Gravity is an important context for adapting the angular vestibulo-ocular reflex (aVOR) by visual-vestibular mismatch. Maximal gain changes occur in the position of adaptation and decrease, as the head is oriented away from this position. The spectral components of the gravity-dependent adaptation are also tuned to the frequency of adaptation. A study is proposed to determine the cellular basis for the gravity and frequency dependent aspects of this adaptation in the vestibular nuclei of the alert monkey. The frequency characteristics of gravity dependent adaptation will first be studied to determine how adaptation at one frequency affects adaptation over a wide range of frequencies and head orientations. This will lay the groundwork for studying adaptive changes in canal-ocular transduction and reorientation of otolith polarization vectors in central vestibular neurons. Such studies will include: 1) Characterizing the cellular changes associated with gravity-dependent adaptation in the otolith polarization vectors of otolith-canal convergent neurons in the vestibular nuclei. 2) Investigating how adaptive changes in the otolith reorientation and canal-ocular transduction of canal-otolith convergent neurons at a particular frequency are related to the spatio-temporal convergence (STC) characteristics of the unit. Preliminary investigations show that when gravity-dependent adaptation is induced at a specific frequency, the canal-otolith convergent neurons developed STC characteristics that also tuned to the frequency of adaptation. 3) Determine how FTN and PVP neurons code the gravity-dependent adaptation. 4) Tie these findings to a neuronal net model, which will form the underlying basis for the study. It is postulated that gravity- dependent adaptation is a spatio-temporal process, which is encoded in the STC characteristics of canal- otolith convergent units. It is further proposed that vestibular-only (VO) neurons in the vestibular nuclei transmit this information through the flocculus to floccular target neurons (FTN) and directly to position- vestibular-pause (PVP) cells that code gravity dependent adaptation.The proposed research will determine the neural mechanism for the dependence of adaptation of the vestibulo-ocular reflex on gravity. This could provide a new approach to explaining difficulties in balance and spatial orientation commonly encountered after lesions of the vestibular system.
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