToM across 151 participants. Individual-participant analyses reveal that all core language regions respond more strongly when participants read vignettes about false beliefs compared to the control vignettes. However, we show that these differences are largely due to linguistic confounds, and no such effects appear in a nonverbal ToM task. These results argue against cognitive and neural overlap between language processing and ToM. In exploratory analyses, we find responses to social processing in the "periphery" of the language network-right-hemisphere homotopes of core language areas and areas in bilateral angular gyri-but these responses are not selectively ToM-related and may reflect general visual semantic processing.