Grids are key technologies to federate data distributed in multiple health institutions, thus enabling large scale multi-centric studies. However, the take up of these technologies is slow due to the difficulty to manipulate sensitive medical data in an open environment and the recognized risk of federated sites to loose control over their valuable data. In this paper we propose a distributed data access control policy, enabling the federation of existing data stores, where local security policies prevail. Our system supports multi-centric neuroscience studies. It achieves a com- promise between enabling collaborative work through data sharing and preventing unauthorized access to data in a competitive environment.