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Biomedical subjects

T C Hain

Publications and source records attributed to T C Hain.

At least 19 recordsLinked to original sources
PubMed2005

Perturbation parameters associated with nonlinear responses of the head at small amplitudes.

The head-neck system has multiple degrees of freedom in both its control and response characteristics, but is often modeled as a single joint mechanical system. In this study, we have attempted to quantify the perturbation parameters that would elicit nonlinear responses in a single degree-of-freedom neuromechanical system at small amplitudes and velocities of perturbation. Twelve healthy young adults seated on a linear sled randomly received anterior-posterior sinusoidal translations with +/-15 mm and +/-25 mm peak displacements at 0.81, 1.76, and 2.25 Hz. Head angular velocity and angular position data were examined using a nonlinear phase-plane analysis. Poincare sections of the phase plane were computed and Lyapunov exponents calculated to measure divergence (chaotic behavior) or convergence (stable behavior) of system dynamics. Variability of head angular position and velocity across the entire phase plot was compared to that of the Poincare sections to quantify spatial-temporal irregularity. Multiple equilibrium points and positive Lyapunov exponents revealed chaotic behavior at 0.81 Hz at both amplitudes whereas responses at 1.76 and 2.25 Hz exhibited periodic oscillations, clustered phase points, and negative Lyapunov exponents. However, intersubject variability increased at the lowest frequency and a few subjects presented chaotic behavior at all frequencies. An inverted pendulum with position and velocity threshold nonlinearity was adopted as a simplistic model of the head and neck. Simulations with the model resulted in features similar to those observed in the experimental data. Our principal finding was that increasing the perturbation amplitude had a stabilizing effect on the behavior across frequencies. Nonlinear behaviors observed at the lowest stimulus frequency might be attributed to fluctuations in control between the multiple sensory inputs. Although this study has not conclusively pointed toward any single mechanism as responsible for the responses observed, it has revealed clear directions for further investigation. To examine if changing the sensory modalities would elicit a significant change in the nonlinear behaviors observed here, further experiments that target a patient population with some sort of sensory deficit are warranted.

Acceleration↗
PubMed2002

Effects of predictive mechanisms on head stability during forward trunk perturbation.

While much is known about reflex and mechanical contributions to the control of head stability, little is known about predictive control. The goal of this experiment was to determine the contribution of predictive mechanisms to head stability in space, in the pitch plane, during forward trunk perturbations. Eleven standing healthy subjects had their trunk pulled forward by a load-pulley apparatus. The perturbation was either self-triggered or imposed (triggered by the experimenter). Subjects were exposed to two loads: 2% and 4% of their body weight. The contributions of torques acting on the head-neck system were inferred from head and trunk kinematics, neck muscle EMG, and the torques acting on the head, which were computed using inverse dynamics. The results showed that both the head and trunk moved less during the self-triggered than imposed condition during both loads for most of the participants. There was no evidence of predictive neck countertorque or increased neck muscle co-contraction during the self-triggered condition. These findings suggest that most of the subjects improved head stability in the self-triggered condition by reducing trunk motion and the associated interactive torque that perturbed the head.

Adaptation, Physiological↗
PubMed2002

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.

Algorithms↗
PubMed2001

Tullio phenomenon with dehiscence of the superior semicircular canal.

HYPOTHESIS: The goal of the investigation was to determine if vector analysis of nystagmus in a patient with the Tullio phenomenon could determine the source of the nystagmus. BACKGROUND: The Tullio phenomenon consists of the combination of vertigo and abnormal eye and/or head movements provoked by sound. Dehiscence of the superior semicircular canal can be found in certain patients with the Tullio phenomenon. METHODS: The patient was tested with pure tones ranging from 250 to 3,000 Hz at 95dB HL. The time course of the three-dimensional vector of eye movement, including torsion and vertical and horizontal displacement angles was determined by individual stop-frame analysis of digitized video. RESULTS: Torsion amplitude varied from 1 to 7 degrees; vertical amplitude varied from 1 to 5 degrees; and horizontal amplitude varied less than 1.5 degrees. The maximal response occurred on stimulation of the right ear with a 1,250-Hz 95-dB HL tone. This elicited a reliable counterclockwise torsional and down-beating fast phase nystagmus as seen from the examiner's point of view. Comparison of the nystagmus with known canal vectors identified the right superior semicircular canal as the source of stimulation. High-resolution computed tomography scan of the temporal bone showed a definite right superior canal dehiscence. CONCLUSION: The origin of nystagmus from the Tullio phenomenon can be identified by calculating the three-dimensional vector of the observed nystagmus. We show that vector analysis of the observed eye movement can be used to infer the source of nystagmus in these patients. The development of real-time, three-dimensional vector analysis of nystagmus is desirable.

Audiometry, Pure-Tone↗
PubMed2001

Effects of delayed auditory feedback (DAF) on the pitch-shift reflex.

Changes in voice pitch auditory feedback to vocalizing subjects elicit compensatory changes in voice fundamental frequency (F0). The neural mechanisms responsible for this behavior involve the auditory and vocal-motor systems, collectively known as the audio-vocal system. Previous work [Burnett et al., J. Acoust. Soc. Am. 103, 3153-3161 (1998); Hain et al., Exp. Brain Res. 130, 133-141 (2000); Larson et al., J. Acoust. Soc. Am. 107, 559-564 (2000)] indicated that this system operates using negative feedback to cancel out low-level errors in voice F0 output. By introducing delays in the auditory feedback pathway, we hoped to transiently "open" the feedback loop and learn which components of the response are most closely related to the timing of the auditory feedback signal. Subjects were presented with pitch-shift stimuli that were paired with a delay of 0, 50, 100, 200, 300, or 500 ms. Delayed auditory feedback did not affect voice F0 response latency or magnitude, but it delayed the timing of later parts of the response. As a further test of the audio-vocal control system, a second experiment was conducted in which delays of 100, 200, or 300 ms were combined with stimuli having onset velocities of 1000 or 330 cents/s. Results confirmed earlier reports that the system is sensitive to velocity of stimulus onset. A simple feedback model reproduced most features of both experiments. These results strongly support previous suggestions that the audio-vocal system monitors auditory feedback and, through closed-loop negative feedback incorporating a delay, adjusts voice F0 so as to cancel unintentional small magnitude fluctuations in F0.

Adolescent↗
PubMed2000

Vibration does not improve results of the canalith repositioning procedure.

OBJECTIVE: To determine whether, in patients with benign paroxysmal positional vertigo (BPPV), the canalith repositioning procedure performed with vibration applied over the mastoid bone of the affected ear is more effective in resolving the symptoms and preventing recurrence of BPPV than the procedure performed without vibration. DESIGN: Retrospective case review. SETTING: Tertiary referral center. PATIENTS: Ninety-four patients diagnosed as having BPPV involving the posterior semicircular canal. INTERVENTIONS: Patients were assigned to one of 2 treatment groups: the canalith repositioning procedure with vibration (n=44) and with no vibration (n=50). MAIN OUTCOME MEASURES: Effectiveness of treatment was determined through clinical reevaluation or reported through a telephone interview 1 week after treatment. Intensity of symptoms was quantified on a scale of 1 to 3 (mild, moderate, or severe); effectiveness of treatment was categorized on a scale of 1 to 4 (cure, much better, better, or no change). Rate of recurrence was determined through later clinical reevaluation or a telephone interview. RESULTS: At 1 week, 57 of the 94 patients were cured and 16 were much better, providing a 78% overall success rate. There was no significant difference in effectiveness of the treatment or the frequency of reoccurrence of BPPV between the vibration and no-vibration groups as determined from the Kaplan-Meier product-limit method and log-rank test. Rate of recurrence was 47% at a maximum follow-up of 5.25 years. CONCLUSIONS: Our results suggest that, while the canalith repositioning procedure is effective in the treatment of BPPV, vibration applied during the maneuver does not significantly affect short-term or long-term outcomes.

Adult↗
PubMed2000

Instructing subjects to make a voluntary response reveals the presence of two components to the audio-vocal reflex.

Previous findings have shown that subjects respond to an alteration, or shift, of auditory feedback pitch with a change in voice fundamental frequency (F0). When pitch shifts exceeding 500 ms in duration were presented, subjects' averaged responses appeared to consist of both an early and a late component. The latency of the second response was long enough to be produced voluntarily. To test the hypothesis that there are two responses to pitch-shift stimuli and to clarify the role of intention, subjects were instructed to change their voice F0 in the opposite direction of the pitch-shift stimulus, in the same direction, or not to respond at all. In a second group, subjects were tested under the above conditions as well as under instructions to raise voice F0 or to lower F0 as rapidly as possible upon hearing a pitch shift. Results showed that, when given instructions to produce a voluntary response, subjects made both an early vocal response (VR1) and a later vocal response (VR2). The second response, VR2, was almost always made in the instructed direction, whereas VR1 was often made incorrectly. The latency of VR1 was reduced under instructions to respond to feedback pitch shifts by changing voice F0 in the opposite direction, compared with that when told to ignore the pitch shifts. Latency and amplitude measures of VR2 differed under the various experimental conditions. These results demonstrate that there are two responses to pitch-shift stimuli. The first is relatively automatic but may be modulated by instructions to the participant. The second response is probably a voluntary one.

Acoustic Stimulation↗
PubMed2000

Effects of pitch-shift velocity on voice Fo responses.

Previous studies have shown that voice fundamental frequency (F0) is modified by changes in the pitch of vocal feedback and have demonstrated that the audio-vocal control system has both open- and closed-loop control properties. However, the extent to which this system operates in closed-loop fashion may have been underestimated in previous work. Because the step-type stimuli used were very rapid, and people are physically unable to change their voice F0 as rapidly as the stimuli, feedback responses might have been reduced or suppressed. In the present study, pitch-shift stimuli, consisting of a disparity between voice F0 and feedback pitch of varying ramp onset velocities, were presented to subjects vocalizing a steady /ah/ sound to examine the effect of stimulus onset on voice F0 responses. Results showed that response velocity covaried with stimulus velocity. Response latency and time of the peak response decreased with increases in stimulus velocity, while response magnitude decreased. A simple feedback model reproduced most features of these responses. These results strongly support previous suggestions that the audio-vocal system monitors auditory feedback and, through closed-loop negative feedback, adjusts voice F0 so as to cancel low-level fluctuations in F0.

Adolescent↗
PubMed1999

Effects of T'ai Chi on balance.

OBJECTIVE: To determine if the practice of T'ai Chi significantly improves balance. METHODS: Twenty-two persons with mild balance disorders were studied. Five measures of balance were obtained, including 3 objective measures (moving platform posturography, Romberg testing, and reach testing) and 2 disability questionnaires (Dizziness Handicap Inventory and a modified Medical Outcomes Study general health survey). To be included, patients were required to be able to stand in the eyes-closed regular Romberg position for 30 seconds. The subjects underwent 8 weeks of T'ai Chi training and practice and then were retested. RESULTS: Highly significant improvements were found on both the posturography test and the Dizziness Handicap Inventory questionnaire scores (P<.001 and P=.004, respectively). Trends toward improvement were also noted in Romberg test results and the Medical Outcomes Study survey (P=.03 for both). Reach was not improved. CONCLUSION: These findings suggest that T'ai Chi training improves balance.

Adult↗
PubMed1999

Mal de debarquement.

MAIN OUTCOME MEASURE: Clinical features of mal de debarquement syndrome. RESULTS: Nearly all respondents were middle-aged women (26 of 27; mean age, 49.3 years). The duration of symptoms ranged from 6 months to 10 years (mean, 3.5 years; SD, 2.5 years). The symptoms were constant in 23 (85%) patients. Neither meclizine hydrochloride nor transdermal scopolamine was helpful. Benzodiazepines were of the most benefit. Balance rehabilitation physical therapy was undertaken by 15 patients, who on average reported a small benefit. CONCLUSIONS: More than double the number of previously reported cases of mal de debarquement syndrome were identified by this study. The syndrome usually occurs in middle-aged women following an ocean cruise. Symptoms are often refractory to vestibular suppressants as well as physical therapy.

Adult↗
PubMed1999

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.

Adult↗
PubMed1999

Ototoxicity resulting from combined administration of metronidazole and gentamicin.

HYPOTHESIS: The hypothesis that metronidazole can augment the ototoxicity of gentamicin was tested. BACKGROUND: Metronidazole and gentamicin are antibiotics that are used in combination to provide broad-spectrum antimicrobial coverage. It has been observed clinically that an increased ototoxic effect occurs when these agents are used in combination. METHODS: Groups of guinea pigs were given various doses of gentamicin alone, various doses of gentamicin in combination with metronidazole, or metronidazole alone. Auditory damage was determined electrophysiologically by measurement of the compound action potential. Hair cell damage was quantified by immunofluorescent microscopy. RESULTS: Electrophysiologic data revealed an augmented ototoxic effect when metronidazole was given with both a moderate and a high dose of gentamicin. Thresholds (dB SPLp) for the compound action potential (N1) for animals receiving a medium dose of gentamicin alone (50 mg/kg) were approximately 20-dB SPLp. This threshold increased to approximately 50-dB SPLp when metronidazole (35 mg/kg) was administered along with the medium-dose gentamicin. Additionally, animals receiving high-dose gentamicin (75 mg/kg) alone demonstrated increased N1 thresholds from 85 to 95 when metronidazole (35 mg/kg) was added to the gentamicin regimen. This effect was evident histopathologically by increased cochlear hair cell damage. Outer hair cell loss for animals receiving medium-dose gentamicin alone did not differ from that of controls. When metronidazole (35 mg/kg) was combined, however, outer hair cell loss increased to approximately 50%. CONCLUSIONS: These data support the clinical observation of augmented ototoxicity in patients receiving combined gentamicin and metronidazole. Caution should be used when administering these two agents together. Clinicians should consider other antibiotic strategies whenever possible.

Animals↗
PubMed1999

Predicting control mechanisms for human head stabilization by altering the passive mechanics.

The purpose of this study was to clarify the mechanisms controlling head and neck stabilization in the horizontal (yaw) and vertical (pitch) planes by changing the passive mechanics of the head-neck motor system. Angular velocities of the head and trunk in space were recorded in seated subjects during external perturbations of the trunk with pseudorandom sum-of-sines (SSN) stimuli. Four subjects in yaw and nine subjects in pitch actively stabilized their heads in the dark, and performed a mental distraction task in the dark both with and without a weight atop the head. In yaw, the behavior of the head was found to change relatively little with added inertia. As adding inertia to a passive mechanical system should cause substantial changes in dynamics, we inferred that neural mechanisms were invoked to maintain the constant response dynamics. A mathematical model of head-neck control was applied to predict the relative influence of the vestibulocollic and cervicocollic reflexes, and of inertia, stiffness, and viscosity. Using optimization methods to fit the model to experimental data, we identified stiffness and vestibulocollic reflex gain as the primary contributors to the control of head stabilization in space. In pitch, increasing inertia accentuated phase shifts at higher frequencies. Because our pitch model was insufficiently constrained, we only simulated responses due to passive mechanics. Model simulation predicted unstable head motion at all test frequencies. Subjects were able to compensate for trunk motion at most frequencies, however, suggesting that neural components were modulated to exert compensatory responses both with and without additional weight.

Adaptation, Physiological↗
PubMed1998

Voice F0 responses to manipulations in pitch feedback.

Recent studies have shown that when phonating subjects hear their voice pitch feedback shift upward or downward, they respond with a change in voice fundamental frequency (F0) output. Three experiments were performed to improve our understanding of this response and to explore the effects of different stimulus variables on voice F0 responses to pitch-shift stimuli. In experiment 1, it was found that neither the absolute level of feedback intensity nor the presence of pink masking noise significantly affect magnitude or latency of the voice F0 response. In experiment 2, changes in stimulus magnitude led to no systematic differences in response magnitudes or latencies. However, as stimulus magnitude was increased from 25 to 300 cents, the proportion of responses that changed in the direction opposite that of the stimulus ("opposing" response) decreased. A corresponding increase was observed in the proportion of same direction responses ("following" response). In experiment 3, increases in pitch-shift stimulus durations from 20 to 100 ms led to no differences in the F0 response. Durations between 100 and 500 ms led to longer duration voice F0 responses with greater response magnitude, and suggested the existence of a second F0 response with a longer latency than the first.

Adolescent↗
PubMed1997

Pressure-induced ocular torsion.

We describe 3 patients within whom ocular torsion could be induced by application of positive air pressure to one external ear canal. In all cases, the superior pole of the eye rotated away from the stimulated ear when positive pressure was applied. The amplitude of torsion ranged from 3 degrees to 16 degrees. Exploratory tympanotomy was performed in all 3 patients. In 2 patients, round-window fistulas were found and repaired. In the third patient, no fistula was noted but the oval- and round-window areas were patched. There was no resolution of symptoms after surgery in any patient. Based on these cases, patients presenting with pressure-induced ocular torsion are unlikely to have resolution of their symptoms, even when a perilymphatic fistula is confirmed and repaired. We hypothesize that pressure-induced ocular torsion is caused by an irreversible juxtaposition of the utricle and stapes footplate.

Adult↗
PubMed1996

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.

Computer Simulation↗
PubMed1996

Stimulus parameters and inertial load: effects on the incidence of protective stepping responses in healthy human subjects.

OBJECTIVE: To test the hypotheses that the incidence of protective stepping in response to sudden translations of the support would (1) increase as a function of both the magnitude of surface displacement and velocity of platform movement, and (2) decrease in association with an increase in external loading applied to the body. DESIGN: A log-linear approach was used to analyze the incidence of stepping by testing several models incorporating different platform stimulus parameters (direction, displacement, velocity) and external loading (0% and 20% body weight). SETTING: Institutional-based research laboratory. PARTICIPANTS: Eight healthy younger adult (21 to 28 years) volunteers. MAIN OUTCOME MEASURES: The incidence and number of protective steps served as the primary planned outcome variables. RESULTS: Steps occurred more frequently for anterior (83 steps) versus posterior (45 steps) translations. Step occurrence was generally proportional to platform velocity, and increased with displacements up to 15cm, but then plateaued. External loading was associated with a reduction in the number of steps for lower magnitudes of platform motion but had little effect at higher magnitudes. CONCLUSION: The tendency to step in response to externally applied disturbances to stance appears to be a complex function of direction, velocity, displacement, and inertial load.

Adult↗