Transforming Vestibular Information for Human Action

The vestibular organs in the inner ear provide the brain with important sensory information for many motor and perceptual functions. In the past, research into understanding these mechanisms has been hampered by the inability to selectively stimulate the vestibular system without affecting other sensory systems or affecting the behaviour of interest. In recent years we have pioneered vestibular stimulation techniques that allow us to do this non-invasively in human subjects. We plan to develop these techniques further and use them to investigate some specific mechanisms in the brain that are necessary for using vestibular information to control different types of motor behaviour. Such mechanisms are vital but not very much is known about them. We plan to investigate these mechanisms in human subjects and try to understand how they work, which parts of the brain are involved, and how they are affected by neurological disease to produce functional deficits. First, we will use our vestibular stimulation techniques to develop ways of probing these mechanisms associated with the different motor systems that control the eyes, balance, or voluntary movement. We shall then use this information to study how the mechanisms break down in specific neurological diseases. To do this we will study patients who have had a stroke affecting the parietal cortex, or who have a genetic disease that disrupts the cerebellum. We shall also use a brain stimulation technique to disrupt the normal function of the parietal cortex in healthy subjects. These investigations will tell us about the roles played by these specific parts of the brain in the mechanisms, whether these brain areas control vestibular input to all motor systems equally, and the resulting functional deficits caused by stroke and cerebellar disease.

The vestibular system supplies the brain with a unique and complete description of head motion and orientation in three dimensions. This information is important for many motor and perceptual functions. In recent years we have pioneered a technique for electrically stimulating the vestibular afferent system (galvanic vestibular stimulation; GVS) in isolation from other sensory systems and without interfering with natural free behaviour. This has provided us with a tool for investigating vestibular influences on action. GVS effectively evokes a virtual head motion because it is interpreted by the brain as having arisen from a real movement of the head in space, even though no such movement has occurred. We plan to develop GVS further and to introduce an additional mechanical method of vestibular stimulation (bone-tone stimulation; BTS) with the aim of being able to select from a range of possible virtual head motion vectors. We will then use these techniques to study a class of central processes that transform the vestibular input into coordinate frames appropriate for the behaviour being controlled. These are basic and powerful processes that have evolved to solve the problems introduced by the multi-jointed body, which allows almost any spatial relationship between sensory input and motor output. We will investigate in human subjects the properties and characteristics of these processes. We will also address the questions of which brain structures contribute to the processes and whether the neural substrate is the same for all motor functions and body parts. To achieve these aims we will develop experimental paradigms that allow us to probe the vestibular transformation processes that are used for the control of three distinct motor systems: the oculomotor, balance, and voluntary movement systems. Our hypothesis is that the posterior parietal cortex and the cerebellum form a network that is involved in these transformation processes. To test this hypothesis we will use a lesion-study approach to contrast the effects of specific lesions with control subjects on coordinate transformation performance for each of the three motor systems. We will study: 1) patients with posterior parietal cortex damage due to stroke; 2) patients with genetically-defined lesions of the cerebellum; 3) healthy subjects in whom normal processing of the posterior parietal cortex has been temporarily disrupted by transcranial magnetic stimulation (TMS) techniques that have been designed to produce a temporary virtual lesion.