Method and apparatus for controlling a neural stimulus

An implantable device for controllably applying a neural stimulus, the device comprising:

• a plurality of electrodes including one or more nominal stimulus electrodes and one or more nominal sense electrodes;
• a stimulus source for providing a stimulus to be delivered from the one or more stimulus electrodes to a neural pathway in order to give rise to an evoked action potential on the neural pathway;
• measurement circuitry for recording a neural compound action potential signal sensed at the one or more sense electrodes;
• and
• a control unit configured to:
• control application of a neural stimulus as defined by a set of parameter values;
• measure via the measurement circuitry a neural compound action potential response evoked by the stimulus;
• determine from the measured evoked response a feedback variable;
• compare the feedback variable to a therapy map, the therapy map defining a therapeutic relationship of control variable to feedback variable;
• alter one or more of the stimulus parameter values to effect the required change in the control variable, and
• iteratively perform the applying, measuring, comparing and altering, in order to improve alignment of the feedback variable with the therapy map over time.