The specific aims were to assess pre-, in- and post flight changes in otolith sensitivity, asymmetry, and function; and to compare cervical VEMP, ocular VEMP methods using both vibration and auditory stimuli to assess utricular and saccular function in different postures.
Custom LabView software was developed to implement the head tapper and bone conduction protocols. The head tapper software was adapted from an existing system that was implemented for Dr. Seidler’s Neuromapping study. The bone conduction protocol was adapted from Welgampola and colleagues using a Brüel and Kjær mini-shaker with a custom fiberglass rod interface. Since tapper and bone conduction techniques rely on the operator manually applying a probe and one objective of the study was to incorporate threshold stimuli, the investigators also incorporated a bite-board attached sensor to directly measure skull head acceleration at the various stimulus levels.
Fifteen subjects participated in this experiment. The cVEMP response was recorded by electrodes placed over tensed sternocleidomastoid muscles. The oVEMP response was recorded by electrodes placed just beneath the eyes over the inferior oblique and inferior rectus extraocular muscles. Bilateral cVEMP and oVEMP responses were elicited by a handheld head tapper and a mini-shaker, both set for delivering head acceleration at four different levels. Head acceleration and the resulting VEMP amplitude were measured for each level. In addition, measurements of the subjective visual vertical (SVV) and verbal reports of tilt perception were obtained during unilateral centrifugation involving 10 degrees equivalent roll tilt.
RESULTS: The head tapper was effective at eliciting both oVEMP and cVEMP in 67-90% of the trials with head accelerations ranging from 0.17 to 0.24 g. However, there was a large variability in the head accelerations recorded for each stimulus level. By contrast, the mini-shaker was effective at inducing discrete head accelerations, but at the levels recorded in this study (<0.12 g) produced oVEMP and cVEMP in only 10-30% of the trials. Measurements of subjective vertical during unilateral centrifugation showed normal range of responses; however, the inconsistency of eliciting VEMP responses limited comparisons to the SVV.
While the cVEMPs and oVEMP responses were more robust with the head tapper than the mini-shaker, it is not clear how much this difference was attributable to the stimulus profile (step impulse versus tone burst), magnitude of the head accelerations imparted, and/or other factors related to the hand-held application of the stimuli (e.g., direction of the resultant force relative to the otolith organs). Additional development would be required to validate either method to effectively measure changes in otolith gain during spaceflight, especially at threshold levels. While not used in this ground-based study, the auditory tone method offers advantages of a more consistent application of stimulus levels over a wide range (threshold to maximum response), as well as increased feasibility of self-administered testing to save crew time. There are well-known guidelines for controlling VEMP responses, namely neck muscle contraction for the cVEMP and gaze direction for the oVEMP. For studies using hand-held administered application of bone-conducted stimuli, the results of this ground study suggest that measurements of head acceleration should also be considered to control for response variability.
No datasets exist for this study. A final report was archived.
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