The avian macula lagena is a markedly curved otolithic organ located at the distal tip of the cochlear duct. It is physically distinct from the auditory organ, the basilar papilla. Anatomical descriptions of the sensory organ and its neural projections to the brainstem suggest that the macula lagena may be a gravity receptor similar to the other otolithic organs of the vestibular system. It has been shown that the avian macula lagena does not likely contribute to auditory function, and lagena afferents predominately exhibit a regular spontaneous activity pattern. However, the natural or adequate stimulus for lagenar afferents has not been determined. The purpose of the present research is to test the hypothesis that the avian macula lagena is a gravity receptor. The following questions will be addressed: 1) Are primary afferents of the avian macula lagena responsive to gravity and/or vibration? If so, what are the effects of changing the direction of the gravity vector and/or the amplitude of vibration?; and 2) To what position within the avian macula lagena do characterized neurons project: To do this we will record single unit activity from primary afferents of the lagenar macula in one- to two-week old chickens (Gallus domesticus) and characterize the firing patterns of these neurons to static tilts and vibration. The characterized neurons will then be injected with biocytin to determine the neural projections to the lagena. These data will be compared and contrasted with data available for otolithic receptors of lower and higher vertebrate species. New insights will be gained concerning the avian macula lagena specifically. This in turn will provide the basis for a better understanding of how different gravity receptors might work. A more complete understanding of the functional types of neurons present in the lagena will contribute significantly to our understanding of scalp recorded short latency vestibular evoked potentials. These data will also be used in formulating questions and strategies for future research which will focus on detailed analyses of lagenar physiological responses to gravitational and vibratory stimuli. Future research efforts will be directed towards identifying and comparing physiological roles of the maculae lagena, utriculi, and sacculi, as input sensors to the avian motor control system. In addition, we will seek to understand embryonic development of these organs and factors that influence their ontogeny.