The ethical monitoring of brain-machine interfaces (BMIs) is discussed in connection with the potential impact of BMIs on distinguishing traits of persons, changes of personal identity, and threats to personal autonomy. It is pointed out that philosophical analyses of personhood are conducive to isolating an initial thematic framework for this ethical monitoring problem, but a contextual refinement of this initial framework depends on applied ethics analyses of current BMI models and empirical case-studies. The personal autonomy-monitoring problem is approached by identifying various ways in which the inclusion of a robotic controller in the motor pathway of an output BMI may limit or jeopardize personal autonomy.