Comparison of codes for sensory feedback using electrocutaneous tracking

Ten electrocutaneous codes suitable for sensory feedback were systematically compared using a random tracking task. The representative sample of codes used single and multiple electrode displays, bipolar and monopolar electrocutaneous stimuli, and frequency, intensity, and spatial modulation. The experimental results from 2 preliminary studies using 4 subjects each, and a full scale study, using 21 subjects, suggested that electrocutaneous tracking was a reliable and sensitive indicator of a code's intrinsic effectiveness for transmitting information cutaneously. In addition, multiple electrode codes using spatial modulation were clearly and significantly superior to any single electrode codes examined. Single electrode codes using frequency modulation were superior to intensity modulation codes whether the pulse stimulus used was monopolar or bipolar. Although subjects rated bipolar stimulation as being slightly more comfortable than monopolar stimulation, they performed better when using monopolar stimulation. Furthermore, sensory adaptation to the monopolar stimulus was considerably less than to the bipolar stimulus. The most effective multiple electrode code was Seven Electrodes in a Linear Array on the Abdomen. The most effective single electrode code was the Low Pulse Rate Modulation Code, wherein the sensory information was embedded in pulse rates between 1 and 15 per second. This research was supported by the National Institute of Health through Grant GM 21430-03.     

Effects of “animal hypnosis” on a rhythmic defensive dominant

A defensive dominant was created in rabbits using rhythmic electrocutaneous stimulation of the left forelimb at a frequency of 0.5 Hz. After stimulation ended, the latent excitation state was tested using sound stimuli. Animals responded either with increases in non-rhythmic paw muscle activity or with rhythmic twitching of the paw at a frequency close to that of the electrocutaneous stimulation. After hypnotization, the incidence of rhythmic responses to the stimulation testing the dominant focus increased, while the incidence of non-rhythmic responses decreased. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel’nosti imeni I. P. Pavlova, Vol. 57, No. 1, pp. 43–51, January–February, 2007.     

Estimation of the distribution of intramuscular current during electrical stimulation of the quadriceps muscle

Electrical stimulation is commonly used for strengthening muscle but little evidence exists as to the optimal electrode size, waveform, or frequency to apply. Three male and three female subjects (22–40 years old) were examined during electrical stimulation of the quadriceps muscle. Two self adhesive electrode sizes were examined, 2 cm × 2 cm and 2 cm × 4 cm. Electrical stimulation was applied with square and sine waveforms, currents of 5, 10 and 15 mA, and pulse widths of 100–500 μs above the quadriceps muscle. Frequencies of stimulation were 20, 30, and 50 Hz. Current on the skin above the quadriceps muscle was measured with surface electrodes at five positions and at three positions with needle electrodes in the same muscle. Altering pulse width in the range of 100–500 μs, the frequency over a range of 20–50 Hz, or current from 5 to 15 mA had no effect on current dispersion either in the skin or within muscle. In contrast, the distance separating the electrodes caused large changes in current dispersion on the skin or into muscle. The most significant finding in the present investigation was that, while on the surface of the skin current dispersion was not different between sine and square wave stimulation, significantly more current was transferred deep in the muscle with sine versus square wave stimulation. The use of sine wave stimulation with electrode separation distances of less then 15 cm is recommended for electrical stimulation with a sine wave to achieve deep muscle stimulation.     

Effects of training on human tracking of electrocutaneous signals

The effects of training on a person's ability to perceive, interpret and utilize information presented via the tactile sense were examined by using a dual-channel electrocutaneous tracking approach. The electrocutaneous code studied was linear pulse rate encoding of information. The stimulus had a pulse width of 200 μs and a pulse rate between 2 and 50 pulsed per second. Three tracking tasks-visual tracking, onedimensional electrocutaneous tracking and two-dimensional electrocutaneous tracking-were performed by 20 subjects during eight to nine daily training sessions. The effects of this training regimen were found to be highly significant for both electrocutaneous tracking tasks (p<0.00005). The overall average improvement between successive training sessions was 21.6% for one-dimensional electrocutaneous tracking and 22.9% for two-dimensional electrocutaneous tracking. Furthermore, the rate of improvement was fastest during the initial training sessions with a slower rate of improvement seen in most subjects beginning with their fifth session. The cumulative effects of training were also reflected in the lesser amounts of practice and review required by the subjects with each succeeding test session. Results from this study can be used to estimate the amount and rate of improvement that one can expect using a typical training program for tactile sensory aids.     

Attenuation of pathological tremors by functional electrical stimulation I: Method

In this study we explored the possibility of suppressing pathological tremors using closed-loop functional electrical stimulation (FES) to activate the tremorogenic muscles out-of-phase. A displacement signal monitored with a transducer was filtered so as to be “tuned” to the tremor frequency at the wrist or elbow. The filtered signal was used to amplitude-modulate the electrical stimulation. The design process was based on measurements of the open-loop frequency response characteristics of the forearm and hand to stimulation of the elbow and wrist flexors and extensors in a number of subjects. These data allowed us to identify closed-loop configurations, which attenuated 2–5 Hz tremors substantially, while only minimally attenuating functional movements in the 0–1 Hz range. There was a fairly delicate balance between efficacy and the risk of instability. However, designs were identified that offered enough tremor suppression and adequate immunity to muscle/load variations for the technique to be considered seriously for clinical application.     

Characterization of formation and trace rhythm reproduction by rabbit hippocampal neurons during early and late ontogeny

The formation and reproduction of memory traces by hippocampal neurons were studied and a relationship between the number of presented series of periodical electrocutaneous stimulation and degree of trace acquisition of the rhythm, on the one hand, and rabbit age (6–30 days, 1, 4–5, and 7 years), on the other, was detected. The hippocampus of 6–7-day and 7-year-old rabbits is characterized by low neuron activity and inability to trace acquisition of rhythm. The pulse frequency and trace acquisition of the rhythm in animals aged 8–14 days and 4–5 years (middle age) formed slower than in adult animals (after 2–4 stimulation series on days 2–4 of experiment) and could not be reproduced on the next day without reminding. In rabbits aged 25–30 days and 1 year the basal activity reached the optimum level and trace acquisition of rhythm was observed after 1–2 series on days 1–2 of experiment and was reproduced without reminding on the next day. The detected physiological stages are in good correlation with the morphochemical organization of the rabbit hippocampus at the stages of early and late ontogeny. __________ Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 143, No. 3, pp. 254–257, March, 2007     

Electrotactile two point discrimination as a function of frequency,pulse width and pulse time delay

The feasibility of frequency modulated two-point discrimination as a design concept for electrocutaneous sensory substitution displays has been investigated further. Two new parameters have been tested on human subjects for their effect on the two-point threshold: pulse width and pulse time delay (phase shift). The pulse width study has shown that 100-μsec pulses resulted in the lowest threshold for spatial stimulation while 10-μsec pulses yielded the lowest threshold for temporal stimulation. It was also shown that pulse phase shifts of 0 to 180° result in different threshold values. Phase shifts of 0 to 135° showed slight threshold improvement. The 180° phase shift yielded substantial improvement. Supported by Veterans Administration Grant V101 (134)P-330.     

Direct Electrical Stimulation Using a Battery-Operated Device for Induction and Modulation of Colonic Contractions in Pigs

Direct electrical stimulation of the colon offers a promising approach for the induction of propulsive colonic contractions by using an implantable device. The objective of this study was to assess the feasibility to induce colonic contractions using a commercially available battery-operated stimulator (maximum pulse width of 1 ms and maximum amplitude of 10 V). Three pairs of pacing electrodes were inserted into the cecal seromuscular layer of anesthetized pigs. During a first set of in vivo experiments conducted on six animals, a pacing protocol leading to cecum contractions was determined: stimulation bursts with 1 ms pulse width, 10 V amplitude (7–15 mA), 120 Hz frequency, and 30-s burst duration, repeated every 2–5 min. In a second testing phase, an evaluation of the pacing protocol was performed in four animals (120 stimulation bursts in total). By using the battery-operated stimulator, contractions of the cecum and movement of contents could be induced in 92% of all stimulations. A cecal shortening of about 30% and an average intraluminal pressure increase of 10.0 ± 6.0 mmHg were observed.     

Electrocutaneous code pairs for artificial sensory communication systems

Pairs of electrocutaneous codes suitable for dual-channel sensory communication systems were compared using a dual-channel electrocutaneous tracking task. The tracking task required the test subject to dynamically respond to changes in the tactile sensation being modulated by two independent pseudorandom signals, one for each channel. The rule (or method) by which the signals changed the tactile sensations was called an electrocutaneous code. Four frequency variation codes and two intensity variation codes were paired in different combinations and then checked as to their effectiveness for sensory communications. The experimental protocol used a balanced incomplete block design which involved 24 subjects testing 3 of 8 code pairs each. Although the variance in the tracking performances between subjects was larger than the differences between the code pairs, learning rates for the various pairs were significantly different. The easiest one to learn was the Low Pulse Rate Modulation Code paired with itself. Other findings included the general superiority of monophasic stimulation code pairs over biphasic stimulation code pairs, the need for placement of the two electrodes on different dermatomes in order to achieve satisfactory dual-channel communications, and the greater sensitivity to electrocutaneous stimulation of the ventral side of the forearm versus its dorsal side. This research was supported by a National Science Foundation Grant, ENG7908197 and completed in the Department of Biomedical Engineering, Louisiana Tech University, Ruston, Louisiana.     

The effects of stochastic galvanic vestibular stimulation on human postural sway

 Galvanic vestibular stimulation serves to modulate the continuous firing level of the peripheral vestibular afferents. It has been shown that the application of sinusoidally varying, bipolar galvanic currents to the vestibular system can lead to sinusoidally varying postural sway. Our objective was to test the hypothesis that stochastic galvanic vestibular stimulation can lead to coherent stochastic postural sway. Bipolar binaural stochastic galvanic vestibular stimulation was applied to nine healthy young subjects. Three different stochastic vestibular stimulation signals, each with a different frequency content (0–1 Hz, 1–2 Hz, and 0–2 Hz), were used. The stimulation level (range 0.4–1.5 mA, peak to peak) was determined on an individual basis. Twenty 60-s trials were conducted on each subject – 15 stimulation trials (5 trials with each stimulation signal) and 5 control (no stimulation) trials. During the trials, subjects stood in a relaxed, upright position with their head facing forward. Postural sway was evaluated by using a force platform to measure the displacements of the center of pressure (COP) under each subject’s feet. Cross-spectral measures were used to quantify the relationship between the applied stimulus and the resulting COP time series. We found significant coherency between the stochastic vestibular stimulation signal and the resulting mediolateral COP time series in the majority of trials in 8 of the 9 subjects tested. The coherency results for each stimulation signal were reproducible from trial to trial, and the highest degree of coherency was found for the 1- to 2-Hz stochastic vestibular stimulation signal. In general, for the nine subjects tested, we did not find consistent significant coherency between the stochastic vestibular stimulation signals and the anteroposterior COP time series. This work demonstrates that, in subjects who are facing forward, bipolar binaural stochastic galvanic stimulation of the vestibular system leads to coherent stochastic mediolateral postural sway, but it does not lead to coherent stochastic anteroposterior postural sway. Our finding that the coherency was highest for the 1- to 2-Hz stochastic vestibular stimulation signal may be due to the intrinsic dynamics of the quasi-static postural control system. In particular, it may result from the effects of the vestibular stimulus simply being superimposed upon the quiet-standing COP displacements. By utilizing stochastic stimulation signals, we ensured that the subjects could not predict a change in the vestibular stimulus. Thus, our findings indicate that subjects can act as ”responders” to galvanic vestibular stimulation. Received: 13 March 1998 / Accepted: 8 October 1998     

Short and medium latency muscle responses evoked by electrical vestibular stimulation are a composite of all stimulus frequencies

Electrical vestibular stimulation produces biphasic responses in muscles maintaining balance. The two components of these muscle responses (termed the short latency and medium latency components) are believed to be independent and elicited by vestibular stimuli of different frequencies. We tested these hypotheses by determining (a) if frequency-specific stimulation protocols could evoke independently the short and medium latency responses and (b) whether these two components are triggered by distinct brain regions with a fixed time delay, interacting around 10 Hz. First, subjects were provided 10–25 Hz, 0–10 Hz, and 0–25 Hz vestibular stimuli to selectively modulate the short latency, medium latency, or both components of the response; and second, they were provided twenty sinusoidal stimuli from 1 to 20 Hz with a 0–20 Hz control trial, designed to determine whether an interaction between the short and medium latency responses occurs at a specific stimulation frequency. Both the 0–10 Hz and 10–25 Hz vestibular stimuli elicited multiphasic waveforms, suggesting the short and medium latency components were not modulated independently by the frequency-specific stimuli. Sinusoidal vestibular stimuli evoked responses at the stimulated frequency but no evidence of a reflex component interaction was observed. Instead, summation of the responses evoked by each of the sinusoidal stimuli resembled the biphasic response to broad bandwidth stimuli. Due to the lack of interaction and linear contribution of all stimulus frequencies to both the short and medium latency responses, the present results support the use of broad bandwidth electrical vestibular signal for physiological or clinical testing.     

Coordination between breathing and forearm movements during sinusoidal tracking

Time relationships (coordination) between breathing and rhythmical limb movements were analyzed during sinusoidal tracking movements of the forearm in 11 healthy subjects. The tracking rate was varied systematically between 0.1 and 1.0 Hz in 0.1-Hz steps. The aim of the study was to elucidate whether rhythmical tracking movements can entrain breathing, and whether this entrainment depends upon the movement rate. Subjects exhibited coordination between tracking movements and breathing at various rate ratios (1:1, 1:2, 1:3). At tracking rates of between 0.2 and 0.6 Hz, 1:1 coordination occurred with a maximum at 0.3 Hz; this rate range was called the 1:1 entrainment band. Coordination of 1:2 occurred at between 0.5 and 1.0 Hz (the 1:2 coordination band) with a maximum at 0.7 Hz. Coordination of 1:3 could be detected at between 0.5 and 1.0 Hz. Different subjects showed 1:n entrainment bands at similar locations but different widths of the rate range studied. The breathing rate during tracking was significantly higher than at rest, and it was correlated positively with tracking rate. This correlation, however, depended upon the width of the entrainment bands. Breathing rates varied between 0.2 and 0.6 Hz for all coordination patterns. We conclude that the occurrence of fixed time relationships is an expression of the strength of central nervous system coupling between the two processes. The frequency of coordination between breathing and rhythmical tracking movements depends critically upon the movement rate. Accepted: 7 October 1999     

Bioelectrical cochlear noise and its contralateral suppression: relation to background activity of the eighth nerve and effects of sedation and anesthesia

 The bioelectrical activity of the cochlea, without any ipsilateral acoustic stimulation, was recorded in awake guinea pigs (GPs) between electrodes chronically implanted at the round window (RW) and the skull. Measuring its power in the band centered around 1.0 kHz (0.5–2.5 kHz) provided an indirect measure of the ensemble background (EBA) activity of the eighth nerve. Contralateral white-noise (CLWN) stimulation reduced this EBA, presumably by activation of medial olivocochlear fibers. The aim of the investigation was to validate measurements of EBA and of its contralateral suppression in order to study the medial efferent function. The first goal was to find the best conditions for recording the EBA in the absence of ipsilateral stimulation and for studying its suppression by contralateral acoustic stimulation, which implies that no noise was generated by the experimental animal. Thus recordings were compared in normal, awake GPs and in GPs under sedation with xylazine, anesthetized with a combination of xylazine and ketamine, and with and without temperature regulation. In order to monitor the effects of sedation and anesthesia, the recordings were analyzed not only in the 0.5- to 2.5-kHz frequency band but also in the other frequency bands, 5–50 Hz, 50–150 Hz, and 150–500 Hz, which presumably include general central and neuromuscular contributions. The results show that sedation with xylazine accompanied by regulation of body temperature does not affect the EBA value nor its contralateral suppression. Nevertheless, anesthesia should be avoided, even with control of body temperature. The second goal of this study was to identify the specific cochlear contribution to the raw RW signal. Thus recordings were performed in normal and deafened animals and analyzed in the frequency band 0.5–2.5 kHz and also in the other frequency bands of 5–50 Hz, 50–150 Hz, and 150–500 Hz. The results indicate that most of the cochlear activity lies in the frequency band 0.5–2.5 kHz, with also some minor contribution coming from the 150- to 500-Hz band. Analysis and comparison of power values in the different conditions indicate that specific cochlear EBA power was about 60 μV². From a commonly accepted mean background discharge rate of 50 spikes/s (sp/s), the EBA power without CLWN should have been around 4.4 μV² if the fibers’ activity was random. This difference suggests that there is probably some degree of synchrony between individual fibers. There was a reduction of approximately 45% during CLWN stimulation. This suppression might correspond to a reduction in both discharge rate and synchrony of the fibers. Received: 28 January 1996 / Accepted: 10 April 1997     

The reorganization of tremulous movements in the upper limb due to finger tracking maneuvers

In light of the interplay between limb oscillatory outputs and the outcome performance of movement effectors, this study was undertaken to investigate neuromotor control in the upper limb during position tracking and posture holding. Sixteen volunteers conducted a postural pointing task and two index tracking maneuvers at 0.3 and 0.6 Hz with an outstretched arm. Limb acceleration in the index finger, hand, forearm, arm, and C7 spinal process were monitored to correlate functionally with the accuracy of index rhythmic displacements. The results showed that index oscillatory activity multiplied with tracking speed, but hand oscillatory activity declined during tracking movement. The tracking maneuvers also altered spectral distribution of the tremulous activities in the context of a lower spectral peak in the range of 8–12 Hz and suppression of spectral peaks at 2–4 Hz, in reference to that already presented in posture tremor. Consisting of three local maxima around 2–4, 8–12, and 18–22 Hz, the coherence of tremulous activity between the finger and hand during position tracking was nearly identical, but inferior to high coherence below 12 Hz during posture holding. Functionally, better tracking performance was associated with a smaller tremulous activity in the finger and hand, entailing sophisticated release of mechanical couplings in the finger-hand and hand-forearm. In conclusion, inversely related to tracking performance, limb tremulous movements were task-dependently organized, and speed-invariant coherence of limb tremulous movement specified an inter-segmental coordination, which is physiological evidence of the generalized motor program for position tracking.     

Frequency-domain analysis of cerebral autoregulation from spontaneous fluctuations in arterial blood pressure

The dynamic relationship between spontaneous fluctuations of arterial blood pressure (ABP) and corresponding changes in crebral blood flow velocity (CBFV) is studied in a population of 83 neonates. Static and dynamic methods are used to identify two subgroups showing either normal (group A, n=23) or impaired (group B, n=21) cerebral autoregulation. An FFT algorithm is used to estimate the coherence and transfer function between CBFV and ABP. The significance of the linear dependence between these two variables in demonstrated by mean values of squared coherence >0.50 for both groups in the frequency range 0.02–0.50 Hz. However, group A has significanlty smaller coherences than group B in the frequency ranges 0.02–0.10 Hz and 0.33–0.49 Hz. The phase response of group A is also significantly more positive than that of group B, with slopes of 9.3±1.05 and 1.80±1.2 rad Hz⁻¹, respectively. The amplitude frequency response is also significantly smaller for group A in relation to group B for the frequency range 0.25–0.43 Hz. These results suggest that transfer function analysis may be able to identify different components of cerebral autoregulation and also provide a deeper understanding of recent findings by other investigators.     

Relationship between pulse rate and pulse width for a constant-intensity level of electrocutaneous stimulation

The relationship between pulse rate (PR) and pulse width (PW) for a constant level of electrocutaneous stimulation was ascertained using the method of comparative judgments. Twelve volunteer subjects were asked to adjust the PW of a Comparison Stimulus (S₂) until its intensity matched that of a Standard Stimulus (S₁) for which the PW was 200 μs and PR was 10 or 20 pulses per sec (pps). As expected, the experimental results indicate that the PW of a constant-current amplitude pulse train should decrease as its PR increases if a constant level of tactile stimulation intensity is desired. However, PW and PR were not linear-inversely related (p<0.005). Rather, their relationship was best described by a logarithmic equation: log PW=a+b log PR, where PW is in microseconds,a is 2.82,b is −0.412, and PR is between 1 and 100 pps. Utilization of this relationship during electrical stimulation of the skin sense will decouple the intensity component of the tactile sensation from its frequency component, thereby enhancing the potential comfort and clarity of this sensory communication interface.     

Activity of rabbit neocortex and hippocampus neurons in orientational-investigative behavior and freezing

Autocorrelation histograms were used to study the nature of spike activity in neurons recorded bilaterally from the visual and parietal areas of the cortex and hippocampal field CA1 in rabbits in free behavior during exposure to emotionally significant stimuli. Active movement orientational-investigative reactions to stimuli were associated with grouping of discharges and periodicity in the spike activity of most neurons in the cortex and hippocampus, this being dominated by the θ frequency (predominantly 4–5 Hz in the cortex and 4–5 and 6–7 Hz in the hippocampus). As compared with active movement reactions, freezing in response to stimulation was associated with increased numbers of neurons with uniform discharge distributions, while the spike activity of neurons with discharge periodicity showed increases in the intensity of the δ frequency (predominantly from 2 to 4 Hz), while θ intensity decreased. The number of neurons with periodic frequency in the δ range was greater in freezing than in the baseline state of calmly sitting rabbits. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 92, No. 11, pp. 1273–1284, November, 2006.     

Investigating the relationship between fetus EEG,respiratory, and blood pressure signals during maturation using wavelet transform

In this study, we introduce the fast wavelet transform as a method for characterizing maturational changes in electrocortical activity, respiratory activity, and blood pressure in fetal lambs in early (110–122 days), mid (123–135 days), and late (136–144 days) third trimester (term 145 days). Each recording was 2 hr in duration. Wavelet decomposition was performed for six sets of parametersD _2j where 1≤j≤6. The six series wavelet transforms represent the following signal frequency bands: 1. 16–32 Hz; 2. 8–16 Hz; 3. 4–8 Hz; 4. 2–4 Hz; 5. 1–2 Hz; 6. 0.5–1 Hz. In the early group, power in the electrocephalogram (EEG) was highest in the fourth wavelet band, with relatively low power in the other bands. Increase in gestational age was characterized by increased power in all four wavelet bands. Power in the first wavelet band was significantly increased during low-voltage fast activity (LVFA) in the late group. The respiratory and blood pressure signals showed common frequency components with respect to time and were coincident with the LVFA EEG signal. Respiratory activity was only observed during some of the LVFA periods and was completely absent during high-voltage slow activity (HVSA) EEG. The respiratory signal showed dominant power in the fourth wavelet band, and less power in the third and fifth band. The blood pressure signal was also characterized by dominant power in the fourth wavelet band. This power was significantly increased during periods of respiratory activity. These results suggest a strong relationship between fetal EEG, blood pressure, and breathing movements.     

Effects of stimulation parameters on modification of spinal spasticity

Various electrical stimuli with frequencies from 10 Hz to 1000 Hz and pulse widths from 10 μs to 1000 μs were applied to seven spinal-cord injured patients with spasticity of the knee muscle. Spasticity was assessed with the pendulum test and EMG activity in the quadriceps and hamstrings. No universal optimum combination of stimulation parameters could be established but stimuli of 100 Hz and 100 μs pulse width were more effective than other combinations. Subjective remarks of patients regarding the effects over 24 h did not always correlate with measured data obtained within one hour after stimulation.     

Improving balance function using vestibular stochastic resonance: optimizing stimulus characteristics

Stochastic resonance (SR) is a phenomenon whereby the response of a non-linear system to a weak periodic input signal is optimized by the presence of a particular non-zero level of noise. Stochastic resonance using imperceptible stochastic vestibular electrical stimulation, when applied to normal young and elderly subjects, has been shown to significantly improve ocular stabilization reflexes in response to whole-body tilt; improved balance performance during postural disturbances and optimize covariance between the weak input periodic signals introduced via venous blood pressure receptors and the heart rate responses. In our study, 15 subjects stood on a compliant surface with their eyes closed. They were given low-amplitude binaural bipolar stochastic electrical stimulation of the vestibular organs in two frequency ranges of 1–2 and 0–30 Hz over the amplitude range of 0 to ±700 μA. Subjects were instructed to maintain an upright stance during 43-s trials, which consisted of baseline (zero amplitude) and stimulation (non-zero amplitude) periods. Measures of stability of the head and trunk using inertial motion unit sensors attached to these segments and the whole body using a force plate were measured and quantified in the mediolateral plane. Using a multivariate optimization criterion, our results show that the low levels of vestibular stimulation given to the vestibular organs improved balance performance in normal healthy subjects in the range of 5–26% consistent with the stochastic resonance phenomenon. In our study, 8 of 15 and 10 of 15 subjects were responsive for the 1–2- and 0–30-Hz stimulus signals, respectively. The improvement in balance performance did not differ significantly between the stimulations in the two frequency ranges. The amplitude of optimal stimulus for improving balance performance was predominantly in the range of ±100 to ±400 μA. A device based on SR stimulation of the vestibular system might be useful as either a training modality to enhance adaptability or skill acquisition, or as a miniature patch-type stimulator that may be worn by people with disabilities due to aging or disease to improve posture and locomotion function.