Neck motor unit activity displays neural signatures of temporal control during sequential saccade planning

Goal-directed behavior involves the transformation of neural movement plans into appropriate muscle activity patterns. Studies involving single saccades have shown that a rapid, direct pathway links saccade planning in frontal eye fields (FEF) to neck muscle activity. It is unknown if the rapid connection between FEF and neck muscle is maintained during sequential saccade planning. We show that sequence planning signals in the FEF are preserved in the neck EMG, although the activity is delayed specifically for the second saccade. Our results suggest that while the direct link between FEF and neck muscle facilitates downstream continuation of FEF response patterns, an indirect route exists through an inhibitory control center like the basal ganglia, limiting the information flow during processing of saccade sequences. Thus, the indirect and direct pathways from the FEF may function together to enable rapid synchronous, but controlled eye-head responses to sequential gaze shifts.

A gate-and-switch model for head orientation behaviors in C. elegans

SummaryThe nervous system seamlessly integrates perception and action. This ability is essential for stable representation of and appropriate responses to the external environment. How the sensorimotor integration underlying this ability occurs at the level of individual neurons is of keen interest. In Caenorhabditis elegans, RIA interneurons receive input from sensory pathways and have reciprocal connections with head motor neurons. Through separate physiological mechanisms, RIA simultaneously encodes both head orientation and sensory stimuli. Based on these observations, we proposed a model for how RIA may integrate these two signals to detect the spatial distribution of stimuli across head sweeps and generate directional head responses. Here, we show that blocking synaptic release from RIA disrupts head orientation behaviors in response to unilaterally presented stimuli. We found that sensory encoding in RIA is gated according to head orientation. This dependence on head orientation is independent of motor encoding in RIA, suggesting a second, posture-dependent pathway upstream of RIA. This gating mechanism may allow RIA to selectively attend to stimuli that are asymmetric across head sweeps. Attractive odor removal during head bends triggers rapid head withdrawal in the opposite direction. Unlike sensory encoding, this directional response is dependent on motor inputs to and synaptic output from RIA. Together, these results suggest that RIA is part of a sensorimotor pathway that is dynamically regulated according to head orientation at two levels: the first is a gate that filters sensory representations in RIA, and the second is a switch that routes RIA synaptic output to dorsal or ventral head motor neurons.

Modeling eye-head gaze shifts in multiple contexts without motor planning

During gaze shifts, the eyes and head collaborate to rapidly capture a target (saccade) and fixate it. Accordingly, models of gaze shift control should embed both saccadic and fixation modes and a mechanism for switching between them. We demonstrate a model in which the eye and head platforms are driven by a shared gaze error signal. To limit the number of free parameters, we implement a model reduction approach in which steady-state cerebellar effects at each of their projection sites are lumped with the parameter of that site. The model topology is consistent with anatomy and neurophysiology, and can replicate eye-head responses observed in multiple experimental contexts: 1) observed gaze characteristics across species and subjects can emerge from this structure with minor parametric changes; 2) gaze can move to a goal while in the fixation mode; 3) ocular compensation for head perturbations during saccades could rely on vestibular-only cells in the vestibular nuclei with postulated projections to burst neurons; 4) two nonlinearities suffice, i.e., the experimentally-determined mapping of tectoreticular cells onto brain stem targets and the increased recruitment of the head for larger target eccentricities; 5) the effects of initial conditions on eye/head trajectories are due to neural circuit dynamics, not planning; and 6) “compensatory” ocular slow phases exist even after semicircular canal plugging, because of interconnections linking eye-head circuits. Our model structure also simulates classical vestibulo-ocular reflex and pursuit nystagmus, and provides novel neural circuit and behavioral predictions, notably that both eye-head coordination and segmental limb coordination are possible without trajectory planning.

Development and Preliminary Validation of a Parametric Pediatric Head Finite Element Model for Population-Based Impact Simulations

Head injury is the leading cause of pediatric fatality and disability in the United States (1). Although finite element (FE) method has been widely used for investigating head injury under impact, there are only a few 3D pediatric head FE models available in the literature, including a 6-month-old child head model developed by Klinich et al (2), a newborn, a 6-month-old and a 3-year-old child head model developed by Roth et al. (3, 4, 5), and a 1.5-month-old infant head model developed by Coats et al (6). Each of these models only represents a head at a single age with single head geometry. Nowadays, population-based simulations are getting more and more attention. In population-based injury simulations, impact responses for not only an individual but also a group of people can be predicted, which takes into account variations among people thus providing more realistic predictions. However, a parametric pediatric head model capable of simulating head responses for different children at different ages is currently not available. Therefore, the objective of this study is to develop a fast and efficient method to build pediatric head FE models with different head geometries and skull thickness distributions. The method was demonstrated by morphing a 6-month-old infant head FE model into three newborn infant head FE models and by validating three morphed head models against limited cadaveric test data.