Antimicrobial penetration in a dual-species oral biofilm after noncontact brushing: an in vitro study.

OBJECTIVES: Oral biofilm is inevitably left behind, even after powered brushing. As a special feature, powered brushing removes biofilm in a noncontact mode. When the brushing distance becomes too large, biofilm is left behind. We hypothesize that biofilm left behind after brushing has different viscoelastic properties than before brushing, impacting antimicrobial penetration. MATERIALS AND METHODS: In vitro grown dual-species biofilms were subjected to 20 % mechanical deformation before and after powered brushing at 4-mm brushing distance. Biofilm thickness and stress relaxation were measured for unbrushed and brushed biofilms. Stress relaxation was analyzed with a three-element Maxwell model. Antimicrobial penetration from five mouthrinses was microscopically evaluated for unbrushed and brushed biofilms. RESULTS: Thicknesses of unbrushed and brushed biofilms were similar. Brushing decreased the prevalence of fast and increased the prevalence of slow relaxation elements, which was accompanied by deeper penetration of chlorhexidine and cetylpyridinium chloride. Penetration of antimicrobials from other mouthrinses was relatively low in unbrushed and brushed biofilms. CONCLUSIONS: This confirmation of our hypothesis points to an additional advantage of powered toothbrushing in a noncontact mode, changing the viscoelastic properties of biofilm in a direction that increases antimicrobial penetration of chlorhexidine and cetylpyridinium. CLINICAL RELEVANCE: The biofilm left behind after noncontact powered toothbrushing may have less recalcitrance toward penetration of chlorhexidine and cetylpyridinium chloride than prior to brushing.

Modeling head tracking of visual targets.

Control of the head involves somatosensory, vestibular, and visual feedback. The dynamics of these three feedback systems must be identified in order to gain a greater understanding of the head control system. We have completed one step in the development of a head control model by identifying the dynamics of the visual feedback system. A mathematical model of human head tracking of visual targets in the horizontal plane was fit to experimental data from seven subjects performing a visual head tracking task. The model incorporates components based on the underlying physiology of the head control system. Using optimization methods, we were able to identify neural processing delay, visual control gain, and neck viscosity parameters in each experimental subject.

Dynamic and kinematic strategies for head movement control.

This paper describes our analysis of the complex head-neck system using a combination of experimental and modeling approaches. Dynamical analysis of head movements and EMG activation elicited by perturbation of trunk position has examined functional contributions of biomechanically and neurally generated forces in lumped systems with greatly simplified kinematics. This has revealed that visual and voluntary control of neck muscles and the dynamic and static vestibulocollic and cervicocollic reflexes preferentially govern head-neck system state in different frequency domains. It also documents redundant control, which allows the system to compensate for lesions and creates a potential for substantial variability within and between subjects. Kinematic studies have indicated the existence of reciprocal and co-contraction strategies for voluntary force generation, of a vestibulocollic strategy for stabilizing the head during body perturbations and of at least two strategies for voluntary head tracking. Each strategy appears to be executed by a specific muscle synergy that is presumably optimized to efficiently meet the demands of the task.

Predicting vestibular, proprioceptive, and biomechanical control strategies in normal and pathological head movements.

Little is known of the functionality of the vestibulocollic reflex (VCR) and cervico-collic reflex (CCR) during head and neck movements caused by perturbations of the trunk. Previously, we formulated mathematical expressions for these neck reflexes and incorporated them into a model of horizontal plane head movements. The formalism of this neuromechanical model allowed us to examine separately the main components of head movement control. In the present study, we examine selected parameters within the main components of the model, and associate variations of these parameters with disease processes affecting head and neck movements, such as loss of sensory input or modification in central or motor function. Our simulations led us to several conclusions. First, the probable use of the VCR and CCR in yaw plane head movements is to tune the head response. In the time domain, they diminish natural head oscillations (head wobble) related to head mechanics. Equivalently, in the frequency domain, they reduce the amplitude of head wobble (resonances) around 2 Hz. Second, our simulations suggest that the VCR is about ten times stronger than the CCR in normal humans. Moreover, this disproportion is associated with only very minor contributions from the CCR in yaw. Third, head oscillations (or instability) can be generated by mechanical or neural changes in the head and neck system. Finally, readjustments of central nervous system dynamic operations could provide mechanisms to compensate for sensory and motor dysfunction caused by disease.

Spatial alignment of rotational and static tilt responses of vestibulospinal neurons in the cat.

The responses of vestibulospinal neurons to 0.5-Hz, whole-body rotations in three-dimensional space and static tilts of whole-body position were studied in decerebrate and alert cats. The neurons' spatial properties for earth-vertical rotations were characterized by maximum and minimum sensitivity vectors (R(max) and R(min)) in the cat's horizontal plane. The orientation of a neuron's R(max) was not consistently related to the orientation of its maximum sensitivity vector for static tilts (T(max)). The angular difference between R(max) and T(max) was widely distributed between 0 degrees and 150 degrees, and R(max) and T(max) were aligned (i.e., within 45 degrees of each other) for only 44% (14/32) of the neurons. The alignment of R(max) and T(max) was not correlated with the neuron's sensitivity to earth-horizontal rotations, or to the orientation of R(max) in the horizontal plane. In addition, the extent to which a neuron exhibited spatiotemporal convergent (STC) behavior in response to vertical rotations was independent of the angular difference between R(max) and T(max). This suggests that the high incidence of STC responses in our sample (56%) reflects not only canal-otolith convergence, but also the presence of static and dynamic otolith inputs with misaligned directionality. The responses of vestibulospinal neurons reflect a complex combination of static and dynamic vestibular inputs that may be required by postural reflexes that vary depending on head, trunk, and limb orientation, or on the frequency of stimulation.

Modeling learning in brain stem and cerebellar sites responsible for VOR plasticity.

A simple model of vestibuloocular reflex (VOR) function was used to analyze several hypotheses currently held concerning the characteristics of VOR plasticity. The network included a direct vestibular pathway and an indirect path via the cerebellum. An optimization analysis of this model suggests that regulation of brain stem sites is critical for the proper modification of VOR gain. A more physiologically plausible learning rule was also applied to this network. Analysis of these simulation results suggests that the preferred error correction signal controlling gain modification of the VOR is the direct output of the accessory optic system (AOS) to the vestibular nuclei vs. a signal relayed through the cerebellum via floccular Purkinje cells. The potential anatomical and physiological basis for this conclusion is discussed, in relation to our current understanding of the latency of the adapted VOR response.

Interdependence of spatial properties and projection patterns of medial vestibulospinal tract neurons in the cat.

Activity of vestibular nucleus neurons with axons in the ipsi- or contralateral medial vestibulospinal tract was studied in decerebrate cats during sinusoidal, whole-body rotations in many planes in three-dimensional space. Antidromic activation of axon collaterals distinguished between neurons projecting only to neck segments from those with collaterals to C6 and/or oculomotor nucleus. Secondary neurons were identified by monosynaptic activation after labyrinth stimulation. A three-dimensional maximum activation direction vector (MAD) summarized the spatial properties of 151 of 169 neurons. The majority of secondary neurons (71%) terminated above the C6 segment. Of these, 43% had ascending collaterals to the oculomotor nucleus (VOC neurons), and 57% did not (VC neurons). The majority of VOC and VC neurons projected contralaterally and ipsilaterally, respectively. Most C6-projecting neurons could not be activated from oculomotor nucleus (V-C6 neurons) and projected primarily ipsilaterally. All VO-C6 neurons projected contralaterally. The distributions of MADs for secondary neurons with different projection patterns were different. Most VOC (84%) and contralaterally projecting VC (91%) neurons had MADs close to the activation vector of a semicircular canal pair, compared with 54% of ipsilaterally projecting VC (i-VC) and 39% of V-C6 neurons. Many i-VC (44%) and V-C6 (48%) neurons had responses suggesting convergent input from horizontal and vertical canal pairs. Horizontal and vertical gains were comparable for some, making it difficult to assign a primary canal input. MADs consistent with vertical-vertical canal pair convergence were less common. Type II yaw or type II roll responses were seen for 22% of the i-VC neurons, 68% of the V-C6 neurons, and no VOC cells. VO-C6 neurons had spatial properties between those of VOC and V-C6 neurons. These results suggest that secondary VOC neurons convey semicircular canal pair signals to both ocular and neck motor centers, perhaps linking eye and head movements. Secondary VC and V-C6 neurons carry more processed signals, possibly to drive neck and forelimb reflexes more selectively. Two groups of secondary i-VC neurons exhibited vertical-horizontal canal convergence similar to that present on neck muscles. The vertical-vertical canal convergence present on many neck muscles, however, was not present on medial vestibulospinal neurons. Spatial transformations achieved by the vestibulocollic reflex may occur in part on secondary neurons but further combination of canal signals must take place to generate compensatory muscle activity.

Relation between axon morphology in C1 spinal cord and spatial properties of medial vestibulospinal tract neurons in the cat.

Twenty-one secondary medial vestibulospinal tract neurons were recorded intraaxonally in the ventromedial funiculi of the C1 spinal cord in decerebrate, paralyzed cats. Antidromic stimulation in C6 and the oculomotor nucleus identified the projection pattern of each neuron. Responses to sinusoidal, whole-body rotations in many planes in three-dimensional space were characterized before injection of horseradish peroxidase or Neurobiotin. The spatial response properties of 19 neurons were described by a maximum activation direction vector (MAD), which defines the axis and direction of rotation that maximally excites the neuron. The other two neurons had spatio-temporal convergent behavior and no MAD was calculated. Collateral morphologies were reconstructed from serial frontal sections to reveal terminal fields in the C1 gray matter. Axons gave off multiple collaterals that terminated ipsilaterally to the stem axon. Collaterals of individual axons rarely overlapped longitudinally but projected to similar regions in the ventral horn when viewed in transverse sections. The number of primary collaterals in C1 was different for vestibulo-collic, vestibulo-oculo-collic, and C6-projecting neurons: on average one every 1.34, 1.72, and 4.25 mm, respectively. The heaviest arborization and most terminal boutons were seen in the ventral horn, in laminae VIII and IX. Varicosities on terminal branches in lamina IX were observed adjacent to large cell bodies-putative neck motoneurons-in counterstained tissue. Some collaterals had branches that extended dorsally to lamina VII. Neurons with different spatial properties had terminal fields in different regions of the ventral horn. Axons with type I responses and MADs near those of a semicircular canal pair had widely distributed collateral branches and numerous terminations in the dorsomedial, ventromedial, and spinal accessory nuclei and in lamina VIII. Axons with type I responses that suggested convergent canal pair input, with type II responses, and with spatio-temporal convergent behavior had smaller terminal fields. Some neurons with these more complex spatial properties projected to the dorsomedial and spinal accessory but not to the ventromedial nuclei. Others had focused projections to dorsolateral regions of the ventral horn with few branches in the motor nuclei.

Prediction of complex two-dimensional trajectories by a cerebellar model of smooth pursuit eye movement.

A neural network model based on the anatomy and physiology of the cerebellum is presented that can generate both simple and complex predictive pursuit, while also responding in a feedback mode to visual perturbations from an ongoing trajectory. The model allows the prediction of complex movements by adding two features that are not present in other pursuit models: an array of inputs distributed over a range of physiologically justified delays, and a novel, biologically plausible learning rule that generated changes in synaptic strengths in response to retinal slip errors that arrive after long delays. To directly test the model, its output was compared with the behavior of monkeys tracking the same trajectories. There was a close correspondence between model and monkey performance. Complex target trajectories were created by summing two or three sinusoidal components of different frequencies along horizontal and/or vertical axes. Both the model and the monkeys were able to track these complex sum-of-sines trajectories with small phase delays that averaged 8 and 20 ms in magnitude, respectively. Both the model and the monkeys showed a consistent relationship between the high- and low-frequency components of pursuit: high-frequency components were tracked with small phase lags, whereas low-frequency components were tracked with phase leads. The model was also trained to track targets moving along a circular trajectory with infrequent right-angle perturbations that moved the target along a circle meridian. Before the perturbation, the model tracked the target with very small phase differences that averaged 5 ms. After the perturbation, the model overshot the target while continuing along the expected nonperturbed circular trajectory for 80 ms, before it moved toward the new perturbed trajectory. Monkeys showed similar behaviors with an average phase difference of 3 ms during circular pursuit, followed by a perturbation response after 90 ms. In both cases, the delays required to process visual information were much longer than delays associated with nonperturbed circular and sum-of-sines pursuit. This suggests that both the model and the eye make short-term predictions about future events to compensate for visual feedback delays in receiving information about the direction of a target moving along a changing trajectory. In addition, both the eye and the model can adjust to abrupt changes in target direction on the basis of visual feedback, but do so after significant processing delays.

A dynamical model for reflex activated head movements in the horizontal plane.

We present a controls systems model of horizontal-plane head movements during perturbations of the trunk, which for the first time interfaces a model of the human head with neural feedback controllers representing the vestibulocollic (VCR) and the cervicocollic (CCR) reflexes. This model is homeomorphic such that model structure and parameters are drawn directly from anthropomorphic, biomechanical and physiological studies. Using control theory we analyzed the system model in the time and frequency domains, simulating neck movement responses to input perturbations of the trunk. Without reflex control, the head and neck system produced a second-order underdamped response with a 5.2 dB resonant peak at 2.1 Hz. Adding the CCR component to the system dampened the response by approximately 7%. Adding the VCR component dampened head oscillations by 75%. The VCR also improved low-frequency compensation by increasing the gain and phase lag, creating a phase minimum at 0.1 Hz and a phase peak at 1.1 Hz. Combining all three components (mechanics, VCR and CCR) linearly in the head and neck system reduced the amplitude of the resonant peak to 1.1 dB and increased the resonant frequency to 2.9 Hz. The closed loop results closely fit human data, and explain quantitatively the characteristic phase peak often observed.

Phase-shifted direction adaptation of the vestibulo-ocular reflex in cat.

The ability of the vestibulo-ocular reflex (VOR) to alter the phase of the motor output relative to the sensory input is examined. Alert cats were trained for 2 h with 0.25 Hz sinusoidal horizontal vestibular and vertical optokinetic rotational stimuli. In each experiment the optokinetic training stimulus was phase shifted by 0 degree, +45 degrees, -45 degrees, or 90 degrees from the vestibular stimulus. Vertical and horizontal eye movements were measured during horizontal rotations in darkness before and after the training procedure. Phase-advance experiments (+45 degrees) produced an adaptive vertical VOR with a mean phase of +28 degrees. After phase-delay experiments (-45 degrees), the adapted VOR had a mean phase of -19 degrees. The peak adaptive change in VOR gain was at or near the 0.25 Hz training frequency in each experimental group, but the gain depended in a complex manner on the testing frequency and the degree of phase shift of the training stimulus. Training with a 45 degrees phase-delayed optokinetic stimulus produced an adaptive vertical VOR with a gain that was relatively higher at frequencies below the training stimulus than at those that were above. Training with a 45 degrees phase-advanced optokinetic stimulus produced an adaptive vertical VOR with a gain that was higher at frequencies above the training frequency than at those that were below. During training with a phase-shifted optokinetic stimulus, adjustment of the relative efficacies of two neural pathways, a velocity pathway and an integrating pathway, could account for gain dependence on testing frequency and phase shift. This was corroborated by a model of the VOR that incorporates parallel velocity and integrating pathways. Data from 45 degrees phase advances were fit by increasing the gain of the velocity versus integrating pathway, whereas 45 degrees phase delay data were fit by decreasing the gain of the direct versus integrating pathway. The models altered the time constants of either the common oculomotor integrator or the velocity storage mechanism.

Changes in sensitivity of vestibular nucleus neurons induced by cross-axis adaptation of the vestibulo-ocular reflex in the cat.

In alert, chronically-prepared cats, we studied response characteristics of well-isolated vestibular nucleus neurons (n = 9) while pairing yaw rotation with a pitch optokinetic stimulus, resulting in cross-axis adaptation of the horizontal vestibulo-ocular reflex. Each neuron's sensitivity to whole body rotation in a variety of axes in three-dimensional space was determined. When tested in darkness following adaptation, neurons showed statistically significant increases in sensitivity to yaw but not vertical plane rotations, suggesting participation in reflex plasticity.

Predictive smooth pursuit of complex two-dimensional trajectories in monkey: component interactions.

Smooth pursuit eye movements were studied in monkeys tracking target spots that moved two-dimensionally. Complex target trajectories were created by applying either two or three sinusoids to horizontal and vertical axes in various combinations. The chance of observing predictable performance was increased by repeated training on each trajectory. Data analyses were based upon repeated presentations of each trajectory within sessions and on successive days. We wished to determine how accurately monkeys could pursue targets moving along these trajectories and to observe interactions among frequency components. At intermediate frequencies, tracking performance was smooth and consistent during repeated presentations with saccadic corrections that were well integrated with smooth pursuit. The mean gain for eight different sum-of-sines trajectories was 0.83 and the mean magnitude (absolute value) of the phase error was 6 degrees. In light of the long delays that have been associated with the processing of visual information, these values indicate that the monkeys were pursuing predictively. Five factors influenced predictive pursuit performance: (1) there was a decline in performance with increasing frequency; (2) horizontal pursuit was better than vertical pursuit; (3) high-frequency components were tracked with higher gains and phase lags, while lower-frequency components were tracked with lower gains and phase leads; (4) the gain of sinusoidal pursuit was always reduced when a second sinusoid was applied to the same axis or, to a lessor extent, when a second sinusoid of higher frequency was applied to the orthogonal axis; (5) the phase of sinusoidal pursuit shifted from a phase lag to a phase lead when combined with a second sinusoid of higher frequency, but was not affected by the addition of a lower-frequency sinusoid. Findings 1 and 2 confirm, in monkeys, results reported for humans, and 3 extends to monkeys and to two-dimensional pursuit results based upon human subjects. All of these findings demonstrate that complex predictive tracking is controlled by a nonlinear and nonhomogeneous system that uses predictive strategies in concert with feedback control to generate good pursuit.

Preferred axis of rotation of floccular Purkinje cells in the decerebrate cat.

Responses of 35 Purkinje cells in decerebrate cats were recorded during 0.5 Hz rotations in 4-11 vertical planes and the horizontal plane to determine their semicircular canal input. Most neurons received convergent input from two canals (21 neurons) or 3 canals (5 neurons). Few Purkinje cells were maximally sensitive to rotations about an axis appropriate to their inferior olivary input as determined by Gerrits and Voogd [15,24,27,49].

Spatial coordination by descending vestibular signals. 2. Response properties of medial and lateral vestibulospinal tract neurons in alert and decerebrate cats.

Spatial response properties of medial (MVST) and lateral (LVST) vestibulospinal tract neurons were studied in alert and decerebrate cats during sinusoidal angular rotations of the whole body in the horizontal and many vertical planes. Of 220 vestibulospinal neurons with activity modulated during 0.5-Hz sinusoidal rotations, 200 neurons exhibited response gains that varied as a cosine function of stimulus orientation and phases that were near head velocity for rotation planes far from the minimum response plane. A maximum activation direction vector (MAD), which represents the axis and direction of rotation that maximally excites the neuron, was calculated for these neurons. Spatial properties of secondary MVST neurons in alert and decerebrate animals were similar. The responses of 88 of 134 neurons (66%) could be accounted for by input from one semicircular canal pair. Of these, 84 had responses consistent with excitation from the ipsilateral canal of the pair (13 horizontal, 27 anterior, 44 posterior) and 4 with excitation from the contralateral horizontal canal. The responses of the remaining 46 (34%) neurons suggested convergent inputs. The activity of 38 of these was significantly modulated by both horizontal and vertical rotations. Twelve neurons (9%) had responses that were consistent with input from both vertical canal pairs, including 9 cells with MADs near the roll axis. Thirty-two secondary MVST neurons (24%) had type II yaw and/or roll responses. The spatial response properties of 18 secondary LVST neurons, all studied in decerebrate animals, were different from those of secondary MVST neurons. Sixteen neurons (89%) had type II yaw and/or roll responses, and 12 (67%) appeared to receive convergent canal pair input. Convergent input was more common on higher-order vestibulospinal neurons than on secondary neurons. These results suggest that MVST and LVST neurons and previously reported vestibulo-ocular neurons transmit functionally different signals. LVST neurons, particularly those with MADs close to the roll axis, may be involved in the vestibular-limb reflex. The combination of vertical and ipsilateral horizontal canal input on many secondary MVST neurons suggests a contribution to the vestibulocollic reflex. However, in contrast to most neck muscles, very few neurons had maximum vertical responses near pitch.

Mechanisms controlling human head stabilization. I. Head-neck dynamics during random rotations in the horizontal plane.

1. Potential mechanisms for controlling stabilization of the head and neck include voluntary movements, vestibular (VCR) and proprioceptive (CCR) neck reflexes, and system mechanics. In this study we have tested the hypothesis that the relative importance of those mechanisms in producing compensatory actions of the head-neck motor system depends on the frequency of an externally applied perturbation. Angular velocity of the head with respect to the trunk (neck) and myoelectric activity of three neck muscles were recorded in seven seated subjects during pseudorandom rotations of the trunk in the horizontal plane. Subjects were externally perturbed with a random sum-of-sines stimulus at frequencies ranging from 0.185 to 4.11 Hz. Four instructional sets were presented. Voluntary mechanisms were examined by having the subjects actively stabilize the head in the presence of visual feedback as the body was rotated (VS). Visual feedback was then removed, and the subjects attempted to stabilize the head in the dark as the body was rotated (NV). Reflex mechanisms were examined when subjects performed a mental arithmetic task during body rotations in the dark (MA). Finally, subjects performed a voluntary head tracking task while the body was kept stationary (VT). 2. Gains and phases of head velocity indicated good compensation to the stimulus in VS and NV at frequencies < 1 Hz. Gains dropped and phases advanced between 1 and 2 Hz, suggesting interference between neural and mechanical components. Above 3 Hz, the gains of head velocity increased steeply and exceeded unity, suggesting the emergence of mechanical resonance.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms controlling human head stabilization. II. Head-neck characteristics during random rotations in the vertical plane.

1. In this study we have tested the hypothesis that the mechanisms controlling stabilization of the head-neck motor system can vary with both the frequency and spatial orientation of an externally applied perturbation. Angular velocity of the head with respect to the trunk (neck) and myoelectric activity of two neck muscles (semispinalis capitis and sternocleidomastoid) were recorded in eight seated subjects during pseudorandom rotations of the trunk in the vertical (pitch) plane. Subjects were externally perturbed with a random sum-of-sines stimulus at frequencies ranging from 0.35 to 3.05 Hz. Four instructional sets were presented. Voluntary mechanisms were examined by having the subjects actively stabilize the head in the presence of visual feedback as the body was rotated (VS). Visual feedback was then removed, and the subjects attempted to stabilize the head in the dark as the body was rotated (NV). Reflex mechanisms were examined when subjects performed a mental arithmetic task during body rotations in the dark (MA). Finally, subjects performed a voluntary head tracking task while the body was kept stationary (VT). 2. In VS and NV, gains and phases of head velocity indicated good compensation for the perturbation at frequencies up to 2 Hz. Between 2 and 3 Hz, gains dropped slowly and then steeply descended above 3 Hz as phases became scattered. 3. In MA, gains were lower and exhibited more scatter than in VS and NV at frequencies < 1 Hz. Phases around -180 degrees indicated that compensatory activity was occurring even with these low gains. Between 1 and 2 Hz, response gains steeply ascended, implying that reflex mechanisms were becoming the predominant mechanism for compensation in this frequency range. Above 2 Hz, gains dropped off to 0.5 and lower, but phases remained close to -180 degrees, suggesting that the reflex mechanisms were not dominant in this frequency range, but that they were still contributing toward compensation for the trunk perturbation. 4. Neck muscle electromyographic (EMG) responses were similar in VS, NV, and MA, demonstrating decreasing gains between 0.35 and 1.5 Hz, and then increasing beyond the previous high level of activation. This U-shaped response pattern implies an enhanced participation of neural mechanisms, probably of reflex origin, in the higher frequency range. 5. Patterns observed during external perturbations of the trunk were not apparent in the response dynamics of voluntary head tracking. In VT, subjects successfully tracked the stimulus only at the lowest frequencies of head movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial coordination by descending vestibular signals. 1. Reflex excitation of neck muscles in alert and decerebrate cats.

Electromyographic activity of dorsal neck muscles and neck torques was recorded to study vestibulocollic, cervicocollic, and combined reflexes in alert and decerebrate cats during rotations of the whole body, the body except for the head, and the head but not the rest of the body. Cats were rotated about many axes that lay in the frontal, sagittal, and horizontal planes using sinusoidal 0.25-Hz waveforms or sum-of-sinusoid wave-forms. Robust electromyographic responses were recorded from six muscles, with response directionality that in most cases did not show strong dependence on the reflex tested or on other factors including exact neck angle, stimulus amplitude from 5 degrees to 60 degrees, and intact versus decerebrate state. Based on the strength of responses to rotations about all the tested axes, neck muscles could be characterized by maximal activation direction vectors representing the axis and direction of rotation in three-dimensional space that was most excitatory during reflex responses. Responses to rotations about axes that lay in a coordinate plane were predicted by a cosine function of the angle between the axis under test and the maximally excitatory axis in the plane. All muscles were excited by the nose down phase of pitch rotation and by yaw and roll away from the side on which the muscle lay. Biventer cervicis was best activated by rotations with axes near nose-down pitch, and its axis of maximal activation also had small, approximately equal components of yaw and roll toward the contralateral side. Complexus was best excited by rotations with axes nearest roll, but with large components along all three axes. Occipitoscapularis was best excited by rotations about axes near pitch, but with a moderately large contralateral yaw component and a smaller but significant contralateral roll component. Splenius was best excited by rotations with a large component of contralateral yaw, considerable nose-down pitch, and a smaller component of contralateral roll. Rectus major was best excited by rotations near nose-down pitch, but with a substantial contralateral yaw component and smaller contralateral roll component. Obliquus inferior was best excited by rotations with a large component of contralateral yaw, but with considerable contralateral roll and nose-down pitch components. All muscles responded as though they received convergent input from all three semicircular canals. Vestibulocollic and combined reflex responses in alert cats and vestibulocollic, cervicocollic, and combined responses in decerebrate cats appeared to have the same directionality, as evidenced by insignificant shifts in maximal activation vectors.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamics of directional plasticity in the human vertical vestibulo-ocular reflex.

Directional plasticity of the human vestibulo-ocular reflex (VOR) was studied in 10 subjects. The adaptation paradigm coupled 0.25 Hz, 19 degrees/s vertical pitch vestibular rotations with 28 degrees/s horizontal optokinetic oscillations. Electro-oculographic recordings in the dark were taken at 0.05, 0.1, 0.25, 0.5, and 1 Hz pitch rotations before and after training and at 15-minute intervals during 0.25 Hz adaptation. Peak head velocity was kept at 19 degrees/sec for frequencies above 0.1 Hz, while constant amplitude was maintained at +/- 24 degrees for 0.05 and 0.1 Hz. In all subjects, directional training produced slow phase horizontal VOR eye movements that were not present during vertical rotations before adaptation. During the 2-hour training period, the cross-axis VOR gain at 0.25 Hz increased up to 0.16. Adaptive VOR gain was highest at the lowest frequency and reached a tuned peak at the 0.25 Hz training frequency. Cross-axis VOR phase remained around 0 degrees at higher frequencies and lagged at lower frequencies. In all subjects, the cross-axis VOR gain was diminished when subjects were exposed to 0.25 Hz pitch rotations paired with a stationary visual field. The dynamics of the vertical VOR remained constant throughout the experiment. These results are further evidence that the frequency response characteristics of adaptive cross-axis VOR gain are similar in humans and cats, while phase behavior is less complex in humans. The high adaptive gain at low frequencies implicates otolith contributions during cross-axis adaptation.