The experiments proposed in this application will investigate the manner in which rotational head movement signals are encoded probabilistically by individual spikes in the vestibular system and the extent to which dynamical state estimation may play a role in the processing of head movement sense data. Broadband, random stimuli will be used to construct mathematical models describing the transformation of angular velocity stimuli into spike trains by the population of semicircular canal (SC) afferents and second- order vestibular neurons (2[unreadable]VNs) in the vestibular nuclear complex (VNC) of bullfrogs. From these intermediate models, responses consisting of hundreds of thousands of spikes will be simulated in order to determine the conditional probability distribution that describes the likelihood of the head's rotational kinematics being in a particular state when a particular neuron discharges. This distribution is termed the "spike measurement distribution" (SMD) for SC afferents to emphasize that it represents a measurement of the current head state, while it is called the "spike posterior distribution" (SPD) for 2[unreadable]VNs because the spiking in these neurons is hypothesized to represent the posterior distribution of head states. Borrowing from modern optimal control theory, it is thought that this SPD should represent an optimal dynamical state estimate that should be based upon the previous state, the predicted state transition (which would be embedded implicitly in the timing/connectivity/synaptic weights of the neural microcircuit), and the latest measurements arriving from afferents. Such processing should be detected in the form of a decrease in the entropy of the SPDs in comparison with the SMDs, reflecting an information gain. Public Health Relevance: An estimated 20% of Americans suffer from disorders of the vestibular system, which underlies our sense of balance and allows us to keep our eyes fixed on a visual target. The research in this application will improve our basic understanding of how the diverse set of inputs to the vestibular system are processed, which may aid in the design of more specific diagnostic tests and strategies for treating and rehabilitating people suffering from the terrible effects of such disorders.