Morphological evolution of language-relevant brain areas

Predictive coding and dimension-selective attention enhance the lateralization of spoken language processing

Hemispheric lateralization in speech and language processing exemplifies functional brain specialization. Seminal work in patients with left hemisphere damage highlighted the left-hemispheric dominance in language functions. However, speech processing is not confined to the left hemisphere. Hence, some researchers associate lateralization with auditory processing asymmetries: slow temporal and fine spectral acoustic information is preferentially processed in right auditory regions, while faster temporal information is primarily handled by left auditory regions. Other scholars posit that lateralization relates more to linguistic processing, particularly for speech and speech-like stimuli. We argue that these seemingly distinct accounts are interdependent. Linguistic analysis of speech relies on top-down processes, such as predictive coding and dimension-selective auditory attention, which enhance lateralized processing by engaging left-lateralized sensorimotor networks. Our review highlights that lateralization is weaker for simple sounds, stronger for speech-like sounds, and strongest for meaningful speech. Evidence shows that predictive speech processing and selective attention enhance lateralization. We illustrate that these top-down processes rely on left-lateralized sensorimotor networks and provide insights into the role of these networks in speech processing.

Enhanced cortical activity of swallowing under acid stimulation in normal individuals: an fNIRS study

Introduction

The aims of this study are to investigate the activation patterns of the cerebral cortex in healthy individuals during liquid swallowing, as well as the differences in cerebral cortex activation between swallowing distilled water and swallowing acidic solutions, using functional near-infrared spectroscopy (fNIRS).

Methods

Eighteen healthy right-handed volunteers participated in this study. Each volunteer randomly completed two swallowing tasks: swallowing distilled water and swallowing an acidic solution, which differed in taste. By analyzing the average concentration of oxyhemoglobin across various channels, we observed the activation patterns and differences in brain regions when volunteers performed different swallowing tasks.

Results

During the act of swallowing distilled water, the significantly activated brain regions in the prefrontal cortex included the bilateral inferior frontal cortex and the right Broca's area. When swallowing an acidic solution, the significantly activated regions in the prefrontal cortex were the bilateral inferior frontal cortex (IFC), bilateral orbitofrontal cortex (OFC), bilateral dorsolateral prefrontal cortex (DLPFC), right Broca's area, left primary somatosensory cortex (S1), and left premotor/supplementary motor area (PMC/SMA). Paired t-tests revealed that the activation levels during the swallowing of acidic liquid were higher than those during the swallowing of distilled water in the bilateral dorsolateral prefrontal cortex, left primary somatosensory cortex, and left premotor/supplementary motor area.

Conclusion

Functional near-infrared spectroscopy (fNIRS) can be applied to research on brain functions related to swallowing. It has revealed differences in the activation of brain regions between healthy individuals when swallowing distilled water and sour solutions. Swallowing sour liquids activates more brain areas compared to swallowing water, suggesting that sour stimuli effectively activate the swallowing cortical network.

Transcriptomic changes across subregions of the primate cerebellum support the evolution of uniquely human behaviors

Background

Compared to other primates, humans display unique behaviors including language and complex tool use. These abilities are made possible in part by the cerebellum. This region of the hindbrain, comprising the flocculus, vermis, and lateral hemispheres, has expanded throughout primate evolution, particularly in great apes. Given the cerebellum's architecture-differing in connectivity, neuron content, and functions across subregions-examining subregional differences is crucial to understanding its evolutionary trajectory.

Results

We performed bulk RNA-seq across samples from six primate species, representing 40-50 million years of evolutionary history, across four subregions of the cerebellum (vermis, flocculus, right lateral hemisphere, left lateral hemisphere). We analyzed changes in gene expression with respect to evolutionary relationships via the Ornstein-Uhlenbeck model and found that, on average, 8.5% of orthologous genes are differentially expressed in humans relative to other non-human primates. Subregion-specific gene expression patterns reveal that the primate lateral hemispheres exhibit significant differences in synaptic activity and glucose metabolism, which in turn are highly implicated in neural processing.

Conclusions

This study provides a novel perspective on gene expression divergences across cerebellar subregions in multiple primate species, offering valuable insights into the evolution of this brain structure. Our findings reveal distinct subregional transcriptomic patterns, with the lateral hemispheres emerging as key sites of divergence across the six primate species. The enrichment of genes related to synaptic activity, glucose metabolism, locomotion, and vocalization highlights the cerebellum's crucial role in supporting the neural complexity underlying uniquely human and other species-specific primate behaviors.

Gray matter volume and asymmetry in Broca's and Wernicke's area homologs in chimpanzees (Pan troglodytes) using a probabilistic region of interest approach

Broca's and Wernicke's areas are comprised of Brodmann areas 44, 45 and 22 in the human brain. Because of their roles in higher cognitive and linguistic function, there has been historical and contemporary interest in comparative studies on the morphology and cytoarchitectonic organization in Broca's and Wernicke's between primate species. One challenge to comparative morphological studies between human and nonhuman primates for Broca's and Wernicke's areas is the absence in homologous sulci used to define these regions. To address this limitation, we created probabilistic atlas maps of BA44, BA45 and BA22 based on previously reported cytoarchitectonic maps of these regions in chimpanzees. We then applied the maps to segmented gray matter volume to estimate gray matter within each region and hemisphere. Females were found to have significantly higher gray matter volumes for BA44 and BA45 compared males. Significant negative associations were found between age and gray matter volume for BA44 and BA45 but not BA22. Population-level asymmetries were found for BA44, BA45 and BA22 but there are some limitations in the interpretation of these findings. Lastly, using quantitative genetic analyses, we found significant heritability in the average gray matter volume for BA44 and BA45 but not BA22. The sex and age effects found in chimpanzees are consistent with previous studies in humans.

Simultaneous invasive and non-invasive recordings in humans: A novel Rosetta stone for deciphering brain activity

Simultaneous noninvasive and invasive electrophysiological recordings provide a unique opportunity to achieve a comprehensive understanding of human brain activity, much like a Rosetta stone for human neuroscience. In this review we focus on the increasingly-used powerful combination of intracranial electroencephalography (iEEG) with scalp electroencephalography (EEG) or magnetoencephalography (MEG). We first provide practical insight on how to achieve these technically challenging recordings. We then provide examples from clinical research on how simultaneous recordings are advancing our understanding of epilepsy. This is followed by the illustration of how human neuroscience and methodological advances could benefit from these simultaneous recordings. We conclude with a call for open data sharing and collaboration, while ensuring neuroethical approaches and argue that only with a true collaborative approach the promises of simultaneous recordings will be fulfilled.

Brain structure and function: a multidisciplinary pipeline to study hominoid brain evolution

To decipher the evolution of the hominoid brain and its functions, it is essential to conduct comparative studies in primates, including our closest living relatives. However, strong ethical concerns preclude in vivo neuroimaging of great apes. We propose a responsible and multidisciplinary alternative approach that links behavior to brain anatomy in non-human primates from diverse ecological backgrounds. The brains of primates observed in the wild or in captivity are extracted and fixed shortly after natural death, and then studied using advanced MRI neuroimaging and histology to reveal macro- and microstructures. By linking detailed neuroanatomy with observed behavior within and across primate species, our approach provides new perspectives on brain evolution. Combined with endocranial brain imprints extracted from computed tomographic scans of the skulls these data provide a framework for decoding evolutionary changes in hominin fossils. This approach is poised to become a key resource for investigating the evolution and functional differentiation of hominoid brains.