Foot rotation contribution to trunk and gaze stability during whole-body mediated gaze shifts: a principal component analysis study.

Large gaze displacements are mediated by combined motion of the eye, head, trunk, and foot. We applied principal component analysis (PCA) to examine the degree of variability and linearity in the angular velocity pattern of the various segments involved that participate in this task. Ten normal subjects stood up and had to visually fixate and realign their bodies with LED targets separated 45 degrees apart, ranging from +/-45 to 360 degrees. The outbound movement in this paradigm is unpredictable whereas the return (inbound) movement occurs under spatially predictable conditions. Under such predictable conditions, subjects generate in approximately 15% of the trials gaze shifts, with periods of fairly constant high gaze velocity (single-step gaze shifts). PCA showed that gaze velocity variability did not change if the feet were rotating or not. Foot velocity was variable and showed additional PCs suggestive of non-linear motion components. Trunk and head-in-space velocity showed intermediate levels of variability but its variability decreased during the foot stepping movements. The results suggest that the feet, trunk, and head are less tightly controlled by the central nervous system than gaze velocity. Movements of the feet seem to aid trunk stability and motion rather than gaze control.

Long time-constant behavior of the oculomotor plant in barbiturate-anesthetized primate.

The mechanics of the extraocular muscles and orbital tissue ("oculomotor plant") can be approximated by a small number of viscoelastic (Voigt) elements in series. Recent analysis of the eye's return from displacement in lightly anesthetized rhesus monkeys has suggested a four-element plant model with time constants (TCs) of approximately 0.01, 0.1, 1, and 10 s. To demonstrate directly the presence of long (1,10 s) TC elements and to assess their contribution quantitatively, horizontal eye displacement was induced in Cynomolgus monkeys under deep barbiturate anesthesia that prevented interference from spontaneous eye movements. The displacement was maintained for either a prolonged (30 s) or brief (0.2 s) period before release. Return to resting position took 20-30 s after prolonged displacement but only 1-2 s after brief displacement, consistent with the presence of long TC elements that would only be substantially stretched in the former condition. Quantitative fitting of the release curves after prolonged displacement indicated that the two long TC elements contribute a substantial proportion (approximately 30%) of the total plant compliance. A model based on the estimated compliance values is shown to account quantitatively both for our release data and for Goldstein and Robinson's data on hysteresis of ocular motoneuron firing rates measured after centripetal saccades following prolonged eccentric fixation. Long time-constant elements in the plant thus make a substantial contribution to some types of eye movement, and their inclusion in plant models can help interpret the firing patterns of single units in the oculomotor system.

Estimation of premotor synaptic drives to simulated abducens motoneurons for control of eye position.

The firing rate of an abducens motoneuron (AbMN) is linearly related to eye position with slope K, above recruitment threshold theta. Within the AbMN population K increases as theta increases. It is possible that these properties depend on the synaptic drives generated by the major premotor inputs to AbMNs, namely position-vestibular-pause (PVP) cells and eye and head velocity (EHV) cells in the medial vestibular nucleus, and eye-position and burst-position cells in the nucleus prepositus hypoglossi (NPH). Premotor inputs to AbMNs were therefore modelled by a two-layer net, in which the output nodes represented the AbMNs (with fixed intrinsic properties) and the input nodes the three classes of premotor units ( n=20/class). Conjugate eye-position commands were used to generate the firing rates in premotor units found experimentally. The output of the net was compared with observed AbMN firing rates, and the resultant error used to adjust the magnitude and sign of the connection weights between premotor units and AbMNs. To provide additional constraints on permitted weights, the net was also trained under simulated smooth pursuit, cancellation of the vestibular-ocular reflex, and the vestibulo-ocular reflex itself (all at 0.5 Hz). Since the projections of EHV cells have not been clearly characterized, two versions of the model were trained, corresponding to different assumptions about these projections. In both versions, position-related AbMN firing rates were derived mainly from an excitatory drive from PVP cells and an inhibitory drive from NPH cells with the opposite ON direction. Variation in AbMN threshold theta and position sensitivity K depended on the strength of the drive from the NPH: the stronger the drive, the higher both K and theta. This arrangement was observed in six variants of the basic model with different parameter values, and in a simplified form (constant PVP drive and varying NPH drive) was able to generate qualitatively the observed relationship between K and theta even in the absence of input from EHV cells. It appears to be a robust mechanism for producing the experimentally observed variation in position-related firing of AbMNs, even without a contribution from their intrinsic properties, and predicts that local blocking of the inhibitory drive from cells in the NPH should lower both the position threshold and sensitivity of an individual AbMN. The model also indicates that if EHV cells have ipsilateral inhibitory projections, as has been proposed on the basis of their similarity with cells receiving input from the flocculus, then their role in eye-position control would reinforce that of cells in the NPH.

Neural network simulations of the primate oculomotor system IV. A distributed bilateral stochastic model of the neural integrator of the vertical saccadic system.

The present report examines the performance of a distributed bi-directional neural network that simulates the vertical velocity to position integrator of the primate brain. Consistent with anatomy and physiology, its units receive stochastically weighted input from vertical medium-lead burst neurons. Also consistent with anatomy, units belonging to integrators with opposite on-directions (up or down) are interconnected via the posterior commissure (again in a stochastically weighted manner) and they can be excitatory or inhibitory. To demonstrate that integration can be a one-step process, the output of model units was routed directly to vertical motoneurons. Model units replicate the wide range of saccade-related discharge patterns encountered in the portion of the primate brain that is thought to house the vertical neural integrator (the interstitial nucleus of Cajal) while "lesions" of model units and/or their interconnections replicate the symptoms which follow insults to this brain area.