Environment symmetry drives a multidirectional code in rat retrosplenial cortex

A class of neurons showing bidirectional tuning in a two-compartment environment was recently discovered in dysgranular retrosplenial cortex (dRSC). We investigated here whether these neurons possess a more general environmental symmetry-encoding property, potentially useful in representing complex spatial structure. We report that directional tuning of dRSC neurons reflected environment symmetry in onefold, twofold and fourfold-symmetric environments: this was the case not just globally, but also locally within each sub-compartment. Thus, these cells use environmental cues to organize multiple directional tuning curves, which perhaps sometimes combine via interaction with classic head direction cells. A consequence is that both local and global environmental symmetry are simultaneously encoded even within local sub-compartments, which may be important for cognitive mapping of the space beyond immediate perceptual reach.

Similar stretch reflexes and behavioral patterns are expressed by the dominant and non-dominant arms during postural control

Limb dominance is evident in many daily activities leading to the prominent idea that each hemisphere of the brain specializes in controlling different aspects of movement. Past studies suggest the dominant arm is primarily controlled via an internal model of limb dynamics that enables the nervous system to produce efficient movements. In contrast, the non-dominant arm may be primarily controlled via impedance mechanisms that rely on the strong modulation of sensory feedback from individual joints to control limb posture. We tested whether such differences are evident in behavioral responses and stretch reflexes following sudden displacement of the arm during posture control. Experiment 1 applied specific combinations of elbow-shoulder torque perturbations (the same for all participants). Peak joint displacements, return times, endpoint accuracy, and the directional tuning and amplitude of stretch reflexes in nearly all muscles were not statistically different between the two arms. Experiment 2 induced specific combinations of joint motion (the same for all participants). Again, peak joint displacements, return times, endpoint accuracy, and the directional tuning and amplitude of stretch reflexes in nearly all muscles did not differ statistically when countering the imposed loads with each arm. Moderate to strong correlations were found between stretch reflexes and behavioral responses to the perturbations with the two arms across both experiments. Collectively, the results do not support the idea that the dominant arm specializes in exploiting internal models and the non-dominant arm in impedance control by increasing reflex gains to counter sudden loads imposed on the arms during posture control.

The organization of the gravity-sensing system in zebrafish

Motor circuits develop in sequence from those governing fast movements to those governing slow. Here we examine whether upstream sensory circuits are organized by similar principles. Using serial-section electron microscopy in larval zebrafish, we generated a complete map of the gravity-sensing (utricular) system from the inner ear to the brainstem. We find that both sensory tuning and developmental sequence are organizing principles of vestibular topography. Patterned rostrocaudal innervation from hair cells to afferents creates a directional tuning map in the utricular ganglion, forming segregated pathways for rostral and caudal tilt. Furthermore, the mediolateral axis of the ganglion is linked to both developmental sequence and temporal kinetics. Early-born pathways carrying phasic information preferentially excite fast escape circuits, whereas later-born pathways carrying tonic signals excite slower postural and oculomotor circuits. These results demonstrate that vestibular circuits are organized by tuning direction and kinetics, aligning them with downstream motor circuits and behaviors.