In an interesting short paper just published in Trends in Cognitive Science, Caltech neuroscientist Ralph Adolphs offers his thoughts on The Unsolved Problems of Neuroscience.
Here's Adolphs' list of the top 23 questions (including 3 "meta" issues), which, he says, was inspired by Hilbert's famous set of 23 mathematical problems:
Problems that are solved, or soon will be: I. How do single neurons compute? II. What is the connectome of a small nervous system, like that of Caenorhabitis elegans (300 neurons)? III. How can we image a live brain of 100,000 neurons at cellular and millisecond resolution? IV. How does sensory transduction work? Problems that we should be able to solve in the next 50 years: V. How do circuits of neurons compute? VI. What is the complete connectome of the mouse brain (70,000,000 neurons)? VII. How can we image a live mouse brain at cellular and millisecond resolution? VIII. ...