If ontological idealism is correct, a brain is an idea among the others. Then why if we modify (the idea of) a brain, the whole mental world changes accordingly? Why is there a mental representation (the brain) that, if modified or eliminated, brings with it all the others? How can an idealist explain this asymmetry?
"The physical is simply the verifiable contents of perception, a particular category of experience. As such, what we call ‘physical interference with the brain’ is simply the extrinsic appearance of experiential activity external to an alter that, in turn, disrupts the inner experiences of the alter from across its dissociative boundary." Bernardo Kastrup, The Idea of the World (pp. 85-86).