Those are some excellent questions that are hot topics for scientists. Neuroscientists have a good grasp of the basic function of different neurons (nerve cells), like how they fire and how they communicate with each other with chemicals (neurotransmitters). But here’s a lot that no one really knows about how neurons and their supporting cells work together to do complex things like imagining, feeling, and planning.

In terms of development, brains change throughout our lives. Stem cells become a variety of types of neurons and non-neuron brain cells. New connections between cells are built, strengthened, or become more complex. Others weaken or are lost. These connections determine our ability to think, remember, feel, and imagine. We used to think that the neurons did all of the work, while other brain cells just helped to keep the neurons alive, but newer evidence shows that these glial cells are important in higher brain functions. Some may even become neurons. So it would take a lot more than just getting a bunch of stem cells into a particular shape to create a functioning brain. It is still a very interesting idea because it could be one way to understand natural brains.

Let’s forget about the complexity of the human brain and just think about a relatively simple brain, like the brains of a well-researched roundworm, C. elegans. These worms only have 302 neurons in their bodies and all are mapped, meaning that researchers know where each neuron is going to go as the worm develops. Brain function relies on connections between neurons, so that information would be critical. If someone could make an artificial C. elegans brain, they could try different variations to see what is critical for a functioning brain.

If you could insert stem cells into a developed brain, how do you imagine that would influence brain function? What would be some of the drawbacks and advantages?

You might want to become a neurobiologist yourself and try to answer these questions. I can pretty much guarantee that no one is going to answer them before you get to graduate school.

This is really as much of a philosophical question as a scientific question. In this case, why we think the way we do is an extremely broad question. It’s also important to realize that there are thousands of years of philosophy and religion that approach this question from the idea of the soul. A more scientific answer is that we think the way we do because it helps us survive and reproduce. Thought allows for enhanced problem solving which increases survival and allows the species to continue. A byproduct of this survival pressure is abstract thought which has led to many behaviors seemingly irrelevant to survival: art, music, etc. What it is in our brain that allows us to imagine or have emotions is the complexity of the brain.

The average human brains has 86 billion brain cells (called neurons) and each of these cells can communicate with hundreds or thousands of other neurons. The net result of this is that we have about 100 trillion neuron-neuron connections called synapses. These neurons communicate electrically with each other and whether one neuron is connected to another neuron is a piece of information. Multiplying that information by 100 trillion times gives a lot of ways to store information. This is what allows for imagination and emotion, the fact that we have an enormous number of different combinations of neuron synapses to store information. In some ways it’s analogous to a computer. It’s incredible the things that a computer can do as far as storing and creating new information. But computers operate with a much simpler method than the thousands of synapses per neuron in the human brain. So the fact that we can store so much information in our brain and connect that information together to form thoughts allows for abstract stuff like emotions and imagination.

If you could 3D print a brain from stem cells, which is impossible with our current technology and may be for a very, very long time, in theory it could think in the same way as the human brain. Though it’s important to realize that the relation between the human brain and the body is important as well, which a 3D brain would lack.

If we knew the answer to this, then we could program computers that have human emotions and intelligence. We don't, which is why we can't do this.