An optrode arrangement for delivering optical stimulation to target tissue in a patient, the optrode arrangement comprising an implantable optrode comprising at least one electrically powered light emitter and an electrical circuit and control lines for controlling the light emitter, whereby an associated controller has two modes of operation: a stimulation mode in which it is configured to control the light emitter to deliver optical stimulation to the target tissue and a diagnostic mode in which it is configured to determine a condition of the light emitter and/or the optrode.
