The brain is very plastic, it probably would adapt at any consistent input. A most interesting question is, what would you sacrifice?

Because the part that you would use to control your extra tentacle or whatever is probably currently used for something. If you are missing an arm is obvious what part of your brain you would use, otherwise, some sacrifice probably have to happen.

Even reading, it seems, take the space that is used in people that can't read for some detailed visual processing (1).

Maybe you could add some extra processing in the extra arm, so it's more "cheap" to use.

(1) - I learnt that in "Reading in the brain" by Stanislas Dehaene.

I don't think we necessarily have to sacrifice something. We learn new skills and knowledge all the time. Using the new limb could be a skill. Maybe it's not as dexterous as the motor cortex and real hands at first, but there might be some benefits over natural hands.

I think that you sacrifice something when you learn a new skill. The brain have a finite capacity and it just forget not used facts or skills, and the reason it's forgetting is because it's always learning.

For instance, I'm learning German and I can feel how is messing with my English, specially with the spelling (English is not my first language).

I think memory retention has more to do with simplicity / emotional consistency.

The new skill you learn may reinforce existing areas if its components are transferrable. Also, I see schemas as compression in the brain; eidetic memory shows how much some brains are capable of.

Forgetting is very good for clarification. I think your example suggests that your spelling in English hasn't been established deeply enough; I understand why you might have an adverse reaction to learning it though (German spelling is far more reasonable).

But if it has a finite capacity, when does it fill completely and with what? I guess it's entirely possible that only kids who are growing up can learn to use the extra limb and adults won't be able to.

I don't think it never fills completely, it just empties the older entries to make place for the new ones.

Also part of why voice interfaces didn't take off for complex tasks, talking about performing the task interferes too much with the mental process.

The real innovation is when you can find a method to control the physical hand and the virtual hand independently.

I suspect it would be possible if you can do some kind of real-time feedback/tuning of the ML model. Reinforcement learning stuff.

By slowly adjusting the model to reward hitting the ball back in pong, but also to penalize moving the real physical hand, you might achieve it.

Can't the brain alone use visual feedback to close the loop, separating both hands? If thinking in a particular way activates the only the robot hand (trembling), I'd guess that with sufficient plasticity you should be able to start enhancing control of it.

Obviously tactile and proprioceptive feedback through the chip would enable better / quicker control, given the sufficient plasticity hypothesis.

Absolutely. The first test subjects of this technology were chimps who weren't paraplegic, just controlling extra arms.

I bet it is possible but that does not mean that it is going to be easy. Try training yourself to wiggle an ear or to lift a single eyebrow if you cannot do that yet. It's possible but it takes quite some time.

Many monkeys use their tail as an extra arm, so, should not be a problem.

I thought the ctrl-labs guys did that.

this is why we can't have nice things... :)