Symposium on the Role of the Vestibular Organs in Space Exploration (1970) / Chapter Skim
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PROGRESS IN VESTIBULAR MODELING
PROGRESS IN VESTIBULAR MODELING
Pages 353-380
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From page 353... ...
This paper presents the objectives, assumptions, analytic evaluations, and experimental data acquired during the investigations of two such topics which have been examined in some detail at the MIT Man-Vehicle Laboratory. The disparity between the experimentally evaluated time constants of objective and subjective responses to angular accelerations and the hydromechanical time constants of the semicircular canals is further accentuated by a rigorous analysis of the semicircular canals as a damped hydromechanical angular accelerometer.
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From page 354... ...
The influence of cupula dynamics on the sensory capabilities of the semicircular canals can be quantified by evaluation of the influence of an ampullary pressure differential on the position of the cupula. Since a pressure difference across the ampulla results in a torque on the cupula, this functional dependence can be evaluated by a torque balance equation on the cupula.
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From page 355... ...
The overall transfer function, relating cupula position to applied angular acceleration, for the human semicircular canals is determined to be PULA SIDE MEMBRANOUS AMPULLA VESTIBULAR NEURONS AB 0.06 Cm.
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From page 356... ...
In summary, the analyses of this section have shown that the effects of the viscous drag of the endolymph in the duct of the semicircular canals can be accurately represented by a first-order system with a time constant of about 0.05 second. They further show that the drag of the cupula on the wall of the membranous ampulla contributes some additional damping to the cupula-endolymph system.
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From page 357... ...
By comparison of the cupula pressure differential that is generated by a constant angular acceleration applied to a human semicircular canal, and the pressure differential across a restriction in a flexible toroidal duct with dimensions similar to the human semicircular canals, the relationship between "roller-pump" displacement of the cupula and constant angular acceleration of the cupula can be established. This relationship between the rate of angular rotation, the magnitude of the duct restriction, and the equivalent constant angular acceleration that would produce the same steady-state cupular displacement in a l-g acceleration field is for the human semicircular canals where a = 0.015 cm K = 0.3cm p = 1 gm/cm3 /*
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From page 358... ...
EXPERIMENTAL RESULTS To supplement the various experiments of Guedry, Benson, Bodin, Correia, Money, and others, and to investigate the variation of the bias and the amplitude of the sinusoidal component of vestibular nystagmus as a function of rotation rate, the MIT Instrumentation Laboratory precision centrifuge with a rotating platform at a 32-foot radius was fitted with the Man-Vehicle Control Laboratory rotating chair simulator, and six experimental subjects were rotated at 5, 7.5, 10,20,30, and 40 rpm in a 0.3-g horizontal acceleration field. Nystagmus was measured with eyes open in the dark by use of a Biosystems.
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From page 359... ...
In summary, these experiments, which provide a slightly different vestibular stimulation than the "barbecue-spit" experiments of Guedry and of Benson or the "revolution-without-rotation" experiments of Money, further verify the hypothesis that rotation at a constant velocity in a linear acceleration field does provoke vestibular nystagmus. The results of the analysis show that a duct area constriction of only 3 percent provides sufficient roller-pump action to generate the observed bias component of nystagmus.
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From page 360... ...
Guedry's papers, he commented that Hixson has observed, during head-over-heels rotation of the human at a constant angular velocity, that there is a bias component of nystagmus also. Any of these rotations where a canal is out of the plane of the applied linear acceleration should give rise to this roll-pumping action.
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From page 361... ...
Anliker: Yes; 1 fully agree with you. Graybiel: If you subject the horizontal semicircular canals to a very high angular acceleration or deceleration when the head is upright and the canals most responsive, and then you do not get a response but do in the Earthhorizontal axis, some way or other this does not add up.
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From page 362... ...
Anliker: I think maybe we should continue this afterward and not put the patience of the audience to a test. Uillingliiiin: Did you measure the phase angle between the rotating linear acceleration vector and nystagmus; and if so, was it related to the rpm?
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From page 363... ...
Adaptation is represented as a shift of reference level based on the recent history of cupula displacement. This model accounts for the differences in time constants among nystagmus and subjective cupulograms, secondary'nystagmus, and decreased sensitivity to prolonged acceleration.
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From page 364... ...
3. There is ample evidence that the dynamics of the subjective velocity response are fundamentally different from those of the nystagmus: A consistent difference appears in the time constants conventionally determined for the secondorder canal model, depending upon whether they are estimated from eye-movement recording or from measurements of subjective sensation of rotation (cupulograms)
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From page 365... ...
Our approach was to hypothesize two paths for the model output, one for the subjective response and one for the nystagmus slow phase velocity. Different adaptation time constants for each pathway were determined and placed in series with a second-order cupula transfer function resulting in the model shown in figure 2.
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From page 366... ...
Aschan and Bergstedt concluded that since the peak cupula deflection should be the same in all three cases on the basis of the model of Van Egmond et al., the strength of the secondary nystagmus depends particularly on "the length of the duration of the primary nystagmus induced." As shown in figure 4, the results of the Aschan and Bergstedt experiments are predicted by the adaptation model. (Note that equal peak cupula deflections are predicted only with the time constants of the original Van Egmond model.)
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From page 367... ...
-- Adaptation model for subjective response latency to constant angular acceleration. Meiry data from reference 8; Clark and Stewart data from reference 9.
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From page 368... ...
The adaptation model can predict the general form of the average response of a normal, alert individual to angular accelerations in a horizontal plane, provided linear accelerations are not present. Possible effects of bias are not included.
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From page 369... ...
This paper describes theoretical considerations and experimental results which strongly suggest that the secondary response is chiefly due to a quantitatively definable adaptive process which operates whenever a signal is generated in the semicircular canals. THEORETICAL CONSIDERATIONS The mechanical portion of the semicircular canals can be described as a second-order linear system, comparable to a torsion pendulum with heavy viscous damping (ref.
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From page 370... ...
With this system a step change in angular velocity input will lead to an initial response (cupular deflection) followed by an exponential return to zero response with a time constant Tc o 16 seconds estimated by cupulometric nystagmography (refs.
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From page 371... ...
This stimulus was large enough to produce a clear secondary response without generating maximum eye velocities so high as to be limited by eye dynamics. In practice, the ramp velocity was achieved by first taking the subject slowly to the appropriate angular velocity in one direction and leaving him in this steady state for 3 minutes to permit complete cupular restoration.
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From page 372... ...
The accuracy of eye angular velocity measurement was of the same order of magnitude as the size of the dots in these figures. To match the results obtained in this way with the mathematical model, the transfer function defined in equation (7)
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From page 373... ...
As previously mentioned in the theoretical considerations, a stepwise stimulus generates an initial response followed by an exponential decay. The response to a ramp stimulus rises with the same exponential time course as in figure IA to achieve an asymptotic level which is held steady until cessation of the acceleration (fig.
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From page 374... ...
. The upper response is from a sudden change in angular velocity, while the lower one is from a velocity ramp.
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From page 375... ...
This matter will be referred to again in the discussion. Figure 2 illustrates the plot of resulting slow phase eye angular velocity (ordinate)
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From page 376... ...
The values of the time constant of cupular restoration Tc and the adaptive time constant Ta calculated from the relevant potentiometer settings on the analog model of equation (7) are given in table 1 for all subjects and all experiments.
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From page 377... ...
The difference can readily be accounted for by the fact that the earlier values were essentially obtained from data points restricted to the primary response, largely on account of the fact that results were usually plotted on log-linear graph paper. Since from the torsion pendulum model the response to a step change in stimulus angular velocity should be an exponential decay, it has been customary to approximate the plotted response with a straight line, the slope of which then gives the required time constant.
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From page 378... ...
This would be seen only when the subject was rotated from a resting position, and would disappear once he was moved. Should this be the case, the threshold for perception of a change in angular acceleration for a subject who has been at constant angular acceleration should be greater than for a subject who has just previously been exposed to a change in acceleration.
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From page 379... ...
A Form of Apparent Motion Which May Be Observed Following Stimulation of the Semicircular Canals.
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From page 380... ...
S.; AND HOOD, J D.: The Speed of the Slow Component of Ocular Nystagmus Induced by Angular Acceleration of the Head; Its Experimental Determination and Application to the Physical Theory of the Cupular Mechanism.
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