A micro-drive hearing aid: a novel non-invasive hearing prosthesis actuator

The direct hearing device (DHD) is a new auditory prosthesis that combines conventional hearing aid and middle ear implant technologies into a single device. The DHD is located deep in the ear canal and recreates sounds with mechanical movements of the tympanic membrane. A critical component of the DHD is the microactuator, which must be capable of moving the tympanic membrane at frequencies and magnitudes appropriate for normal hearing, with little distortion. The DHD actuator reported here utilized a voice coil actuator design and was 3.7 mm in diameter. The device has a smoothly varying frequency response and produces a precisely controllable force. The total harmonic distortion between 425 Hz and 10 kHz is below 0.5 % and acoustic noise generation is minimal. The device was tested as a tympanic membrane driver on cadaveric temporal bones where the device was coupled to the umbo of the tympanic membrane. The DHD successfully recreated ossicular chain movements across the frequencies of human hearing while demonstrating controllable magnitude. Moreover, the micro-actuator was validated in a short-term human clinical performance study where sound matching and complex audio waveforms were evaluated by a healthy subject.     

Neck proprioceptors contribute to the modulation of muscle sympathetic nerve activity to the lower limbs of humans

Several different strategies have now been used to demonstrate that the vestibular system can modulate muscle sympathetic nerve activity (MSNA) in humans and thereby contribute to the regulation of blood pressure during changes in posture. However, it remains to be determined how the brain differentiates between head-only movements that do not require changes in vasomotor tone in the lower limbs from body movements that do require vasomotor changes. We tested the hypothesis that neck movements modulate MSNA in the lower limbs of humans. MSNA was recorded in 10 supine young adult subjects, at rest, during sinusoidal stretching of neck muscles (100 cycles, 35° peak to peak at 0.37 ± 0.02 Hz) and during a ramp-and-hold (17.5° for 54 ± 9 s) static neck muscle stretch, while their heads were held fixed in space. Cross-correlation analysis revealed cyclical modulation of MSNA during sinusoidal neck muscle stretch (modulation index 45.4 ± 5.3 %), which was significantly less than the cardiac modulation of MSNA at rest (78.7 ± 4.2 %). Interestingly, cardiac modulation decreased significantly during sinusoidal neck displacement (63.0 ± 9.3 %). By contrast, there was no significant difference in MSNA activity during static ramp-and-hold displacements of the neck to the right or left compared with that with the head and neck aligned. These data suggest that dynamic, but not static, neck movements can modulate MSNA, presumably via projections of muscle spindle afferents to the vestibular nuclei, and may thus contribute to the regulation of blood pressure during orthostatic challenges.     

三种不同内固定治疗胫骨平台后内侧骨折的生物力学研究

目的设计一种与胫骨平台后内髁解剖形态匹配的钢板,比较其与5孔L型有限接触动力加压钢板(LCDCP)和7孔直型重建接骨板治疗胫骨平台后内侧骨折的稳定性。方法将18具胫骨平台后内侧骨折的标本随机分为3组,每组6具。其中,A组标本模型采用前内侧5孔L型LC-DCP固定,B组标本模型采用后内侧7孔直型重建接骨板固定,C组标本模型采用新设计的胫骨平台后内侧锁定钢板固定。分别测量各组标本在轴向载荷为500、1000、1500 N下的垂直位移及失效载荷。结果 A组标本分别在500、1000、1500 N下的垂直位移是(1.035±0.140)mm、(1.721±0.149)mm、(2.263±0.134)mm,B组标本分别在500、1000、1500 N下的垂直位移是(0.268±0.702)mm、(0.788±0.507)mm、(1.518±0.111)mm,C组标本分别在500、1000、1500 N下的垂直位移是(0.180±0.049)mm、(0.578±0.103)mm、(0.760±0.055)mm。在同一载荷下,三组标本骨折块垂直位移组间比较差异均有统计学意义(0.05)。A组标本失效载荷为(1234.3±56.009)N,B组标本失效载荷为(2065.8±102.098)N,C组标本失效载荷为(2544.2±92.982)N,三组间比较差异均有统计学意义(0.05)。结论新设计的胫骨平台后内侧锁定钢板在治疗胫骨平台后内侧骨折上,较5孔L型LC-DCP和7孔直型重建接骨板具有更好的生物力学优势。     

Perioperative Brain Shift and Deep Brain Stimulating Electrode Deformation Analysis: Implications for rigid and non-rigid devices

Deep brain stimulation (DBS) efficacy is related to optimal electrode placement. Several authors have quantified brain shift related to surgical targeting; yet, few reports document and discuss the effects of brain shift after insertion. Objective: To quantify brain shift and electrode displacement after device insertion. Twelve patients were retrospectively reviewed, and one post-operative MRI and one time-delayed CT were obtained for each patient and their implanted electrodes modeled in 3D. Two competing methods were employed to measure the electrode tip location and deviation from the prototypical linear implant after the resolution of acute surgical changes, such as brain shift and pneumocephalus. In the interim between surgery and a pneumocephalus free postoperative scan, electrode deviation was documented in all patients and all electrodes. Significant shift of the electrode tip was identified in rostral, anterior, and medial directions (p < 0.05). Shift was greatest in the rostral direction, measuring an average of 1.41 mm. Brain shift and subsequent electrode displacement occurs in patients after DBS surgery with the reversal of intraoperative brain shift. Rostral displacement is on the order of the height of one DBS contact. Further investigation into the time course of intraoperative brain shift and its potential effects on procedures performed with rigid and non-rigid devices in supine and semi-sitting surgical positions is needed.     

Contribution of visual velocity and displacement cues to human balancing of support surface tilt

Vision helps humans in controlling bipedal stance, interacting mainly with vestibular and proprioceptive cues. This study investigates how postural compensation of support surface tilt is compromised by selectively reducing visual velocity cues by stroboscopic illumination of a stationary visual scene. Healthy adult subjects were presented with pseudorandom tilt sequences in the sagittal plane (tilt frequency range 0.017–2.2 Hz; velocity amplitude spectrum constant up to a frequency of 0.6 Hz, angular displacement amplitude spectrum increasing with decreasing frequencies). Center of mass (COM) sway responses were recorded for stroboscopic illuminations at 48, 32, 16, 8, and 4 Hz, as well as under continuous illumination and with eyes closed. With strobe duration (5 ms) and mean luminance (1 lx) kept constant, visual acuity and perceived brightness remained constant and the visual scene was perceived as stationary. Yet, tilt-evoked COM excursions increased with decreasing strobe frequency in a graded way, with largest effects occurring at tilt frequencies where large tilt velocities coincided with small displacements. In addition, COM excursions were reduced at the lowest strobe frequency compared to eyes closed, with the largest effect occurring at tilt frequencies where tilt displacements were large. We conclude that two mechanisms exist, a velocity mechanism that deals with tilt compensation and is foremost affected by the stroboscopic illumination and a displacement mechanism. This compares favorably to previous findings that, transferred to a stance control model, suggest a velocity mechanism for tilt compensation and a position mechanism for gravity compensation.     

The effect of age on post-activation depression of the upper limb H-reflex

Purpose

Post-activation depression (PaD) refers to the inhibition of the H-reflex induced by a preceding conditioning stimulus able to activate the afferents mediating the H-reflex itself. PaD can be investigated assessing the frequency-related depression of the H-reflex. This parameter, which is highly correlated to the severity of spasticity, has been used in the longitudinal assessment of spastic patients, in particular to assess the effect of drugs and rehabilitation over the years. However, in such longitudinal assessment, changes observed might be age related and not only disease related. The aim of this study was to investigate the possible age effects on PaD.

Methods

The frequency-related depression of the flexor carpi radialis (FCR) H-reflex was examined in two groups of young (20 subjects; 28 ± 3 years) and aged (18 subjects; 69 ± 6 years) healthy subjects. PaD was evaluated by comparing the H-reflex amplitudes obtained with a stimulation frequency of 0.1 Hz with those obtained using higher frequencies (0.33–0.5–1–2 Hz).

Results

The results showed that frequency-related depression of the FCR H-reflex is similar in young and elderly subjects at all frequencies, with the exception of 2 Hz.

Conclusion

Our study shows that ageing does not affect the frequency-related depression of the FCR H-reflex at the frequencies of 1 Hz or lower, supporting the reliability of this method to assess PaD in the clinical practice, particularly for the longitudinal assessment of spasticity. A decrease of GABA-ergic presynaptic inhibition seems to be the more likely explanation for the age-related changes that we observed at the frequency of 2 Hz.     

Selective changes in cerebellar-cortical processing following motor training

The aim of this study was to investigate the effect of varying stimulation rate and the effects of a repetitive typing task on the amplitude of somatosensory evoked potential (SEP) peaks thought to relate to cerebellar processing. SEPs (2,000 sweep averages) were recorded following median nerve stimulation at the wrist at frequencies of 2.47, 4.98, and 9.90 Hz from 12 subjects before and after a 20-min repetitive typing task. Typing and error rate were recorded 2-min pre- and post-typing task. Effect of stimulation rate was analysed with ANOVA followed by pairwise comparisons (paired t tests). Typing effects were analysed by performing two-tailed paired t tests. Increasing stimulation frequency significantly decreased the N30 SEP peak amplitude (p < 0.02). Both the 4.98 and 9.90 Hz rates lead to significantly smaller N30 peak amplitudes compared to the 2.47 Hz (p ≤ 0.01). The N24 amplitude significantly increased following the typing task for both 4.98 and 2.47 Hz (p ≤ 0.025). In contrast, there was a highly significant decrease (p < 0.001) in the N18 peak amplitude post-typing at all frequencies. Typing rate increased (p < 0.001) and error rate decreased (p < 0.05) following the typing task. The results suggest that the N24 SEP peak amplitude is best recorded at 4.98 Hz since the N30 amplitude decreases and no longer contaminates the N24 peak, making the N24 visible and easier to measure, while still enabling changes due to repetitive activity to be measured. The decrease in N18 amplitude along with an increase in N24 amplitude with no change in N20 amplitude may be explained by the intervention reducing inhibition at the level of the cuneate nucleus and/or interior olives leading to alterations in cerebellar-cortical processing.     

Compensatory head and eye movements in the frog and their contribution to stabilization of gaze

Summary Compensatory head movements, recorded in unrestrained frogs, were compared to compensatory eye movements recorded from animals that had their head fixed. Movements were evoked by oscillating the animal in the dark (vestibular stimulation) or in the light in front of an earth-fixed, patterned visual background (combined stimulation) or by rotating vertical black and white bars (optokinetic stimulation) around the stationary animal. Oscillations occurred in the horizontal plane at frequencies between 0.025 and 0.5 Hz. Gain and phase values of head and eye movements, relative to stimulus movements were calculated.Evoked eye movements were limited in amplitude to 3–6 °, increasing with the size of the animal. Head movements were limited to ±30–40 °. Resetting fast-phases of both head and eyes were very rarely observed during sinusoidal stimulation and no eye movements were recorded in the absence of intended head movements.Vestibularly evoked head movements exhibited a frequency-dependent threshold that was not observed for vestibulo-ocular responses. Above threshold, the gain of evoked head responses increased and reached a frequency-dependent plateau at which the system behaved approximately linearly. Within the linear range, gain of vestibularly evoked responses increased with frequency (from 0.04 at 0.025 Hz to 0.75 at 0.5 Hz) and phase lead decreased (from about 80 ° to 0 °). Vestibularly evoked eye movements similarly increased in gain from 0.05 to 0.56 and decreased in phase lead from about 56 ° to 10 ° over the same frequency range.Optokinetically evoked head and eye movements had their highest gains (about 0.8 and 0.5) at low constant velocities ( 1–4 °/s) or frequencies ( 0.025 Hz). At higher constant velocities or frequencies the gain dropped. The phase lag increased from close to zero (at 0.025 Hz) to about 60 ° for the head and to about 20 ° for the eye movements (at 0.25 Hz). These phase lags are explained by reaction times of the evoked movements of about 600 ms (head) and 200 ms (eyes).Combined stimulation evoked compensatory head movements with gain and phase values that were frequency-independent in the linear range. Head movements compensated for about 80–90% of the imposed gaze shift with a small phase lag (0–10 °). Evoked eye movements were found to be large enough in amplitude and fast enough in time to enable a frog to stabilize its gaze exclusively with slow phase compensatory movements for a large variety of frequency and amplitude combinations. The two motor systems controlling movements of the head and the eye are matched in such a way that the non-linearities of the evoked eye movements can compensate for the non-linearities of the evoked head movements.Supported by a grant from Deutsche Forschungsgemeinschaft (Pr. 158/2) and Swiss National Science Foundation (3.505.79 and 3.616.80)     

Autonomous control for mechanically stable navigation of microscale implants in brain tissue to record neural activity

Emerging neural prosthetics require precise positional tuning and stable interfaces with single neurons for optimal function over a lifetime. In this study, we report an autonomous control to precisely navigate microscale electrodes in soft, viscoelastic brain tissue without visual feedback. The autonomous control optimizes signal-to-noise ratio (SNR) of single neuronal recordings in viscoelastic brain tissue while maintaining quasi-static mechanical stress conditions to improve stability of the implant-tissue interface. Force-displacement curves from microelectrodes in in vivo rodent experiments are used to estimate viscoelastic parameters of the brain. Using a combination of computational models and experiments, we determined an optimal movement for the microelectrodes with bidirectional displacements of 3:2 ratio between forward and backward displacements and a inter-movement interval of 40 s for minimizing mechanical stress in the surrounding brain tissue. A regulator with the above optimal bidirectional motion for the microelectrodes in in vivo experiments resulted in significant reduction in the number of microelectrode movements (0.23 movements/min) and longer periods of stable SNR (53 % of the time) compared to a regulator using a conventional linear, unidirectional microelectrode movement (with 1.48 movements/min and stable SNR 23 % of the time).     

Ocular stability in the horizontal,frontal and sagittal planes in the rabbit

Summary Eye and head movements in the horizontal, frontal and sagittal planes were recorded in the rabbit with a newly developed technique using dual scleral search coils in a rotating magnetic field. The compensatory eye movements elicited by passive sinusoidal oscillation detoriated for frequencies below 0.1 Hz in the horizontal, but not in the frontal and sagittal planes. In the light gain was relatively independent of frequency in all planes and amounted to 0.82–0.69, 0.92–0.83 and 0.65–0.59 in the horizontal, frontal and sagittal plane, respectively. In freely moving animals, similar input-output relations were found. The stability of the retinal image thus proved to be inversely proportional to the amount of head movements associated with behavioural activity. Maximal retinal image velocities varied between 2–4°/s for a rabbit sitting quietly and 30–40°/s during locomotor activity. Gaze displacements showed different characteristics in the various planes, possibly in relation with the structure of the retinal visual streak. Horizontal gaze changes were mainly effected by saccades. Gaze changes in the frontal plane were relatively rare and effected by non-saccadic, combined head and eye movements with temporary suppression of compensatory eye movements. Eye rotations in the sagittal plane, possibly functioning to adjust the direction of binocular vision vertically, were abundant and effected by large head movements in combination with a low gain of compensatory eye movements in this plane.     

Vestibuloocular reflex dynamics during high-frequency and high-acceleration rotations of the head on body in rhesus monkey

For frequencies >10 Hz, the vestibuloocular reflex (VOR) has been primarily investigated during passive rotations of the head on the body in humans. These prior studies suggest that eye movements lag head movements, as predicted by a 7-ms delay in the VOR reflex pathways. However, Minor and colleagues recently applied whole-body rotations of frequencies < or =15 Hz in monkeys and found that eye movements were nearly in phase with head motion across all frequencies. The goal of the present study was to determine whether VOR response dynamics actually differ significantly for whole-body versus head-on-body rotations. To address this question, we evaluated the gain and phase of the VOR induced by high-frequency oscillations of the head on the body in monkeys by directly measuring both head and eye movements using the magnetic search coil technique. A torque motor was used to rotate the heads of three Rhesus monkeys over the frequency range 5-25 Hz. Peak head velocity was held constant, first at +/-50 degrees /s and then +/-100 degrees /s. The VOR was found to be essentially compensatory across all frequencies; gains were near unity (1.1 at 5 Hz vs. 1.2 at 25 Hz), and phase lag increased only slightly with frequency (from 2 degrees at 5 Hz to 11 degrees at 25 Hz, a marked contrast to the 63 degrees lag at 25 Hz predicted by a 7-ms VOR latency). Furthermore, VOR response dynamics were comparable in darkness and when viewing a target and did not vary with peak velocity. Although monkeys offered less resistance to the initial cycles of applied head motion, the gain and phase of the VOR did not vary for early versus late cycles, suggesting that an efference copy of the motor command to the neck musculature did not alter VOR response dynamics. In addition, VOR dynamics were also probed by applying transient head perturbations with much greater accelerations (peak acceleration >15,000 degrees /s(2)) than have been previously employed. The VOR latency was between 5 and 6 ms, and mean gain was close to unity for two of the three animals tested. A simple linear model well described the VOR responses elicited by sinusoidal and transient head on body rotations. We conclude that the VOR is compensatory over a wide frequency range in monkeys and has similar response dynamics during passive rotation of the head on body as during passive rotation of the whole body in space.     

Under which conditions do the skin and probe decouple during sinusoidal vibrations?

Previous experiments performed on monkey and human fingertips suggested that the skin surface and stimulus probe decouple for sinusoidal displacements applied perpendicularly to the skin surface. From these observations, it was concluded that sinusoidal vibration may not be a suitable stimulus for understanding and modeling the tactile system. We repeated these experiments on human observers using stimulus frequencies ranging from 0.5 to 240 Hz and with displacement amplitudes up to 1 mm peak-to-peak (p-p). The skin and probe movements were measured in the steady-state using stroboscopic illumination and video microscopy. Contrary to previous conclusions, we found that decoupling did not occur for amplitudes less then 0.25 mm p-p, regardless of stimulus frequency. Decoupling was only observed for stimulus amplitudes greater than 0.25 mm over the stimulus-frequency range investigated. To further investigate this effect, a modified stimulus contactor was used, which permitted the measurement of the skin’s movement using reflected light. Measurements were made on both the index fingertip and the thenar eminence. Regardless of body site, no decoupling between the skin and stimulus probe was observed for frequencies ranging from 20 to 100 Hz up to displacements of 0.25 mm p-p. These levels are well within the range used in most human psychophysical experiments performed on these parts of the body. We conclude that sinusoidal vibration can be used reliably to stimulate the tactile system and is an appropriate stimulus for developing models of touch. Received: 12 October 1998 / Accepted: 26 June 1999     

Mapping Slow Waves by EEG Topography and Source Localization: Effects of Sleep Deprivation

Slow waves are a salient feature of the electroencephalogram (EEG) during non-rapid eye movement (non-REM) sleep. The aim of this study was to assess the topography of EEG power and the activation of brain structures during slow wave sleep under normal conditions and after sleep deprivation. Sleep EEG recordings during baseline and recovery sleep after 40 h of sustained wakefulness were analyzed (eight healthy young men, 27 channel EEG). Power maps were computed for the first non-REM sleep episode (where sleep pressure is highest) in baseline and recovery sleep, at frequencies between 0.5 and 2 Hz. Power maps had a frontal predominance at all frequencies between 0.5 and 2 Hz. An additional occipital focus of activity was observed below 1 Hz. Power maps?≤?1 Hz were not affected by sleep deprivation, whereas an increase in power was observed in the maps?≥?1.25 Hz. Based on the response to sleep deprivation, low-delta (0.5–1 Hz) and mid-delta activity (1.25–2 Hz) were dissociated. Electrical sources within the cortex of low- and mid-delta activity were estimated using eLORETA. Source localization revealed a predominantly frontal distribution of activity for low-delta and mid-delta activity. Sleep deprivation resulted in an increase in source strength only for mid-delta activity, mainly in parietal and frontal regions. Low-delta activity dominated in occipital and temporal regions and mid-delta activity in limbic and frontal regions independent of the level of sleep pressure. Both, power maps and electrical sources exhibited trait-like aspects.     

Human vestibulo-ocular responses to rapid,helmet-driven head movements

High-frequency head rotations in the 2–20 Hz range and passive, unpredictable head acceleration impulses were produced by a new technique, utilizing a helmet with a torque motor oscillating a mass. Unrestrained head and eye movements were recorded using magnetic sensor coils in a homogeneous magnetic field. In order to analyze the influence of the visual system on the vestibulo-ocular reflex (VOR), we took measurements under three experimental conditions: (1) with a stationary visual target; (2) in total darkness with the subject imagining the stationary target; and (3) with a head-fixed target. The results in 15 healthy subjects were highly consistent. At 2 Hz, VOR gain was near unity; above 2 Hz, VOR gain started to decrease, but this trend reversed beyond 8 Hz, where the gain increased continuously up to 1.1–1.3 at 20 Hz. Phase lag increased with frequency, from a few deg at 2 Hz to about 45 degrees at 20 Hz. Above 2 Hz, VOR gain was not significantly different for the three experimental conditions. Head acceleration impulses produced a VOR with near-unity gain in both directions. We also tested three subjects with clinically total bilateral loss of labyrinthine functions. These labyrinthine-defective subjects showed, in comparison to the normal subjects, strikingly lower gains and much longer delays in the VOR during sinu-soidal and step-like head movements. These results suggest that our new torque-driven helmet technique is effective, safe and convenient, enabling the assessment of the VOR at relatively high frequencies where both visual and mental influences are minimized.     

Normal performance and expression of learning in the vestibulo-ocular reflex (VOR) at high frequencies

The rotatory vestibulo-ocular reflex (VOR) keeps the visual world stable during head movements by causing eye velocity that is equal in amplitude and opposite in direction to angular head velocity. We have studied the performance of the VOR in darkness for sinusoidal angular head oscillation at frequencies ranging from 0.5 to 50 Hz. At frequencies of > or = 25 Hz, the harmonic distortion of the stimulus and response were estimated to be <14 and 22%, respectively. We measured the gain of the VOR (eye velocity divided by head velocity) and the phase shift between eye and head velocity before and after adaptation with altered vision. Before adaptation, VOR gains were close to unity for frequencies < or = 20 Hz and increased as a function of frequency reaching values of 3 or 4 at 50 Hz. Eye velocity was almost perfectly out of phase with head velocity for frequencies < or = 12.5 Hz, and lagged perfect compensation increasingly as a function of frequency. After adaptive modification of the VOR with magnifying or miniaturizing optics, gain showed maximal changes at frequencies <12.5 Hz, smaller changes at higher frequencies, and no change at frequencies larger than 25 Hz. Between 15 and 25 Hz, the phase of eye velocity led the unmodified VOR by as much as 50 degrees when the gain of the VOR had been decreased, and lagged when the gain of the VOR had been increased. We were able to reproduce the main features of our data with a two-pathway model of the VOR, where the two pathways had different relationships between phase shift and frequency.     

Head stabilization during various locomotor tasks in humans

Summary This experiment, which extends a previous investigation (Pozzo et al. 1990), was undertaken to examine how head position is controlled during natural locomotor tasks in both normal subjects (N) and patients with bilateral vestibular deficits (V). 10 normals and 7 patients were asked to perform 4 locomotor tasks: free walking (W), walking in place (WIP), running in place (R) and hopping (H). Head and body movements were recorded with a video system which allowed a computed 3 dimensional reconstruction of selected points in the sagittal plane. In order to determine the respective contribution of visual and vestibular cues in the control of head angular position, the 2 groups of subjects were tested in the light and in darkness. In darkness, the amplitude and velocity of head rotation decreased for N subjects; these parameters increased for V subjects, especially during R and H. In darkness, compared to the light condition, the mean position of a line placed on the Frankfort plane (about 20–30° below the horizontal semi-circular canal plane) was tilted downward in all conditions of movement, except during H, for N subjects. In contrast, this flexion of the head was not systematic in V subjects: the Frankfort plane could be located above or below earth horizontal. In V subjects, head rotation was not found to be compensatory for head translation and the power spectrum analysis shows that head angular displacements in the sagittal plane contain mainly low frequencies (about 0.3–0.8 Hz). The respective contribution of visual and vestibular cues in the control of the orientation and the stabilization of the head in space is discussed.     

The Importance of Properly Compensating for Head Movements During MEG Acquisition Across Different Age Groups

Unlike EEG sensors, which are attached to the head, MEG sensors are located outside the head surface on a fixed external device. Subject head movements during acquisition thus distort the magnetic field distributions measured by the sensors. Previous studies have looked at the effect of head movements, but no study has comprehensively looked at the effect of head movements across age groups, particularly in infants. Using MEG recordings from subjects ranging in age from 3 months through adults, here we first quantify the variability in head position as a function of age group. We then combine these measured head movements with brain activity simulations to determine how head movements bias source localization from sensor magnetic fields measured during movement. We find that large amounts of head movement, especially common in infant age groups, can result in large localization errors. We then show that proper application of head movement compensation techniques can restore localization accuracy to pre-movement levels. We also find that proper noise covariance estimation (e.g., during the baseline period) is important to minimize localization bias following head movement compensation. Our findings suggest that head position measurement during acquisition and compensation during analysis is recommended for researchers working with subject populations or age groups that could have substantial head movements. This is especially important in infant MEG studies.     

Visually-induced adaptive plasticity in the human vestibulo-ocular reflex

Summary The vestibulo-ocular reflex (VOR) is under adaptive control which corrects VOR performance when visual-vestibular mismatch arises during head movements. However, the dynamic characteristics of VOR adaptive plasticity remain controversial. In this study, eye movements (coil technique) were recorded from normal human subjects during sinusoidal rotations in darkness before and after 8 h. of adaptation to 2X binocular lenses. The VOR was studied at 7 frequencies between 0.025 and 4.0 Hz at 50°/s peak head velocity (less for 2.5–4 Hz). For 0.025 and 0.25 Hz, the VOR was tested at 4 peak head velocities between 50 and 300° /s. Before 2X lens adaptation, VOR gain was around 0.9 at 2.5–4.0 Hz and dropped gradually with decreasing frequency to under 0.6 at 0.025 Hz. Phase showed a small lead at the highest frequencies which declined to 0° as frequency decreased to 0.5–0.25 Hz, but then rose to 14° by 0.025 Hz. VOR gain was independent of head velocity in the range 50–300°/s at both 0.025 and 0.25 Hz. However, Phase lead rose with increasing head velocity, more so at 0.025 than at 0.25 Hz. After 2X lens adaptation, gain rose across the frequency bandwidth. However, the proportional gain enhancement was frequency dependent; it was greatest at 0.025 Hz (44%), and declined with increasing frequency to reach a minimum at 4 Hz (19%). Phase lead increased after 2X lens adaptation at lower frequencies, but decreased at higher frequencies. New velocity-dependent gain nonlinearities also developed which were not present prior to adaptation; gain declined as peak head velocity increased from 50 to 300°/s at both 0.025 (23% drop) and 0.25 Hz (15% drop). This may suggest an amplitude-dependent limitation in VOR adaptive plasticity. Results indicate both frequency and amplitude dependent nonlinearities in human VOR response dynamics before and after adaptive gain recalibration.     

Wavelet analysis of lumbar muscle oxygenation signals during whole-body vibration: implications for the development of localized muscle fatigue

The objective of this study was to assess the effects of whole-body vibration (WBV) on lumbar muscle oxygenation oscillations in healthy men based on the wavelet transform of near-infrared spectroscopy signals. Twelve healthy participants were exposed to WBV at frequencies of 3, 4.5 and 6 Hz while muscle oxygenation signal was monitored before, during and recovery from WBV. With spectral analysis based on wavelet transform of NIR signal, six frequency intervals were identified (I, 0.005–0.0095 Hz; II, 0.0095–0.02 Hz; III, 0.02–0.06 Hz; IV, 0.06–0.16 Hz; V, 0.16–0.40 Hz and VI, 0.40–2.0 Hz). It was found that the muscle oxygenation oscillations at 4.5 Hz in the frequency intervals I, II and III was lower during WBV compared with that of at 3 Hz. Present results demonstrated WBV at 4.5 Hz induced lower oscillatory activities than that of at 3 Hz. The lower oscillatory activities might indicate a decrease in the efficiency of oxygen supply to the oxygenated tissue and such mechanism might contribute to the development of local muscle fatigue.     

Head movements elicited by electrical stimulation of the anteromedial cortex of the rat

Electrical stimulation, 5-sec trains, 0.5 msec rectangular pulses, 20 Hz pulse frequency, was applied to the anteromedial cortex of awake rats (N = 28) to determine if head orienting behavior could be elicited. Following recovery from surgery in which up to six monopolar electrodes were implanted, the rats were adapted to and tested on a small platform surrounded by water. Elicited behavior patterns reliable at 150 microA or less in Experiment 1 (66 sites) and at 100 microA or less in Experiment 2 (32 sites) were analyzed. The most common response was a single head turn in a direction contralateral to the side of stimulation. Positive sites occurred throughout the anterior cingulate and the medial precentral regions. Coordinated forelimb movements were associated with contralateral head movements at dorsal anterior cingulate sites. Ipsilateral head turns were elicited at sites in the most rostral and ventral parts of the anteromedial cortex, including the prelimbic area. Lateral scanning sites were widespread but tended to concentrate at the mergence of the contralateral and ipsilateral movement fields. Vertical movements were often elicited with ipsilateral movements; dorsal sites supported downward head movements and ventral sites supported upward head movements. Sites supporting forward head movements were concentrated in the anterior cingulate region caudal to the genu of the corpus callosum. Neck extension usually appeared in conjunction with lateral head movements and frequently with forelimb movements.