Neural Integration of Eye and Head Movements

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DESCRIPTION (provided by applicant): The long-term objective of the research program is to understand the neural integration of multiple motor systems. The oculomotor system has long served as a model for the study of neural control of movement. In the natural environment, however, an orienting response consists of multiple oculomotor and skeletomotor actions combined to form a complex yet coordinated movement. An integrated action ideal for scientific investigation is a coordinated eye-head movement because it builds on our advanced knowledge of the oculomotor system. The research also offers diagnostic value for spatial orientation deficits resulting from oculomotor, vestibular and cervical disorders. Recent experiments suggest that familiar oculomotor structures, such as the superior colliculus (SC) and pontomedullary reticular formation (PMRF), output a motor command to displace the line of sight (gaze shift) by a desired amplitude and direction. The gaze shift can be executed as a coordinated eye-head movement, implying that the outputs of these oculomotor structures also control premotor circuits that innervate the neck muscles. An association between neural discharge and extraocular motor neurons is demonstrated by recording activity during head-restrained saccades. In contrast, a relationship between spikes and neck motor neurons is only inferred by observing activity during coordinated eye-head movements. A direct evaluation of the activity with head movements has not been performed yet because, under ordinary circumstances, the eye and head components of the coordinated movement are temporally correlated. Hence, four specific aims are proposed to characterize activity and identify neurons associated with head movement control. Using behavioral tasks developed to temporally uncouple head movements from saccadic eye movements, SC neurons will be recorded for head movement related activity, both when no gaze shift is required (Specific Aim 1) and when a gaze shift is planned or being executed (Specific Aim 2). Similar manipulations will be employed to investigate the distribution of PMRF neurons that encode eye-only, head-only and coordinated eye-head movements (Specific Aim 3). Finally, anatomical techniques will be used to determine whether SC neurons send divergent axon collaterals or segregated projections to the eye and head control regions of PMRF (Specific Aim 4).