| Abstract No.: | A-D1137 |
| Country: | Canada |
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| Title: | MECHANISMS FOR VESTIBULAR COMPENSATION: EXTRA-VESTIBULAR SIGNALS ENHANCE PERFORMANCE DURING ACTIVE HEAD MOVEMENTS |
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| Authors/Affiliations: | 2 Soroush Sadeghi*; 1 Lloyd Minore; 2 Kathleen Cullen;
1 Johns Hopkins University, Baltimore, MD, USA; 2 McGill University, Montreal, QC, Canada
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| Content: | Objectives: Following unilateral labyrinthectomy, the gain of the vestibulo-ocular reflex (VOR) decreases acutely, but returns to normal values in ~1 week in response to rotations at low frequencies and velocities (e.g., Sadeghi et al. 2006). Furthermore, the VOR gain is even higher for active head rotations (e.g., Newlands et al. 2001). Previous studies have investigated the central changes at the level of contralesional vestibular nuclei (VN) (e.g., Smith and Curthoys 1989). However, there are certain shortcomings in these studies: most were conducted on anesthetised rodents, neurons were not characterized based on their physiological discharge properties or projections, and responses were studied only for passive whole body rotations. We investigated compensatory changes in the contralesional VN of alert macaque in response to passive and active rotations in the acute and chronic stages after unilateral labyrinthectomy.
Materials and Methods: We recorded from position-vestibular-pause (PVP) and vestibular-only (VO) neurons in the contralesional VN in one rhesus monkey before and 1-60 days after lesion. We used 4 sets of paradigms: 1) Vestibular sensitivity was tested using whole body rotation (WBR) (0.5 Hz). 2) Neck sensitivity was tested by rotating the body under a fixed head (BUH). 3) Integration of neck and vestibular signals was tested by passive head-on-body rotations (PHB). 4) Finally, we studied the responses of cells to active head-on-body rotations.
Results: We recoded 67 PVPs and 81 VOs. The resting rates of all neurons were in the normal range, even on day 1. Vestibular sensitivities: On day 1, neurons showed a decrease in their sensitivity. In particular, type I PVPs showed a 50% decrease, corresponding to a similar decrease in VOR gain. With the exception of type II VOs, responses improved over the next 60 days and reached ~100% of the normal values. Neck sensitivities: ~50% of neurons showed modulation in response to BUH paradigm on week 1. From week 2-8, the percentage of PVPs with neck sensitivity decreased by ~30%. Response to PHB: The average response to PHB rotations could be predicted by summation of neck and vestibular sensitivity vectors. Response to active head-on-body movements: During the first week, the estimated and predicted gains for responses of type I PVP neurons were similar. After week 1, the estimated gain was ~20% larger than the predicted gain. For type I VOs, similar to normal animals, the response was attenuated by ~80%.
Conclusion: 1) The acute decrease in the sensitivity of type I contralesional PVPs and its recovery later on corresponds to the ~60% acute decrease in the vestibulo-ocular reflex gain and its recovery in ~1 month. 2) Extra-vestibular signals play a role in the vestibular compensation: i) about 50% of neurons carry a neck-related signal acutely, which adds linearly to the vestibular signal during head-on-body movements; ii) during active head movements, signals from higher centers are reflected as an increase in the sensitivity of type I PVPs and higher VOR gains. |
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