Using high-resolution qMRI, motor cortex subdivisions controlling the leg, hand, and face in humans and chimpanzees are compared and consistently higher myelin and iron content in the hand-knob in both species are found, suggesting an evolutionarily conserved role.
This forum article investigates the theoretical implications of the latest findings regarding the evolution of the arcuate fascicle (AF), the main white matter fibre tract for language. Thanks to ultra-high-resolution post-mortem tractography, a weak, left-lateralised connection between the inferior frontal gyrus and the middle temporal gyrus (MTG) connection was demonstrated in wild and captive chimpanzees. This proposed white matter substrate for the Faculty of Language in the Narrow sense (FLN) was previously thought to be unique to humans along with FLN itself. This finding suggests that the last common ancestor of humans and chimpanzees already possessed this neural architecture, casting into doubt the idea of a saltatory, human-specific emergence. The AF-MTG connection may therefore represent a missing link in language evolution, with strength and lateralisation that may have increased during human evolution, pointing to a more continuous evolution of language.
A central question in motor neuroscience is how the brain represents the state of the limbs to guide volitional movements. While the primate motor cortex is known to encode movement kinematics, such as velocity and direction, whether it also maintains a direct and explicit representation of hand position in 3D space remains debated. To address this, we recorded the activity of single neurons in the primary motor cortex (M1) and dorsal premotor cortex (PMd) of two male rhesus macaques performing a naturalistic, self-paced 3D reach-and-grasp task. We found significant populations of neurons in both M1 (36.2%) and PMd (21.3%) that are robustly tuned to the instantaneous 3D position of the hand. In these neurons, the tuning for hand position-characterized by localized, elongated fields-coexists with tunings for other kinematic variables, reflecting the principle of mixed selectivity. Critically, the spatial organization of these representations differs between the two areas: M1 fields are systematically oriented along cardinal axes and exhibit multi-scale spatial clustering, whereas PMd fields are more randomly organized. Furthermore, a small subset of these hand position-tuned cells is sufficient to decode the hand's 3D trajectory with high fidelity. Our findings demonstrate that an explicit and functionally organized representation of 3D hand position is a fundamental component of primate motor cortex, complementing dynamic motor signals to support high-fidelity motor control.Significance Statement To guide skilled actions, the brain must track the hand's location. While the motor cortex is known for controlling movement commands, we reveal it also creates an explicit, highly organized 3D map of hand position. This neural representation is systematically structured, differing between primary and premotor areas. This discovery reshapes our understanding of motor control, showing the brain merges spatial information ("where") with motor commands ("how") of the hand in the same areas. These insights are crucial for creating more effective brain-computer interfaces for individuals with paralysis.
Shenghao Cao 曹盛浩, Kaixi Tian 田凯茜, Shan Yu 余山· Journal of Neuroscience· 0 citations
The anterior cingulate cortex (ACC)—a component of both the limbic system and the frontal-executive network—is composed of three anatomically distinct subregions with diverse roles in processing cognitive and emotional information: rostral area 32 (A32), dorsal area 24 (A24), and ventral area 25 (A25). Pyramidal neurons (PYRs) as the neural substrates of cortical communication govern signal processing and integration differently across brain regions, layers, and pathways. Their properties and diversity across the heterogeneous ACC subregions have yet to be comprehensively characterized in primates. Here, we compared the biophysical and morphological properties of PYRs in layers 2-3 (L2-3) and 5-6 (L5-6) across ACC subregions of young adult rhesus monkeys (8 females, 11 males) using in vitro whole-cell patch-clamp recording with intracellular filling. Results showed the more pronounced laminar differences in biophysical properties of PYRs in A24 and A32 than in A25, with L2-3 PYRs exhibiting lower excitability than L5-6 PYRs. Further, A32 contained a higher proportion of PYRs receiving high-frequency-sEPSC and an enrichment of mushroom spines in L2-3 compared with L5-6. This heterogeneity in laminar differentiation aligns with cytoarchitectural differences across ACC subregions and suggests layer-specific signaling and excitatory drive in A24 and A32. In contrast, A25 showed cellular heterogeneity in firing patterns as well as different inhibitory signaling dynamics from other ACC subregions. Together, these results highlight the heterogeneity in laminar differentiation of PYR properties across ACC subregions, contributing to their diverse circuits and roles in cognitive-emotional integration and disruption in disease.
Yuxin Zhou, Maxine Hsiung, Angela L. Capriglione et al.· Journal of Neuroscience· 0 citations
The human extrastriate visual cortex contains fine-scale columns selectively responsive to motion, disparity, and color. However, the developmental interplay between these functional modules remains poorly understood. Using high-resolution functional MRI, we compared the mesoscale organization of the extrastriate cortex in 16 individuals with normal vision and 15 participants with amblyopia (PwA) caused by strabismus (n = 8) or anisometropia (n = 7). In controls, the cortical territory occupied by disparity-selective columns exhibited a competitive relationship with that of motion- and color-selective columns. In PwA, we witnessed a reduction in the size of disparity-selective columns accompanied by expansion of the cortical territory allocated to motion- and color-selective columns, while the interdigitated organization of these sites remained unchanged. At the macroscale, this phenomenon simply manifested as weaker disparity- plus stronger motion- and color-selective responses in PwA than controls. Our results show that the mesoscale modules are rivals in development allowing intact functions to usurp those that are compromised.
S. Nasr, J. Skerswetat, B. Kennedy et al.· Proceedings of the National...· 0 citations
The Cairo spiny mouse (Acomys cahirinus), a promising animal model in neuroscience, lacks a detailed neuroanatomical map of its cerebral cortex. This study aims to delineate the location and internal subdivision of the sensorimotor cortex, specifically the region controlling the hindlimb. For this purpose, retrograde tracing from the lumbar spinal cord using Fast Blue was combined with immunohistochemical characterization using neuronal markers (NeuN, SMI‐32, and calbindin 28 kDa) in adult animals. Fast Blue‐labeled neurons were identified in layer V within the cortical region defined as the sensorimotor cortex responsible for hindlimb control. Within this region, the general cytoarchitecture was defined using NeuN, which allowed visualization of all cortical layers. The border between the lateral and medial sub‐areas was clearly identified by SMI‐32 immunostaining, which showed denser labeling in the lateral zone (presumably the primary somatosensory cortex) compared to the medial zone (presumably the secondary motor cortex). Within the defined layers and sub‐areas, a comprehensive analysis of the calbindin‐expressing interneuron population was performed, revealing significant interareal differences in soma size and cellular density of calbindin‐positive neurons exclusively within layers III–IV. These findings provide the first detailed map of the hindlimb sensorimotor cortex in spiny mice, establishing a crucial neuroanatomical foundation for future studies using this novel model in sensorimotor research.
Anton Beljajev, A. Veshchitskii, Aleksandr Mikhalkin et al.· The Journal of comparative n...· 0 citations
Functional asymmetry between the cerebral hemispheres is a defining feature of the sensorimotor system, with the dominant hemisphere playing a central role in motor control. Whether motor learning is similarly lateralized, however, remains unresolved. To tackle this question, we combined a comprehensive meta-analysis (114 datasets) with a series of well-powered, preregistered experiments (N = 526) to test two core behavioral predictions of hemispheric lateralization in sensorimotor adaptation, a canonical form of motor learning: (1) adaptation is preferentially expressed in the dominant hand and (2) transfers asymmetrically between limbs. Across both approaches, we found that adaptation and interlimb transfer were strikingly symmetric. Together, these findings support a fundamental dissociation in the neural organization of skilled behavior: whereas motor control is lateralized to the dominant hemisphere, motor learning is supported by a neural architecture that functions symmetrically.
