What if the communication were stored on a digital device, embedded in a person's skull, which they were able to interface their brain with?

That may sound like an unrealistic scenario, but there are already people with medical implants that record digital data, and there has been a case of using Fitbit data to help convict someone:

https://www.nytimes.com/2018/10/03/us/fitbit-murder-arrest.h...

Interesting question yourself. I am not a lawyer, but: I think this would hinge on the definitions of "compelled" and "witness" in the fifth amendment. I think the courts would rule that being "a witness against himself" would require synthesizing thoughts into a statement then made under oath. I think searching thoughts would be considered a search.

It would be interesting to see how the courts would handle the fact that many thoughts are fleeting and not indicative of belief or intent.

It might also be allowed by the fourth amendment, provided the warrant was legitimate, depending on the interpretation of "in their persons."