If we are to understand how the brain performs different integrated functions in cellular terms, we need both to understand all relevant levels of analysis from the molecular to the behavioural and cognitive levels and to realize an integration of such levels. This is currently a major challenge for neuroscience. Most research, whether dealing with perception, action or learning, focuses on a few levels of organization, for instance the molecular level and brain imaging, and leaves other crucial areas practically untouched. To reach the level of understanding that we desire, a multi-level approach is required in which the different levels link into each other. It is possible to bridge across the different levels for one system, and this has been demonstrated, for example, in the lamprey in generation of goal-directed locomotion. It can be argued that an integrated analysis of any neural system cannot be performed without the aid of a close interaction between experiments and modelling. The dynamic processing within any neural system is such that an intuitive interpretation is rarely sufficient.