Where is the problem?
The chief purpose of these pages is to pin down where different people believe the unresolved problems lie. An argument can be made that the difficult problems lie outside of the circuit from sensory information to motor output (or equivalent) that usually happens several times a second. In that timeframe the brain does something rather simple. In the case of 3D vision, the difficult problem is to understand how images change when an observer moves through a static scene and how those changes might be represented, including the problem of relating unseen remembered objects to the current optic array. This is an example of a problem that is 'outside the brain', in the sense that a computer vision implementation would be very helpful in understanding the solution to it. The interesting aspects of the solution are likely to be at this algorithmic level, not the complexities of the instantiation in the brain.
There are some tricky problems. It is worth keeping a list as we go: Some tricky problems