EFFECTS OF ACTIVITY ON POSTURAL STABILITY IN ELDERLY

University Of Rochester, Rochester NY

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Abstract

An extensive study of the translational LVOR frequency response during interaural (horizontal) and dorsoventral (vertical) oscillation has been completed in squirrel monkeys. We have found that the now well-known influence of vergence on LVOR sensitivity is remarkably limited to frequencies above around 0.5 Hz. Further, phase angle displays an increasingly large lead as frequency declines from 4 to 0.5 Hz. These findings suggest that the translational LVOR behaves with high-pass filtering characteristics, as does the AVOR, but with a cut-off at a much higher frequency (0.5 in the LVOR as opposed to 0.01 in the AVOR). The LVOR was also shown to behave linearly with stimulus amplitude over the range, 0.1-0.7 g peak acceleration. This remarkable performance suggests that the LVOR operates in a bandwidth that is almost exclusively above the capabilities of vision. We have also completed a sub-project on eccentric rotation, which activates both the AVOR and LVOR simultaneously. When the subject is displaced eccentrically from the rotation axis (e.g., 40 cm), and with the nose facing out, the effect of vergence on response gain at 4 Hz increases dramatically. This reflects the addition of the LVOR to the AVOR. The LVOR is driven by interaural (with the head upright) or dorsoventral (with the head on its side) tangential acceleration induced by eccentric rotation. This component adds to the horizontal or vertical AVOR, respectively, stimulated by the rotatory motion (head yaw if upright or pitch if on side). When the subject is turned around to face in toward the rotation axis, the LVOR component of the response is reversed and subtracts from the AVOR. We then observe a decline in gain with decreasing viewing distance. At distances closer than the rotation axis, the overall ocular response reverses direction; this is so because the LVOR component overwhelms the smaller AVOR component under these viewing conditions. The frequency-dependent behavior of the VOR during eccentric rotation has been addressed in depth, and for both horizontal and vertical responses. In concert with our new understanding of the LVOR, we observe the same frequency limitations of its influence on combined AVOR-LVOR responses. That is, the LVOR component of respouses to eccentric rotation is limited to frequencies above around 0.5 Hz. Further, the AVOR and LVOR have been shown to interact linearly during complex motion, as vector additions.