Gradient descent decomposition of force-field motor primitives optogenetically elicited for motor mapping of the murine lumbosacral spinal cord.

Generating diverse motor behaviors critical for survival is a challenge that confronts the central nervous system (CNS) of all animals. During movement execution, the CNS performs complex calculations to control a large number of neuromusculoskeletal elements. The theory of modular motor control proposes that spinal interneurons are organized in discrete modules that can be linearly combined to generate a variety of behavioral patterns. These modules have been previously represented as stimulus-evoked force fields (FFs) comprising isometric limb-endpoint forces across workspace locations. Here, we ask whether FFs elicited by different stimulations indeed represent the most elementary units of motor control or are themselves the combination of a limited number of even more fundamental motor modules. To probe for potentially more elementary modules, we optogenetically stimulated the lumbosacral spinal cord of intact and spinalized Thy1-ChR2 transgenic mice ( n=21), eliciting FFs from as many single stimulation loci as possible (20-70 loci per mouse) at minimally necessary power. We found that the resulting varieties of FFs defied simple categorization with just a few clusters. We used gradient descent to further decompose the FFs into their underlying basic force fields (BFFs), whose linear combination explained FF variability. Across mice, we identified 4-5 BFFs with partially localizable but overlapping representations along the spinal cord. The BFFs were structured and topographically distributed in such a way that a rostral-to-caudal traveling wave of activity across the lumbosacral spinal cord may generate a swing-to-stance gait cycle. These BFFs may represent more rudimentary submodules that can be flexibly merged to produce a library of motor modules for building different motor behaviors.

Finger pressure adjustments to various object configurations during precision grip in humans and monkeys.

In this study, we recorded the pressure exerted onto an object by the index finger and the thumb of the preferred hand of 18 human subjects and either hand of two macaque monkeys during a precision grasping task. The to-be-grasped object was a custom-made device composed by two plates which could be variably oriented by a motorized system while keeping constant the size and thus grip dimension. The to-be-grasped plates were covered by an array of capacitive sensors to measure specific features of finger adaptation, namely pressure intensity and centroid location and displacement. Kinematic measurements demonstrated that for human subjects and for monkeys, different plate configurations did not affect wrist velocity and grip aperture during the reaching phase. Consistently, at the instant of fingers-plates contact, pressure centroids were clustered around the same point for all handle configurations. However, small pressure centroid displacements were specifically adopted for each configuration, indicating that both humans and monkeys can display finger adaptation during precision grip. Moreover, humans applied stronger thumb pressure intensity, performed less centroid displacement and required reduced adjustment time, as compared to monkeys. These pressure patterns remain similar when different load forces were required to pull the handle, as ascertained by additional measurements in humans. The present findings indicate that, although humans and monkeys share common features in motor control of grasping, they differ in the adjustment of fingertip pressure, probably because of skill and/or morphology divergences. Such a precision grip device may form the groundwork for future studies on prehension mechanisms.

ViSA: a neurodynamic model for visuo-spatial working memory, attentional blink, and conscious access.

Two separate lines of study have clarified the role of selectivity in conscious access to visual information. Both involve presenting multiple targets and distracters: one simultaneously in a spatially distributed fashion, the other sequentially at a single location. To understand their findings in a unified framework, we propose a neurodynamic model for Visual Selection and Awareness (ViSA). ViSA supports the view that neural representations for conscious access and visuo-spatial working memory are globally distributed and are based on recurrent interactions between perceptual and access control processors. Its flexible global workspace mechanisms enable a unitary account of a broad range of effects: It accounts for the limited storage capacity of visuo-spatial working memory, attentional cueing, and efficient selection with multi-object displays, as well as for the attentional blink and associated sparing and masking effects. In particular, the speed of consolidation for storage in visuo-spatial working memory in ViSA is not fixed but depends adaptively on the input and recurrent signaling. Slowing down of consolidation due to weak bottom-up and recurrent input as a result of brief presentation and masking leads to the attentional blink. Thus, ViSA goes beyond earlier 2-stage and neuronal global workspace accounts of conscious processing limitations.

The role of the motor system in discriminating normal and degraded speech sounds.

Listening to speech recruits a network of fronto-temporo-parietal cortical areas. Classical models consider anterior, motor, sites involved in speech production whereas posterior sites involved in comprehension. This functional segregation is more and more challenged by action-perception theories suggesting that brain circuits for speech articulation and speech perception are functionally interdependent. Recent studies report that speech listening elicits motor activities analogous to production. However, the motor system could be crucially recruited only under certain conditions that make speech discrimination hard. Here, by using event-related double-pulse transcranial magnetic stimulation (TMS) on lips and tongue motor areas, we show data suggesting that the motor system may play a role in noisy, but crucially not in noise-free environments, for the discrimination of speech signals.

Vocal pitch discrimination in the motor system.

Speech production can be broadly separated into two distinct components: Phonation and Articulation. These two aspects require the efficient control of several phono-articulatory effectors. Speech is indeed generated by the vibration of the vocal-folds in the larynx (F0) followed by ''filtering" by articulators, to select certain resonant frequencies out of that wave (F1, F2, F3, etc.). Recently it has been demonstrated that the motor representation of articulators (lips and tongue) participates in the discrimination of articulatory sounds (lips- and tongue-related speech sounds). Here we investigate whether the results obtained on articulatory sounds discrimination could be extended to phonation by applying a dual-pulse TMS protocol while subjects had to discriminate F0-shifted vocal utterances [a]. Stimulation over the larynx motor representation, compared to the control site (tongue/lips motor cortex), induced a reduction in RT for stimuli including a subtle pitch shift. We demonstrate that vocal pitch discrimination, in analogy with the articulatory component, requires the contribution of the motor system and that this effect is somatotopically organized.

The motor somatotopy of speech perception.

Listening to speech recruits a network of fronto-temporo-parietal cortical areas. Classical models consider anterior (motor) sites to be involved in speech production whereas posterior sites are considered to be involved in comprehension. This functional segregation is challenged by action-perception theories suggesting that brain circuits for speech articulation and speech perception are functionally dependent. Although recent data show that speech listening elicits motor activities analogous to production, it's still debated whether motor circuits play a causal contribution to the perception of speech. Here we administered transcranial magnetic stimulation (TMS) to motor cortex controlling lips and tongue during the discrimination of lip- and tongue-articulated phonemes. We found a neurofunctional double dissociation in speech sound discrimination, supporting the idea that motor structures provide a specific functional contribution to the perception of speech sounds. Moreover, our findings show a fine-grained motor somatotopy for speech comprehension. We discuss our results in light of a modified "motor theory of speech perception" according to which speech comprehension is grounded in motor circuits not exclusively involved in speech production.