Neural activity during a simple reaching task in macaques is counter to gating and rebound in basal ganglia–thalamic communication

Task-related activity in the ventral thalamus, a major target of basal ganglia output, is often assumed to be permitted or triggered by changes in basal ganglia activity through gating- or rebound-like mechanisms. To test those hypotheses, we sampled single-unit activity from connected basal ganglia output and thalamic nuclei (globus pallidus-internus [GPi] and ventrolateral anterior nucleus [VLa]) in monkeys performing a reaching task. Rate increases were the most common peri-movement change in both nuclei. Moreover, peri-movement changes generally began earlier in VLa than in GPi. Simultaneously recorded GPi-VLa pairs rarely showed short-time-scale spike-to-spike correlations or slow across-trials covariations, and both were equally positive and negative. Finally, spontaneous GPi bursts and pauses were both followed by small, slow reductions in VLa rate. These results appear incompatible with standard gating and rebound models. Still, gating or rebound may be possible in other physiological situations: simulations show how GPi-VLa communication can scale with GPi synchrony and GPi-to-VLa convergence, illuminating how synchrony of basal ganglia output during motor learning or in pathological conditions may render this pathway effective. Thus, in the healthy state, basal ganglia-thalamic communication during learned movement is more subtle than expected, with changes in firing rates possibly being dominated by a common external source

Case Studies in Forcing Small Motor Neural Circuits

The goal of this work is to explore a variety of forcing scenarios in small motor control neural circuits. Numerous motor circuits interact to produce coordinated movement, and forcing represents a convenient way to abstract the activity of other circuits so that the analysis remains tractable. First, we analyze a pacemaking circuit responsible for joint co-ordination. This circuit receives periodic bottom-up forcing from ground contact signalling. We then modulate the circuit’s pacemaking properties through top-down input, and examine the range in which this circuits output remains entrained to the forcing, allowing the animal to keep its limbs coordinated during changing pace. Next, we turn our attention to a patho-logical case of motor tics, where an unknown forcing signal produces stereotyped outputs. We build a detailed neural model which expresses a great variety of observed behavior, and carefully analyze how different current channels in the model can account for these different behaviors. We then link it to another neuron model in a small, indirectly coupled circuit and study the response to different forcing profiles, and explain the model responses to this forcing, offering mechanisms by which the motor tic may be explained. Finally, we observe that during sustained, but modulated, forcing, the detailed model exhibits chaotic behavior. We simplify the model to the key components in generating the chaotic firing, and perform mathematical analysis utilizing tools from dynamical systems to examine the route to chaos through examining its bifurcation structure and Lyapunov exponent approximations

The roles of ascending sensory signals and top-down central control in the entrainment of a locomotor CPG

Previous authors have proposed two basic hypotheses about the factors that form the basis of locomotor rhythms in walking insects: sensory feedback only or sensory feedback together with rhythmic activity of small neural circuits called central pattern generators (CPGs). Here we focus on the latter. Following this concept, to generate functional outputs, locomotor control must feature both rhythm generation by CPGs at the level of individual joints and coordination of their rhythmic activities, so that all muscles are activated in an appropriate pattern. This work provides an in-depth analysis of an aspect of this coordination process based on an existing network model of stick insect locomotion. Specifically, we consider how the control system for a single joint in the stick insect leg may produce rhythmic output when subjected to ascending sensory signals from other joints in the leg. In this work, the core rhythm generating CPG component of the joint under study is represented by a classical half-center oscillator constrained by a basic set of experimental observations. While the dynamical features of this CPG, including phase transitions by escape and release, are well understood, we provide novel insights about how these transition mechanisms yield entrainment to the incoming sensory signal, how entrainment can be lost under variation of signal strength and period or other perturbations, how entrainment can be restored by modulation of tonic top-down drive levels, and how these factors impact the duty cycle of the motor output