The soul is created directly by God and is united with the body at conception. Among other things, one can consider the soul to be the “life force” that makes us live - from that very first moment of our existence in our mother’s womb until it is separated from the body at our physical death. In any case, it is given to us at our conception, when none of us had a brain of any type at all. Therefore, questions regarding lack of a functioning brain are irrelevant. The child, however incapacitated mentally, is still a human being, endowed by God with an immortal soul.
Good, logical response. Thanks.