Improved Feedback Control of Neuromodulation

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 at least one stimulus parameter;
• measure via the measurement circuitry a neural compound action potential response evoked by the stimulus;
• determine from the measured evoked response a feedback variable;
• complete a feedback loop by using the feedback variable to control the at least one stimulus parameter value;
• and
• adaptively compensate for changes in a gain of the feedback loop caused by electrode movement relative to the neural pathway, by applying a compensating transfer function to the feedback variable, the compensating transfer function being configured to compensate for both (i) a distance-dependent transfer function of stimulation and (ii) a distance dependent transfer function of measurement which is distinct from (i).

A non-transitory computer readable medium for controllably applying a neural stimulus, comprising the following instructions for execution by one or more processors:

• computer program code means for applying the neural stimulus to a neural pathway in order to give rise to an evoked action potential on the neural pathway, the stimulus being applied as defined by at least one stimulus parameter;
• computer program code means for measuring a neural compound action potential response evoked by the stimulus and deriving from the measured evoked response a feedback variable;
• computer program code means for completing a feedback loop by using the feedback variable to control the at least one stimulus parameter value;
• and
• computer program code means for adaptively compensating for changes in a gain of the feedback loop caused by electrode movement relative to the neural pathway, by applying a compensating transfer function to the feedback variable, the compensating transfer function being configured to compensate for both (i) a distance-dependent transfer function of stimulation and (ii) a distance dependent transfer function of measurement which is distinct from (i).