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. What causes psychiatric and neurological illness? IX. How do learning and memory work? X. Why do we sleep and dream? XI. How do we make decisions? XII. How does the brain represent abstract ideas? Problems that we should be able to solve, but who knows when: XIII. How does the mouse brain compute? XIV. What is the complete connectome of the human brain (80,000,000,000 neurons)? XV. How can we image a live human brain at cellular and millisecond resolution? XVI. How could we cure psychiatric and neurological diseases? XVII. How could we make everybody’s brain function best? Problems we may never solve: XVIII. How does the human brain compute? XIX. How can cognition be so flexible and generative? XX. How and why does conscious experience arise? Meta-questions: XXI. What counts as an explanation of how the brain works? (and which disciplines would be needed to provide it?) XXII. How could we build a brain? (how do evolution and development do it?) XXIII. What are the different ways of understanding the brain? (what is function, algorithm, implementation?)

While Adolphs' suggestions are sensible and thought-provoking, I have doubts about some of them, especially the "complete connectome" questions. The set of connections within the brain is surely unique to each individual, and moreover, it changes over time (plasticity). So there will not be one complete mouse or human connectome. Instead, we ought to seek out the processes by which connections are formed and changed.

Adolphs R (2015). The unsolved problems of neuroscience. Trends in cognitive sciences PMID: 25703689