Eye movements are amongst the most precisely controlled and at the same time very elementary organized movement forms in man and animal. They are of primary importance for vision. A large part of everyday brain activity is concerned in some way or another with eye movements. Moreover, in man and in animals with frontally placed eyes, the brain very precisely coordinates the movement of the two eyes, a precondition for the intriguing capacity of spatial vision and stereoperception.
One of the big challenges of the brain in controlling eye movements for the sake of vision is to stabilize the gaze lines during passive and active movements of the head. This task is by no means trivial because even small disturbances of head position, for example by the all time present heart beats, can make reading of a newspaper impossible, as observable after acute loss of the sensory inputs from the balance organs in the inner ear.
One oft the main objectives of our project is to investigate how the sensory signals from the balance organs in the inner ear interact with the mechanisms underlying a stable and robust perception of the visual world (that is independent of active or passive self-motion) and with associated gaze orienting movements. Along these lines a series of experiments focus on the stabilization mechanisms that are at work during the execution of simple behavioral tasks while head and body orientation relative to earth vertical passively change. While performing a particular, visually demanding task, for example fixation of a stationary or moving object relative to a complex visual background, the subjects are sitting inside an artificial visual environment that can be tilted and moved in various ways relative to earth vertical. By means of such experimental manipulations we aim at dissociating orienting responses, which normally include movements of the eyes, head and torso or body, from associated mechanisms that underlie the generation of a stable and robust perception of the visual surround. The obtained information about the interaction and central processing of multisensory motion cues, including those from the balance organs in the inner ear, in a given experimental configuration allows us to formulate testable hypotheses that can be followed up by invasive physiological techniques (single unit recordings, reversible focal inactivation of brain areas etc) exploring the function of brain areas that are known to be involved in balance control and stabilizing visual perception.
A detailed knowledge of the neural mechanisms involved in the generation of orienting responses is of great clinically interest. Our experiments in particular aim at showing how gravity-dependent transformations of gaze-control signals are implemented at various levels in the brain. For this, we try to better characterize the pathways and mechanisms in the brainstem/cerebellum that are involved in orienting behavior. The results will lead to a better understanding of the pathology of ocular control mechanisms and their interactions with vision. It is, for example, still not well understood how patients with lower brain-stem infarctions may show deviations in the perception of earth vertical or even experience attacks of up-side-down inversions of vision (room tilt illusions) although the traditional visual pathways are not affected.
Animal models in general will continue to be an indispensable tool for studying brain functions at both the behavioral, cellular network and single nerve cell level. Such models provide important guidelines in the search for a better understanding and development of new diagnostic tools and treatments of human orientation disorders (see Disorders of the Vestibular System, edited by Robert Baloh & Michael Halmagyi, Oxford University Press, 1996; Th. Brandt: Vertigo - Its multisensory Syndroms, Springer, 1999). To answer the questions addressed in our project it is advantageous to work with non-human primates because of the many similarities in the behavioral as well as neural organization of eye/head movements and the visual system.