The highest that lower limit can be is at the level where we can simulate each neuron in a human brain; at that point, we can build an AI merely by simulation.

The highest that lower limit can be is at the level where we can simulate each neuron in a human brain; at that point, we can build an AI merely by simulation. This assumes, of course, that we'll be able to observe every possible action of a neuron, and that these actions are all finitely computable.