Before we can start thinking about how to build a brain, we must have a theory of what it's for. (NB: When I say brain, I really mean the entire nervous system)
What broad functions does the brain perform? I make the assumption, that all nervous system activity serves two broad goals: management of the body and generation of observable behaviour.
1. Zombie-like Management: There are many processes that need to be monitored, regulated or initiated in a stereotypical manner. These include critical life processes like breathing, heart-beats, peristalsis, controlling blood circulation, etc. There are also stereotyped responses to external inputs like reflexes, eye blinking, pupil control, smiles, etc. There are probably hundreds of such processes hidden amongst more complex behaviour. Many of these are probably evolutionary older systems.
I'm calling these zombie-like processes primarily because of their stereotyped nature. Either they are genetically programmed algorithms, or are very quickly learnt and then remain relatively stable. Very complex behaviour can emerge out of such stereotyped algorithms. But, however complex, this behaviour will not be flexible and is probably not under voluntary control.
2. Sleep and Wakefulness: The generation, maintenance, and regulation of arousal and sleep. While the mechanisms and functions for arousal and sleep themselves are not clear (for example, we aren't even sure what they mean at the neural level) we know in practice that both arousal and sleep are crucial for normal behaviour and health. The system to regulate this is probably another zombie system but it appears to be much more complex.
3. Perception: A well-publicised function of the brain is 'perception', which essentially refers to the process of building internal representations of aspects of the environment (both external and internal) based on information captured by the senses. There are two important unanswered questions about perception - a) what is/are it's end product/s? (internal representation is rather vague) b) Are other aspects of brain function like attention, emotion, etc distinct functions or mechanisms within perception?
4. Memory: Another well-publicised function of the brain is memory storage, maintenance and retreival. The brain is capable of storing information from past sensory experiences for both short and long terms. Further, it is able to selectively maintain this stored information and allow it's rapid retreival. Memory appears to be a very fundamental function and happens at very low-level systems. So, cells and synapses 'learn'. However, there is also 'explicit memory', which is an elaborate system of storing, maintaining and retrieving large streams of information in a flexible manner.
5. Executive: The really sexy 'executive' functions of the brain are the loosely described congitive functions of reasoning, intelligence, thought, problem solving, decision-making, mental imagery. Largely, this refers to the ability of the brain to think, take decisions, and make plans in a flexible and voluntary way.
7. I: A difficult to classify function of the brain is the generation of a self. Whether this should be classed within either 5 or 3 or keep it separate is not clear. From a subjective viewpoint, we have this dramatic sense of the ego. However, whether the production of the ego is critical to function or a consequence of the way perception and planning/decision-making is organised is not clear.
6. The Infantry: Finally, the brain performs the function controlling motor responses of the millions of muscles in the body.
NB: It may seem that I have 'left out' several important aspects like consciousness, qualia, language, sleep, etc. but in my opinion they are mechanisms that help perform the above functions. Language is likely to be a specialised mechanism within the 'executive' functions. So, language may really be a form of thinking. Similarly, consciousness may be a mechanism to aid perception, or decision-making. Qualia is still puzzling. One may think of qualia as symbols instantiating meaning, but why they have their particular form is an absolute puzzle.
