Changes in the horizontal vestibulo-ocular reflex of the rhesus monkey with behavioral and pharmacological alerting.

It is well known that eye movements are influenced by an animal's state of arousal. Alterations in the dynamic characteristics of the horizontal vestibulo-ocular reflex of adolescent rhesus monkeys induced by changes in the animal's state of arousal were studied using linear system analysis employing both single frequency sinusoidal and white noise rotational stimulations. Arousal changes were induced by a behavioral task and/or the administration of amphetamines (0.5 mg/kg). Results indicate that highly alert animals display vestibulo-ocular reflex gains significantly different from less alert animals. Specifically, the gain of the vestibulo-ocular reflex is closer to unity over a wider range of frequencies in more alert animals. These changes were independent of the method used to maintain a high level of arousal.

Symposium on low frequency harmonic acceleration, the rotatory chair. Responses to low-frequency harmonic acceleration in patients with acoustic neuromas.

There has been increased interest in harmonic acceleration (HA) for the evaluation of vestibular pathology in recent years. The purpose of this study was to determine the value of harmonic acceleration testing in 24 patients with surgically proven acoustic neuromas. All patients were tested preoperatively and postoperatively, and none were lost to follow-up. Thirteen were tested one year or more after surgery. For rotational evaluation, each patient was seated in an enclosed chair driven by a D.C. torque motor turntable system and rotated utilizing five low frequency sinusoids (.01 - .16 Hz). Slow eye movements in response to acceleration were analyzed by analog and digital computer techniques. Output measures were the phase relationships between the input stimulus velocity and the slow phase eye movement velocity and labyrinthine preponderance (LP) or asymmetry. Preoperative results showed that caloric examination identified 79% of patients with tumors and HA 67% (2 standard deviation criteria). When both tests were evaluated together they identified 91% of all patients with tumors. Of the 4 small tumors (less than 2 cm), HA identified 4 and caloric examination 2. All patients were tested approximately 7 days postoperatively and had statistically significant shifts in their phase and LP measures. The shifts in phase were stable up to 3 years after surgery; however, the LP returned toward normal values. These findings indicate that the vestibular system can compensate but not repair itself after the loss of one labyrinth. In those patients who continued to be symptomatic 1 year or more after surgery, the LP did not return to normal values. Harmonic acceleration testing complemented caloric testing in the identification of patients with tumors and objectively demonstrated patients with continued symptomatology after surgery.

Monocular nystagmic responses to caloric stimulation.

Twenty-five normal subjects and 173 clinical patients received standard bithermal caloric testing. Vestibular nystagmus was evaluated for cumulative slow phase velocity from the summated horizontal eye recording and independent recording of the left and right eye. These data revealed that cold water stimulation produced more intense activation of the ipsilateral eye. Simultaneous closed-circuit video and D.C. electro-oculographic recordings from eight normal rhesus monkeys in response to cold water irrigations confirmed the fact that this stimulus leads to differential activation of the extraocular muscles. A possible explanation for this finding is discussed.

Vestibular responses to bithermal caloric and harmonic acceleration.

Vestibulo-oculomotor responses from 33 patients with either peripheral labyrinthine or central pathology were compared using the standardized Fitzgerald-Hallpike bithermal caloric and harmonic accelerations. In numerous cases (40%), patients with normal caloric responses reflected abnormalities in either their phase relationships or labyrinthine preponderance to sinusoidal acceleration. Results clearly indicate that harmonic acceleration provides additional information to the clinician in the evaluation of vestibulo-oculomotor pathology that is not reflected in the responses to standard caloric tests.

Cross-power spectral density analysis of pursuit tracking. Evaluation of central and peripheral pathology.

Twenty-one normal subjects were evaluated for pursuit tracking performance at frequencies of .2, .4, .8, 1.2 and 1.6 Hz. Measures of phase (lead or lag of target), gain (amplitude of response) and spectral purity (distortion of smooth tracking) were obtained by a cross-power spectrum analysis of the stimulus input and the eye movement output. It was determined that normal subjects could quite adequately track a target moving at .4 Hz. After establishing this parameter for normals, 43 patients with central and peripheral pathology related to the vestibulo-oculomotor system were tested at this frequency. Data revealed that not only central lesions but also peripheral labyrinthine impairment could cause a marked decrement in pursuit tracking ability. Also, marked changes in tracking were evident following both central and peripheral surgical procedures.

Dynamic range of the frequency response of the horizontal vestibulo-ocular reflex of the alert rhesus monkey.

The vestibulo-ocular reflex (VOR) of the alert rhesus monkey was studied using rotational acceleration in the horizontal plane. Both sinusoidal and pseudorandom binary sequence (PRBS) stimuli were used to perform parameters. Results from sinusoidal studies over the frequency range 0.01 to 1.28 Hz, with stimulus amplitudes ranging from 16 to 256 degrees/second were consistent with a linear model of the VOR. Results PRBS stimuli whose bandwidths extended from 0.008 to 0.45 Hz with amplitudes of 4 to 256 degrees corroborated the sinusoidal results. Sinusoidal data obtained at high rotational frequencies indicated that a peak VOR gain occurred at approximately 4 Hz. Close agreement between sinusoidal and PRBS data suggested that the rhesus VOR does not manifest the predictive behavior reported in studies of smooth pursuit eye movements.

Modulation of mean eye position during vestibular induced eye movements of the rhesus monkey.

Vestibular induced nystagmus in the dark was measured during sinusoidal rotational stimulation in three male monkeys using electrooculography. A periodic modulation of mean eye position was observed. This periodic modulation was quantified by least squares fitting through the nystagmus a sinusoid whose frequency was the same as that of the stimulus. The magnitude and the phase of the best fitting sinusoid were used to determine gain and phase delay at various combinations of stimulus amplitudes and frequency. Results indicate that the mean eye position in the alert animals is predominantly modulated in the direction of the fast component of nystagmus. The magnitude of this modulation of mean eye position is independent of the amplitude of the stimulus. The phase relationship of this modulation is dependent on the frequency of stimulation. In drowsy animals, the modulation of mean eye position is predominantly in the direction of the slow component of nystagmus. We conclude that the modulation of mean eye position during vestibular induced nystagmus operates as an automatic gain control system which is sensitive to the animal's level of alertness. The quantitative data in this report are useful in refining current models of vestibular nystagmus.

Comparison of subjective symptomatology and responses to harmonic acceleration in patients with Menière's disease.

Vestibular function testing employing caloric stimulation has been of limited value in the evaluation of patients with Menière's disease, with its cardinal shortcoming being poor repeatability. This modality has not been useful in determining appropriate treatment for these individuals. We have used low frequency harmonic acceleration to study patients with vestibular pathology and have noted a high level of repeatability of responses to this type of stimulation. This study was designed to determine if the clinical status of patients with Menière's disease could be correlated with their responses to harmonic acceleration. Thirty-six patients with typical symptoms of Menière's disease were evaluated and divided into four groups based on severity of symptoms. Analysis of variance of the phase shifts in asymmetry (or preponderance) showed a significant difference between the groups. Responses to this type of stimulation correlated with the clinical status of the patients to a significant degree. These data indicate that harmonic acceleration can be of value to the otologist in the evaluation of patients with Menière's disease.

Long-term effects of microgravity and possible countermeasures.

It is well known that long-term exposure to microgravity causes a number of physiological and biochemical changes in humans; among the most significant are: 1) negative calcium balance resulting in the loss of bone; 2) atrophy of antigravity muscles; 3) fluid shifts and decreased plasma volume; and 4) cardiovascular deconditioning that leads to orthostatic intolerance. It is estimated that a mission to Mars may require up to 300 days in a microgravity environment; in the case of an aborted mission, the astronauts may have to remain in reduced gravity for up to three years. Although the Soviet Union has shown that exercise countermeasures appear to be adequate for exposures of up to one year in space, it is questionable whether astronauts could or should have to maintain such regimes for extremely prolonged missions. Therefore, the NASA Life Sciences Division has initiated a program designed to evaluate a number of methods for providing an artificial gravity environment.

Frequency response analysis of human saccadic eye movements: estimation of stochastic muscle forces.

A frequency response method is used to estimate parameters of a fourth-order model of the oculomotor system and the active state tensions during a saccadic eye movement. The lateral and medial rectus muscle of each eye is modeled as a parallel combination of an active state tension generator with a viscosity and elastic element, connected to a series elastic element. The eyeball is modeled as a sphere connected to a viscosity and elastic element. Each of these elements is assumed to be ideal and linear. The active state tension for each muscle is modeled by a low-pass filtered pulse-step waveform. Initial estimates of the oculomotor mechanical components are based on physiological evidence. Initial estimates of the active state tension are based on an extrapolation of the eye movement trajectory. Horizontal saccadic eye movements were recorded from infrared signals reflected from the anterior surface of the cornea and then digitized. Parameter estimates were calculated for the model by using a conjugate gradient search program which minimizes the integral of the absolute value of the squared error between the model and the data. The predictions of the model are shown to be in good agreement with the data. Final estimates of motoneuronal activity demonstrate that the agonist muscle is maximally stimulated during the early portion of a saccadic eye movement regardless of the amplitude of the saccade; only the duration of the maximal stimulation affects the size of the saccade. The antagonist muscle is completely inhibited during the period of maximum agonist muscle stimulation. Furthermore, it is demonstrated that saccade motoneuronal activity is a stochastic phenomenon.

Analog processing of vestibular nystagmus for on-line cross- correlation data analysis.

An analog processing circuit is described which allow accurate measurement of the phase relationships between input angular acceleration and resulting eye velocity. Vestibular nystagmic data are processed via analog technics to yield slowphase eye velocity. The turntable velocity input is cross-correlated with the eye velocity output, using a Nicolet MED-80 minicomputer system. The resulting correlograms are further processed to obtain precise phase information. Test data analysis shows a system resolution within 1 degree. Data from human and animal subjects are portrayed.

A modeling approach to the assessment of smooth pursuit eye movement.

We have proposed a quasilinear model of the oculomotor system in the performance of a smooth pursuit tracking task. Model parameters (gain, phase, and spectral purity) have been measured for 21 normal subjects at stimulus frequencies of 0.2, 0.4, 0.8, 1.2, and 1.6 Hz. The normal oculomotor system was found to be quite linear when tracking at the three lowest frequencies, with up to 95% of the output power being linearly correlated with the input stimulus. Data from patients with known pathology are presented to demonstrate model parameter sensitivity to oculomotor dysfunction.

Cases from the aerospace medicine residents' teaching file. Case #2. An aviator with acoustic neuroma.

From the Aerospace medicine residents' teaching file: on aviator with acoustic neuroma. The clinical presentation, evaluation and disposition of a patient with an acoustic neuroma are discussed.

Single sinusoids compared with a multiple-sinusoids technique for evaluating horizontal semicircular canal function.

The vestibulo-ocular reflex (VOR) of 162 normal subjects was evaluated using two rotational testing procedures. Data from 79 subjects were collected using a set of five single-frequency stimuli (0.01 to 0.16 Hz) and 83 subjects were tested using multifrequency stimuli (0.01 to 0.27 Hz). The use of multifrequency stimuli in place of single frequencies reduced testing time from 75 min to 20 min per subject. The diagnostically useful phase response of the VOR, as estimated by the two tests, were not statistically different. Equivalent phase measures may be obtained within a shorter testing period using the multifrequency procedure.

Neurosciences research in space: future directions.

Future research in the neurosciences can best be understood in the context of NASA's life sciences goals in the near term (1990-95), mid term (1995-2000), and long term (2000 and beyond). Since NASA is planning short-duration Spacelab and International Microgravity Laboratory (IML) flights for many years to come, the acute effects of exposure to microgravity will continue to be of experimental and operational interest in the near term. To this end, major new areas of research will be devoted to ground-based studies of preflight adaptation trainers and their efficacy in preventing or reducing the incidence of space motion sickness. In addition, an extensive series of studies of the vestibular system will be conducted inflight on the IML-1 mission The IML-2 mission will emphasize behavior and performance, biological rhythms, and further vestibular studies. In the mid-term period, Spacelab missions will employ new technology such as magnetic recording techniques in order to evaluate changes in the processing of sensory and motor inputs at the brainstem and cortical level during exposure to microgravity. Two Space Life Sciences (SLS) missions planned for the mid to late 1990's, SLS-4 and SLS-5, will utilize an onboard centrifuge facility that will enable investigators to study the effects of partial gravity on sensory and motor function. In the long term (2000 and beyond), Space Station Freedom and long-duration missions will provide opportunities to explore new options in the neurosciences, such as sensory substitution and augmentation, through the use of physical sensors to provide three-dimensional tactile-visual, tactile-auditory and tactile-somatosensory inputs. The use of this technology will be extremely important in the area of robotic telepresence. Finally, Space Station Freedom and proposed LifeSat missions will provide neuroscientists the opportunity to study the effects of partial gravity and microgravity on neuronal plasticity.