CARDIOVASCULAR & NEUROVESTIBULAR
SYNERGY PROJECT
Context-Specific Adaptation of Gravity-Dependent Vestibular Reflex Responses
Principal Investigator: Thomas J. Mullen, Ph.D.
PROJECT OVERVIEW
This general objective of this study is to evaluate, in human subjects, the roles of visual and vestibular systems in modulating autonomic cardiovascular reflexes and to quantitatively compare the neck barocuff method with CSI (Cardiovascular System Identification) in estimating baroreflex sensitivity. Specifically, we will test the following hypotheses:
That vestibular stimulation
induced by true rotation in yaw modulates the HR baroreflex gain
as measured by CSI and the neck barocuff technique.
That visually induced illusions
of rotation in yaw produce modulation of the HR baroreflex as
measured by CSI and the neck barocuff technique.
That CSI estimates of HR Baroreflex
sensitivity correlate with those determined by the neck barocuff
technique.
That visually induced illusions
of a change in perceived orientation with respect to gravity
evoke a change in autonomic modulation of cardiovascular reflexes.
We will conduct a two-part study in sixteen healthy, normotensive, non-smoking human subjects (8 male, 8 female, ages 20-50 yr.).
Session 1: Yaw Rotation Protocol
In the first session, each subject will be seated in the NASA Off-Vertical Axis Rotator while wearing the Virtual Environment Generator (VEG) head mounted display. For neck barocuff measurements, a tightly sealing Silastic chamber will be strapped to each subject's anterior neck A blood pressure cuff controlled by a Dynamap system will be applied to one arm. For CSI, an electrocardiogram will be recorded using a standard limb lead (typically Lead II), respiration will be measured by two-belt inductance plethysmography and arterial pressure will be measured using a Finapres finger cuff system (applied to the opposite limb of the Dynamap System). Eye movements will be monitored by electro-oculography (EOG) from electrodes placed at the outer canthus of each eye and above and below the right eye. Cardiovascular and EOG data will be collected under each of the following conditions:
Subject stationary, visual
scene stationary.
Subject rotating sinusoidally
at 0.25 Hz (peak accel 125º/s2, peak vel. 60º/s)
Subject stationary - virtual
scene rotating sinusoidally as in (2) (or at reduced frequency
(0.1Hz) for development of stronger angular motion illusion)
Subject rotating and virtual
scene rotating as in (2) (visual-vestibular agreement)
Schematic of the Data Acquistion
Apparatus. The data acquired by the instrumentation
worn by the subject is processed through a computer. The output
System Identification is shown in more detail below. Click
on this picture, or any of the ones below, to view an enlarged
version.
Results of the System Identification performed by the computer. Click on this picture, or any of the nes below, to view an enlarged version.
The Virtual Reality Generator (VEG). The
head mounted display (HMD) shown on this picture is capable of
showing virtual scenarios thanks to two LCD screens placed in
front of each eye. The VEG system flew on the STS-90 Neurolab
Mission in April 1998. The Neurolab Experiment focoused on visual
cues and orientation when the "downward cue" of gravity
is missing. Click on this picture, or any of the ones below,
to view an enlarged version.
Session 2: Tilt Protocol
In the second session, subjects will be positioned supine on a tilt table while wearing the VEG head mounted display. Baroreflex sensitivity will once again be measured both by barocuff and by CSI. Subjects will be instrumented and data will be collected in the same manner as described in Session 1 with the exception that EOG data will not be recorded and an additional stroke volume measurement will be made during CSI data collection. Beat-to-beat stroke volume will be quantified using Ultrasound Doppler measurements of aortic flow made from a transducer focused from the suprasternal notch (Eriksen and Walløe, 1990). The additional Stoke Volume data will be incorporated into an extended CSI model to allow for quantification of resistance baroreflex responses. Data will be collected under each of the following conditions:
Subject supine- virtual environment
supine.
Subject supine- virtual environment
upright
Subject erect- virtual environment
upright
Subject erect - virtual environment
supine
A brief period will be allowed for hemodynamic equilibration between each change in real or simulated posture. In conditions 2 and 3, the MIT VEG will be used to visually present a virtual environment simulating an upright posture. Subjects will report their perceived orientation both verbally, and by setting a joystick controlled virtual "down" pointer to their perceived vertical. Repeat recordings will be obtained as time and instrumentation permits. Analyses of cardiovascular data will be completed as described for Session 1. In addition, the resistance baroreflex will be evaluated using an extended CSI model incorporating cardiac output estimates from stroke volume data.
The VEG Hardware Box.
The real "heart" of the EG system, this
hardare box is connected to the HMD which displays the virtual
environments. Click on this picture, or any of the ones below,
to view an enlarged version.
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